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

Abstract

The application discloses a modified graphite felt electrode for a vanadium redox flow battery and a preparation method thereof, and belongs to the technical field of electrode materials of a flow battery. The preparation method comprises the following steps: electrochemical oxidation etching treatment of graphite felt; self-assembly of a conductive layer on the graphite felt after the oxidation etching, so that graphite felt with the conductive layer is obtained; and loading of a metal@nitrogen-doped carbon (M@NC) nano catalytic layer on the graphite felt with the conductive layer, so that the modified graphite felt electrode is obtained. The application realizes comprehensive improvement of various performances of the graphite felt through substrate treatment, structure change and loading of a catalyst, adopts mild electrochemical oxidation etching to protect the structure of carbon fibers, forms stable conductive channels through the self-assembly of a net-shaped conductive layer, significantly increases the specific surface area, adopts a nitrogen-doped carbon shell to wrap metal nanodots to form the M@NC catalytic layer, the catalytic layer is not easy to be dissolved out and the capacity attenuation rate is obviously improved, the loading amount of the catalytic layer is larger on the net-shaped structure conductive layer with a larger specific surface area, and meanwhile, the process is simple and low in cost.

Description

Technical Field

[0001] This invention belongs to the field of flow battery electrode material technology, specifically relating to a modified graphite felt electrode for all-vanadium redox flow batteries and its preparation method. Background Technology

[0002] Vanadium redox flow batteries possess intrinsic safety, long lifespan, large capacity, 4-10 hours of storage, and power-energy decoupling, making them considered the most suitable technology for large-scale, long-term energy storage. 5+ / V 4+ and V 2+ / V 3+ Redox couples achieve charging and discharging by gaining and losing electrons on the electrode surface, with a proton exchange membrane separating the positive and negative electrolytes. Electrodes are among the most critical materials affecting electrode reaction activity and rate, battery cycle efficiency, and lifespan. Currently, widely researched and applied graphite felt electrodes possess advantages such as a high-porosity three-dimensional structure, high conductivity, excellent mechanical properties, and good chemical stability. However, unmodified graphite felt electrodes exhibit low electrocatalytic activity, poor fiber surface wettability, small electrochemical specific surface area, and poor long-cycle stability, resulting in slow electrode reaction rates, high reaction polarization, low battery energy storage efficiency, and low electrolyte utilization, making it difficult to achieve excellent energy storage performance and economic benefits.

[0003] Current methods for modifying the surface of graphite felt mainly include surface oxidation (acid / heat treatment), doping, electrocatalyst loading, and fiber surface composite. Traditional surface oxidation can improve the hydrophilicity of the electrode surface fibers and increase the electrochemical specific surface area, but it can also cause excessive etching of the fiber surface, leading to increased resistance, damage to the fiber structure, and a significant reduction in the mechanical stability of the electrode material. Loaded electrocatalysts often use noble metals such as Pt, Ir, W, Ce, and Ga, as well as Pb-polluting metals, which cannot meet cost and environmental protection requirements. Furthermore, the loaded metal elements are not effectively protected and are easily dissolved in acidic environments, leading to electrode deactivation (efficiency decrease of >8% after 100 cycles). Patent CN119230857A discloses a nano-carbon layer modified graphite felt electrode for vanadium redox flow batteries, its preparation method, and its application, using heat treatment and magnetron sputtering to modify the surface of the graphite felt electrode. However, single surface modification methods only improve the electrode's characteristics and functions, failing to achieve multifunctionality for graphite felt electrodes. Patent CN113809338A discloses a method for preparing electrode materials for vanadium redox flow batteries, which involves modification using dopamine doping. While heteroelement doping and fiber-surface composite carbon materials initially significantly improve electrode surface activity and achieve extremely high redox efficiency, the single composite method without synergistic multi-processing can easily lead to functional group or composite deintercalation and deactivation after long-term cycling. To improve the overall performance of vanadium redox flow batteries, new graphite felt electrode modification strategies still need to be researched. Summary of the Invention

[0004] To address the shortcomings of existing technologies in graphite felt modification, this invention proposes a modified graphite felt electrode for vanadium redox flow batteries and its preparation method. Through surface oxidation, mild etching, electrostatic crosslinking, network bonding, conductivity compensation, encapsulation protection, and catalytic loading, the surface properties of graphite felt fibers are synergistically improved, simultaneously enhancing the electrode's hydrophilicity, conductivity, catalytic activity, and long-term stability. This achieves multifunctionality of the graphite felt electrode and comprehensively improves battery energy efficiency and cycle life.

[0005] The technical solution provided by this invention is as follows:

[0006] In a first aspect, the present invention provides a method for preparing a modified graphite felt electrode for a vanadium redox flow battery, comprising the following steps:

[0007] S1. Electrochemical oxidation etching treatment of graphite felt: The graphite felt is placed in an electrolyte under constant potential or constant current conditions for electrochemical oxidation etching, ultrasonic cleaning, and vacuum drying.

[0008] S2. A conductive layer is self-assembled on the oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer.

[0009] S3. A metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is loaded onto a graphite felt with an attached conductive layer to obtain a modified graphite felt electrode.

[0010] Furthermore, the constant potential condition is 0.9-1.2 V; the constant current condition is a current density of 2-5 mA / cm². 2 Under the conditions.

[0011] Furthermore, the electrolyte comprises an electrolytic matrix of 0.4-0.6 mol / L, an H2O2 oxidant of 0.05-0.2 mol / L, an organic acid etchant of 0.05-0.1 mol / L, and deionized water.

[0012] The electrolytic matrix is ​​a sulfate, including Na2SO4, K2SO4, and CaSO4.

[0013] The organic acid etchant is one of malic acid, citric acid, tartaric acid, and acetic acid.

[0014] Furthermore, the electrochemical oxidation etching time is 15-30 min.

[0015] Furthermore, the ultrasonic cleaning uses ethanol and water in a volume ratio of 1:(1-2) as the cleaning agent, and cleans for 10-15 minutes under ultrasonic conditions with a power of 100-200 W and a frequency of 30-60 kHz.

[0016] Furthermore, the vacuum drying temperature is 50-80℃.

[0017] The modification mechanism of step S1 is that H2O2 generates active oxygen free radicals under an electric field, which selectively oxidize the fiber edges. The generated -COOH / -OH oxygen-containing groups attach to the fiber surface to increase its hydrophilicity. The organic acid etchant chelates and etches the amorphous carbon on the fiber surface, thereby exposing more graphite crystal edges and significantly increasing the number of active sites.

[0018] Further, step S2 involves self-assembling a conductive layer on the oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer, including: placing the oxidized and etched graphite felt in an impregnation solution, oscillating at 60-70 ℃ for 4-6 h, and heat-curing at 120-150 ℃ for 2-3 h.

[0019] Furthermore, the impregnation solution comprises 2-6 wt% of a P-source crosslinking agent, 2-3 g / L of an N-source conductivity enhancer, 0.8-1.2 mg / mL of an O-source support agent, 0.05-0.15 mol / L of an electrostatic crosslinking accelerator, and deionized water.

[0020] Furthermore, the P-source crosslinking agent is a nucleotide or an inositol phosphate derivative; the nucleotide includes adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP); the inositol phosphate derivative includes inositol hexaphosphate (IP6, i.e., phytic acid), inositol pentaphosphate (IP5), and inositol triphosphate (IP3).

[0021] Furthermore, the N-source conductive enhancer is polyethyleneamine (PVAm), polyethyleneimine (PEI), or polyallylamine (PAA).

[0022] Furthermore, the O source support is reduced graphene oxide (RGO).

[0023] Furthermore, the electrostatic crosslinking promoter is an ammonium salt, including ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), ammonium carbonate ((NH4)2CO3), and ammonium bicarbonate (NH4HCO3).

[0024] Step S2 involves self-assembling a composite network conductive layer on the oxidized and etched graphite felt to increase the specific surface area, dope with impurity elements, and improve conductivity. The modification mechanism is as follows: P-source crosslinking agent and N-source conductive reinforcing agent provide doping elements to further regulate the electronic structure, promote the delocalization migration of π electrons in the graphite layer, reduce the fiber interface resistance, form conductive channels, and allow for faster charge transfer; the electrostatic crosslinking promoter ammonium salt can protonate the amine groups (-NH2, -NH3) in the N-source conductive reinforcing agent. +The positive charge is enhanced, and a stable composite system is formed with the P-source crosslinking agent through "electrostatic attraction-hydrogen bond assistance", which significantly improves the electrostatic attraction between the two and forms a stable crosslinked structure. The O-source support agent reduces graphene oxide, which can reduce or bridge the gaps between graphite felt fibers to form a proton-electron dual-channel network structure, significantly increasing the specific surface area of ​​the graphite felt electrode and reducing the surface resistance.

[0025] Further, step S3 involves loading a metal@nitrogen-doped carbon (M@NC) nanocatalytic layer onto a graphite felt with an attached conductive layer to obtain a modified graphite felt electrode. This includes: placing the graphite felt with the attached conductive layer in a growth solution, adjusting the pH to 9.0-9.6, and reacting fully at 50-80 °C for 3-5 h; slowly rinsing with a 1-2% nitric acid ethanol solution to remove uncoordinated metal ions from the growth solution; and annealing at 280-320 °C for 2-3 h in an argon atmosphere to form a stable loaded metal@nitrogen-doped carbon (M@NC) nanocatalytic layer, thus obtaining the modified graphite felt electrode.

[0026] The 1-2% nitric acid ethanol solution refers to nitric acid accounting for 1-2% of the volume of ethanol, with a nitric acid mass fraction of 68%.

[0027] Furthermore, the growth solution comprises 0.03-0.08 mol / L of a metal source, 0.1-0.4 mol / L of a metal-organic framework (MOF) ligand, 0.05-0.2 mol / L of a pH adjuster, and deionized water.

[0028] Furthermore, the metal source includes a nickel source, a bismuth source, and a cobalt source; the nickel source is nickel nitrate Ni(NO3)2, nickel chloride NiCl2, nickel sulfate NiSO4, nickel acetate (CH3COO)2Ni, or nickel acetylacetonate C. 10 H 14 One of NiO4; the bismuth source is one of bismuth nitrate Bi(NO3)3, bismuth oxychloride BiOCl, bismuth subcarbonate Bi2(CO3)3, and bismuth trioxide Bi2O3; the cobalt source is one of cobalt chloride CoCl2, cobalt nitrate Co(NO3)2, cobalt sulfate CoSO4, cobalt acetate (CH3COO)2Co, and cobalt acetylacetone C. 10 H 14 One of the CoO4 types.

[0029] Furthermore, the metal-organic framework ligand is one of 2-methylimidazole, terephthalic acid, 2-ethylimidazole, trimesoic acid, and 5-aminotetrazole.

[0030] Furthermore, the pH adjuster is an organic amine, including monoethanolamine, diethanolamine, triethanolamine, and triisopropanolamine.

[0031] Step S3 involves loading a core-shell structured metal@nitrogen-doped carbon (M@NC) nanocatalytic layer onto a graphite felt with an attached conductive layer, significantly enhancing the redox catalytic activity of the graphite felt electrode. The modification mechanism is the presence of metal ions (Ni). 2+ Bi 3+ Co 3+ All possess rich coordination modes and affinity for various functional groups, exhibiting extremely high flexibility in constructing metal-organic frameworks (M-MOFs). Nitrazolium and imidazole diligands possess derived hollow structures, which can coordinate with metal ions in alkaline growth solutions to form stable M-MOFs. Pyrolysis in argon at 280-320℃ generates carbon-shell-encapsulated metal nanoparticles (2-4 nm in diameter), thereby blocking the dissolution of metal ions and effectively protecting the metal particles by embedding nitrogen-doped carbon shells, catalyzing VOCs. 2+ / VO2 + Vanadium ion electrode reaction suppresses side reactions and improves cycle stability.

[0032] Secondly, the present invention provides a modified graphite felt electrode for a vanadium redox flow battery, which is prepared by the above-described method.

[0033] Thirdly, the present invention provides an all-vanadium redox flow battery comprising the above-described modified graphite felt electrode.

[0034] The beneficial effects of this invention are:

[0035] This invention achieves a comprehensive improvement in the various properties of graphite felt through matrix treatment, structural modification, and catalytic loading. The matrix treatment employs H₂O₂ / organic acid electrochemical oxidation, a gentle etching process that protects the carbon fiber structure while achieving matrix oxidation and etching. In contrast, existing technologies using concentrated sulfuric acid oxidation and etching result in a significant increase in fiber resistance, and the difficulty in controlling the etching degree leads to fiber structural damage. The structural modification involves a self-assembled network conductive layer on the graphite felt fiber structure. The N-source conductive reinforcing agent and P-source crosslinking agent, under the action of the ammonium salt accelerator, enhance electrostatic attraction, forming stable conductive channels. Simultaneously, the O-source support agent reduces or bridges the gaps between graphite felt fibers, significantly increasing the specific surface area. Existing technologies use single composite graphene or carbon nanotube materials, or dopants of single heteroelements, offering limited performance improvements. Supported catalysis utilizes a metal@nitrogen-doped carbon (M@NC) nanocatalyst layer, encapsulating metal nanodots within a nitrogen-doped carbon shell. This process is simple, low-cost, and the active metal catalyst layer is less prone to dissolution, significantly improving the capacity decay rate. Furthermore, the catalyst layer can be loaded onto a larger specific surface area network of conductive layers. In contrast, existing technologies often use precious metals or single elements as catalysts without effective encapsulation protection, resulting in higher process costs, lower long-term stability of the catalyst layer, and easy dissolution after prolonged operation, leading to rapid efficiency decay. Additionally, the catalyst layer is often directly loaded onto the fiber surface, resulting in a smaller loading capacity, weaker loading bonding strength, and easy detachment of the active layer.

[0036] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the specific embodiments indicated in the description. Attached Figure Description

[0037] Figure 1 This is a flowchart of the preparation method of the modified graphite felt electrode of the present invention;

[0038] Figure 2 The wettability contact angle test diagram of the modified graphite felt electrode prepared in Example 1;

[0039] Figure 3 SEM micrograph of the modified graphite felt electrode prepared in Example 1;

[0040] Figure 4 The graph shows the charge-discharge efficiency of the modified graphite felt electrode in Example 1 and the comparative example in a flow battery. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the implementation schemes of this application will be described in detail below with reference to specific embodiments. Unless otherwise defined, the technical and scientific terms used in this invention have the meanings commonly understood by those skilled in the art. Without departing from the concept of this invention, those skilled in the art can make various improvements and changes to the specific embodiments described in this specification, all of which fall within the protection scope of this invention. Unless otherwise specified, all raw materials used in this invention are commercially available.

[0042] Example 1

[0043] The method for preparing modified graphite felt electrodes includes the following steps:

[0044] S1. Electrochemical oxidation etching treatment of graphite felt:

[0045] Prepare 1000 mL of electrolyte solution containing 0.5 mol / L Na2SO4 + 0.1 mol / L H2O2 + 0.05 mol / L citric acid using deionized water;

[0046] The cleaned graphite felt was placed in an electrolyte at a constant potential of 1.0 V for electrochemical oxidation etching for 20 min; then cleaned for 10 min under ultrasonic conditions of 120 W power and 40 kHz frequency using ethanol and water in a volume ratio of 1:1; and finally vacuum dried at 60℃ for 5 h.

[0047] S2. A conductive layer is self-assembled on the oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer, including:

[0048] Prepare 1000 mL of a mixed impregnation solution containing 4 wt% adenosine triphosphate (ATP), 2 g / L polyethyleneimine (PEI), 1.0 mg / mL reduced graphene oxide (RGO), and 0.1 mol / L ammonium chloride (NH4Cl) using deionized water.

[0049] The oxidized and etched graphite felt was placed in an impregnation solution and reacted with shaking at 70 °C for 6 h, followed by heat treatment and curing at 130 °C for 3 h to form an ATP-PEI-RGO self-assembled network conductive layer.

[0050] S3. A Ni@NC nanocatalyst layer is loaded onto a graphite felt with an attached conductive layer to obtain a modified graphite felt electrode, including:

[0051] Prepare a mixed growth solution of 0.05 mol / L Ni(NO3)3 + 0.2 mol / L 2-methylimidazole + 0.1 mol / L diethanolamine using deionized water;

[0052] The graphite felt with the conductive layer attached was placed in a growth solution at pH 9.2 and reacted at 60 °C for 4 h. Afterwards, the graphite felt was slowly rinsed with a 1% nitric acid ethanol solution to remove uncoordinated Ni. 2+ Then, the graphite felt electrode with a stable Ni@NC nanocatalytic layer is annealed at 300 °C for 2 h in an argon atmosphere to form a graphite felt electrode.

[0053] Figure 2 The contact angle test diagram of the modified graphite felt electrode prepared in Example 1 shows that the droplet is absorbed by the graphite felt at the moment of falling, exhibiting excellent hydrophilic wettability, with a contact angle close to 0°.

[0054] Figure 3 The image shows the SEM morphology of the modified graphite felt electrode prepared in Example 1. The indentations on the fiber surface after electrochemically coordinated oxidation etching can be clearly distinguished. The self-assembled ATP-PEI-RGO network conductive layer on the graphite felt fiber surface and the Ni@NC catalytic layer with nitrogen-doped carbon shell encapsulating Ni are also clearly visible. The multi-level modification measures greatly enrich the active sites and structural characteristics on the surface of the graphite felt fiber, significantly improving its redox activity and stability.

[0055] Example 2

[0056] The method for preparing modified graphite felt electrodes includes the following steps:

[0057] S1. Electrochemical oxidation etching treatment of graphite felt:

[0058] Prepare 1000 mL of electrolyte solution containing 0.4 mol / L Na2SO4 + 0.15 mol / L H2O2 + 0.1 mol / L citric acid using deionized water;

[0059] The cleaned graphite felt was placed under a current density of 3 mA / cm². 2 Electrochemical oxidation etching was performed in an electrolyte under constant current for 25 min; cleaning was performed for 15 min using ethanol and water at a volume ratio of 1:1.1 as cleaning agents under ultrasonic conditions at a power of 200 W and a frequency of 30 kHz; and vacuum drying was carried out at 60℃ for 5 h.

[0060] S2. A conductive layer is self-assembled on the oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer, including:

[0061] Prepare 1000 mL of a mixed impregnation solution containing 5 wt% adenosine triphosphate (ATP), 2.5 g / L polyethyleneimine (PEI), 1.2 mg / mL reduced graphene oxide (RGO), and 0.15 mol / L ammonium chloride (NH4Cl) using deionized water.

[0062] The oxidized and etched graphite felt was placed in an impregnation solution and reacted with shaking at 60 °C for 6 h, followed by heat treatment and curing at 150 °C for 3 h to form an ATP-PEI-RGO self-assembled network conductive layer.

[0063] S3. A Ni@NC nanocatalyst layer is loaded onto a graphite felt with an attached conductive layer to obtain a modified graphite felt electrode, including:

[0064] Prepare a mixed growth solution of 0.08 mol / L Ni(NO3)3 + 0.15 mol / L 2-methylimidazole + 0.12 mol / L diethanolamine using deionized water;

[0065] The graphite felt with the conductive layer attached was placed in a growth solution at pH 9.0 and reacted at 70 °C for 4 h. Afterwards, the graphite felt was slowly rinsed with a 1% nitric acid ethanol solution to remove uncoordinated Ni. 2+ Then, the graphite felt electrode with a stable Ni@NC nanocatalytic layer is annealed at 250 °C for 2 h in an argon atmosphere to form a graphite felt electrode.

[0066] Example 3

[0067] The method for preparing modified graphite felt electrodes includes the following steps:

[0068] S1. Electrochemical oxidation etching treatment of graphite felt:

[0069] Prepare 1000 mL of electrolyte solution containing 0.5 mol / L K2SO4 + 0.1 mol / L H2O2 + 0.05 mol / L tartaric acid using deionized water;

[0070] The cleaned graphite felt was placed in an electrolyte at a constant potential of 1.0 V for electrochemical oxidation etching for 20 min; then, it was cleaned with an ultrasonic cleaning agent of ethanol and water in a volume ratio of 1:1 at a power of 120 W and a frequency of 40 kHz for 10 min; and finally, it was vacuum dried at 60 °C for 5 h.

[0071] S2. A conductive layer is self-assembled on the oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer, including:

[0072] Prepare 1000 mL of a mixed impregnation solution containing 4 wt% phytic acid IP6 + 2 g / L polyethyleneimine PEI + 1 mg / mL reduced graphene oxide (RGO) + 0.1 mol / L ammonium chloride (NH4Cl) using deionized water.

[0073] The oxidized and etched graphite felt was placed in an impregnation solution and reacted with vibration at 70°C for 6 h. Then, it was heat-treated and cured at 130°C for 3 h to form an IP6-PEI-RGO self-assembled mesh conductive layer.

[0074] S3. A Bi@NC nanocatalyst layer is loaded onto a graphite felt with an attached conductive layer to obtain a modified graphite felt electrode, including:

[0075] Prepare a mixed growth solution of 0.05 mol / L BiOCl + 0.2 mol / L 2-ethylimidazole + 0.1 mol / L triethanolamine using deionized water;

[0076] The graphite felt with the conductive layer attached was placed in a growth solution at pH 9.2 and reacted at 70 °C for 6 h. Afterwards, the graphite felt was slowly rinsed with a 1% nitric acid ethanol solution to remove uncoordinated Bi. 3+ Then, the graphite felt electrode with a stable Bi@NC nanocatalytic layer is annealed at 300 °C for 2 h in an argon atmosphere to form a graphite felt electrode.

[0077] Example 4

[0078] The difference from Example 1 is that the P-source crosslinking agent replaces adenosine triphosphate (ATP) with phytic acid IP6.

[0079] Example 5

[0080] The difference from Example 1 is that the concentration of the oxidant H2O2 is replaced by 0.2 mol / L instead of 0.1 mol / L.

[0081] Example 6

[0082] The difference from Example 1 is that the polyethyleneimine (PEI) of the N source and the conductive enhancer is replaced with polyallylamine (PAA).

[0083] Example 7

[0084] The difference from Example 1 is that the NH4Cl electrostatic crosslinking promoter is replaced from 0.1 mol / L to 0.15 mol / L.

[0085] Example 8

[0086] The difference from Example 1 is that the MOF ligand is replaced by pyromellitic acid instead of 2-methylimidazole.

[0087] Example 9

[0088] The difference from Example 1 is that the pH of the growth medium was adjusted from 9.2 to 9.6.

[0089] Example 10

[0090] The difference from Example 1 is that the annealing at 300 °C for 2 h in an argon atmosphere is replaced with annealing for 3 h.

[0091] Example 11

[0092] The difference from Example 1 is that Ni in the metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is replaced with Bi.

[0093] Example 12

[0094] The difference from Example 1 is that Ni in the metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is replaced with Co.

[0095] Comparative Example 1

[0096] Unlike Example 1, the electrochemical oxidation etching treatment of the graphite felt in step S1 was not performed.

[0097] Comparative Example 2

[0098] Unlike Example 1, the conductive layer was not assembled in step S2.

[0099] Comparative Example 3

[0100] Unlike Example 1, the supported Bi@NC catalyst layer did not undergo step S3.

[0101] Vanadium redox flow battery systems were assembled using graphite felt electrodes prepared in the examples and comparative examples, and charge-discharge tests were conducted. The test results are shown in Table 1. Vanadium redox flow battery system: The battery electrode reaction area is 6 × 8 cm². 2The electrolyte contains 1.65 mol / L vanadium ions and 4.0 mol / L sulfate ions; the battery's constant current charge / discharge current density is 200 mA / cm². 2 The charge / discharge cutoff values ​​for each individual cell are 1.55 V and 1.0 V, respectively; the positive and negative electrolyte volumes are each 300 mL.

[0102] Table 1: Performance comparison of graphite felt electrodes in flow batteries between the examples and comparative examples.

[0103]

[0104] Figure 4 The table shows the charge-discharge efficiency of the modified graphite felt electrodes in Example 1 and the comparative examples in a flow battery. Referring to Table 1, the modified graphite felt electrode in Example 1 exhibited the best conductivity, specific surface area, and contact angle, demonstrating the best cycle efficiency in the flow battery charge-discharge test. Examples 2 and 3, by changing the process parameters and the composition of different solutions, showed a slight decrease in the physicochemical properties and charge-discharge efficiency of the modified graphite felt compared to Example 1, but still maintained excellent overall performance. Comparative Examples 1, 2, and 3, by omitting the electrochemical oxidation etching treatment, the composite self-assembled network conductive layer, and the Ni@NC catalyst layer loading steps, respectively, showed a significant decrease in the physicochemical properties and charge-discharge test efficiency of the modified graphite felt electrodes. Comparative Example 1, lacking electrochemical oxidation etching treatment, showed significantly insufficient wettability of the graphite felt fiber matrix. Comparative Example 2, lacking the composite self-assembled network conductive layer, exhibited the lowest conductivity, resulting in increased ohmic polarization and the lowest charge-discharge cycle voltage efficiency. Comparative Example 3 has no supported Ni@NC catalyst layer, therefore its redox activity is significantly insufficient, and its charge-discharge cycle coulombic efficiency is the lowest.

[0105] Furthermore, the inventors compared the embodiments of the present invention with the prior art, using a 6×8 cm... 2 Graphite felt electrode sample, 1.65 MV 3+ / V 4+ The electrolyte and the test results are shown in Table 2.

[0106] Existing technology 1: Using concentrated sulfuric acid to oxidize and etch graphite felt electrodes. The raw graphite felt is immersed in 98% concentrated sulfuric acid for 8-12 hours, then taken out and rinsed with deionized water 2-3 times, and dried in an oven at 100-120℃ for 0.5-1 hour.

[0107] Existing technology 2: Carbon nanotube composite on fiber surface. A carbon nanotube (CNT) dispersion with a concentration of 0.5-5 mg / mL is prepared, mixed with ethanol as a dispersant, and ultrasonically dispersed for 30-60 minutes (power 200-500 W). The cleaned graphite felt is immersed in the dispersion and allowed to stand at room temperature for 1-4 hours. After removal, it is vacuum dried at 80-120℃ for 2-4 hours, and then annealed at 300-500℃ for 1-2 hours in an inert N2 atmosphere.

[0108] Existing technology 3: Supported platinum (Pt) catalyst. A three-electrode pulse electrodeposition method is employed, with a graphite felt as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte is a 0.5-2 mmol / L H₂PtCl₆ solution in 0.5 mol / L H₂SO₄. A relatively high peak current density of 12-36 mA / cm² is used in the first stage. 2 Deposition lasts 30-120 seconds. Stage II utilizes a low peak current density of 4-10 mA / cm³. 2 The total deposition charge is 1-2.5 C / cm³. 2 The electrodeposition temperature is 20-60℃.

[0109] Existing technology 4: Electrochemical oxidation. Prepare an electrolyte solution with sulfuric acid concentration of 0.2-0.5 mol / L, phosphoric acid concentration of 0.6-1.0 mol / L, and ammonium nitrate concentration of 0.05-0.15 mol / L. After cleaning and drying the graphite felt, use it as the anode and a platinum mesh as the cathode. Immerse the graphite felt in the electrolyte solution and perform electrochemical oxidation for 5-30 minutes under a voltage range of 1.5-30 V and a current range of 0.1-2 A. Then, remove the graphite felt, clean it ultrasonically, and dry it in an oven.

[0110] Existing technology 5: N / P heteroelement doping. A precursor solution is prepared by mixing 0.5-0.8 mol / L dopamine hydrochloride, 0.4-0.7 mol / L 1-hydroxyethane-1,1-diphosphonic acid, and 0.5-1.0 mol / L Tris-HCl buffer. Graphite felt is preheated at 150-200℃ for 20-30 min, then immersed in the precursor solution, and placed in a high-pressure reactor for hydrothermal reaction at 160-200℃ for 8-12 h. Subsequently, the graphite felt is carbonized at 700-900℃ in a nitrogen atmosphere for 3-5 h to obtain nitrogen-phosphorus co-doped graphite felt.

[0111] Existing technology 6: Composite graphene. A modifier, benzylamine, is mixed with a 10 mg / ml aqueous dispersion of graphene oxide at a mass ratio of 1:50-100. Graphite felt is immersed in the mixture and subjected to a hydrothermal reaction at 100-180℃ for 20-24 h. After the reaction, the graphite felt is washed to obtain composite graphene graphite felt.

[0112] Existing technology 7: Heat treatment. Graphite felt is subjected to gradient calcination in an inert gas atmosphere (such as nitrogen, argon and helium). First, it is calcined at 300℃-500℃ and held for 1-3 h, and then calcined at 800℃-1000℃ and held for 2-5 h to obtain graphite felt electrodes.

[0113] Table 2:

[0114]

[0115] In summary, the graphite felt electrode in this invention, through synergistic modification in a solution with optimal proportions and composition, can achieve excellent physicochemical properties, significantly improving the energy storage efficiency and economic benefits of vanadium redox flow batteries.

[0116] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a modified graphite felt electrode for a vanadium redox flow battery, characterized in that, Includes the following steps: Electrochemical oxidation etching treatment of graphite felt: The graphite felt is placed in an electrolyte under constant potential or constant current conditions for electrochemical oxidation etching, ultrasonic cleaning, and vacuum drying; the electrolyte includes 0.4-0.6 mol / L of electrolytic matrix, 0.05-0.2 mol / L of H2O2 oxidant, 0.05-0.1 mol / L of organic acid etchant, and deionized water; A conductive layer is self-assembled on an oxidized and etched graphite felt to obtain a graphite felt with an attached conductive layer. The process includes: placing the oxidized and etched graphite felt in an impregnation solution, vibrating it at 60-70°C for 4-6 hours, and then heat-curing it at 120-150°C for 2-3 hours. The impregnation solution comprises 2-6 wt% of a P-source crosslinking agent, 2-3 g / L of an N-source conductive enhancer, 0.8-1.2 mg / mL of an O-source support agent, 0.05-0.15 mol / L of an electrostatic crosslinking accelerator, and deionized water. The P-source crosslinking agent is one of a nucleotide or an inositol phosphate derivative; the N-source conductive enhancer is polyethyleneamine, polyethyleneimine, or polyallylamine; the O-source support agent is reduced graphene oxide; and the electrostatic crosslinking accelerator is an ammonium salt. A modified graphite felt electrode was obtained by loading a metal@nitrogen-doped carbon (M@NC) nanocatalytic layer onto a graphite felt with an attached conductive layer.

2. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 1, characterized in that, The constant potential condition is 0.9-1.2V; the constant current condition is a current density of 2-5mA / cm². 2 .

3. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 1, characterized in that, The electrolytic matrix is ​​a sulfate; the organic acid etchant is one of malic acid, citric acid, tartaric acid, and acetic acid.

4. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 1, characterized in that, The electrochemical oxidation etching time is 15-30 min; the vacuum drying temperature is 50-80℃.

5. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 1, characterized in that, The modified graphite felt electrode is obtained by loading a metal@nitrogen-doped carbon (M@NC) nanocatalytic layer onto a graphite felt with an attached conductive layer, comprising: placing the graphite felt with the attached conductive layer in a growth solution, adjusting the pH to 9.0-9.6, reacting fully at 50-80℃ for 3-5 hours, and then rinsing; annealing at 280-320℃ in an argon atmosphere for 2-3 hours to form a stable loaded M@NC nanocatalytic layer, thereby obtaining the modified graphite felt electrode.

6. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 5, characterized in that, The growth solution comprises 0.03-0.08 mol / L of a metal source, 0.1-0.4 mol / L of a metal-organic framework (MOF) ligand, 0.05-0.2 mol / L of a pH adjuster, and deionized water.

7. The method for preparing a modified graphite felt electrode for a vanadium redox flow battery according to claim 6, characterized in that, The metal source includes a nickel source, a bismuth source, and a cobalt source; The nickel source is one of nickel nitrate, nickel chloride, nickel sulfate, nickel acetate, and nickel acetylacetonate; the bismuth source is one of bismuth nitrate, bismuth oxychloride, bismuth subcarbonate, and bismuth trioxide; the cobalt source is one of cobalt chloride, cobalt nitrate, cobalt sulfate, cobalt acetate, and cobalt acetylacetonate. The metal-organic framework ligand is one of 2-methylimidazolium, terephthalic acid, 2-ethylimidazolium, trimesic acid, and 5-aminotetrazole. The pH adjuster is an organic amine.

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

9. A vanadium redox flow battery, characterized in that, It includes the modified graphite felt electrode for a full vanadium redox flow battery as described in claim 8.

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