Modified fly ash for zinc-bromine flow battery positive electrode and preparation method and application thereof

Abstract

This invention discloses a modified fly ash for the positive electrode of a zinc-bromine flow battery, its preparation method, and its application, belonging to the field of flow battery technology. The method includes: ball milling and uniformly mixing fly ash with a carbon source, nitrogen and sulfur sources, and molten salt; then subjecting the mixture to high-temperature heat treatment in a mixed atmosphere containing ammonia; the fly ash undergoes a carbothermic reduction reaction to generate conductive silicon carbide nanocrystals; simultaneously, nitrogen and sulfur elements are co-doped to form catalytic active sites; the product is washed and dried to obtain the modified fly ash for the positive electrode of a zinc-bromine flow battery. Through the integrated process of molten salt-assisted carbothermic reduction and nitrogen-sulfur co-doping, in-situ generation of conductive silicon carbide nanocrystals and efficient doping of nitrogen and sulfur heteroatoms are achieved, overcoming the contradiction between conductivity and retention of active sites. At 20 mA / cm²... 2 The zinc-bromine flow battery produced under these conditions has a voltage efficiency of 88.5%–90.8% and an energy efficiency of 85.0%–87.6%, achieving high-value utilization of industrial waste and significant performance improvement.

Description

Technical Field

[0001] This invention belongs to the field of flow battery technology, specifically relating to a modified fly ash for the positive electrode of a zinc-bromine flow battery, its preparation method, and its application. Background Technology

[0002] Zinc-bromine flow batteries, due to their high theoretical energy density, low cost, and long cycle life, show great promise for large-scale electrochemical energy storage. Their cathode reaction is based on Br₂ / Br₂. - The redox couple, whose kinetics directly determine the rate performance and energy conversion efficiency of a battery, is a crucial element in battery life. Currently, commercial zinc-bromine flow batteries primarily use carbon-plastic bipolar plates as the cathode. While this material effectively resists bromine corrosion and possesses good mechanical strength, its inherently limited catalytically active sites lead to a low Br2 / Br2 ratio. - The reaction exhibits poor reversibility and high overpotential. Furthermore, the high resistivity of the carbon-plastic composite material degrades battery performance. This issue limits the operating current density of the zinc-bromine flow battery to 20 mA / cm². 2 The low operating current density, resulting in insufficient power density of the battery stack and high material costs, severely restricts the commercialization and large-scale energy storage applications of this battery technology. Therefore, developing cathode materials with both high conductivity and high bromine catalytic activity has become a key technological bottleneck that urgently needs to be overcome in the field of zinc-bromine flow batteries.

[0003] On the other hand, fly ash is a large amount of solid waste generated during the coal combustion process in thermal power plants. Its main components are inorganic oxides such as silicon dioxide (SiO2) and aluminum oxide (Al2O3). Traditionally, fly ash has been mainly used in low-value-added fields such as cement and building materials, failing to fully release its potential value of valuable elements such as silicon and aluminum. In recent years, researchers have attempted to transform fly ash into high-value-added functional materials, such as extracting high-purity aluminum hydroxide, preparing molecular sieves, and developing environmentally friendly adsorption materials. However, fly ash itself is an insulator and lacks catalytic activity for bromine redox reactions. To transform it into high-performance electrochemical energy storage materials, it faces the dual technical obstacles of poor intrinsic conductivity and lack of catalytic sites. This has become the main difficulty in expanding the high-value utilization of fly ash into the energy storage field. Preliminary research has been conducted on the electrochemical modification and application of fly ash. In terms of conductivity modification, high-temperature carbothermic reduction treatment is performed after mixing fly ash with a carbon source to convert the SiO2 on the surface of the fly ash into conductive SiC nanocrystals, thereby improving the conductivity of the material. For example, Chinese patent application CN121573968A discloses a method for preparing conductive ceramics, the resulting product, and its applications, using fly ash as the matrix raw material and employing a carbothermic reduction process to prepare conductive ceramics. However, this patent application only focuses on improving the single property of conductivity and does not involve the construction of catalytic active sites. In terms of catalytic applications, other studies combine fly ash with nitrogen or sulfur sources and prepare catalytic particle electrodes through processes such as doping and high-temperature activation for electrocatalytic oxidation in wastewater treatment. For example, Chinese patent application CN106064962A discloses a method for preparing catalytic particle electrodes using sludge and fly ash and its applications, using sewage sludge and fly ash to prepare catalytic particle electrodes through doping and microwave high-temperature activation. However, its application is limited to the water treatment field, and its catalytic mechanism is completely different from the bromine oxidation-reduction reaction. In addition, there have been research reports on nitrogen-sulfur co-doped biochar materials, but the doping method of these materials is in-situ doping during the biomass pyrolysis process. Their active sites are targeted at microbial metabolism rather than bromine oxidation-reduction reaction, and they do not involve the construction of conductive phases.

[0004] Existing research on fly ash modification has the following significant shortcomings: (1) Conductivity modification and catalytic modification are usually carried out in steps or only for a single performance index. There is no integrated technical solution that simultaneously achieves the construction of a highly conductive carbon network and the introduction of heteroatom catalytic active sites in the same heat treatment process. (2) In the existing technology, although high-temperature heat treatment can significantly improve the conductivity of the material, this process will also cause the decomposition or deactivation of the doped nitrogen, sulfur and other catalytic active sites. Conversely, if low-temperature treatment is used to retain the active sites, the conductivity improvement is insufficient, and it is difficult to achieve both. (3) There are no patent reports on modifying fly ash specifically for use as a cathode catalytic material for zinc-bromine flow batteries and systematically improving its voltage efficiency and energy efficiency. Therefore, there is an urgent need to develop a simple, efficient, and low-cost one-step synergistic modification technology that can simultaneously address the dual problems of poor intrinsic conductivity and lack of bromine catalytic active sites in fly ash during a single heat treatment process. This technology should overcome the technical contradiction between high-temperature carbonization and the retention of active sites, thereby transforming fly ash, a solid waste from thermal power plants, into a zinc-bromine flow battery cathode material with both high conductivity and high catalytic activity. This has significant economic and social implications for significantly improving the performance of zinc-bromine flow batteries and realizing the high-value-added resource utilization of fly ash. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a modified fly ash for the positive electrode of zinc-bromine flow battery, its preparation method and application, so as to solve the technical problems of poor intrinsic conductivity of fly ash, lack of bromine catalytic activity, and difficulty in balancing high-temperature carbonization and retention of active sites in existing modification methods.

[0006] To achieve the above objectives, the present invention employs the following technical solution: This invention discloses a method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery, comprising: ball milling and mixing fly ash with a carbon source, a nitrogen and sulfur source and molten salt to form a composite precursor; placing the composite precursor in a mixed atmosphere containing ammonia for high-temperature heat treatment; and then washing and drying the product obtained from the high-temperature heat treatment to obtain modified fly ash for the positive electrode of a zinc-bromine flow battery. During high-temperature heat treatment, fly ash undergoes a carbothermic reduction reaction to generate conductive silicon carbide nanocrystals, while nitrogen and sulfur elements are co-doped to form catalytic active sites.

[0007] Preferably, the mass ratio of carbon source, nitrogen and sulfur source, molten salt and fly ash is (0.5-1.2):(0.1-0.5):(0.8-1.8):1.

[0008] Preferably, the carbon source is at least one of glucose, sucrose, starch, and cellulose; The nitrogen and sulfur source is at least one of thiourea, cysteine ​​and thioacetamide; The molten salt is at least one of the following: a eutectic salt of lithium chloride and potassium chloride, a eutectic salt of sodium chloride and potassium chloride, and a eutectic salt of calcium chloride and sodium chloride. The melting point of the molten salt is lower than the temperature of the high-temperature heat treatment. The drying temperature is 60-100℃, and the drying time is 6-24 hours.

[0009] Preferably, the ball milling mixing time is 0.5-5 hours; The ammonia-containing mixed atmosphere is a mixture of ammonia and an inert gas, wherein the volume percentage of ammonia is 10%-50%; the flow rate of the mixed atmosphere is 50-300 mL / min.

[0010] Preferably, the heating rate of the high-temperature heat treatment is 2-10℃ / min; the temperature of the high-temperature heat treatment is 700-1000℃; and the time of the high-temperature heat treatment is 1-6 hours.

[0011] The present invention also discloses a modified fly ash for the positive electrode of a zinc-bromine flow battery, which is prepared by the above-mentioned method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery.

[0012] Preferably, the modified fly ash used in the positive electrode of the zinc-bromine flow battery is a silicon carbide nanocrystal-nitrogen-sulfur co-doped carbon composite powder with a three-dimensional conductive network structure, including: silicon carbide nanocrystals, nitrogen-sulfur co-doped carbon layers, and incompletely converted fly ash particle cores; wherein, nitrogen mainly exists in the form of pyridine nitrogen or pyrrole nitrogen, and sulfur mainly exists in the form of thiophene sulfur.

[0013] The present invention also discloses the application of the modified fly ash for zinc-bromine flow battery cathode in the preparation of zinc-bromine flow battery cathode, wherein the zinc-bromine flow battery cathode includes: a current collector and a catalyst layer coated on the surface of the current collector, the catalyst layer comprising the modified fly ash for zinc-bromine flow battery cathode described above.

[0014] Preferably, the method for preparing the catalyst layer includes: dispersing modified fly ash of the zinc-bromine flow battery positive electrode in ethanol or deionized water, ultrasonically treating it to obtain a slurry; uniformly spraying the slurry onto the surface of the current collector, with the distance between the spray gun nozzle and the surface of the current collector being 15-30 cm and the spraying pressure being 0.2-0.5 MPa; and drying it to obtain the catalyst layer. The ultrasonic treatment time is 10-60 minutes; the slurry concentration is 5-30 mg / mL; the drying temperature is 60-120℃, and the drying time is 1-6 hours; the loading of modified fly ash for the zinc-bromine flow battery cathode in the catalyst layer is 2-6 mg / cm³. 2 The current collector is a carbon-plastic bipolar plate.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery. Fly ash, a carbon source, a nitrogen-sulfur source, and molten salt are ball-milled and then subjected to high-temperature heat treatment. During this heat treatment, the molten salt first forms an ionic liquid phase, dissolving and transporting the carbon source to the surface of the fly ash particles, improving the solid-phase reaction interface. This catalyzes the carbothermic reduction reaction between silicon dioxide (SiO2) and carbon, allowing silicon carbide (SiC) nanocrystals to nucleate and grow in situ at a lower temperature, forming a three-dimensional conductive framework. This promotes the reaction of active gases such as ammonia (NH3) and hydrogen sulfide (H2S) generated from the decomposition of the nitrogen-sulfur source with the carbon layer, achieving uniform doping of pyridine nitrogen, thiophene sulfur, and other sites. Simultaneously, externally introduced NH3 is decomposed into H radicals and N radicals under the catalysis of metal oxides or molten salt. N radicals are embedded into the carbon framework to supplement nitrogen doping, while H radicals selectively etch amorphous carbon to form mesopores. This integrated process avoids the inherent problem of active site decomposition caused by the first doping followed by high-temperature carbonization in traditional stepwise modification. Data from Example 6 shows that the zinc-bromine flow battery with modified fly ash cathode, assembled using this method, achieves a voltage efficiency of 90.8% and an energy efficiency of 87.6%, significantly higher than the voltage efficiency of 81.5% and energy efficiency of 75.4% for unmodified bipolar plates, respectively. The preparation method disclosed in this invention uses solid waste fly ash as raw material, completing the construction of the conductive phase and the introduction of catalytic sites in one step. The process is simple, requires no templates, and involves no additional etching steps, making it suitable for industrial production.

[0016] This invention discloses a modified fly ash for the positive electrode of a zinc-bromine flow battery, which exhibits a Raman spectrum at 760 cm⁻¹. -1 and 934cm -1 The presence of characteristic peaks belonging to 3C-SiC, with a redshift compared to standard crystal peaks, indicates the formation of nanocrystals with a size of approximately 20-50 nm. These nanocrystals are distributed on the surface of fly ash particles, forming a three-dimensional conductive network together with the residual carbon layer. Nitrogen exists primarily as pyridine nitrogen and pyrrole nitrogen, while sulfur exists as thiophene sulfur. These two functional groups affect the Br2 / ... - Redox reactions exhibit specific catalytic activity, capable of lowering the activation energy and increasing the exchange current density. In Examples 1-8, the zinc-bromine flow batteries assembled using the modified fly ash as the positive electrode of the zinc-bromine flow battery of this application all maintained a stable voltage efficiency of over 88.5%. Simultaneously, some unreacted fly ash cores were retained, providing skeletal support and preventing the catalyst layer from peeling off due to volume changes.

[0017] This invention discloses the application of modified fly ash for zinc-bromine flow battery cathodes in the preparation of zinc-bromine flow battery cathodes. The modified fly ash is directly used as a coating for carbon-plastic bipolar plates. The preparation process is extremely simple, requiring only the material to be dispersed in water or ethanol, ultrasonically formed into a slurry, sprayed, and then dried. This coating exhibits excellent stability in the operating environment of zinc-bromine flow batteries, providing sufficient catalytic active sites without increasing ion transport resistance due to excessive coating thickness. At 20 mA / cm²... 2 At the specified current density, the battery voltage efficiency using this cathode reaches 88.5%-90.8%, indicating that the coating effectively suppresses bromine shuttle. In contrast, the unmodified carbon-plastic bipolar plate in Comparative Example 1, lacking catalytic sites, has a voltage efficiency of only 81.5%. Therefore, the zinc-bromine flow battery cathode provided in this application is a direct and effective solution to the problems of low current density and low energy efficiency in zinc-bromine flow batteries.

[0018] The application of the zinc-bromine flow battery positive electrode disclosed in this invention in the preparation of zinc-bromine flow batteries, at 20 mA / cm 2 At the operating current density, the voltage efficiency reaches 88.5%-90.8%, and the energy efficiency reaches 85.0%-87.6%. This is significantly better than currently commercially available carbon-plastic bipolar plate assembled batteries (voltage efficiency 81.5%, energy efficiency 75.4%), and also better than most reported uncatalyst-modified carbon-based electrode systems. The SiC nanocrystalline network reduces electron transport resistance, and the nitrogen-sulfur co-doped carbon layer reduces the Br2 / Br2 ratio. - The electrochemical polarization of the reaction; the improvement in energy efficiency depends on the simultaneous improvement of voltage efficiency and coulombic efficiency. Example 6: At 850°C, with ammonia volume percentage of 20% and a loading of 4 mg / cm³... 2 Optimal performance was achieved under the given conditions (voltage efficiency 90.8%, energy efficiency 87.6%). Attached Figure Description

[0019] Figure 1 This is a scanning electron microscope image of the modified fly ash used as the positive electrode of the zinc-bromine flow battery prepared in Example 6 of the present invention. Figure 2 The Raman spectrum of the modified fly ash used as the positive electrode of the zinc-bromine flow battery prepared in Example 6 of this invention; Figure 3 X-ray photoelectron spectra of modified fly ash used as the positive electrode of the zinc-bromine flow battery prepared in Example 6 of the present invention; wherein, (a) is the full spectrum and (b) is the high-resolution C1s spectrum; Figure 4 This is a comparison of the electrical performance test results of the zinc-bromine flow batteries prepared in Examples 1-8 and Comparative Examples 1-4 of the present invention. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0022] Unless otherwise specified, all the technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0023] In this invention, unless otherwise specified, percentage (%) or parts refer to weight percentage or parts relative to the composition.

[0024] Unless otherwise specified, the components or preferred components involved in this invention can be combined with each other to form new technical solutions.

[0025] In this invention, unless otherwise specified, the numerical range "a~b" is an abbreviation for any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "6~22" indicates that all real numbers between "6~22" have been listed in this document, and "6~22" is simply an abbreviation for these numerical combinations.

[0026] The "scope" disclosed in this invention can be in the form of a lower limit and an upper limit, and can be one or more lower limits and one or more upper limits, respectively.

[0027] The term “and / or” as used in this invention refers to any combination of one or more of the associated listed items, as well as all possible combinations, and includes such combinations.

[0028] Unless otherwise stated, the technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art. Furthermore, any methods or materials similar to or equivalent to those described herein may also be used in this invention.

[0029] This invention provides a method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery, comprising the following steps: Step 1: Precursor Preparation Fly ash, carbon source, nitrogen and sulfur source are mixed with molten salt ball milling to form a composite precursor.

[0030] The mass ratio of carbon source, nitrogen and sulfur source, molten salt and fly ash is (0.5-1.2):(0.1-0.5):(0.8-1.8):1.

[0031] The carbon source is at least one of glucose, sucrose, starch, and cellulose.

[0032] The nitrogen and sulfur source is at least one of thiourea, cysteine ​​and thioacetamide.

[0033] The molten salt is at least one of the following: a eutectic salt of lithium chloride and potassium chloride, a eutectic salt of sodium chloride and potassium chloride, and a eutectic salt of calcium chloride and sodium chloride. In the eutectic salt of lithium chloride and potassium chloride, the mass ratio of lithium chloride to potassium chloride is 44%:56%. In the eutectic salt of sodium chloride and potassium chloride, the mass ratio of sodium chloride to potassium chloride is 44%:56%. In the eutectic salt of calcium chloride and sodium chloride, the mass ratio of calcium chloride to sodium chloride is 63.7%:36.3%. Its melting point is lower than the subsequent heat treatment temperature.

[0034] The ball milling time is 0.5-5 hours, preferably 1-3 hours; the ball milling speed is 200-500 rpm; and the ball-to-material ratio is 10:1.

[0035] Step 2: High-temperature annealing modification The composite precursor is placed in a mixed atmosphere containing ammonia and subjected to high-temperature heat treatment with the assistance of molten salt medium, so that fly ash undergoes a carbothermic reduction reaction to generate conductive silicon carbide nanocrystals, while simultaneously achieving co-doping of nitrogen and sulfur elements to form active sites.

[0036] The ammonia-containing mixed atmosphere is a mixture of ammonia and an inert gas, wherein the volume percentage of ammonia is 10%-50%, preferably 20%-30%. The inert gas is nitrogen or argon.

[0037] The flow rate of the mixed atmosphere is 50-300 mL / min, preferably 100-200 mL / min.

[0038] The temperature for high-temperature heat treatment is 700-1000℃, preferably 800-900℃.

[0039] The holding time for high-temperature heat treatment is 1-6 hours, preferably 1-4 hours.

[0040] The heating rate to the high-temperature heat treatment temperature is 2-10℃ / min, preferably 3-7℃ / min.

[0041] Step 3, Post-processing The heat-treated product was filtered, washed, and dried to obtain modified fly ash.

[0042] The process involves three water washes to remove molten salt.

[0043] The drying process is vacuum drying, with a drying temperature of 60-100℃ and a drying time of 6-24 hours.

[0044] The modified fly ash prepared by the above method has a three-dimensional conductive network structure, which consists of: silicon carbide nanocrystals, nitrogen and sulfur co-doped carbon layers, and incompletely converted fly ash particle cores.

[0045] The present invention also provides a modified fly ash for zinc-bromine flow battery cathode prepared by the above-mentioned method for preparing modified fly ash for zinc-bromine flow battery cathode. The modified fly ash for zinc-bromine flow battery cathode is a silicon carbide nanocrystal-nitrogen-sulfur co-doped carbon composite powder, wherein the nitrogen element exists in the form of pyridine nitrogen and pyrrole nitrogen, and the sulfur element exists in the form of thiophene sulfur.

[0046] The present invention also provides a zinc-bromine flow battery positive electrode, comprising a current collector and a catalyst layer coated on the surface of the current collector, the catalyst layer comprising the modified fly ash for the zinc-bromine flow battery positive electrode described above.

[0047] The catalyst layer is prepared by spraying. The specific steps are as follows: the modified fly ash of the zinc-bromine flow battery cathode is dispersed in ethanol or deionized water and ultrasonically treated for 10-60 minutes to obtain a uniformly dispersed slurry. The concentration of the modified fly ash of the zinc-bromine flow battery cathode in the slurry is 5-30 mg / mL. The slurry is uniformly sprayed onto the surface of the current collector using a spray gun. The distance between the spray gun nozzle and the surface of the current collector is 15-30 cm, and the spraying pressure is 0.2-0.5 MPa. Then, it is dried at 60-120℃ for 1-6 hours.

[0048] Based on the geometric area of ​​the current collector, the loading of modified fly ash in the catalyst layer for the zinc-bromine flow battery cathode is 2-6 mg / cm². 2 Preferably 3-5 mg / cm 2 The current collector is a carbon-plastic bipolar plate.

[0049] The present invention also provides a zinc-bromine flow battery comprising the above-described zinc-bromine flow battery positive electrode. At 20 mA / cm² 2 At the specified current density, the voltage efficiency of this zinc-bromine flow battery can reach 88.5%-90.8%, and the energy efficiency can reach 85.0%-87.6%.

[0050] This invention discloses a modified fly ash for the cathode of a zinc-bromine flow battery, based on a synergistic strategy of molten salt-assisted carbothermic reduction and nitrogen-sulfur co-doping. First, fly ash is mixed with a carbon source, a nitrogen-sulfur source, and a low-melting-point molten salt and ball-milled to form a uniform composite precursor. During high-temperature heat treatment, the molten salt melts first to form an ionic liquid phase, promoting the uniform penetration of the carbon source into the surface of the fly ash particles and catalyzing the carbothermic reduction reaction. The reaction equation is SiO2 + 3C → SiC + 2CO, resulting in the in-situ generation of highly conductive SiC nanocrystals. Simultaneously, the nitrogen-sulfur source (such as thiourea) decomposes at 600-750℃, releasing NH3 and H2S, which efficiently dope the carbon network under molten salt catalysis, forming catalytically active sites such as pyridine nitrogen, pyrrole nitrogen, and thiophene sulfur. At high temperatures, the liquid-phase environment formed by molten salt provides physical coating and isolation for the surface of carbon materials, inhibiting the removal and decomposition of doped nitrogen and sulfur heteroatoms from the carbon framework in the form of NH3, H2S, etc. Simultaneously, the Lewis acid properties of the molten salt cations can moderately polarize heteroatoms, reducing their thermal desorption rate. Molten salt cation K + molten salt cation Na + Weak interactions with heteroatoms, such as polarization effects, can further enhance the thermal stability of active sites, thus resolving the traditional technical contradiction that high-temperature carbonization improves conductivity but destroys active sites. The final product, after water washing to remove molten salt, yields SiC nanocrystals-nitrogen-sulfur co-doped carbon three-dimensional conductive powder. This stabilizing effect only occurs during heat treatment; after subsequent water washing to remove molten salt, the nitrogen and sulfur are firmly embedded in the carbon framework.

[0051] Through the above-mentioned synergistic modification, this invention achieves the following beneficial effects: fly ash is transformed from an intrinsic insulator into a highly conductive material; the nitrogen-sulfur doped carbon layer provides abundant Br2 / ... - The material provides catalytic sites for redox reactions and a three-dimensional conductive network structure that accelerates electron transport and reduces polarization overpotential. When this material is sprayed onto the surface of a carbon-plastic bipolar plate as the positive electrode catalyst layer in a zinc-bromine flow battery, it achieves a performance at 20 mA / cm². 2 At current density, the battery voltage efficiency reaches 88.5%-90.8%, and the energy efficiency reaches 85.0%-87.6%, significantly better than commercial carbon-plastic bipolar plates (voltage efficiency 81.5%, energy efficiency 75.4%). Simultaneously, using fly ash, a solid waste from thermal power plants, as raw material achieves high-value-added resource utilization of industrial waste, reducing battery manufacturing costs.

[0052] To address the dual shortcomings of fly ash—poor intrinsic conductivity and lack of bromine catalytic activity—existing stepwise modification methods suffer from the contradiction between high-temperature carbonization and retention of active sites, as well as the low voltage and energy efficiency of existing zinc-bromine flow battery cathode materials. This invention employs a one-step synergistic modification method to simultaneously achieve in-situ generation of conductive silicon carbide nanocrystals and efficient doping with nitrogen and sulfur heteroatoms, thereby obtaining a composite material with both high conductivity and high bromine catalytic activity. Applying this composite material to the cathode of a zinc-bromine flow battery significantly improves battery performance.

[0053] This invention employs an integrated molten salt-assisted carbothermal reduction-nitrogen-sulfur co-doping technology to simultaneously achieve three aspects of modification during a single high-temperature heat treatment process: a highly conductive carbon network is generated in situ on the fly ash surface from the carbon source; SiO2 is reduced by carbothermal reduction to generate SiC nanocrystals, further enhancing conductivity; and the nitrogen and sulfur sources decompose under molten salt catalysis, releasing nitrogen- and sulfur-containing gases to efficiently dope the carbon network, forming catalytically active sites such as pyridine nitrogen and thiophene sulfur. The molten salt medium (such as LiCl-KCl eutectic salt) forms an ionic liquid phase during the heating process, promoting the uniform penetration of the carbon source into the fly ash surface and catalyzing the heteroatom doping reaction. This cleverly solves the traditional technical contradiction of high-temperature carbonization improving conductivity but destroying active sites.

[0054] The positive electrode of the zinc-bromine flow battery of the present invention is coated with modified fly ash on the surface of a carbon-plastic bipolar plate as a positive electrode catalyst layer, at 20 mA / cm 2 At the specified current density, the battery voltage efficiency reaches 88.5%-90.8%, and the energy efficiency reaches 85.0%-87.6%, significantly outperforming commercial carbon-plastic bipolar plates (voltage efficiency 81.5%, energy efficiency 75.4%). This indicates that the material of this invention possesses both excellent electron conductivity and Br2 / Br2 ratio. - Catalytic activity in redox reactions.

[0055] This invention uses fly ash, a solid waste from thermal power plants, as the main raw material. Fly ash is widely available and inexpensive, and it is transformed into a high-performance electrochemical energy storage material. This not only solves the environmental pressure caused by fly ash storage, but also significantly reduces the manufacturing cost of zinc-bromine flow battery cathode materials, resulting in significant economic and social benefits.

[0056] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0057] Example 1 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 5 g of glucose, 1 g of thiourea, and 8 g of a eutectic salt of lithium chloride and potassium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.5:0.1:0.8:1. Place the above mixture in a ball mill jar and ball mill at 300 rpm for 2 hours to obtain a homogeneous composite precursor.

[0058] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 20% of the volume and a total flow rate of 150 mL / min. The temperature is increased to 700℃ at a rate of 5℃ / min and held for 3 hours.

[0059] Step 3, Post-processing. The heat-treated product was washed three times with deionized water and then vacuum dried at 80°C for 12 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0060] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: Step 1: Cathode Preparation. The above-mentioned zinc-bromine flow battery cathode was dispersed in ethanol with modified fly ash and ultrasonically treated for 10 minutes to obtain a slurry with a concentration of 5 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun. The distance between the spray gun nozzle and the current collector surface was 15 cm, and the spraying pressure was 0.3 MPa. It was subsequently dried at 60°C for 6 hours. The catalyst loading was 2 mg / cm³. 2 .

[0061] Step 2: Assembly of the zinc-bromine flow battery. The modified electrode is used as the positive electrode (with the coated side facing the separator), the unmodified carbon-plastic bipolar plate electrode is used as the negative electrode, and a commercially available porous membrane is used as the separator. The electrolyte consists of 2 M zinc bromide, 1.5 M potassium chloride, and 0.3 M 1-methyl-1-ethylpyrrolidine bromide.

[0062] Step 3: Battery performance testing. Connect the zinc-bromine flow battery to the battery testing system, and set the charge / discharge current density to 20 mA / cm². 2 The test temperature was 25℃, the charging time was 4 h, and the discharge cutoff voltage was 0.5 V. Under these conditions, the circuit was cycled 30 times, and the voltage-time curve, coulombic efficiency, voltage efficiency, and energy efficiency were recorded.

[0063] Example 2 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 12 g of sucrose, 5 g of cysteine, and 18 g of a eutectic salt of sodium chloride and potassium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 1.2:0.5:1.8:1. Ball mill the mixture for 3 hours at 350 rpm.

[0064] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and nitrogen is introduced, with ammonia accounting for 30% of the volume and a total flow rate of 200 mL / min. The temperature is increased to 1000℃ at 7℃ / min and held for 4 hours.

[0065] Step 3, Post-processing. The heat-treated product was washed five times with deionized water and then vacuum dried at 80°C for 12 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0066] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 60 minutes to obtain a slurry with a concentration of 30 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 25 cm and the spraying pressure being 0.5 MPa. It was subsequently dried at 120°C for 1 hour. The catalyst loading was 6 mg / cm³. 2 .

[0067] Example 3 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 7 g of starch, 2 g of thioacetamide, and 14 g of a eutectic salt of calcium chloride and sodium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.7:0.2:1.4:1. Ball mill the mixture for 0.5 hours at 400 rpm.

[0068] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 10% of the volume and a total flow rate of 100 mL / min. The temperature is increased to 850℃ at a rate of 2℃ / min and held for 2 hours.

[0069] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 70°C for 18 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0070] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in ethanol with modified fly ash and ultrasonically treated for 30 minutes to obtain a slurry with a concentration of 15 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 20 cm and the spraying pressure being 0.4 MPa. It was subsequently dried at 80°C for 1 hour. The catalyst loading was 4 mg / cm³. 2 .

[0071] Example 4 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 9 g of cellulose, 3 g of thiourea, and 16 g of a eutectic salt of lithium chloride and potassium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.9:0.3:1.6:1. Ball mill the mixture for 5 hours at 300 rpm.

[0072] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 50% of the volume and a total flow rate of 250 mL / min. The temperature is increased to 900℃ at a rate of 10℃ / min and held for 1 hour.

[0073] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 90°C for 8 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0074] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in ethanol with modified fly ash and ultrasonically treated for 20 minutes to obtain a slurry with a concentration of 20 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 25 cm and the spraying pressure being 0.5 MPa. It was subsequently dried at 100°C for 1 hour. The catalyst loading was 5 mg / cm³. 2 .

[0075] Example 5 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 6 g of glucose, 2.5 g of cysteine, and 10 g of a eutectic salt of sodium chloride and potassium chloride. Control the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.6:0.25:1.0:1. Ball mill the mixture for 1.5 hours at 350 rpm.

[0076] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 25% of the volume and a total flow rate of 180 mL / min. The temperature is increased to 800℃ at a rate of 6℃ / min and held for 2 hours.

[0077] Step 3, Post-processing. The heat-treated product was washed five times with deionized water and then vacuum dried at 60°C for 24 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0078] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 40 minutes to obtain a slurry with a concentration of 10 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 18 cm and the spraying pressure being 0.3 MPa. It was subsequently dried at 120°C for 4 hours. The catalyst loading was 3 mg / cm³. 2 .

[0079] Example 6 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 8 g of sucrose, 4 g of thioacetamide, and 12 g of a eutectic salt of calcium chloride and sodium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.8:0.4:1.2:1. Ball mill the mixture for 2 hours at 350 rpm.

[0080] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 20% of the volume and a total flow rate of 150 mL / min. The temperature is increased to 850℃ at a rate of 5℃ / min and held for 3 hours.

[0081] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 100°C for 6 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0082] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 30 minutes to obtain a slurry with a concentration of 25 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 22 cm and the spraying pressure being 0.45 MPa. It was subsequently dried at 80°C for 3 hours. The catalyst loading was 4 mg / cm³. 2 .

[0083] Example 7 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 5.5 g of sucrose, 1.5 g of thioacetamide, and 9 g of a eutectic salt of calcium chloride and sodium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.55:0.15:0.9:1. Ball mill the mixture for 2 hours at 350 rpm.

[0084] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 15% of the volume and a total flow rate of 50 mL / min. The temperature is increased to 750℃ at a rate of 3℃ / min and held for 6 hours.

[0085] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 80°C for 12 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0086] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 20 minutes to obtain a slurry with a concentration of 8 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 30 cm and the spraying pressure being 0.3 MPa. It was subsequently dried at 70°C for 5 hours. The catalyst loading was 2.5 mg / cm³. 2 .

[0087] Example 8 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 11 g of sucrose, 4.5 g of thioacetamide, and 17 g of a eutectic salt of calcium chloride and sodium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 1.1:0.45:1.7:1. Ball mill the mixture for 4 hours at 400 rpm.

[0088] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 40% of the volume and a total flow rate of 300 mL / min. The temperature is increased to 950℃ at 8℃ / min and held for 3.5 hours.

[0089] Step 3, Post-processing. The heat-treated product was washed five times with deionized water and then vacuum dried at 85°C for 10 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0090] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 50 minutes to obtain a slurry with a concentration of 28 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 15 cm and the spraying pressure being 0.2 MPa. It was subsequently dried at 110°C for 2 hours. The catalyst loading was 5.5 mg / cm³. 2 .

[0091] Comparative Example 1 A method for preparing a zinc-bromine flow battery using an unmodified carbon-plastic bipolar plate includes the following steps: Step 1: Assembly of the zinc-bromine flow battery. The zinc-bromine flow battery was assembled using unmodified carbon-plastic bipolar plates as the positive and negative electrodes, and a commercially available porous membrane as the separator. The electrolyte consisted of 2 M zinc bromide, 1.5 M potassium chloride, and 0.3 M 1-methyl-1-ethylpyrrolidine bromide.

[0092] Step 2: Battery performance testing. Connect the zinc-bromine flow battery to the battery testing system, and set the charge / discharge current density to 20 mA / cm². 2 The test temperature was 25℃, the charging time was 4 h, and the discharge cutoff voltage was 0.5 V. Under these conditions, the circuit was cycled 30 times, and the voltage-time curve, coulombic efficiency, voltage efficiency, and energy efficiency were recorded.

[0093] Comparative Example 2 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 8 g of sucrose and 4 g of thioacetamide, and control the mass ratio of carbon source, nitrogen and sulfur source to fly ash to be 0.8:0.4:1. Do not add molten salt. Ball mill the mixture for 2 hours at 350 rpm.

[0094] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 20% of the volume and a total flow rate of 150 mL / min. The temperature is increased to 850℃ at a rate of 5℃ / min and held for 3 hours.

[0095] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 100°C for 6 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0096] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 30 minutes to obtain a slurry with a concentration of 25 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 15 cm and the spraying pressure being 0.3 MPa. It was subsequently dried at 80°C for 3 hours. The catalyst loading was 4 mg / cm³. 2 .

[0097] Comparative Example 3 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 8 g of sucrose, 4 g of thioacetamide, and 12 g of a eutectic salt of calcium chloride and sodium chloride, controlling the mass ratio of carbon source, nitrogen and sulfur source, molten salt, and fly ash to be 0.8:0.4:1.2:1. Ball mill the mixture for 2 hours at 350 rpm.

[0098] Step 2: High-temperature annealing modification. Place the precursor in a tube furnace and introduce a pure argon atmosphere (without adding ammonia) at a total flow rate of 150 mL / min. Increase the temperature to 850℃ at a rate of 5℃ / min and hold for 3 hours.

[0099] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 100°C for 6 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0100] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 30 minutes to obtain a slurry with a concentration of 25 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 15 cm and the spraying pressure being 0.3 MPa. It was subsequently dried at 80°C for 3 hours. The catalyst loading was 4 mg / cm³. 2 .

[0101] Comparative Example 4 A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery includes the following steps: Step 1: Precursor preparation. Take 10 g of fly ash, add 8 g of sucrose and 12 g of a eutectic salt of calcium chloride and sodium chloride. Do not add nitrogen or sulfur sources. Control the mass ratio of carbon source, molten salt and fly ash to be 0.8:1.2:1. Ball mill the mixture for 2 hours at 350 rpm.

[0102] Step 2: High-temperature annealing modification. The precursor is placed in a tube furnace, and a mixed atmosphere of ammonia and argon is introduced, with ammonia accounting for 20% of the volume and a total flow rate of 150 mL / min. The temperature is increased to 850℃ at a rate of 5℃ / min and held for 3 hours.

[0103] Step 3, Post-processing. The heat-treated product was washed four times with deionized water and then vacuum dried at 100°C for 6 hours to obtain modified fly ash for the positive electrode of zinc-bromine flow batteries.

[0104] The method for preparing a zinc-bromine flow battery using the modified fly ash as the positive electrode of the zinc-bromine flow battery includes the following steps: The difference from Example 1 is as follows: Step 1, positive electrode preparation. The above-mentioned zinc-bromine flow battery positive electrode was dispersed in deionized water with modified fly ash and ultrasonically treated for 30 minutes to obtain a slurry with a concentration of 25 mg / mL. This slurry was then uniformly sprayed onto the surface of a carbon-plastic bipolar plate using a spray gun, with the distance between the spray gun nozzle and the current collector surface being 15 cm and the spraying pressure being 0.3 MPa. It was subsequently dried at 80°C for 3 hours. The catalyst loading was 4 mg / cm³. 2 .

[0105] Table 1 summarizes the coulombic efficiency, voltage efficiency, and energy efficiency of Examples 1-8 and Comparative Examples 1-4 under the same test conditions.

[0106] Table 1. Comparison of electrical performance test results of zinc-bromine flow batteries prepared in Examples 1-8 and Comparative Examples 1-4

[0107] Figure 4 This table compares the electrical performance test results of the zinc-bromine flow batteries prepared in Examples 1-8 and Comparative Examples 1-4 of the present invention; as shown in Table 1 and... Figure 4 It can be seen that, compared with the unmodified carbon-plastic bipolar plate (Comparative Example 1), the zinc-bromine flow battery assembled with the positive electrode prepared by the modified fly ash of the present invention has significantly improved coulombic efficiency, voltage efficiency and energy efficiency.

[0108] In Comparative Example 1, the surface of the unmodified carbon-plastic bipolar plate lacks catalytic active sites, and the Br2 / Br ratio is low. - The high reaction overpotential results in a voltage efficiency of only 81.5%; at the same time, the material has a weak physical adsorption capacity for bromine, exhibiting bromine shuttle phenomenon, with a coulombic efficiency of 92.5% and an energy efficiency of 75.4%.

[0109] To further verify the synergistic mechanism of the three elements—molten salt, ammonia atmosphere, and nitrogen and sulfur sources—in the technical solution of this invention, comparative examples 2-4 were added. Comparative example 2 was prepared without the addition of molten salt. Without molten salt, the carbon source could not spread and penetrate evenly on the fly ash surface, resulting in the carbothermic reduction reaction being limited to local contact points, leading to insufficient and uneven SiC formation. Simultaneously, the discontinuous formation of the carbon layer resulted in a lack of a uniform carbon substrate for subsequent nitrogen and sulfur doping, causing uneven distribution of doping sites. Its voltage efficiency dropped to 84.2%, and its energy efficiency was only 79.8%, indicating that molten salt is a key reaction medium for constructing a three-dimensional conductive network and promoting efficient doping. Comparative example 3 underwent heat treatment in a pure argon atmosphere. Although thioacetamide could provide some nitrogen and sulfur sources, the lack of a continuous supply of active N radicals from external ammonia resulted in a severe deficiency of pyridine nitrogen / pyrrole nitrogen active sites, with a voltage efficiency of only 83.8% and an energy efficiency of 79.6%. This demonstrates that ammonia is not only a nitrogen doping source but also a dynamic supplementary source for maintaining high-density active sites. Comparative Example 4 was prepared without the addition of nitrogen and sulfur sources. Although partial nitrogen doping was achieved with the assistance of ammonia and the SiC conductive network was generated with the assistance of molten salt, the synergistic catalytic effect of sulfur and carbon-derived nitrogen was lacking, resulting in an extreme scarcity of surface catalytic active sites and a low Br2 / Br2 ratio. -With increased reaction polarization, the voltage efficiency was only 82.0%, and the energy efficiency was only 76.4%, almost the same as the unmodified Comparative Example 1. These comparative examples illustrate that single or two-component modification alone cannot achieve a significant performance improvement. However, by synergistically combining molten salt, ammonia atmosphere, and nitrogen and sulfur sources in a single high-temperature heat treatment process, the technical contradiction between improving conductivity and constructing active sites can be resolved simultaneously.

[0110] In all embodiments of the present invention, the modified fly ash used for the positive electrode of the zinc-bromine flow battery exhibits superior battery performance compared to all comparative examples. Among them, Example 6 demonstrates the best overall performance, with a voltage efficiency of 90.8%, a coulombic efficiency of 96.5%, and an energy efficiency of 87.6%.

[0111] All process parameters in Example 6 are within the preferred range: heat treatment temperature is 850℃, ammonia volume percentage is 20%, holding time is 3 h, and catalyst loading is 4 mg / cm³. 2 The ball milling time is 2 hours.

[0112] Figure 1 The image shows a scanning electron microscope image of the modified fly ash used as the positive electrode of the zinc-bromine flow battery prepared in Example 6 of this invention. As can be seen from the image, the material forms a uniform three-dimensional porous conductive network. The SiC nanocrystals are about 20-50 nm in size, with uniform particle distribution, no obvious agglomeration, and good interconnectivity of the porous structure.

[0113] Figure 2 The image shows the Raman spectrum of the modified fly ash used as the positive electrode for the zinc-bromine flow battery prepared in Example 6 of this invention. As can be seen from the image, SiC nanocrystals were successfully generated at 760 cm⁻¹. -1 The characteristic peak at 934 cm⁻¹ corresponds to the transverse optical vibration mode (TO) of SiC. -1 The peak at this point corresponds to the longitudinal optical vibration mode (LO); and is located at 796 cm⁻¹, the same as the Γ point of cubic silicon carbide (3C-SiC) crystal. -1 (TO), 972 cm -1Compared to the (LO) optical phonon mode, the characteristic peak shows a slight redshift, originating from nanoscale effects and lattice defects, confirming that SiC exists in nanocrystalline form (BECHELANY M, BRIOUDE A, CORNU D, et al. A Raman Spectroscopy Study of Individual SiC Nanowires. Advanced Functional Materials, Vol. 17, No. 6, March 2007, pp. 939-943; NAKASHIMA S, HARIMA H. Raman Investigation of SiC Polytypes. Physica Status Solidi (a), Vol. 162, No. 1, July 1997, pp. 39-64). Under these conditions, the carbon source is fully carbonized to form a highly conductive carbon network, the carbothermic reduction reaction generates sufficient SiC nanocrystals, and the nitrogen and sulfur sources achieve efficient doping under molten salt catalysis, forming abundant pyridine nitrogen and thiophene sulfur catalytic active sites, thus obtaining optimal battery performance.

[0114] Figure 3 X-ray photoelectron spectroscopy (XPS) of the modified fly ash used as the cathode in the zinc-bromine flow battery prepared in Example 6 of this invention is shown. (a) is the full spectrum, and (b) is the high-resolution C1s spectrum. In the full spectrum, characteristic peaks at 284.8 eV, 401.0 eV, 530.0 eV, and 160.0 eV are observed, corresponding to C1s, N1s, O1s, and S2p, respectively. The O1s signal mainly originates from unavoidable oxygen adsorption or surface oxidation during the testing process. Charge correction was performed using the C1s spectrum corresponding to 284.8 eV as a reference. Peak fitting of the high-resolution C1s spectrum revealed three main components: the peak with a binding energy of 284.8 eV belongs to the C-C bond or C=C bond; the peak at 285.8 eV belongs to the CS bond; and the peak at 288.8 eV belongs to the CN bond. These results confirm that nitrogen and sulfur elements have been successfully doped into the carbon framework structure.

[0115] This invention presents a modified fly ash for zinc-bromine flow battery cathodes prepared using an integrated molten salt-assisted carbothermal reduction-nitrogen-sulfur co-doping technology. This modified fly ash significantly enhances the catalytic activity and conductivity of the zinc-bromine flow battery cathode, effectively suppresses bromine shuttle, and simultaneously improves the battery's voltage efficiency, coulombic efficiency, and energy efficiency. The process parameter combination used in Example 6 is the optimal solution.

[0116] In summary, this invention provides a modified fly ash for the positive electrode of a zinc-bromine flow battery, its preparation method, and its application. Addressing the problems of poor intrinsic conductivity and lack of bromine catalytic activity in fly ash, as well as the difficulty in simultaneously achieving high-temperature carbonization and retaining active sites in existing modification methods, this invention offers a one-step synergistic modification technique: fly ash is mixed with a carbon source, nitrogen and sulfur sources, and molten salt and ball-milled to obtain a composite precursor. This precursor is then subjected to high-temperature heat treatment in an ammonia-containing mixed atmosphere, causing a carbothermic reduction reaction in the fly ash to generate conductive silicon carbide nanocrystals. Simultaneously, nitrogen and sulfur co-doping is achieved, forming catalytically active sites such as pyridine nitrogen and thiophene sulfur. After washing and drying, the modified fly ash for the positive electrode of a zinc-bromine flow battery is obtained. This material is sprayed onto the surface of a carbon-plastic bipolar plate as a positive electrode catalyst layer, and a zinc-bromine flow battery is assembled. At 20 mA / cm²... 2 At the specified current density, the battery voltage efficiency reaches 88.5%-90.8%, and the energy efficiency reaches 85.0%-87.6%, significantly outperforming unmodified carbon-plastic bipolar plates. This invention achieves high-value utilization of industrial waste fly ash while significantly improving the performance of zinc-bromine flow batteries. The process is simple, low-cost, and suitable for large-scale applications.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery, characterized in that, include: Fly ash was ball-milled and mixed with carbon source, nitrogen and sulfur source and molten salt to form a composite precursor. The composite precursor was placed in a mixed atmosphere containing ammonia for high-temperature heat treatment. The product obtained from the high-temperature heat treatment was then washed and dried in sequence to obtain modified fly ash for zinc-bromine flow battery cathode. During the high-temperature heat treatment process, fly ash undergoes a carbothermic reduction reaction to generate conductive silicon carbide nanocrystals, while nitrogen and sulfur elements are co-doped to form catalytic active sites.

2. The method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that, The mass ratio of the carbon source, nitrogen and sulfur source, molten salt and fly ash is (0.5-1.2):(0.1-0.5):(0.8-1.8):

1.

3. The method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that, The carbon source is at least one of glucose, sucrose, starch, and cellulose; The nitrogen and sulfur source is at least one of thiourea, cysteine ​​and thioacetamide; The molten salt is at least one of the following: a eutectic salt of lithium chloride and potassium chloride, a eutectic salt of sodium chloride and potassium chloride, and a eutectic salt of calcium chloride and sodium chloride. The melting point of the molten salt is lower than the temperature of the high-temperature heat treatment. The drying temperature is 60-100℃, and the drying time is 6-24 hours.

4. The method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that, The ball milling mixing time is 0.5-5 hours; The ammonia-containing mixed atmosphere is a mixture of ammonia and an inert gas, wherein the volume percentage of ammonia is 10%-50%; the flow rate of the mixed atmosphere is 50-300 mL / min.

5. The method for preparing modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that, The heating rate of the high-temperature heat treatment is 2-10℃ / min; the temperature of the high-temperature heat treatment is 700-1000℃; and the time of the high-temperature heat treatment is 1-6 hours.

6. A modified fly ash for the positive electrode of a zinc-bromine flow battery, characterized in that, The modified fly ash used for the positive electrode of the zinc-bromine flow battery as described in any one of claims 1-5 is prepared.

7. The modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 6, characterized in that, The modified fly ash used as the positive electrode of the zinc-bromine flow battery is a silicon carbide nanocrystal-nitrogen-sulfur co-doped carbon composite powder with a three-dimensional conductive network structure, including: silicon carbide nanocrystals, nitrogen-sulfur co-doped carbon layers, and incompletely converted fly ash particle cores; wherein, nitrogen mainly exists in the form of pyridine nitrogen or pyrrole nitrogen, and sulfur mainly exists in the form of thiophene sulfur.

8. The application of the modified fly ash for the zinc-bromine flow battery cathode as described in claim 6 or 7 in the preparation of the zinc-bromine flow battery cathode, characterized in that, The zinc-bromine flow battery positive electrode includes: a current collector and a catalyst layer coated on the surface of the current collector, the catalyst layer comprising modified fly ash for the zinc-bromine flow battery positive electrode.

9. The application of the modified fly ash for the positive electrode of a zinc-bromine flow battery according to claim 8 in the preparation of the positive electrode of a zinc-bromine flow battery, characterized in that, The method for preparing the catalyst layer includes: dispersing modified fly ash of the zinc-bromine flow battery positive electrode in ethanol or deionized water, ultrasonically treating it to obtain a slurry; uniformly spraying the slurry onto the surface of the current collector, with the distance between the spray gun nozzle and the surface of the current collector being 15-30 cm and the spraying pressure being 0.2-0.5 MPa; and drying it to obtain the catalyst layer. The ultrasonic treatment time is 10-60 minutes; the slurry concentration is 5-30 mg / mL; the drying temperature is 60-120℃, and the drying time is 1-6 hours; in the catalyst layer, the loading of modified fly ash for the zinc-bromine flow battery cathode is 2-6 mg / cm³. 2 The current collector is a carbon-plastic bipolar plate.

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