Preparation method of hollow carbon spheres embedded with titanium dioxide, electrode catalytic material and application of electrode catalytic material

Abstract

The invention discloses a preparation method of hollow carbon spheres embedded with titanium dioxide, an electrode catalytic material and application of the electrode catalytic material, and belongs to the technical field of all-vanadium redox flow batteries. According to the preparation method, silicon dioxide is taken as a template, phenolic resin is taken as a carbon source to uniformly coat the silicon dioxide, high-temperature carbonization is carried out in an inert gas atmosphere, and hollow carbon spheres are obtained through a chemical etching method; the preparation method comprises the following steps: introducing a hydrolyzable titanium source into a hollow carbon sphere, washing to remove titanium on the surface of the hollow carbon sphere, washing and hydrolyzing titanium in the hollow carbon sphere, and repeatedly washing and drying to obtain a hollow carbon sphere embedded with amorphous titanium dioxide; and performing high-temperature treatment on the embedded amorphous titanium dioxide hollow carbon spheres in an inert gas atmosphere, and regulating and controlling the temperature to obtain the titanium dioxide embedded hollow carbon spheres with different crystal forms. The material is used as an electrode catalytic material for a mixed acid system all-vanadium redox flow battery, reversible oxidation reduction of Cl <-> / Cl2 in a positive electrode electrolyte is realized, chlorine is adsorbed and prevented from being volatilized into air, and the safety and stability of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of all-vanadium redox flow batteries, and particularly relates to a preparation method of hollow carbon spheres embedded with titanium dioxide, an electrode catalytic material, and their applications. Background Art

[0002] At present, with the increasing popularity of environmental protection concepts, the full development and utilization of green energy such as wind energy and solar energy have become the focus of attention from all sectors of society. However, such energy is easily affected by natural environments and geographical conditions, is unevenly distributed in time and space and difficult to accurately predict. If directly incorporated into the power grid, it will pose a huge hidden danger to the safe and stable operation of the power grid. The continuously developing energy storage system is precisely the key to unlocking this huge energy treasure house of renewable energy. Among many energy storage batteries, all-vanadium redox flow batteries have stood out with their unique advantages such as being safe and non-explosive, having an ultra-long cycle life, being environmentally friendly, having flexible assembly, and having a fast response speed, and their share in the large-scale energy storage market has been continuously increasing.

[0003] In recent years, hydrochloric acid and sulfuric acid as the supporting electrolyte for all-vanadium redox flow battery energy storage media have gradually replaced the pure sulfuric acid system. This system has the advantages of high power density and good energy efficiency. However, the hydrochloric acid concentration in the mixed acid system is usually higher than 5 mol / L. The introduction of high-concentration hydrochloric acid makes this system face a huge risk of chlorine evolution. Chlorine is highly toxic and has strong corrosive properties on battery devices. More importantly, when the chlorine reaches a certain concentration, it may cause an explosion, which is a fatal defect for large-scale energy storage. There have been several reports of explosions in commercial mixed acid system all-vanadium redox flow batteries.

[0004] However, there is still no reliable and effective solution to the problem of chlorine evolution at the positive electrode of the mixed acid system all-vanadium redox flow battery at present. Summary of the Invention

[0005] Aiming at the lack of an effective solution to the problem of chlorine evolution at the positive electrode of the mixed acid system all-vanadium redox flow battery in the current existing technology, the object of the present invention is a preparation method of hollow carbon spheres embedded with titanium dioxide. Hollow carbon spheres embedded with titanium dioxide of different crystal forms are prepared and can be used as electrode catalytic materials and applied to the mixed acid system all-vanadium redox flow battery. Titanium dioxide is used to adsorb the evolved chlorine and store it in the hollow carbon spheres, preventing it from diffusing into the air; at the same time, the reversibility of the Cl - / Cl2 redox reaction is increased, avoiding the accumulation of chlorine, thereby greatly reducing the harm of chlorine.

[0006] The object of the present invention is achieved through the following technical solutions: The present invention provides a method for preparing hollow carbon spheres embedded with titanium dioxide. Using silica as a template, phenolic resin as a carbon source to uniformly coat silica, after high-temperature carbonization in an inert gas atmosphere, hollow carbon spheres are obtained by chemical etching; a hydrolyzable titanium source is introduced into the hollow carbon spheres, the titanium on the surface of the hollow carbon spheres is removed by washing, and then the titanium inside the hollow carbon spheres is washed and hydrolyzed. After repeated washing and drying, hollow carbon spheres embedded with amorphous titanium dioxide are obtained; in an inert gas atmosphere, the hollow carbon spheres embedded with amorphous titanium dioxide are subjected to high-temperature treatment, and hollow carbon spheres embedded with titanium dioxide of different crystal forms are obtained by controlling the temperature.

[0007] Further, using silica as a template and phenolic resin as a carbon source to uniformly coat silica is achieved by synthesizing silica through the hydrolysis and condensation reaction of tetraethyl orthosilicate, gradually adding formaldehyde and resorcinol under stirring, continuously stirring, transferring the formed mixture to a hydrothermal reaction kettle for hydrothermal treatment, filtering, and drying to collect the mixture.

[0008] Further, the temperature of high-temperature carbonization is 700°C - 900°C, and the time is 1h - 4h.

[0009] Further, the chemical etching method uses hydrofluoric acid with an analytical pure mass concentration ≥ 40%.

[0010] Further, the hydrolyzable titanium source introduced into the hollow carbon spheres is titanium sulfate, and continuous stirring is carried out until uniform.

[0011] Furthermore, the mass ratio of the hollow carbon spheres to titanium sulfate is 1:(1.5 - 1.7).

[0012] Further, the titanium on the surface of the hollow carbon spheres is removed by washing using absolute ethanol, and the mass-volume ratio of the hollow carbon spheres to the absolute ethanol used for washing and removing titanium is 1:(500 - 700) g / mL; The titanium inside the hollow carbon spheres is washed and hydrolyzed using distilled water or deionized water, and the mass-volume ratio of the hollow carbon spheres to the water used for washing and hydrolysis is 1:(400 - 800) g / mL.

[0013] Further, the temperature for high-temperature treatment of the hollow carbon spheres embedded with amorphous titanium dioxide is 500°C - 900°C, and the time is 1h - 4h.

[0014] The present invention also provides an electrode catalytic material, using the hollow carbon spheres embedded with titanium dioxide of different crystal forms prepared by the method for preparing the hollow carbon spheres embedded with titanium dioxide as the electrode catalytic material.

[0015] The present invention also provides an application of the electrode catalytic material. The hollow carbon spheres embedded with titanium dioxide of different crystal forms are used as the electrode catalytic material and sprayed on a carbon felt to be used as the positive and negative electrodes of a mixed acid system all-vanadium redox flow battery.

[0016] Advantages and effects of the present invention: The hollow carbon spheres embedded with titanium dioxide of different crystal forms prepared by the present invention are used as electrode catalytic materials in a mixed acid system all-vanadium redox flow battery, which can realize the reversible oxidation and reduction of Cl - / Cl2 in the positive electrolyte of the battery, and at the same time adsorb chlorine gas to prevent it from volatilizing into the air, greatly reducing the risk brought by the evolution of highly toxic and explosive chlorine gas and improving the safety and stability of the battery; The electrode catalytic material of the present invention can improve the reaction activity of the active substance vanadium ions in the positive and negative electrolytes of the all-vanadium redox flow battery, and improve the battery performance. Description of the drawings

[0017] Figure 1 Scanning electron microscope (SEM) image of the hollow carbon spheres prepared in Example 1; Figure 2 Transmission electron microscope (TEM) image of the hollow carbon spheres prepared in Example 1; Figure 3 Scanning electron microscope (SEM) image of the hollow carbon spheres embedded with amorphous titanium dioxide prepared in Example 1; Figure 4 Transmission electron microscope (TEM) image of the hollow carbon spheres embedded with amorphous titanium dioxide prepared in Example 1; Figure 5 X-ray diffraction (XRD) pattern of the hollow carbon spheres embedded with amorphous titanium dioxide prepared in Example 1; Figure 6 Scanning electron microscope (SEM) image of the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide prepared in Example 1; Figure 7 Transmission electron microscope (TEM) image of the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide prepared in Example 1; Figure 8 X-ray diffraction (XRD) pattern of the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide prepared in Example 1; Figure 9 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide prepared in Example 1 in a blank mixed acid system containing only sulfuric acid and hydrochloric acid, where: (a) is the hollow carbon, and (b) is the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide; Figure 10 Positive cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide prepared in Example 1; Figure 11Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase / rutile phase titanium dioxide prepared in Example 1 at the negative electrode; Figure 12 Battery performance test diagrams of the hollow carbon spheres, the hollow carbon spheres embedded with amorphous titanium dioxide, and the hollow carbon spheres embedded with anatase / rutile phase titanium dioxide prepared in Example 1 as electrode catalytic materials in a mixed acid vanadium redox flow battery system, where: (a) is the blank control group, (b) is the hollow carbon spheres, (c) is the hollow carbon spheres embedded with amorphous titanium dioxide, and (d) is the hollow carbon spheres embedded with anatase / rutile phase titanium dioxide; Figure 13 Scanning electron microscopy (SEM) image of the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2; Figure 14 Transmission electron microscopy (TEM) image of the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2; Figure 15 X-ray diffraction (XRD) pattern of the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2; Figure 16 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2 in a blank mixed acid system containing only sulfuric acid and hydrochloric acid; Figure 17 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2 at the positive electrode; Figure 18 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2 at the negative electrode; Figure 19 Battery performance test diagram of the hollow carbon spheres embedded with anatase phase titanium dioxide prepared in Example 2 as an electrode catalytic material in a mixed acid vanadium redox flow battery system; Figure 20 Scanning electron microscopy (SEM) image of the hollow carbon spheres embedded with rutile phase titanium dioxide prepared in Example 3; Figure 21 Transmission electron microscopy (TEM) image of the hollow carbon spheres embedded with rutile phase titanium dioxide prepared in Example 3; Figure 22 X-ray diffraction (XRD) pattern of the hollow carbon spheres embedded with rutile phase titanium dioxide prepared in Example 3; Figure 23 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with rutile phase titanium dioxide prepared in Example 3 in a blank mixed acid system containing only sulfuric acid and hydrochloric acid; Figure 24 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with rutile titanium dioxide prepared in Example 3 for the positive electrode; Figure 25 Cyclic voltammetry (CV) curves of the hollow carbon spheres and the hollow carbon spheres embedded with rutile titanium dioxide prepared in Example 3 for the negative electrode; Figure 26 Battery performance test chart of the hollow carbon spheres embedded with rutile titanium dioxide prepared in Example 3 as an electrode catalytic material in a mixed acid vanadium redox flow battery system. Detailed implementation mode

[0018] The present invention will be described in detail below with reference to the embodiments.

[0019] Example 1 A method for preparing hollow carbon spheres embedded with titanium dioxide of the present invention. Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h to uniformly coat the phenolic resin on the silica; filter and dry the mixture, and collect it. In an argon atmosphere, carbonize at 900 °C for 2 h, with a heating rate of 5 °C / min. Treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry it, as Figure 1 and 2 shown to obtain hollow carbon spheres; Add 48 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h. Then, add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; then first wash with 15 mL of anhydrous ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then wash with 12 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. Wash the obtained product repeatedly and dry it in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide, as Figure 3 and 4 shown. The morphology of the hollow carbon spheres can be seen, as Figure 5 shown. The XRD diffraction pattern does not show the characteristic peaks of the crystalline form of titanium dioxide. Place the prepared hollow carbon spheres embedded with amorphous titanium dioxide in an alumina crucible, and then heat it in a tubular furnace at a heating rate of 5 °C / min to 700 °C in an argon atmosphere for 3 h to obtain hollow carbon spheres embedded with anatase / rutile titanium dioxide, as Figure 6 and 7 shown. The morphology of the hollow carbon spheres can be seen, as Figure 8As shown, the XRD diffraction pattern shows the characteristic peaks of two titanium dioxide crystal forms, one is anatase PDF #21-1272 and the other is rutile PDF #21-1276.

[0020] An electrode catalytic material of the present invention uses the anatase / rutile crystal form titanium dioxide hollow carbon spheres prepared in Example 1.

[0021] An electrode catalytic material of Example 1, the anatase / rutile crystal form titanium dioxide hollow carbon spheres, is applied to a mixed acid system all-vanadium redox flow battery.

[0022] Performance test: (1)Cl - Reversibility analysis of the Cl / Cl2 redox reaction: Cyclic voltammetry test conditions: A three-electrode system is adopted: the working electrode is a glassy carbon electrode (GCE, with a diameter of 3 mm), the counter electrode is a graphite rod, and the reference electrode is a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material is coated on the working electrode, and then the working electrode, counter electrode, and reference electrode are respectively installed in a three-necked electrolytic cell, and relevant electrochemical tests are carried out in a blank mixed acid system containing only sulfuric acid and hydrochloric acid.

[0023] As Figure 9 (a) shows, only an oxidation peak exists when the hollow carbon spheres are used as the electrode catalytic material, while as Figure 9 (b) shows, obvious reduction peaks are generated when the anatase / rutile crystal form titanium dioxide hollow carbon spheres are used as the electrode catalytic material, indicating effective inhibition of the precipitation of Cl2 and promotion of the reversible reaction of Cl2 to Cl - .

[0024] (2)Catalytic performance analysis: Cyclic voltammetry test conditions: A three-electrode system is adopted: the working electrode is a glassy carbon electrode (GCE, with a diameter of 3 mm), the counter electrode is a graphite rod, and the reference electrode is a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material is coated on the working electrode, and then the working electrode, counter electrode, and reference electrode are respectively installed in a three-necked electrolytic cell, and relevant electrochemical tests are carried out respectively in the positive electrode electrolyte of 2M VO 2+ , 2M SO4 2- and 5M Cl - and the negative electrode electrolyte of 2M V 3+ , 2M SO4 2- and 5M Cl - .

[0025] As Figure 10The cyclic voltammetry curve of the positive electrode is shown below. The solid line represents the hollow carbon spheres, and the dashed line represents the hollow carbon spheres embedded with anatase / rutile titanium dioxide. It can be seen that the hollow carbon spheres embedded with anatase / rutile titanium dioxide have a larger oxidation-reduction peak current, indicating that the electrochemical activity of the hollow carbon spheres increases after the embedding of anatase / rutile titanium dioxide, and anatase / rutile titanium dioxide plays a role in it.

[0026] As Figure 11 shown in the cyclic voltammetry curve of the negative electrode, the solid line represents the hollow carbon spheres, and the dashed line represents the hollow carbon spheres embedded with anatase / rutile titanium dioxide. It can be seen that the hollow carbon spheres embedded with anatase / rutile titanium dioxide have a larger oxidation-reduction peak current, indicating that the electrochemical activity of the hollow carbon spheres increases after the embedding of anatase / rutile titanium dioxide, and anatase / rutile titanium dioxide plays a role in it.

[0027] (3) Battery performance analysis: Test conditions for the all-vanadium redox flow battery in a mixed acid system: The positive and negative electrodes are carbon felt electrodes with an area of 800 cm 2 . Both the positive and negative electrodes use the electrolyte of the mixed acid all-vanadium redox flow battery. The electrolyte mainly contains 1M V 3+ , 1M VO 2+ , 2M SO4 2- and 5M Cl - ; Hollow carbon spheres, hollow carbon spheres embedded with amorphous titanium dioxide, and hollow carbon spheres embedded with anatase / rutile titanium dioxide are respectively sprayed on the carbon felt electrodes as electrode catalytic materials; Charge-discharge cycling is carried out at a current density of 80 mA / cm 2 . The charging cut-off condition is that the voltage does not exceed 1.6V, and the discharging cut-off condition is that the voltage is not lower than 0.1V.

[0028] As Figure 12 (a) shows, for the blank control group without using an electrode catalytic material, the Coulomb efficiency is 94.67%, the voltage efficiency is 87.49%, and the energy efficiency is 82.83%; As Figure 12 (b) shows, when using hollow carbon spheres as the electrode catalytic material for the all-vanadium redox flow battery in a mixed acid system, the Coulomb efficiency is 95.88%, the voltage efficiency is 87.23%, and the energy efficiency is 83.64%; As Figure 12 (c) shows, when using hollow carbon spheres embedded with amorphous titanium dioxide as the electrode catalytic material for the all-vanadium redox flow battery in a mixed acid system, the Coulomb efficiency of the battery is 96.47%, the voltage efficiency is 87.41%, and the energy efficiency is 84.33%; As Figure 12As shown in (d), anatase / rutile TiO₂-embedded hollow carbon spheres are used as the electrode catalytic material for a mixed-acid system vanadium redox flow battery. The Coulombic efficiency of the battery is 97.44%, the voltage efficiency is 89.52%, and the energy efficiency is 87.23%, which are 2.93%, 2.32%, and 5.31% higher than those of the blank control group, 1.63%, 2.63%, and 4.29% higher than those of the hollow carbon spheres, and 1.01%, 2.41%, and 3.44% higher than those of the amorphous TiO₂-embedded hollow carbon spheres, respectively.

[0029] Example 2 A method for preparing TiO₂-embedded hollow carbon spheres of the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h to uniformly coat phenolic resin on silica; filter and dry to collect the mixture, carbonize it at 900 °C for 2 h in an argon atmosphere with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and deionized water and dry it to obtain hollow carbon spheres; Add 48 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h, then add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; then first wash with 18 mL of absolute ethanol to remove titanium sulfate on the surface of the hollow carbon spheres, and then wash with 18 mL of deionized water to hydrolyze titanium sulfate inside the hollow carbon spheres, wash the obtained product repeatedly, and dry it in a vacuum oven at 60 °C for 6 h to obtain amorphous TiO₂-embedded hollow carbon spheres; Place the prepared amorphous TiO₂-embedded hollow carbon spheres in an alumina crucible, and then heat them in a tubular furnace at a heating rate of 5 °C / min to 500 °C in an argon atmosphere for 4 h to obtain anatase TiO₂-embedded hollow carbon spheres, as Figure 13 and 14 shown. The morphology of the hollow carbon spheres can be seen, as Figure 15 shown. The XRD diffraction pattern shows the characteristic peaks of anatase TiO₂ crystal form (PDF #21-1272).

[0030] An electrode catalytic material of the present invention uses the anatase TiO₂-embedded hollow carbon spheres prepared in Example 2.

[0031] Apply an electrode catalytic material of Example 2, anatase TiO₂-embedded hollow carbon spheres, to a mixed-acid system vanadium redox flow battery.

[0032] Performance test: (1)Analysis of the reversibility of the Cl - / Cl2 redox reaction: Cyclic voltammetry test conditions: A three - electrode system was adopted: the working electrode was a glassy carbon electrode (GCE, 3 mm in diameter), the counter electrode was a graphite rod, and the reference electrode was a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material was coated on the working electrode, and then the working electrode, counter electrode, and reference electrode were respectively installed in a three - necked electrolytic cell, and relevant electrochemical tests were carried out in a blank mixed acid system containing only sulfuric acid and hydrochloric acid.

[0033] As Figure 16 shown, the embedded anatase - type titanium dioxide hollow carbon spheres as the electrode material produced an obvious reduction peak, indicating effective inhibition of the evolution of Cl2 and promotion of the reversible reaction of Cl2 to Cl - .

[0034] (2)Analysis of catalytic performance: Cyclic voltammetry test conditions: A three - electrode system was adopted: the working electrode was a glassy carbon electrode (GCE, 3 mm in diameter), the counter electrode was a graphite rod, and the reference electrode was a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material was coated on the working electrode, and then the working electrode, counter electrode, and reference electrode were respectively installed in a three - necked electrolytic cell, and relevant electrochemical tests were carried out respectively in the positive electrolyte of 2M VO 2+ , 2M SO4 2- and 5M Cl - and in the negative electrolyte of 2M V 3+ , 2M SO4 2- and 5M Cl - .

[0035] As Figure 17 shown in the positive cyclic voltammogram curve, the solid line is for the hollow carbon spheres and the dashed line is for the embedded anatase - type titanium dioxide hollow carbon spheres. It can be seen that the embedded anatase - type titanium dioxide hollow carbon spheres have a larger oxidation - reduction peak current, indicating that the electrochemical activity of the hollow carbon spheres increases after embedding anatase - type titanium dioxide, and anatase - type titanium dioxide plays a role in it.

[0036] As Figure 18 shown in the negative cyclic voltammogram curve, the solid line is for the hollow carbon spheres and the dashed line is for the embedded anatase - type titanium dioxide hollow carbon spheres. It can be seen that the embedded anatase - type titanium dioxide hollow carbon spheres have a larger oxidation - reduction peak current, indicating that the electrochemical activity of the hollow carbon spheres increases after embedding anatase - type titanium dioxide, and anatase - type titanium dioxide plays a role in it.

[0037] (3) Battery performance analysis: Test conditions for the all-vanadium redox flow battery in the mixed acid system: The positive and negative electrodes are carbon felt electrodes with an area of 800 cm 2 , and the positive and negative electrodes both use the electrolyte of the mixed acid all-vanadium redox flow battery. The electrolyte mainly contains 1M V 3+ , 1M VO 2+ , 2M SO4 2- , and 5M Cl - ; Hollow carbon spheres and hollow carbon spheres embedded with anatase-type titanium dioxide are respectively sprayed on the carbon felt electrodes as electrode catalytic materials; The charge-discharge cycle is carried out at a current density of 80 mA / cm 2 . The charging cut-off condition is that the voltage does not exceed 1.6V, and the discharging cut-off condition is that the voltage does not fall below 0.1V.

[0038] As Figure 19 shown, when using hollow carbon spheres embedded with anatase-type titanium dioxide as the electrode catalytic material for the all-vanadium redox flow battery in the mixed acid system, the Coulomb efficiency of the battery is 96.84%, the voltage efficiency is 89.37%, and the energy efficiency is 86.55%; It is increased by 2.29%, 2.15% and 4.49% respectively compared with the blank control group, increased by 1.00%, 2.45% and 3.48% respectively compared with the hollow carbon spheres, and increased by 0.38%, 2.24% and 2.63% respectively compared with the hollow carbon spheres embedded with non-fixed titanium dioxide.

[0039] Example 3 A preparation method of different embedded crystal forms of titanium dioxide hollow carbon spheres of the present invention. Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution. After stirring for 1 h, gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring and stir for 12 h; Then transfer the obtained mixture to a hydrothermal reaction kettle and carry out hydrothermal treatment at 120 °C for 12 h to uniformly coat the phenolic resin on the silica; Filter and dry, then collect the mixture. In an argon atmosphere, carbonize at 900 °C for 2 h, and the heating rate is 5 °C / min. Treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry it to obtain hollow carbon spheres; 48 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres were added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 21 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 24 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 900 °C in an argon atmosphere for 1 h to obtain hollow carbon spheres embedded with rutile-type titanium dioxide, as Figure 20 and 21 shown. It can be seen the morphology of the hollow carbon spheres, as Figure 22 shown. The XRD diffraction pattern shows the characteristic peaks of rutile titanium dioxide crystal form (PDF #21-1276).

[0040] An electrode catalytic material of the present invention uses the hollow carbon spheres embedded with rutile-type titanium dioxide prepared in Example 3.

[0041] The electrode catalytic material of Example 3, the hollow carbon spheres embedded with rutile-type titanium dioxide, was applied to a mixed acid system all-vanadium redox flow battery.

[0042] Performance test: (1) Reversibility analysis of the Cl - / Cl2 redox reaction: Cyclic voltammetry test conditions: A three-electrode system was adopted: the working electrode was a glassy carbon electrode (GCE, diameter 3 mm), the counter electrode was a graphite rod, and the reference electrode was a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material was coated on the working electrode, and then the working electrode, counter electrode, and reference electrode were respectively installed in a three-necked electrolytic cell, and relevant electrochemical tests were carried out in a blank mixed acid system containing only sulfuric acid and hydrochloric acid.

[0043] As Figure 23 shown, the hollow carbon spheres embedded with rutile-type titanium dioxide as the electrode material produced an obvious reduction peak, representing effective inhibition of the precipitation of Cl2 and promotion of the reversible reaction of Cl2 to Cl - .

[0044] (2) Catalytic performance analysis: Cyclic voltammetry test conditions: A three - electrode system was adopted: the working electrode was a glassy carbon electrode (GCE, with a diameter of 3 mm), the counter electrode was a graphite rod, and the reference electrode was a saturated calomel electrode (SCE, 0.24 V vs. SHE); the electrode catalytic material was coated on the working electrode, and then the working electrode, counter electrode, and reference electrode were respectively installed in a three - necked electrolytic cell, and relevant electrochemical tests were carried out under the positive electrolyte of 2M VO 2+ , 2M SO4 2- and 5M Cl - as well as the negative electrolyte of 2M V 3+ , 2M SO4 2- and 5M Cl - .

[0045] As Figure 24 shown in the positive cyclic voltammogram curve, the solid line is for the hollow carbon spheres, and the dashed line is for the hollow carbon spheres embedded with rutile - type titanium dioxide. It can be seen that the hollow carbon spheres embedded with rutile - type titanium dioxide have larger oxidation - reduction peak currents, indicating that the electrochemical activity of the hollow carbon spheres increases after embedding rutile - type titanium dioxide, and rutile - type titanium dioxide plays a role in it.

[0046] As Figure 25 shown in the negative cyclic voltammogram curve, the solid line is for the hollow carbon spheres, and the dashed line is for the hollow carbon spheres embedded with rutile - type titanium dioxide. It can be seen that the hollow carbon spheres embedded with rutile - type titanium dioxide have larger oxidation - reduction peak currents, indicating that the electrochemical activity of the hollow carbon spheres increases after embedding rutile - type titanium dioxide, and rutile - type titanium dioxide plays a role in it.

[0047] (3) Battery performance analysis: Test conditions for the mixed - acid system all - vanadium redox flow battery: the positive and negative electrodes were carbon felt electrodes with an area of 800 cm 2 . The positive and negative electrodes both used the electrolyte of the mixed - acid all - vanadium redox flow battery, and the electrolyte mainly contained 1M V 3+ , 1M VO 2+ , 2M SO4 2- and 5M Cl - ; hollow carbon spheres and hollow carbon spheres embedded with rutile - type titanium dioxide were respectively sprayed on the carbon felt electrodes as electrode catalytic materials; charge - discharge cycling was carried out at a current density of 80 mA / cm 2 . The charge cut - off condition was that the voltage was not higher than 1.6 V, and the discharge cut - off condition was that the voltage was not lower than 0.1 V.

[0048] As Figure 26As shown, the rutile-type titanium dioxide-embedded hollow carbon spheres are used as the electrode catalytic material for the mixed acid system vanadium redox flow battery. The Coulombic efficiency of the battery is 96.78%, the voltage efficiency is 89.34%, and the energy efficiency is 86.47%. These are increased by 2.23%, 2.11% and 4.39% respectively compared with the blank control group, by 0.94%, 2.42% and 3.38% respectively compared with the hollow carbon spheres, and by 0.32%, 2.21% and 2.54% respectively compared with the amorphous titanium dioxide-embedded hollow carbon spheres.

[0049] Example 4 A preparation method of different embedded crystal form titanium dioxide hollow carbon spheres of the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, after stirring for 1 h, gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and carry out hydrothermal treatment at 120 °C for 12 h; filter and dry to collect the mixture, in an argon atmosphere, carbonize at 900 °C for 1 h, with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash with ethanol and distilled water several times and dry. Add 48 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h, then add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; first wash with 15 mL of anhydrous ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, then wash with 21 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres, wash the obtained product repeatedly, and dry in a vacuum oven at 60 °C for 6 h to obtain amorphous titanium dioxide-embedded hollow carbon spheres. Place the prepared amorphous titanium dioxide-embedded hollow carbon spheres in an alumina crucible, and then heat in a tubular furnace at a heating rate of 5 °C / min to 700 °C in an argon atmosphere for 1 h to obtain anatase / rutile-type titanium dioxide-embedded hollow carbon spheres.

[0050] Example 5 A preparation method of different embedded crystal form titanium dioxide hollow carbon spheres of the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, after stirring for 1 h, gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and carry out hydrothermal treatment at 120 °C for 12 h; filter and dry to collect the mixture, in an argon atmosphere, carbonize at 800 °C for 3 h, with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash with ethanol and distilled water several times and dry. 48 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 15 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 24 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 700 °C in an argon atmosphere for 2 h to obtain hollow carbon spheres embedded with anatase / rutile crystalline titanium dioxide.

[0051] Example 6 A preparation method of different hollow carbon spheres embedded with crystalline titanium dioxide according to the present invention. 2.5 mL of tetraethyl orthosilicate was added to 45 mL of ethanol and stirred for 3 min. Then, 40 mL of water and 5 mL of ammonia water solution were added. After stirring for 1 h, 0.6 g of resorcinol and 0.8 mL of formaldehyde were gradually added under stirring and stirred for 12 h. Then, the obtained mixture was transferred to a hydrothermal reaction kettle and hydrothermally treated at 120 °C for 12 h. The mixture was collected after filtration and drying, carbonized at 700 °C for 4 h in an argon atmosphere at a heating rate of 5 °C / min. The product was treated with hydrofluoric acid solution for 20 h to remove the silica template, and then washed with ethanol and distilled water several times and dried to obtain hollow carbon spheres. 48 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 15 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 12 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 500 °C in an argon atmosphere for 1 h to obtain hollow carbon spheres embedded with anatase crystalline titanium dioxide.

[0052] Example 7 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention. Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and carry out hydrothermal treatment at 120 °C for 12 h; filter and dry, collect the mixture, carbonize it at 900 °C for 3 h in a nitrogen atmosphere with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry to obtain hollow carbon spheres; Add 48 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h. Then, add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; then first wash with 18 mL of anhydrous ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then wash with 15 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. Wash the obtained product repeatedly and dry it in a vacuum oven at 60 °C for 6 h to obtain embedded amorphous titanium dioxide hollow carbon spheres; place the prepared embedded amorphous titanium dioxide hollow carbon spheres in an alumina crucible, and then heat them in a tubular furnace at a heating rate of 5 °C / min to 500 °C in a nitrogen atmosphere for 2 h to obtain embedded rutile crystalline titanium dioxide hollow carbon spheres.

[0053] Example 8 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention. Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and carry out hydrothermal treatment at 120 °C for 12 h; filter and dry, collect the mixture, carbonize it at 800 °C for 2 h in a helium atmosphere with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry to obtain hollow carbon spheres; 48 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 21 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 18 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain amorphous titanium dioxide-embedded hollow carbon spheres. The prepared amorphous titanium dioxide-embedded hollow carbon spheres were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 900 °C in a helium atmosphere for 1 h to obtain rutile-type titanium dioxide-embedded hollow carbon spheres.

[0054] Example 9 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention. 2.5 mL of tetraethyl orthosilicate was added to 45 mL of ethanol and stirred for 3 min. Then, 40 mL of water and 5 mL of ammonia water solution were added, and after stirring for 1 h, 0.6 g of resorcinol and 0.8 mL of formaldehyde were gradually added under stirring and stirred for 12 h. Then, the obtained mixture was transferred to a hydrothermal reaction kettle and hydrothermally treated at 120 °C for 12 h. The mixture was collected after filtration and drying, carbonized at 800 °C for 3 h in an argon atmosphere with a heating rate of 5 °C / min, and the product was treated with hydrofluoric acid solution for 20 h to remove the silica template, and then washed with ethanol and distilled water several times and dried to obtain hollow carbon spheres. 48 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 15 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 21 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain amorphous titanium dioxide-embedded hollow carbon spheres. The prepared amorphous titanium dioxide-embedded hollow carbon spheres were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 900 °C in an argon atmosphere for 2 h to obtain rutile-type titanium dioxide-embedded hollow carbon spheres.

[0055] Example 10 Preparation method of titanium dioxide hollow carbon spheres with different embedded crystal forms according to the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h; filter and dry, collect the mixture, carbonize in an argon atmosphere at 800 °C for 4 h, with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash with ethanol and distilled water several times and dry to obtain hollow carbon spheres; Add 45 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h, then add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; then first wash with 15 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then wash with 12 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. Wash the obtained product repeatedly and dry in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. Place the prepared hollow carbon spheres embedded with amorphous titanium dioxide in an alumina crucible, and then heat in a tubular furnace at a heating rate of 5 °C / min to 700 °C in an argon atmosphere for 4 h to obtain hollow carbon spheres embedded with anatase / rutile crystal form titanium dioxide.

[0056] Example 11 Preparation method of titanium dioxide hollow carbon spheres with different embedded crystal forms according to the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h; filter and dry, collect the mixture, carbonize in an argon atmosphere at 700 °C for 3 h, with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash with ethanol and distilled water several times and dry to obtain hollow carbon spheres; 51 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres were added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 18 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 12 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 700 °C in an argon atmosphere for 4 h to obtain hollow carbon spheres embedded with anatase / rutile crystalline titanium dioxide.

[0057] Example 12 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention. 2.5 mL of tetraethyl orthosilicate was added to 45 mL of ethanol and stirred for 3 min. Then, 40 mL of water and 5 mL of ammonia water solution were added, and after stirring for 1 h, 0.6 g of resorcinol and 0.8 mL of formaldehyde were gradually added under stirring and stirred for 12 h. Then, the obtained mixture was transferred to a hydrothermal reaction kettle and hydrothermally treated at 120 °C for 12 h. The mixture was collected after filtration and drying, carbonized at 700 °C for 2 h in an argon atmosphere at a heating rate of 5 °C / min. The product was treated with a hydrofluoric acid solution for 20 h to remove the silica template, and then washed with ethanol and distilled water several times and dried to obtain hollow carbon spheres. 45 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres were added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 18 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 18 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 500 °C in an argon atmosphere for 4 h to obtain hollow carbon spheres embedded with anatase crystalline titanium dioxide.

[0058] Example 13 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; Then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h; Filter and dry, then collect the mixture, carbonize it at 900 °C for 2 h in an argon atmosphere with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry to obtain hollow carbon spheres; Add 51 mg of titanium sulfate to 20 mL of sulfuric acid (3 mol / L) solution and stir for 1 h. Then, add 30 mg of the prepared hollow carbon spheres to the solution and continuously stir at room temperature for 12 h; First, wash with 21 mL of absolute ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then wash with 18 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. Wash the obtained product repeatedly and dry it in a vacuum oven at 60 °C for 6 h to obtain embedded amorphous titanium dioxide hollow carbon spheres; Place the prepared embedded amorphous titanium dioxide hollow carbon spheres in an alumina crucible, and then heat them in a tubular furnace at a heating rate of 5 °C / min to 500 °C in an argon atmosphere for 4 h to obtain embedded anatase crystalline titanium dioxide hollow carbon spheres.

[0059] Example 14 A method for preparing different embedded crystalline titanium dioxide hollow carbon spheres of the present invention: Add 2.5 mL of tetraethyl orthosilicate to 45 mL of ethanol and stir for 3 min, then add 40 mL of water and 5 mL of ammonia water solution, stir for 1 h, and gradually add 0.6 g of resorcinol and 0.8 mL of formaldehyde under stirring, and stir for 12 h; Then transfer the obtained mixture to a hydrothermal reaction kettle and perform hydrothermal treatment at 120 °C for 12 h; Filter and dry, then collect the mixture, carbonize it at 900 °C for 2 h in an argon atmosphere with a heating rate of 5 °C / min, treat the product with hydrofluoric acid solution for 20 h to remove the silica template, and then wash it several times with ethanol and distilled water and dry to obtain hollow carbon spheres; 45 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 21 mL of ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 24 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 900 °C in an argon atmosphere for 4 h to obtain hollow carbon spheres embedded with rutile-type titanium dioxide.

[0060] Example 15 A method for preparing different types of hollow carbon spheres embedded with crystalline titanium dioxide according to the present invention. 2.5 mL of tetraethyl orthosilicate was added to 45 mL of ethanol and stirred for 3 min. Then, 40 mL of water and 5 mL of ammonia water solution were added. After stirring for 1 h, 0.6 g of resorcinol and 0.8 mL of formaldehyde were gradually added under stirring and stirred for 12 h. Then, the obtained mixture was transferred to a hydrothermal reaction kettle and hydrothermally treated at 120 °C for 12 h. The mixture was collected after filtration and drying, carbonized at 900 °C for 2 h in an argon atmosphere at a heating rate of 5 °C / min. The product was treated with hydrofluoric acid solution for 20 h to remove the silica template, and then washed with ethanol and distilled water several times and dried to obtain hollow carbon spheres. 51 mg of titanium sulfate was added to 20 mL of sulfuric acid (3 mol / L) solution and stirred for 1 h. Then, 30 mg of the prepared hollow carbon spheres was added to the solution and continuously stirred at room temperature for 12 h. Then, it was first washed with 15 mL of ethanol to remove the titanium sulfate on the surface of the hollow carbon spheres, and then washed with 24 mL of distilled water to hydrolyze the titanium sulfate inside the hollow carbon spheres. The obtained product was washed repeatedly and dried in a vacuum oven at 60 °C for 6 h to obtain hollow carbon spheres embedded with amorphous titanium dioxide. The prepared hollow carbon spheres embedded with amorphous titanium dioxide were placed in an alumina crucible, and then heated in a tubular furnace at a heating rate of 5 °C / min to 900 °C in an argon atmosphere for 4 h to obtain hollow carbon spheres embedded with rutile-type titanium dioxide.

Claims

1. A method for preparing a hollow carbon sphere embedded with titanium dioxide, characterized in that, Using silica as a template, phenolic resin as a carbon source to uniformly coat silica, after high-temperature carbonization in an inert gas atmosphere, hollow carbon spheres are obtained through chemical etching; a hydrolyzable titanium source is introduced into the hollow carbon spheres, the titanium on the surface of the hollow carbon spheres is removed by washing, and then the titanium inside the hollow carbon spheres is washed and hydrolyzed. After repeated washing and drying, hollow carbon spheres embedded with amorphous titanium dioxide are obtained; in an inert gas atmosphere, the hollow carbon spheres embedded with amorphous titanium dioxide are heat-treated at a controlled temperature to obtain hollow carbon spheres embedded with titanium dioxide of different crystal forms.

2. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that, The method of using silica as a template and phenolic resin as a carbon source to uniformly coat silica is to synthesize silica through the hydrolysis and condensation reaction of tetraethyl orthosilicate. Formaldehyde and resorcinol are gradually added under stirring, and continuous stirring is carried out. The formed mixture is transferred to a hydrothermal reaction kettle for hydrothermal treatment, and the mixture is collected after filtration and drying.

3. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that, The temperature of high-temperature carbonization is 700 °C - 900 °C, and the time is 1 h - 4 h.

4. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that, The chemical etching method uses hydrofluoric acid with an analytical purity mass concentration ≥ 40%.

5. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that, The hydrolyzable titanium source introduced into the hollow carbon spheres is titanium sulfate, and continuous stirring is carried out until uniform.

6. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 5, characterized in that, The mass ratio of the hollow carbon spheres to titanium sulfate is 1:(1.5 - 1.7).

7. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that The titanium on the surface of the hollow carbon spheres is removed by washing with absolute ethanol, and the mass-volume ratio of the hollow carbon spheres to the absolute ethanol used for washing and removing titanium is 1:(500 - 700) g / mL; The titanium inside the hollow carbon spheres is washed and hydrolyzed with distilled water or deionized water, and the mass-volume ratio of the hollow carbon spheres to the water used for washing and hydrolyzing is 1:(400 - 800) g / mL.

8. The preparation method of a hollow carbon sphere embedded with titanium dioxide according to claim 1, characterized in that, The temperature for heat-treating the hollow carbon spheres embedded with amorphous titanium dioxide is 500 °C - 900 °C, and the time is 1 h - 4 h.

9. An electrode catalytic material, characterized in that, Using the hollow carbon spheres embedded with titanium dioxide of different crystal forms prepared by the preparation method of the hollow carbon spheres embedded with titanium dioxide according to any one of claims 1 - 8 as an electrode catalytic material.

10. Use of the electrode catalytic material according to claim 9, characterized in that, Spraying the hollow carbon spheres embedded with titanium dioxide of different crystal forms as an electrode catalytic material on a carbon felt and using it as the positive and negative electrodes of a mixed acid system all-vanadium redox flow battery.

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