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

Abstract

The invention discloses a modified graphite felt electrode for an all-vanadium redox flow battery and a preparation method of the modified graphite felt electrode, and belongs to the technical field of redox flow battery electrode materials. The preparation method comprises the following steps: graphite felt electrochemical oxidation etching treatment; self-assembling a conductive layer on the oxidized and etched graphite felt to obtain the graphite felt attached with the conductive layer; and loading a metal and nitrogen-doped carbon (M and NC) nano catalyst layer on the graphite felt attached with the conductive layer to obtain the modified graphite felt electrode. Various properties of the graphite felt are comprehensively improved through matrix treatment, structure change and load catalysis, mild electrochemical oxidation etching is adopted, a carbon fiber structure is protected, a self-assembled net-shaped conductive layer forms a stable conductive channel, the specific surface area is remarkably increased, a nitrogen-doped carbon shell is adopted to wrap metal nanodots to form an M-NC catalytic layer, and the specific surface area is remarkably increased. The catalyst layer is not easy to dissolve out, the capacity fading rate is obviously improved, the loading capacity of the catalyst layer on the reticular structure conducting layer with a larger specific surface area is larger, and meanwhile, the process is simple and low in cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of liquid flow battery electrode materials, and particularly relates to a modified graphite felt electrode for a full vanadium liquid flow battery and a preparation method thereof. BACKGROUND

[0002] The full vanadium liquid flow battery has intrinsic safety, long service life, large capacity, 4-10 h storage, power and energy decoupling, and is considered to be the most suitable technology for large-scale long-time energy storage. The full vanadium liquid flow battery uses different valence V 5+ / V 4+ and V 2+ / V 3+ Oxidation-reduction couples lose or gain electrons on the electrode surface to realize charging and discharging. The positive / negative electrolyte is separated by a proton exchange membrane. The electrode is one of the most critical materials affecting the electrode reaction activity and rate, battery cycle efficiency and service life, etc. The graphite felt electrode widely researched and applied at present has advantages of high porosity three-dimensional structure, high conductivity, excellent mechanical properties and good chemical stability. However, the unmodified graphite felt electrode has low electrocatalytic activity, poor fiber surface wettability, small electrochemical specific surface area and poor long-period cycle stability, resulting in slow electrode reaction rate, high reaction polarization degree, low battery storage efficiency, low electrolyte utilization rate, making it difficult for the battery to obtain excellent storage performance and economic benefits.

[0003] Current graphite felt surface modification measures mainly include surface oxidation (acid / heat treatment), doping, electrocatalyst loading and fiber surface composite, etc. Traditional surface oxidation can improve the hydrophilicity of the electrode surface fibers and the electrochemical specific surface area, but will cause the fiber surface to be excessively etched, the resistance to rise, the fiber structure to be damaged and the mechanical stability of the electrode material to be severely reduced. The loaded electrocatalysts are mostly noble metals such as Pt, Ir, W, Ce and Ga and Pb pollution metals, which cannot meet the cost and environmental protection requirements, and the loaded metal elements are not effectively protected and are easily dissolved out in the acidic environment to cause the electrode to be deactivated (100 cycle efficiency decreases by >8%). Patent CN119230857A discloses a nano carbon layer modified graphite felt electrode for a full vanadium liquid flow battery and a preparation method and application thereof, and adopts heat treatment and magnetron sputtering to modify the surface of the graphite felt electrode. Single surface modification measure improves the characteristics and functions of the electrode in a single way, and cannot realize the multifunctionalization of the graphite felt electrode. Patent CN113809338A discloses a preparation method of an electrode material for a full vanadium liquid flow battery, and adopts a dopamine doping method for modification. Although the doping of hetero-elements and the composite of carbon materials on the fiber surface initially significantly improve the electrode surface activity and obtain extremely high oxidation-reduction efficiency, the single composite and the lack of multi-process synergy easily cause the de-embedding and deactivation of the functional groups or the composite after long-term cycling of the electrode. In order to improve the comprehensive performance of the full vanadium liquid flow battery, new graphite felt electrode modification strategies still need to be researched. SUMMARY

[0004] In view of the deficiencies of the prior art in the modification of graphite felt, the application provides a modified graphite felt electrode for a vanadium redox flow battery and a preparation method thereof, which improves the surface properties of graphite felt fibers through surface oxidation, mild etching, electrostatic crosslinking, network connection, conductive compensation, packaging protection, and catalytic loading, simultaneously improves the hydrophilicity, conductivity, catalytic activity, and long-term stability of the electrode, realizes the multifunctionalization of the graphite felt electrode, and comprehensively improves the energy efficiency and cycle life of the battery.

[0005] The technical scheme provided by the application is as follows: In a first aspect, the application provides a preparation method of a modified graphite felt electrode for a vanadium redox flow battery, comprising the following steps: S1, electrochemical oxidation etching treatment of graphite felt: placing the graphite felt in an electrolyte under constant potential conditions or constant current conditions for electrochemical oxidation etching, ultrasonic cleaning, and vacuum drying; S2, self-assembling a conductive layer on the graphite felt after oxidation etching to obtain graphite felt attached with the conductive layer; S3, loading a metal@nitrogen-doped carbon (M@NC) nanocatalytic layer on the graphite felt attached with the conductive layer to obtain a modified graphite felt electrode.

[0006] Further, the constant potential condition is 0.9-1.2 V; and the constant current condition is under a current density of 2-5 mA / cm 2 .

[0007] Further, the electrolyte comprises 0.4-0.6 mol / L of an electrolyte base, 0.05-0.2 mol / L of an H2O2 oxidant, 0.05-0.1 mol / L of an organic acid etchant, and deionized water.

[0008] The electrolyte base is a sulfate salt, and the sulfate salt comprises Na2SO4, K2SO4, and CaSO4.

[0009] The organic acid etchant is one of malic acid, citric acid, tartaric acid, and acetic acid.

[0010] Further, the electrochemical oxidation etching time is 15-30 min.

[0011] Further, the ultrasonic cleaning uses ethanol and water in a volume ratio of 1: (1-2) as a cleaning agent, and the graphite felt is cleaned under ultrasonic conditions with a power of 100-200 W and a frequency of 30-60 kHz for 10-15 min.

[0012] Further, the vacuum drying temperature is 50-80℃.

[0013] The modification mechanism of step S1 is that H2O2 generates active oxygen free radicals under the electric field, selectively oxidizing the fiber edge. The generated -COOH / -OH oxygen-containing groups adhere 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 microcrystal edges and significantly increasing the number of active sites.

[0014] Furthermore, step S2 self-assembles a conductive layer on the oxidatively etched graphite felt to obtain a graphite felt with an attached conductive layer, comprising: placing the oxidatively etched graphite felt in an impregnation solution, shaking at 60-70°C for 4-6 hours, and heat-treating and curing at 120-150°C for 2-3 hours.

[0015] Furthermore, the impregnation solution includes 2-6 wt% of a P source crosslinking agent, 2-3 g / L of a N source conductivity enhancer, 0.8-1.2 mg / mL of an O source proppant, 0.05-0.15 mol / L of an electrostatic crosslinking accelerator, and deionized water.

[0016] Furthermore, the P source cross-linking 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).

[0017] Furthermore, the N-source conductivity enhancer is polyvinylamine (PVAm), polyethyleneimine (PEI) or polyallylamine (PAA).

[0018] Furthermore, the O source proppant is reduced graphene oxide (RGO).

[0019] Furthermore, the electrostatic crosslinking accelerator is an ammonium salt, which includes ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), ammonium carbonate ((NH4)2CO3), and ammonium bicarbonate (NH4HCO3).

[0020] Step S2 is to self-assemble a composite mesh conductive layer on the graphite felt after oxidation etching to increase the specific surface area, dope miscellaneous elements, and improve the conductivity. The modification mechanism is as follows: the P source crosslinker and the N source conductive enhancer provide doping elements to further regulate the electronic structure and promote the delocalized migration of the π electrons in the graphite layer, thereby reducing the fiber interface resistance and forming a conductive channel to make the charge transfer faster; the electrostatic crosslinking promoter ammonium salt can protonate the amine groups (-NH2, -NH3 +), enhances its positive charge, and at the same time, forms a stable complex system with the P source crosslinking agent through "electrostatic attraction-hydrogen bond auxiliary", so that the electrostatic attraction between the two is significantly improved, forming a stable crosslinked structure; the O source proppant reduces the graphene oxide to reduce or bridge the gap between the graphite felt fibers, forming a proton-electron double-channel network structure, significantly improving the specific surface area of the graphite felt electrode, while reducing the surface resistance.

[0021] Further, the step S3 loads a metal@nitrogen-doped carbon (M@NC) nano catalytic layer on the graphite felt with the conductive layer attached, to obtain a modified graphite felt electrode, comprising: placing the graphite felt with the conductive layer attached in a growth solution, adjusting the pH to 9.0-9.6, and reacting fully at 50-80 ℃ for 3-5 h; slowly flushing with 1-2% nitric acid ethanol solution to remove uncoordinated metal ions in the growth solution; annealing at 280-320 ℃ for 2-3 h in an argon atmosphere to form a stable metal@nitrogen-doped carbon (M@NC) nano catalytic layer, thereby obtaining a modified graphite felt electrode.

[0022] The 1-2% nitric acid ethanol solution refers to 1-2% nitric acid by volume of ethanol, and the mass fraction of nitric acid is 68%.

[0023] Further, the growth solution comprises 0.03-0.08 mol / L metal source, 0.1-0.4 mol / L metal organic framework (MOF) ligand, 0.05-0.2 mol / L pH adjuster, and deionized water.

[0024] Further, the metal source comprises a nickel source, a bismuth source, and a cobalt source; the nickel source is one of nickel nitrate Ni(NO3)2, nickel chloride NiCl2, nickel sulfate NiSO4, nickel acetate (CH3COO)2Ni, and nickel acetylacetone C 10 H 14 NiO4; the bismuth source is one of bismuth nitrate Bi(NO3)3, bismuth oxychloride BiOCl, bismuth subcarbonate Bi2(CO3)3, and bismuth trioxide Bi2O3; and 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 CoO4.

[0025] Further, the metal organic framework ligand is one of 2-methyl imidazole, terephthalic acid, 2-ethyl imidazole, trimesic acid, and 5-amino tetrazole.

[0026] Further, the PH adjuster is an organic amine, and the organic amine comprises monoethanolamine, diethanolamine, triethanolamine, and triisopropanolamine.

[0027] Step S3 loads the metal@nitrogen-doped carbon (M@NC) nano-catalytic layer with core-shell structure on the graphite felt with the attached conductive layer, significantly improving the redox catalytic activity of the graphite felt electrode. The modification mechanism is that the metal ions Ni 2+ , Bi 3+ , Co 3+ all have rich coordination modes and affinity with various functional groups, and exhibit high flexibility in the construction of metal-organic frameworks (M-MOFs). The nitrogen azole and imidazole double ligands have a hollow structure, which can be coordinated with metal ions in an alkaline growth solution to form stable M-MOFs. Pyrolysis at 280-320℃ in argon generates metal nanopoints (particle size 2-4 nm) wrapped in a carbon shell, thereby blocking the dissolution of metal ions and embedding the nitrogen-doped carbon shell to effectively protect the metal particles, catalyze the VO 2+ / VO2 + vanadium ion electrode reaction, inhibit side reactions, and improve the cycle stability.

[0028] In a second aspect, the present application provides a modified graphite felt electrode for a vanadium redox flow battery, which is prepared by the above preparation method.

[0029] In a third aspect, the present application provides a vanadium redox flow battery comprising the modified graphite felt electrode described above.

[0030] The beneficial effects of the present application are: The present application comprehensively improves the performance of graphite felt by substrate treatment, structure change, and loading catalysis. The substrate treatment uses H2O2 / organic acid electrochemical oxidation, which is mild etching and protects the carbon fiber structure while achieving the purpose of substrate oxidation etching. In the prior art, concentrated sulfuric acid oxidation etching leads to a significant increase in fiber resistance, and the etching degree is difficult to control, resulting in damage to the fiber structure. The structure change is a self-assembled network conductive layer on the graphite felt fiber structure. The electrostatic attraction of the N source conductive enhancer and the P source crosslinking agent is enhanced under the action of the promoter ammonium salt, forming a stable conductive channel. Meanwhile, the O source supporting agent can reduce or bridge the gap between graphite felt fibers, significantly increasing the specific surface area. In the prior art, single composite graphene or carbon nanotubes and other materials or single element doping are used, which has limited performance improvement. The loading catalysis uses a nitrogen-doped carbon shell to encapsulate metal nanoparticles through a metal@nitrogen-doped carbon (M@NC) nano-catalytic layer, which is simple in process and low in cost. The metal active catalytic layer is not easy to dissolve out, and the capacity decay rate is significantly improved. Meanwhile, the loading amount of the catalytic layer is larger on the network structure conductive layer with a larger specific surface area. In the prior art, catalysts are mostly noble metals and single elements, and are not effectively encapsulated and protected, resulting in high process cost and low long-term stability of the catalytic layer. The catalytic layer is easy to dissolve out after long-term operation, and the efficiency decay rate is fast. Meanwhile, the catalytic layer is mostly directly loaded on the fiber surface, with a small loading amount and low loading strength, and the active layer is easy to fall off.

[0031] Other features and advantages of the present application will be set forth in the following specification, and in part will be apparent from the description, or can be learned by practice of the application. The objects and other advantages of the application will be realized and attained by the specific embodiments described hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 Preparation method flow chart of modified graphite felt electrode of the present application; Figure 2 Modified graphite felt electrode prepared for Example 1, test chart of wettability contact angle; Figure 3 SEM micrograph of modified graphite felt electrode prepared for Example 1; Figure 4 Charge-discharge efficiency chart of modified graphite felt electrode of Example 1 and comparative example in flow battery. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present application more clear, the embodiments of the present application will be described in detail below, and the technical terms and scientific terms used in the present application have the meanings commonly understood by the ordinary skilled in the art of the present application without other definitions. Without departing from the concept of the present application, those skilled in the art can make various improvements and changes to the specific embodiments of the present application, which are all within the scope of protection of the present application. The raw materials used in the present application are commercially available unless otherwise specified.

[0034] Example 1 Preparation method of modified graphite felt electrode, comprising the following steps: S1, electrochemical oxidation etching treatment of graphite felt: Prepare 1000 mL of electrolyte of 0.5 mol / L Na2SO4+ 0.1 mol / L H2O2+ 0.05 mol / L citric acid with deionized water; Place the cleaned graphite felt in the electrolyte at a constant potential of 1.0 V for electrochemical oxidation etching for 20 min; use ethanol and water in a volume ratio of 1:1 as cleaning agent, clean under the conditions of power 120 W and frequency 40 kHz ultrasonic for 10 min; vacuum dry at 60℃ for 5h.

[0035] S2, self-assemble conductive layer on the graphite felt after oxidation etching, to obtain graphite felt with attached conductive layer, comprising: A 4 wt% adenosine triphosphate ATP + 2 g / L polyethyleneimine PEI + 1.0 mg / mL reduced graphene oxide (RGO) + 0.1 mol / L ammonium chloride (NH4Cl) mixed impregnation solution 1000 mL was prepared with deionized water; The oxidized and etched graphite felt was placed in the impregnation solution, oscillated and reacted at 70°C for 6 h, and then heat-treated and solidified at 130°C for 3 h to form an ATP-PEI-RGO self-assembled reticular conductive layer.

[0036] S3, loading a Ni@NC nano-catalytic layer on the graphite felt with the conductive layer to obtain a modified graphite felt electrode, comprising: A 0.05 mol / L Ni(NO3)3+ 0.2 mol / L 2-methylimidazole + 0.1 mol / L diethanolamine mixed growth solution was prepared with deionized water; The graphite felt with the conductive layer was placed in the growth solution with a pH of 9.2 and reacted fully at 60°C for 4 h; then the graphite felt was slowly rinsed with 1% nitric acid ethanol solution to remove uncoordinated Ni 2+ , and then annealed at 300°C for 2 h in an argon atmosphere to form a graphite felt electrode with a stable Ni@NC nano-catalytic layer.

[0037] Figure 2 The modified graphite felt electrode prepared in Example 1 was tested for contact angle, and it was found that the liquid drop was immediately absorbed by the graphite felt, showing excellent hydrophilic wettability, with a contact angle close to 0°.

[0038] Figure 3 The modified graphite felt electrode prepared in Example 1 was tested for contact angle, and it was found that the liquid drop was immediately absorbed by the graphite felt, showing excellent hydrophilic wettability, with a contact angle close to 0°.

[0039] Example 2 The preparation method of the modified graphite felt electrode comprises the following steps: S1, graphite felt electrochemical oxidation etching treatment: A 0.4 mol / L Na2SO4+ 0.15 mol / L H2O2+ 0.1 mol / L citric acid electrolyte 1000 mL was prepared with deionized water; The cleaned graphite felt was placed in an electrolyte with a current density of 3 mA / cm 2Electrochemical oxidation etching in electrolyte under constant current for 25 min; cleaning for 15 min under ultrasonic conditions with power of 200 W and frequency of 30 kHz using ethanol and water in a volume ratio of 1:1.1 as cleaning agent; vacuum drying at 60 ℃ for 5 h.

[0040] S2, self-assembling a conductive layer on the graphite felt after oxidation etching to obtain graphite felt with attached conductive layer, comprising: Preparation of 5 wt% adenosine triphosphate ATP + 2.5 g / L polyethyleneimine PEI + 1.2 mg / mL reduced graphene oxide (RGO) + 0.15 mol / L ammonium chloride (NH4Cl) mixed impregnation solution 1000 mL with deionized water; Placing the graphite felt after oxidation etching in the impregnation solution, oscillating reaction at 60 ℃ for 6 h, and then heat treatment and solidification at 150 ℃ for 3 h to form an ATP-PEI-RGO self-assembled reticular conductive layer.

[0041] S3, loading Ni@NC nano-catalytic layer on the graphite felt with attached conductive layer to obtain modified graphite felt electrode, comprising: Preparation of 0.08 mol / L Ni(NO3)3+ 0.15 mol / L 2-methylimidazole + 0.12 mol / L diethanolamine mixed growth solution with deionized water; Placing the graphite felt with attached conductive layer in the growth solution with pH of 9.0, fully reacting at 70 ℃ for 4 h; then slowly flushing the graphite felt with 1% nitric acid ethanol solution to remove uncoordinated Ni 2+ , and then annealing at 250 ℃ for 2 h in an argon atmosphere to form a graphite felt electrode with stable Ni@NC nano-catalytic layer.

[0042] Example 3 The preparation method of the modified graphite felt electrode comprises the following steps: S1, graphite felt electrochemical oxidation etching treatment: Preparation of 0.5 mol / L K2SO4+ 0.1 mol / L H2O2+ 0.05 mol / L tartaric acid electrolyte 1000 mL with deionized water; Electrochemical oxidation etching of the cleaned graphite felt in the electrolyte under constant potential of 1.0 V for 20 min; cleaning for 10 min under ultrasonic conditions with power of 120 W and frequency of 40 kHz using ethanol and water in a volume ratio of 1:1 as cleaning agent; vacuum drying at 60 ℃ for 5 h.

[0043] S2, self-assembling a conductive layer on the graphite felt after oxidation etching to obtain graphite felt with attached conductive layer, comprising: A 4 wt% phytic acid IP6 + 2 g / L polyethyleneimine PEI + 1 mg / mL reduced graphene oxide (RGO) + 0.1 mol / L ammonium chloride (NH4Cl) mixed impregnation solution 1000 mL was prepared with deionized water; The oxidized and etched graphite felt was placed in the impregnation solution, oscillated and reacted at 70°C for 6 h, and then heat-treated and solidified at 130°C for 3 h to form an IP6-PEI-RGO self-assembled net-like conductive layer.

[0044] S3, loading a Bi@NC nano-catalytic layer on the graphite felt with the attached conductive layer to obtain a modified graphite felt electrode, comprising: A 0.05 mol / L BiOCl + 0.2 mol / L 2-ethyl imidazole + 0.1 mol / L triethanolamine mixed growth solution was prepared with deionized water; The graphite felt with the attached conductive layer was placed in the growth solution with a pH of 9.2 and reacted fully at 70°C for 6 h; then the graphite felt was slowly rinsed with 1% nitric acid ethanol solution to remove uncoordinated Bi 3+ , and then annealed at 300°C for 2 h in an argon atmosphere to form a graphite felt electrode with a stable loaded Bi@NC nano-catalytic layer.

[0045] Example 4 The difference from Example 1 is that the P source crosslinking agent replaces adenosine triphosphate (ATP) with phytic acid IP6.

[0046] Example 5 The difference from Example 1 is that the concentration of the oxidizing agent H2O2 is replaced from 0.1 mol / L to 0.2 mol / L.

[0047] Example 6 The difference from Example 1 is that the N source and conductive enhancer polyethyleneimine (PEI) is replaced with polyallylamine (PAA).

[0048] Example 7 The difference from Example 1 is that the electrostatic crosslinking promoter NH4Cl is replaced from 0.1 mol / L to 0.15 mol / L.

[0049] Example 8 The difference from Example 1 is that the MOF ligand is replaced from 2-methyl imidazole to trimesic acid.

[0050] Example 9 The difference from Example 1 is that the pH of the growth solution is adjusted from 9.2 to 9.6.

[0051] Example 10 The difference from Example 1 is that the annealing at 300 ℃ for 2 h in argon atmosphere is replaced by annealing for 3 h.

[0052] Example 11 The difference from Example 1 is that Ni in the metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is replaced by Bi.

[0053] Example 12 The difference from Example 1 is that Ni in the metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is replaced by Co.

[0054] Comparative Example 1 The difference from Example 1 is that the electrochemical oxidation etching treatment of the graphite felt in step S1 is not performed.

[0055] Comparative Example 2 The difference from Example 1 is that the assembly of the conductive layer in step S2 is not performed.

[0056] Comparative Example 3 The difference from Example 1 is that the loading of the Bi@NC catalytic layer in step S3 is not performed.

[0057] The graphite felt electrodes prepared in the examples and comparative examples are assembled into a full vanadium redox flow battery system, and charge-discharge tests are performed. The test results are shown in Table 1. The full vanadium redox flow battery system: the battery electrode reaction area is 6×8 cm 2 ; the vanadium ion concentration in the electrolyte is 1.65 mol / L, and the sulfate concentration is 4.0 mol / L; the current density of the battery constant current charge-discharge is 200 mA / cm 2 ; the single battery charge-discharge cut-off battery is 1.55 V and 1.0 V, respectively; the positive / negative electrolyte volume is 300 mL each.

[0058] Table 1: Performance comparison of graphite felt electrodes in the examples and comparative examples in the flow battery

[0059] Figure 4For the charge-discharge efficiency of the modified graphite felt electrodes of Example 1 and Comparative Examples in the flow battery, in combination with Table 1, the modified graphite felt electrode of Example 1 exhibited the optimal conductivity, specific surface area and contact angle, and exhibited the best cycle efficiency in the charge-discharge test of the flow battery. Examples 2 and 3 changed the process parameters and the composition of different solutions, respectively, and the physicochemical properties and charge-discharge efficiency of the modified graphite felt obtained were slightly lower than those of Example 1, but the overall performance was still excellent. Comparative Examples 1, 2 and 3 omitted the steps of electrochemical oxidation etching treatment, composite self-assembled network conductive layer and loading of Ni@NC catalytic layer, respectively, and the physicochemical properties and charge-discharge test efficiency of the modified graphite felt electrodes were significantly reduced. Among them, Comparative Example 1 had no electrochemical oxidation etching treatment, and the wettability of the graphite felt fiber matrix was obviously insufficient. Comparative Example 2 had no composite self-assembled network conductive layer, and the graphite felt electrode exhibited the lowest conductivity, so the ohmic polarization was intensified, and the charge-discharge cycle voltage efficiency was also the lowest. Comparative Example 3 had no loading of Ni@NC catalytic layer, so the redox activity was obviously insufficient, and the charge-discharge cycle coulombic efficiency was the lowest.

[0060] In addition, the inventors compared the embodiments of the present application with the prior art, and used a 6x8 cm 2 graphite felt electrode sample, 1.65 M V 3+ / V 4+ electrolyte, and the test results are shown in Table 2.

[0061] Prior Art 1: The graphite felt electrode was treated by concentrated sulfuric acid oxidation etching. The original graphite felt was soaked in concentrated sulfuric acid with a concentration of 98% for 8-12 hours, washed with deionized water for 2-3 times, and dried in an oven at 100-120°C for 0.5-1 hour.

[0062] Prior Art 2: Carbon nanotubes were composited on the surface of the fibers. A carbon nanotube CNTs dispersion solution with a concentration of 0.5-5 mg / mL was prepared, mixed with a dispersant ethanol and ultrasonically dispersed for 30-60 minutes (power 200-500 W). The cleaned graphite felt was immersed in the dispersion solution at room temperature for 1-4 hours, taken out and vacuum dried at 80-120°C for 2-4 hours, and then annealed at 300-500°C for 1-2 hours in an inert gas N2 atmosphere.

[0063] Prior Art 3: Platinum Pt catalyst was loaded. A three-electrode system pulse electrodeposition method was used, the graphite felt was used as the working electrode, the platinum electrode was used as the counter electrode, and the saturated calomel electrode (SCE) was used as the reference electrode. The electrolyte was a 0.5 mol / L H2SO4 solution containing 0.5-2 mmol / L H2PtCl6. In stage I, a larger peak current density of 12-36 mA / cm 2 was used for deposition for 30-120 s. In stage II, a small peak current density of 4-10 mA / cm 2, the total deposition charge is 1-2.5 C / cm 2 , the electrodeposition temperature is 20-60℃.

[0064] Prior Art 4: Electrochemical Oxidation. Prepare an electrolyte solution containing 0.2-0.5 mol / L sulfuric acid, 0.6-1.0 mol / L phosphoric acid, and 0.05-0.15 mol / L ammonium nitrate. Wash and dry a graphite felt as the anode, and a platinum mesh as the cathode. Place the graphite felt in the electrolyte solution and electrochemically oxidize it for 5-30 minutes at a voltage of 1.5-30 V and a current of 0.1-2 A. Remove the material, ultrasonically clean it, and then dry it in an oven.

[0065] Prior Art 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°C for 20-30 minutes, then immersed in the precursor solution and placed in an autoclave at 160-200°C for 8-12 hours for a hydrothermal reaction. The graphite felt is then carbonized at 700-900°C in a nitrogen atmosphere for 3-5 hours to obtain nitrogen-phosphorus co-doped graphite felt.

[0066] Prior Art 6: Composite Graphene. A modifier, benzylamine, is mixed uniformly 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°C for 20-24 hours. After the reaction, the composite graphene felt is washed.

[0067] Prior Art 7: Heat Treatment: Graphite felt is subjected to a gradient calcination process in an inert gas atmosphere (such as nitrogen, argon, and helium), first at 300°C-500°C for 1-3 hours, then at 800°C-1000°C for 2-5 hours, to obtain a graphite felt electrode.

[0068] Table 2:

[0069] In summary, the graphite felt electrode in the present invention is synergistically modified in a solution with an optimal ratio and composition, which can obtain excellent physical and chemical properties and significantly improve the energy storage efficiency and economic benefits of the all-vanadium redox flow battery.

[0070] Finally, it should be noted that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art will appreciate that the technical solutions described in the foregoing embodiments can be modified or equivalent replacements can be made to some of the technical features, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery, characterized in that: The following steps are involved: Electrochemical oxidation etching treatment of graphite felt: placing the graphite felt in an electrolyte under constant potential or constant current conditions for electrochemical oxidation etching, ultrasonic cleaning, and vacuum drying; self-assembling a conductive layer on the graphite felt after oxidation etching to obtain a graphite felt with a conductive layer attached; A metal@nitrogen-doped carbon (M@NC) nanocatalytic layer is loaded on a graphite felt with a conductive layer attached to obtain a modified graphite felt electrode.

2. The method for preparing a modified graphite felt electrode for an all-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 / cm 2 .

3. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that: 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.

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

5. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that: The electrochemical oxidation etching time is 15-30 minutes; the vacuum drying temperature is 50-80°C.

6. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that: The method of self-assembling a conductive layer on the oxidatively etched graphite felt to obtain a graphite felt with an attached conductive layer comprises placing the oxidatively etched graphite felt in an impregnation solution, shaking at 60-70° C. for 4-6 hours, and heat-treating and curing at 120-150° C. for 2-3 hours.

7. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 6, characterized in that: The impregnation solution includes 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 proppant, 0.05-0.15 mol / L of an electrostatic crosslinking accelerator, and deionized water.

8. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 7, characterized in that: The P source cross-linking agent is one of nucleotides or inositol phosphate derivatives; The N source conductivity enhancer is polyethylene amine, polyethylene imine or polyallylamine; The O source proppant is reduced graphene oxide; The electrostatic crosslinking accelerator is an ammonium salt.

9. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that: The method of loading a metal@nitrogen-doped carbon (M@NC) nanocatalytic layer on a graphite felt with a conductive layer attached to obtain a modified graphite felt electrode includes: placing the graphite felt with a conductive layer attached in a growth solution, adjusting the pH to 9.0-9.6, fully reacting at 50-80°C for 3-5 hours, and then rinsing; and annealing at 280-320°C for 2-3 hours in an argon atmosphere to form a stably loaded M@NC nanocatalytic layer to obtain a modified graphite felt electrode.

10. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 9, characterized in that: The growth solution includes 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 regulator, and deionized water.

11. The method for preparing a modified graphite felt electrode for an all-vanadium redox flow battery according to claim 10, 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-methylimidazole, terephthalic acid, 2-ethylimidazole, trimesic acid, and 5-aminotetrazole; The pH regulator is an organic amine.

12. A modified graphite felt electrode for all-vanadium redox flow battery, characterized in that: Prepared by the preparation method according to any one of claims 1 to 11.

13. An all-vanadium redox flow battery, characterized in that: The modified graphite felt electrode for the all-vanadium redox flow battery comprises the modified graphite felt electrode for the all-vanadium redox flow battery according to claim 12.

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