Spinel nanofiber composite graphite felt electrode and preparation method thereof

Abstract

The invention relates to the technical field of all-vanadium redox flow battery electrodes, and discloses a spinel nanofiber composite graphite felt electrode and a preparation method thereof, and the preparation method comprises the following steps: graphite felt pretreatment; preparing a spinel precursor solution; preparing spinel nanofibers; performing surface reconstruction treatment; preparing a composite graphite felt electrode; by regulating A-site and B-site metals of spinel, the conductivity of the composite graphite felt electrode is improved, activation polarization and ohmic polarization are reduced, energy loss generated by side reaction in battery operation is reduced, and through surface reconstruction, the spinel structure is stabilized, meanwhile, the surface roughness of nanofibers is increased, the active sites of the electrode are increased, and the conductivity of the composite graphite felt electrode is improved. And the efficiency of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of all-vanadium redox flow battery electrodes, and in particular to a spinel nanofiber composite graphite felt electrode and a preparation method thereof. Background Art

[0002] Among various flow batteries, all-vanadium flow batteries (abbreviated as vanadium batteries) are considered to be one of the most promising large-scale energy storage batteries because they reduce the impact of cross-contamination between positive and negative electrode materials, have high electrochemical reversibility, and greatly extend the service life of the electrolyte.

[0003] However, the graphite felt electrodes commonly used in all-vanadium flow batteries still suffer from low electrochemical activity and a small specific surface area, resulting in low vanadium battery efficiency. Furthermore, because the vanadium electrolyte is highly acidic and corrosive, the corrosion resistance and stability of the graphite felt electrodes in long-term contact with the vanadium electrolyte play a key role in the battery's cycle stability and service life. Summary of the Invention

[0004] The present invention provides a spinel nanofiber composite graphite felt electrode and a preparation method thereof to overcome the above problems.

[0005] A first aspect of the present invention provides a method for preparing a spinel nanofiber composite graphite felt electrode, comprising the following steps: Step S1: dispersing a metal salt in a mixed solvent and adding a chelating agent to react to obtain a spinel precursor solution; Step S2: mixing the spinel precursor solution with a polymer solution to obtain spinel nanofibers; Step S3: placing the spinel nanofibers in a reducing agent solution for reduction etching to obtain spinel nanofibers with a heterogeneous structure; Step S4: the heterogeneous structured spinel nanofibers are dispersed in a perfluorosulfonic acid resin solution to obtain an impregnation solution, and the pretreated graphite felt is placed in the impregnation solution for physical impregnation, and dried to obtain a spinel nanofiber composite graphite felt electrode.

[0006] Preferably, in step S1: The volume ratio of water to organic solvent in the mixed solvent is 1:(3-10), wherein the organic solvent is at least one selected from anhydrous ethanol, acetone, propanol, isopropanol and ethylene glycol; The chelating agent is selected from at least one of citric acid, EDTA, ascorbic acid, glycerol and alanine; The metal salt is composed of strontium nitrate, manganese nitrate and cobalt nitrate, and the total concentration of metal ions of the metal salt dispersed in the mixed solvent is 0.03-0.3 mol / L; wherein the molar concentration ratio of the strontium nitrate, the manganese nitrate and the cobalt nitrate is x:(1-x):2, wherein 0.1≤x≤0.9; The molar concentration ratio of the total molar concentration of the metal ions in the metal salt to the molar concentration of the chelating agent is 1:(1.1-2).

[0007] The strontium nitrate, manganese nitrate, and cobalt nitrate in the metal salts are further sintered to form a spinel structure, with strontium and manganese serving as the A-site metals of the spinel, and cobalt serving as the B-site metal. By controlling the concentration of strontium nitrate and regulating the amount of strontium doped, the conductivity of the composite graphite felt electrode can be improved, reducing the activation polarization and ohmic polarization during the battery's redox reaction, thereby improving the battery's voltage efficiency and energy efficiency. By controlling the concentration of manganese nitrate and regulating the amount of manganese doped, the spinel on the electrode maintains structural stability during battery operation, which is beneficial to the electrode's corrosion resistance and stability. The introduction of cobalt can effectively accelerate the kinetic rate of the battery's redox reaction, catalyzing the redox ability of vanadium, reducing the reaction energy barrier, and improving the battery's energy efficiency.

[0008] Preferably, after adding the chelating agent for reaction in step S1, the preparation method further comprises: Stirring in a water bath and adjusting the pH value with an alkaline solution; wherein, The water bath stirring temperature is 50-80°C and the stirring time is 0.5-3h; The alkaline solution is selected from at least one of sodium hydroxide solution, potassium hydroxide solution and ammonia solution; The concentration of the alkaline solution is 1-4 mol / L; The pH value of the pH adjustment is 6-8.

[0009] Preferably, in step S2, the mixing treatment includes degassing, wet spinning, and sintering; wherein, The wet spinning process comprises: placing the spinning precursor solution obtained after degassing in a spinning machine, setting the spinning pump spinning speed to 5-20 m / min, extruding the spinning solution through the capillary pores of the spinneret into a coagulant, diffusing and precipitating to form nascent fibers, and then taking the solution out and drying it naturally in air at room temperature; The sintering process is as follows: calcining at 600-900° C. for 2-6 hours in an air atmosphere, with a heating rate of 2-8° C. / min and a cooling rate of 2-6° C. / min.

[0010] Preferably, in step S2: The polymer in the polymer solution is selected from at least one of polyacrylonitrile, chitosan, cellulose, polyvinyl alcohol and polyvinyl chloride; The solvent in the polymer solution is selected from at least one of dimethyl sulfoxide, NN-dimethylformamide, dimethylacetamide, sodium thiocyanate and nitric acid.

[0011] Preferably, the coagulant is selected from at least one of isobutanol, ethanol, isopropanol, water and acetic acid; The volume ratio of the polymer in the polymer solution, the solvent in the polymer solution and the coagulant is 1:(2-4):(10-30).

[0012] Preferably, in step S3: The reduction etching conditions are as follows: reacting in a 0.1-1 mol / L reducing agent solution for 10-60 min; The reducing agent solution is selected from at least one of a sodium borohydride solution, a lithium aluminum hydride solution, a diisobutylaluminum hydride solution, a lithium borohydride solution and a zinc borohydride solution.

[0013] The spinel surface is subjected to reduction etching using a reducing agent to reconstruct the spinel surface into a core-shell structure, which can increase the surface roughness of the nanofibers and thereby increase the specific surface area and active sites.

[0014] Preferably, in step S4: The mass fraction of the perfluorosulfonic acid resin solution is 5%; The physical immersion time is 6 to 24 hours, and the temperature is 20 to 60°C.

[0015] Preferably, in step S4: The pretreated graphite felt is obtained by placing the graphite felt in an atmosphere furnace for heat treatment, then washing the heat-treated graphite felt with dilute hydrochloric acid and ultrapure water, and drying; wherein, The heat treatment process is as follows: in a nitrogen atmosphere, keep the temperature at 350-500°C for 2-4 hours, and increase the temperature at a rate of 3-6°C / min; The concentration of the dilute hydrochloric acid is 2 mol / L; The drying condition is oven drying at 50°C.

[0016] A second aspect of the present invention provides a spinel nanofiber composite graphite felt electrode, which is prepared using the above-mentioned method for preparing the spinel nanofiber composite graphite felt electrode.

[0017] The beneficial effects of the present invention are:

[0018] 1. The present invention improves the conductivity of the composite graphite felt electrode, reduces activation polarization and ohmic polarization, and improves the voltage efficiency and energy efficiency of the battery by regulating the doping amount of A-site metal Sr.

[0019] 2. The present invention regulates the doping amount of the A-site metal Mn to make the spinel structure stable during battery operation, which is beneficial to the corrosion resistance and stability of the spinel material.

[0020] 3. The present invention accelerates the reaction kinetics by introducing the B-site metal Co, catalyzes the redox ability of vanadium, reduces the reaction energy barrier, and improves the energy efficiency of the battery.

[0021] 4. The present invention uses a reducing agent to perform reduction etching on the spinel surface, so that the spinel surface is reconstructed to form a core-shell structure, which can increase the surface roughness of the nanofiber, thereby increasing the specific surface area and active sites, enhancing the interfacial interaction ability between the electrode surface and the electrolyte, and thus reducing the energy barrier of the vanadium ion redox reaction, improving the catalytic ability of the electrode's redox, and improving the efficiency of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the preparation of spinel nanofibers in Example 1 of the present invention.

[0023] Figure 2 TEM and Mapping images of the spinel nanofibers in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. However, it should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the present invention.

[0025] The present invention provides a method for preparing a spinel nanofiber composite graphite felt electrode, comprising the following steps: Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace for heat treatment, washing the treated graphite felt with 2 mol / L hydrochloric acid and ultrapure water, and drying in a 50°C oven; the heat treatment process is: nitrogen atmosphere, keeping warm at 350-500°C for 2-4 hours, and a heating rate of 3-6°C / min.

[0026] Step S2, spinel precursor solution preparation: metal salt is dissolved in water and organic solvent in proportion, stirred in a water bath, a chelating agent is added, stirred for 30min, an alkaline solution is added to stabilize the pH value, stirring is continued, and after naturally cooling to room temperature, a spinel precursor solution is obtained. Wherein the metal salt is: strontium nitrate, manganese nitrate and cobalt nitrate, and the total metal ion concentration is 0.03~0.3mol / L, and the molar concentration ratio strontium nitrate: manganese nitrate: cobalt nitrate=x:(1~x):2,0.1≤x≤0.9, the organic solvent described in the step S3 is: any one of anhydrous ethanol, acetone, propanol, isopropyl alcohol and ethylene glycol, and the chelating agent is: any one of citric acid, EDTA, ascorbic acid, glycerol and alanine, and the molar concentration ratio metal lithium ion total concentration: chelating agent=1:(1.1~2). The alkaline solution is any one of sodium hydroxide, potassium hydroxide and ammonia water, with a concentration of 1-5 mol / L, a pH of 6-8, a water bath temperature of 50-80°C, and a continuous stirring time of 0.5h-3h.

[0027] Step S3, preparation of spinel nanofibers: a polymer and a solvent are mixed in proportion to obtain a polymer solution, the spinel precursor solution prepared in step S2 is poured into the polymer solution, and the solution is continuously stirred at room temperature to obtain a spinning solution, the spinning solution is subjected to a vacuum degassing treatment, and then wet spinning is performed to obtain a nascent fiber, and the obtained precursor is placed in an atmosphere furnace for high-temperature sintering to obtain spinel nanofiber Sr x Mn (1-x) Co2O4. The stirring time is 6 to 24 hours. The wet spinning process is as follows: a spinning precursor solution is placed in a spinning machine, a spinning pump is set to a spinning speed of 5 to 20 m / min, the spinning solution is extruded through the capillaries of the spinneret and flows into a coagulant, where it diffuses and precipitates to form nascent fibers, which are then taken out and dried naturally in air at room temperature. The polymer described in step S3 is any one of polyacrylonitrile, chitosan, cellulose, polyvinyl alcohol, and polyvinyl chloride; the solvent is any one of dimethyl sulfoxide, NN-dimethylformamide, dimethylacetamide, sodium thiocyanate, and nitric acid; and the coagulant is isobutanol, ethanol, isopropanol, water, and acetic acid. The volume ratio of polymer, solvent, and coagulant is 1:(2 to 4):(10 to 30). The fiber sintering process is as follows: calcination at 600 to 900°C for 2 to 6 hours in an air atmosphere, with a heating rate of 2 to 8°C / min and a cooling rate of 2 to 6°C / min until the temperature drops to 25°C.

[0028] Step S4, surface reconstruction treatment: placing the spinel nanofiber composite graphite felt in a reducing agent for reduction etching to obtain a heterogeneous structure of spinel nanofiber Co(OH)2 / Sr x Mn (1-x)The reduction etching conditions are as follows: reacting in a 0.1-1 mol / L reducing agent for 10-60 min, wherein the reducing agent is any one of sodium borohydride, lithium aluminum hydride, diisobutylaluminum hydride, lithium borohydride, and zinc borohydride.

[0029] Step S5, preparing a composite graphite felt electrode: The nanofibers obtained in step S4 are dispersed in a 5 wt% perfluorosulfonic acid resin solution to obtain an impregnation solution; the pretreated graphite felt is immersed in the impregnation solution for physical impregnation, and then dried in a 50°C oven to obtain a spinel nanofiber composite graphite felt electrode. The physical impregnation time is 6 to 24 hours, and the physical impregnation temperature is 20 to 60°C.

[0030] Example 1: Using the following technical solutions Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace at 350°C for 4 hours with a heating rate of 6°C / min, washing the treated graphite felt with 2 mol / L hydrochloric acid and ultrapure water, and drying in a 50°C oven.

[0031] Step S2, preparation of spinel precursor solution: ultrapure water and anhydrous ethanol are blended in a volume ratio of 1:3, strontium nitrate, manganese nitrate and cobalt nitrate are added to obtain a mixed solution of 0.001 mol / L strontium nitrate, 0.009 mol / L manganese nitrate and 0.02 mol / L cobalt nitrate, and stirred in a water bath at 50°C, 0.033 mol / L citric acid is added, stirred for 30 minutes, and then 1 mol / L sodium hydroxide is slowly added to adjust the pH value of the solution to 6. After continuous stirring for 0.5 hours, it is naturally cooled to room temperature to obtain a spinel precursor solution.

[0032] Step S3, preparation of spinel nanofibers: polyacrylonitrile and dimethyl sulfoxide are mixed in a volume ratio of 1:2 to obtain a polymer solution, the spinel precursor solution obtained in step S2 is poured into it, and the solution is continuously stirred at room temperature for 6 hours. The spinning solution is placed in a spinning machine after vacuum degassing treatment, and the spinning pump is set to a spinning speed of 5m / min. The spinning solution is extruded through the capillary pores of the spinneret and flows into isobutanol with a volume of 10 times that of polyacrylonitrile, diffused and precipitated to form nascent fibers, and naturally dried in air at room temperature. The obtained nascent fibers are placed in an air atmosphere and sintered at a high temperature of 600°C for 6 hours, with a heating rate of 2°C / min and a cooling rate of 2°C / min, to obtain Sr 0.1 Mn 0.9 Co2O4.

[0033] Step S4, surface reconstruction: Sr 0.1 Mn 0.9 Co2O4 was placed in 0.1 mol / L sodium borohydride and reduced and etched for 60 min to obtain the spinel nanofiber composite graphite felt electrode material Co(OH)2 / Sr containing heterogeneous structure.0.1 Mn 0.9 Co2O4.

[0034] Step S5, preparation of composite graphite felt electrode: disperse spinel nanofibers in 5wt% perfluorosulfonic acid resin solution, immerse at 60°C for 6h to obtain an impregnation solution; immerse the pretreated graphite felt in the impregnation solution, and use a physical impregnation method to composite the spinel nanofibers on the surface of the graphite felt. After impregnation, place the mixture in a 50°C oven and dry it completely to obtain a spinel nanofiber composite graphite felt electrode material.

[0035] like Figure 1 As shown, spinel nanofibers (Sr 0.1 Mn 0.9 After the Co2O4 is etched by the reducing agent, the surface Co(III) is reduced to Co(II), forming the second phase material Co(OH)2, and finally forming a new spinel nanofiber structure (Co(OH)2 / Sr 0.1 Mn 0.9 Co2O4).

[0036] like Figure 2 The spinel nanofibers (Co(OH)2 / Sr 0.1 Mn 0.9 Co2O4) TEM and Mapping images. It can be seen from the figure that the surface of the spinel nanofiber is covered with a heterogeneous layer to form a core-shell structure.

[0037] Example 2: Using the following technical solution Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace at 380°C for heat treatment for 3.5 hours with a heating rate of 5°C / min. The treated graphite felt is washed with 2 mol / L hydrochloric acid and ultrapure water and dried in a 50°C oven.

[0038] Step S2, preparation of spinel precursor solution: ultrapure water and acetone are mixed in a volume ratio of 1:5, strontium nitrate, manganese nitrate and cobalt nitrate are added to obtain a mixed solution of 0.0075 mol / L strontium nitrate, 0.0225 mol / L manganese nitrate and 0.06 mol / L cobalt nitrate, and stirred in a water bath at 60°C. 0.117 mol / L EDTA is added and stirred for 30 minutes. Then, 2 mol / L sodium hydroxide is slowly added to adjust the pH value of the solution to 6.5. After continuous stirring for 1 hour, it is naturally cooled to room temperature to obtain a spinel precursor solution.

[0039] Step S3, preparation of spinel nanofibers: chitosan and N, N-dimethylformamide are mixed in a volume ratio of 1:2.5 to obtain a polymer solution, the spinel precursor solution obtained in step S2 is poured into it, and the solution is continuously stirred at room temperature for 10 hours. The spinning solution is placed in a spinning machine after vacuum degassing treatment, and the spinning pump is set to a spinning speed of 8m / min. The spinning solution is extruded through the capillary pores of the spinneret and flows into ethanol with a volume of 15 times that of chitosan, diffuses and precipitates to form nascent fibers, and naturally dries in air at room temperature. The obtained nascent fibers are placed in an air atmosphere and sintered at a high temperature of 650°C for 5 hours, with a heating rate of 3°C / min and a cooling rate of 3°C / min to obtain Sr 0.25 Mn 0.75 Co2O4.

[0040] Step S4, surface reconstruction: Sr 0.25 Mn 0.75 Co2O4 was placed in 0.3 mol / L lithium aluminum hydride and reduced and etched for 50 min to obtain a spinel nanofiber composite graphite felt electrode material Co(OH)2 / Sr containing a heterogeneous structure. 0.25 Mn 0.75 Co2O4. Step S5, preparation of composite graphite felt electrode: disperse spinel nanofibers in a 5wt% perfluorosulfonic acid resin solution, immerse at 40°C for 10 hours to obtain an impregnation solution; immerse the pretreated graphite felt in the impregnation solution, and composite the spinel nanofibers onto the surface of the graphite felt by a physical impregnation method. After impregnation, dry the graphite felt in a 50°C oven to obtain a spinel nanofiber composite graphite felt electrode material.

[0041] Example 3: Using the following technical solution Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace at 400°C for heat treatment for 3 hours with a heating rate of 4.5°C / min. The treated graphite felt is washed with 2 mol / L hydrochloric acid and ultrapure water and dried in a 50°C oven.

[0042] Step S2, preparation of spinel precursor solution: ultrapure water and propanol are mixed in a volume ratio of 1:7, strontium nitrate, manganese nitrate and cobalt nitrate are added to obtain a mixed solution of 0.02 mol / L strontium nitrate, 0.02 mol / L manganese nitrate and 0.08 mol / L cobalt nitrate, and stirred in a water bath at 65°C, 0.18 mol / L ascorbic acid is added, and after stirring for 30 minutes, 3 mol / L potassium hydroxide is slowly added to adjust the pH value of the solution to 7. After continuous stirring for 1.5 hours, it is naturally cooled to room temperature to obtain a spinel precursor solution.

[0043] Step S3, preparation of spinel nanofibers: cellulose and dimethylacetamide are mixed in a volume ratio of 1:3 to obtain a polymer solution, the spinel precursor solution obtained in step S2 is poured into it, and the solution is continuously stirred at room temperature for 15 hours. The spinning solution is placed in a spinning machine after vacuum degassing treatment, and the spinning pump is set to a spinning rate of 12m / min. The spinning solution is extruded through the capillary pores of the spinneret and flows into isopropanol with a volume of 20 times that of cellulose. The solution diffuses and precipitates to form nascent fibers, which are naturally dried in air at room temperature. The obtained nascent fibers are placed in an air atmosphere and sintered at 700°C for 4 hours, with a heating rate of 5°C / min and a cooling rate of 4°C / min to obtain Sr 0.5 Mn 0.5 Co2O4.

[0044] Step S4, surface reconstruction: Sr 0.5 Mn 0.5 Co2O4 was placed in 0.5 mol / L diisobutylaluminum hydride and reduced and etched for 30 min to obtain a spinel nanofiber composite graphite felt electrode material containing a heterogeneous structure Co(OH)2 / Sr 0.5 Mn 0.5 Co2O4.

[0045] Step S5, preparation of composite graphite felt electrode: disperse spinel nanofibers in 5wt% perfluorosulfonic acid resin solution, immerse at 30°C for 15h to obtain an impregnation solution; immerse the pretreated graphite felt in the impregnation solution, and use a physical impregnation method to composite the spinel nanofibers on the surface of the graphite felt. After impregnation, place the mixture in a 50°C oven and dry it completely to obtain a spinel nanofiber composite graphite felt electrode material.

[0046] Example 4: Using the following technical solutions Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace at 450°C for heat treatment for 2.5 hours with a heating rate of 4°C / min. The treated graphite felt is washed with 2 mol / L hydrochloric acid and ultrapure water and dried in a 50°C oven.

[0047] Step S2, preparation of spinel precursor solution: ultrapure water and isopropanol are mixed in a volume ratio of 1:8, strontium nitrate, manganese nitrate and cobalt nitrate are added to obtain a mixed solution of 0.0375 mol / L strontium nitrate, 0.0125 mol / L manganese nitrate and 0.1 mol / L cobalt nitrate, and stirred in a water bath at 70°C. 0.27 mol / L propylene glycol is added and stirred for 30 minutes. Then, 4 mol / L ammonia water is slowly added to adjust the pH value of the solution to 7.5. After continuous stirring for 2 hours, the solution is naturally cooled to room temperature to obtain a spinel precursor solution.

[0048] Step S3, preparation of spinel nanofibers: polyvinyl alcohol and sodium bisulfate are mixed in a volume ratio of 1:3.5 to obtain a polymer solution, the spinel precursor solution obtained in step S2 is poured into it, and the solution is continuously stirred at room temperature for 20 hours. The spinning solution is placed in a spinning machine after vacuum degassing treatment, and the spinning pump is set to a spinning speed of 15m / min. The spinning solution is extruded through the capillary pores of the spinneret and flows into water 25 times the volume of polyvinyl alcohol, diffuses and precipitates to form nascent fibers, and naturally dries in air at room temperature. The obtained nascent fibers are placed in an air atmosphere and sintered at 800℃ for 3 hours, with a heating rate of 6℃ / min and a cooling rate of 5℃ / min to obtain Sr 0.75 Mn 0.25 Co2O4.

[0049] Step S4, surface reconstruction: Sr 0.75 Mn 0.25 Co2O4 was placed in 0.7 mol / L lithium borohydride and reduced and etched for 20 min to obtain a spinel nanofiber composite graphite felt electrode material Co(OH)2 / Sr containing a heterogeneous structure. 0.75 Mn 0.25 Co2O4.

[0050] Step S5, preparation of composite graphite felt electrode: disperse spinel nanofibers in 5wt% perfluorosulfonic acid resin solution, immerse at 25°C for 20h to obtain an impregnation solution; immerse the pretreated graphite felt in the impregnation solution, and use a physical impregnation method to composite the spinel nanofibers on the surface of the graphite felt. After impregnation, place the mixture in a 50°C oven and dry it completely to obtain a spinel nanofiber composite graphite felt electrode material.

[0051] Example 5: Using the following technical solution Step S1, graphite felt pretreatment: placing the graphite felt in an atmosphere furnace at 500°C for heat treatment for 2 hours with a heating rate of 3°C / min. The treated graphite felt is washed with 2 mol / L hydrochloric acid and ultrapure water and dried in a 50°C oven.

[0052] Step S2, preparation of spinel precursor solution: ultrapure water and ethylene glycol are blended in a volume ratio of 1:10, strontium nitrate, manganese nitrate and cobalt nitrate are added to obtain a mixed solution of 0.09 mol / L strontium nitrate, 0.01 mol / L manganese nitrate and 0.2 mol / L cobalt nitrate, and stirred in a water bath at 80°C, 0.6 mol / L alanine is added, stirred for 30 minutes, and then 4 mol / L ammonia water is slowly added to adjust the pH value of the solution to 8. After continuous stirring for 3 hours, it is naturally cooled to room temperature to obtain a spinel precursor solution.

[0053] Step S3, preparation of spinel nanofibers: polyvinyl chloride and nitric acid are mixed in a volume ratio of 1:4 to obtain a polymer solution, the spinel precursor solution obtained in step S2 is poured into it, and the solution is continuously stirred at room temperature for 24 hours. The spinning solution is placed in a spinning machine after vacuum degassing treatment, and the spinning pump is set to a spinning speed of 20m / min. The spinning solution is extruded through the capillary pores of the spinneret and flows into acetic acid with a volume of 30 times that of polyvinyl chloride. The solution diffuses and precipitates to form nascent fibers, which are naturally dried in air at room temperature. The obtained nascent fibers are placed in an air atmosphere and sintered at 900°C for 2 hours, with a heating rate of 8°C / min and a cooling rate of 6°C / min to obtain Sr 0.9 Mn 0.1 Co2O4.

[0054] Step S4, surface reconstruction: Sr 0.9 Mn 0.1 Co2O4 was placed in 1 mol / L zinc borohydride and reduced and etched for 10 min to obtain a spinel nanofiber composite graphite felt electrode material containing a heterogeneous structure Co(OH)2 / Sr 0.9 Mn 0.1 Co2O4.

[0055] Step S5, preparation of composite graphite felt electrode: disperse spinel nanofibers in 5wt% perfluorosulfonic acid resin solution, immerse at 20°C for 24h to obtain an impregnation solution; immerse the pretreated graphite felt in the impregnation solution, and use a physical impregnation method to composite the spinel nanofibers on the surface of the graphite felt. After impregnation, place it in a 50°C oven and dry it completely to obtain a spinel nanofiber composite graphite felt electrode material.

[0056] Comparative Example 1: Using the following technical solutions This comparative example is a blank control group, that is, untreated graphite felt is used as the electrode.

[0057] Comparative Example 2: Using the following technical solutions The preparation process of this comparative example differs from that of Example 1 only in that step S4 is not performed.

[0058] Comparative Example 3: Using the following technical solutions The preparation process of this comparative example differs from that of the embodiment only in that steps S3 and S4 are not performed, the precursor solution prepared in step S2 is dried in an oven at 70°C for 12 hours, and then sintered in an air atmosphere at 600°C for 6 hours with a heating rate of 2°C / min and a cooling rate of 2°C / min to obtain spherical Sr 0.1 Mn 0.9 Co2O4.

[0059] Comparative Example 4: Using the following technical solutions The preparation process of this comparative example differs from that of Example 1 only in that strontium nitrate is not added in step S1.

[0060] Comparative Example 5: Using the following technical solutions The preparation process of this comparative example differs from that of Example 1 only in that cobalt nitrate is not added in step S1.

[0061] Comparative Example 6: Using the following technical solution The preparation process of this comparative example differs from that of Example 1 only in that manganese nitrate is not added in step S1.

[0062] The graphite felts prepared in Examples 1-5 and Comparative Examples 1-4 were assembled into battery stacks and subjected to charge and discharge tests under the same test conditions. The battery coulombic efficiency, voltage efficiency, energy efficiency, and voltage efficiency after 500 cycles were recorded. The test results are shown in Table 1: Table 1 Battery charge and discharge test results

[0063] As shown in Table 1, Examples 1-5 have higher energy and voltage efficiencies than Comparative Example 1, indicating that the present invention effectively improves the reaction activity of the electrode. x Mn (1-x) Co2O4 and has a higher reaction active area, optimizes the adsorption of vanadium ions in the flow battery, catalyzes the redox ability of vanadium, reduces the reaction energy barrier, and improves the energy efficiency of the battery. Compared with Example 4, Examples 1-5 have higher energy and voltage efficiency, indicating that the doping amount of A-site metal Sr is regulated to improve the conductivity of the composite graphite felt electrode, reduce activation polarization and ohmic polarization, and improve the voltage efficiency and energy efficiency of the battery. Compared with Example 5, Examples 1-5 have higher energy and voltage efficiency, indicating that the introduction of B-site metal Co accelerates the reaction kinetics, catalyzes the redox ability of vanadium, reduces the reaction energy barrier, and improves the energy efficiency of the battery. Compared with Examples 1-2 and Comparative Examples 2-3, they have higher energy and voltage efficiency, indicating that the use of a reducing agent to reduce and etch the spinel surface causes the spinel surface to reconstruct a core-shell structure, which can increase the surface roughness of the nanofiber, thereby increasing the specific surface area and active sites, enhancing the interface interaction ability between the electrode surface and the electrolyte, thereby reducing the vanadium ion redox reaction energy barrier, improving the electrode's redox catalytic ability, and improving the efficiency of the battery. Compared with Comparative Examples 1 and 6, Examples 1-5 have higher voltage efficiency stability after 500 cycles, indicating that the introduction of metal Mn can make the spinel structure stable during battery operation, which is beneficial to the corrosion resistance and stability of the spinel material.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a spinel nanofiber composite graphite felt electrode, characterized in that: The steps include: Step S1: dispersing a metal salt in a mixed solvent and adding a chelating agent to react to obtain a spinel precursor solution; Step S2: mixing the spinel precursor solution with a polymer solution to obtain spinel nanofibers; Step S3: placing the spinel nanofibers in a reducing agent solution for reduction etching to obtain spinel nanofibers with a heterogeneous structure; Step S4: the heterogeneous structured spinel nanofibers are dispersed in a perfluorosulfonic acid resin solution to obtain an impregnation solution, and the pretreated graphite felt is placed in the impregnation solution for physical impregnation, and dried to obtain a spinel nanofiber composite graphite felt electrode.

2. The preparation method according to claim 1, characterized in that In the step S1: The volume ratio of water to organic solvent in the mixed solvent is 1:(3-10), wherein the organic solvent is at least one selected from anhydrous ethanol, acetone, propanol, isopropanol and ethylene glycol; The chelating agent is selected from at least one of citric acid, EDTA, ascorbic acid, glycerol and alanine; The metal salt is composed of strontium nitrate, manganese nitrate and cobalt nitrate, and the total concentration of metal ions of the metal salt dispersed in the mixed solvent is 0.03-0.3 mol / L; wherein the molar concentration ratio of the strontium nitrate, the manganese nitrate and the cobalt nitrate is x:(1-x):2, wherein 0.1≤x≤0.9; The molar concentration ratio of the total molar concentration of the metal ions in the metal salt to the molar concentration of the chelating agent is 1:(1.1-2).

3. The preparation method according to claim 1, characterized in that After the chelating agent is added for reaction in step S1, the preparation method further comprises: Stirring in a water bath and adjusting the pH value with an alkaline solution; wherein, The water bath stirring temperature is 50-80°C and the stirring time is 0.5-3h; The alkaline solution is selected from at least one of sodium hydroxide solution, potassium hydroxide solution and ammonia solution; The concentration of the alkaline solution is 1-4 mol / L; The pH value of the pH adjustment is 6-8.

4. The preparation method according to claim 1, characterized in that In step S2, the mixing process includes degassing, wet spinning, and sintering; wherein, The wet spinning process comprises: placing the spinning precursor solution obtained after degassing in a spinning machine, setting the spinning pump spinning speed to 5-20 m / min, extruding the spinning solution through the capillary pores of the spinneret into a coagulant, diffusing and precipitating to form nascent fibers, and then taking the solution out and drying it naturally in air at room temperature; The sintering process is as follows: calcining at 600-900° C. for 2-6 hours in an air atmosphere, with a heating rate of 2-8° C. / min and a cooling rate of 2-6° C. / min.

5. The preparation method according to claim 1, characterized in that In the step S2: The polymer in the polymer solution is selected from at least one of polyacrylonitrile, chitosan, cellulose, polyvinyl alcohol and polyvinyl chloride; The solvent in the polymer solution is selected from at least one of dimethyl sulfoxide, NN-dimethylformamide, dimethylacetamide, sodium thiocyanate and nitric acid.

6. The preparation method according to claim 4, characterized in that The coagulant is selected from at least one of isobutanol, ethanol, isopropanol, water and acetic acid; The volume ratio of the polymer in the polymer solution, the solvent in the polymer solution and the coagulant is 1:(2-4):(10-30).

7. The preparation method according to claim 1, characterized in that In the step S3: The reduction etching conditions are as follows: reacting in a 0.1-1 mol / L reducing agent solution for 10-60 min; The reducing agent solution is selected from at least one of a sodium borohydride solution, a lithium aluminum hydride solution, a diisobutylaluminum hydride solution, a lithium borohydride solution and a zinc borohydride solution.

8. The preparation method according to claim 1, characterized in that In the step S4: The mass fraction of the perfluorosulfonic acid resin solution is 5%; The physical immersion time is 6 to 24 hours, and the temperature is 20 to 60°C.

9. The preparation method according to claim 1, characterized in that In the step S4: The pretreated graphite felt is obtained by placing the graphite felt in an atmosphere furnace for heat treatment, then washing the heat-treated graphite felt with dilute hydrochloric acid and ultrapure water, and drying; wherein, The heat treatment process is as follows: in a nitrogen atmosphere, keep the temperature at 350-500°C for 2-4 hours, and increase the temperature at a rate of 3-6°C / min; The concentration of the dilute hydrochloric acid is 2 mol / L; The drying condition is oven drying at 50°C.

10. A spinel nanofiber composite graphite felt electrode, characterized in that: The spinel nanofiber composite graphite felt electrode is prepared by the preparation method of any one of claims 1 to 9.

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