All-vanadium electrolyte, its preparation method and all-vanadium redox flow battery

Abstract

The present application discloses a vanadium-based electrolyte, a preparation method thereof, and a vanadium redox flow battery, belonging to the technical field of batteries. The method includes: S1: performing alkaline leaching treatment on vanadium slag with an alkaline leaching agent to obtain an alkaline leaching product and separating a vanadium enrichment; S2: mixing the vanadium enrichment, a first ammonium salt, and a first solvent to carry out reaction II to obtain a vanadium precipitation product, separating to obtain an ammonium vanadate deposit; performing a first calcination heat treatment on the ammonium vanadate deposit to obtain a vanadium oxide; then mixing the vanadium oxide with a first reducing agent and a second solvent to carry out reaction III to obtain a first solution containing tetravalent vanadium; S3: under the action of a hydrogen evolution catalyst, the first solution containing tetravalent vanadium undergoes reaction IV to obtain a solution containing trivalent vanadium. This method can efficiently extract vanadium-based electrolytes with different valence states, realize the resource utilization of vanadium slag solid waste, effectively reduce production costs, and reduce environmental protection risks; at the same time, with the help of a hydrogen evolution electrocatalyst, the reduction rate of vanadium ions is increased, and no other reducing agents are introduced, improving the operation safety and reliability.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and more particularly, to a vanadium-based electrolyte, a preparation method thereof, and a vanadium redox flow battery. Background Art

[0002] The preparation process of vanadium-based electrolyte for vanadium redox flow battery (VFB) has a significant impact on the battery performance and service life. Currently, the main problem existing in this preparation process is that the reaction kinetics of reducing V 4+ to V 3+ is relatively slow, which results in a high energy consumption during the preparation process, thereby increasing the production cost of vanadium-based electrolyte and restricting the development and commercial application of VFB.

[0003] Vanadium ions, as the active substances of the electrolyte, store and release electrical energy through the change of ionic valence states. The preparation methods of vanadium-based electrolyte mainly include physical method, chemical method and electrolysis method, and their raw materials are all high-purity VOSO4 or V2O5, which are expensive. Among them, the physical method is to directly dissolve high-purity VOSO4 in sulfuric acid to prepare vanadium-based electrolyte. The process is simple, but the high price of high-purity VOSO4 and poor process economy limit its large-scale industrial production. The chemical reduction method has an advantage in raw material cost when preparing low-concentration vanadium-based electrolyte, but the overall process flow is long, and impurities are easily introduced into the electrolyte during the preparation process, resulting in low electrochemical activity of the prepared electrolyte and being unfavorable for the industrialization process. The electrolysis method uses V2O5 as the main raw material, and divides the electrolytic cell of the stack into positive and negative electrode areas with a diaphragm. Add a H2SO4 solution containing V2O5 to the negative electrode area and reduce it to V 3+ or V 4+ ions. The electrolysis method has a simple operation process, but the solubility of vanadium ions in the electrolyte is limited (especially when the temperature is higher than 40 °C, V 5+ is easy to precipitate, and when the temperature is lower than 10 °C, V 2+ , V 3+ , V 4+ are easy to precipitate), and factors such as slow reaction rate limit its development. All in all, there are still many technical problems to be overcome in the preparation of vanadium-based electrolyte, such as the preparation, reduction, electrolysis and other processes of high-purity vanadium oxides. The preparation process flow is long and the energy consumption is high; in addition, preparing vanadium-based electrolyte with vanadium oxides as raw materials will produce high-salt wastewater, resulting in great environmental protection pressure and difficulty in large-scale production applications. Currently, some researchers have prepared vanadium-based electrolyte by adding hydrogen evolution platinum (Pt)-based electrocatalysts and achieved certain results. However, precious metal Pt is not only rare and limited in reserves, but also when the Pt-based catalyst is used for catalytic reduction to prepare vanadium-based electrolyte, since it needs to be immersed in an acidic electrolyte, it not only needs to have a certain corrosion resistance, but also needs to have strong electrocatalytic activity, which limits its large-scale application.

[0004] The prior art proposes to use vanadium slag as a raw material to prepare a vanadyl sulfate electrolyte through a process combining "impurity removal, extraction - back extraction, oil removal, and electro - adsorption for deep desalination". Although this method shortens the traditional process flow, there are still some problems. In the extraction - back extraction process, organic reagents are introduced, which may bring in additional impurities, thus increasing the risk of electrolyte contamination. At the same time, the introduction of electro - adsorption desalination results in a low recovery rate of vanadium in the vanadyl sulfate solution, leading to low preparation efficiency and high energy consumption.

[0005] The prior art proposes to use ammonium vanadate compounds as raw materials to remove impurities by heating with an acidic (HCl + H2SO4) solvent at a temperature above 338 °C. The vanadium - containing compound is dissolved under hot - pressing conditions after introducing a reducing agent, thereby preparing a vanadium - containing electrolyte. In the impurity removal step of this method, the addition of impurity - removing agents is avoided, improving the purity of the treated material. However, in the high - temperature evaporation process, the heating temperature exceeds 300 °C, which requires extremely high corrosion resistance and airtightness of the equipment. At the same time, high - temperature heating also leads to high energy consumption.

[0006] The prior art also proposes to use ammonium metavanadate as a raw material to prepare high - purity vanadium pentoxide through the method of "redissolution - crystallization - dissolution in water - ammonium precipitation - calcination". However, this method has a low crystallization temperature (2 - 5 °C) and a long crystallization time (20 - 40 h), which requires additional refrigeration equipment to maintain a low - temperature environment, increasing equipment costs and energy consumption.

[0007] The prior art also proposes to use sodium vanadate solution as a raw material to prepare calcium vanadate solid by chemical precipitation. Under strong acidic conditions, a reduction reaction occurs with oxalic acid, and pentavalent vanadate ions are reduced to tetravalent vanadyl ions, forming a tetravalent vanadyl sulfate solution. At this time, calcium ions and sulfate ions form calcium sulfate precipitation, and the tetravalent vanadium solution is obtained through solid - liquid separation. The tetravalent vanadium solution is electrolyzed to obtain a 3.5 - valent vanadium electrolyte. Although this method shortens the preparation process flow of vanadium electrolyte to a certain extent, it consumes excessive sulfuric acid during electrolysis and generates a large amount of waste acid and acid mist, which not only increases the raw material cost but also poses an environmental pollution hazard.

[0008] The prior art also proposes to reduce high - purity vanadium pentoxide under a reducing gas to obtain a low - valence vanadium oxide. Then, the low - valence vanadium oxide is mixed with an activator and heated for activation to obtain a vanadium - containing paste - like electrolyte. Finally, the vanadium - containing paste - like electrolyte is dissolved in water to obtain a vanadium electrolyte with an average valence state of vanadium between + 3 and + 4. This method has certain advantages, being relatively fast and effective. However, it still requires vanadium pentoxide as a reactant, with a high cost, and this method cannot accurately obtain a 3.5 - valent vanadyl sulfate electrolyte, which may be limited in some application scenarios with strict requirements for the specific valence state of the electrolyte.

[0009] In the prior art methods for purification, vanadium oxide is used as a raw material, and vanadium electrolyte is prepared through "activation - dissolution - catalytic reduction". The electrolyte prepared by this method has a high concentration. Compared with the process of "calcination - chemical reduction - electrochemical reduction", the process is short and the cost is low. However, in the catalytic reduction process, hydrogen is inevitably introduced as a reducing gas. On the one hand, storing hydrogen in gaseous form requires high - pressure containers or cryogenic storage tanks, which is accompanied by safety problems and high infrastructure costs; on the other hand, hydrogen is flammable, posing a safety hazard; because hydrogen is odorless and colorless, its flame is almost invisible, making leakage detection challenging.

[0010] Therefore, a method for preparing 3.5 - valent vanadium electrolyte with a fast reduction rate, economy and high efficiency is an urgent problem to be solved in the all - vanadium redox flow battery industry. Summary of the Invention

[0011] The main object of the present application is to provide an all - vanadium electrolyte, a preparation method thereof and an all - vanadium redox flow battery, so as to solve the problems of high cost, complex process and low reaction rate in the preparation of all - vanadium electrolyte in the prior art.

[0012] To achieve the above object, according to one aspect of the present application, a method for preparing an all - vanadium electrolyte is provided, including the following steps:

[0013] Step S1: Using an alkaline leaching agent to perform alkaline leaching on vanadium slag for Reaction I to obtain an alkaline leaching product. After solid - liquid separation of the alkaline leaching product, a vanadium - enriched material and a filtrate are obtained;

[0014] Step S2: Mixing the vanadium - enriched material, a first ammonium salt and a first solvent to perform Reaction II to obtain a vanadium precipitation product. After solid - liquid separation of the vanadium precipitation product, a sediment containing ammonium vanadate is obtained; performing a first calcination heat treatment on the sediment containing ammonium vanadate to obtain vanadium oxide; mixing the vanadium oxide and a first reducing agent in a second solvent to perform Reaction III to obtain a first solution containing tetravalent vanadium;

[0015] Step S3: Performing Reaction IV on the first solution containing tetravalent vanadium under the electrocatalytic reduction conditions of a hydrogen evolution catalyst to obtain a solution containing trivalent vanadium.

[0016] Further, the first solution containing tetravalent vanadium obtained in Step S2 is divided into two parts; performing Reaction IV on one part of the first solution containing tetravalent vanadium under the electrocatalytic reduction conditions of a hydrogen evolution catalyst to obtain a solution containing trivalent vanadium; performing the process of Step S4 after Step S3 on the remaining part of the first solution containing tetravalent vanadium;

[0017] Among them, the process of Step S4 includes: mixing the remaining part of the first solution containing tetravalent vanadium and the solution containing trivalent vanadium and dissolving them in a sulfuric acid solution to obtain a 3 - 4 - valent vanadium electrolyte.

[0018] Further, the molar ratio of the sulfuric acid solution to the divanadium(IV) ions in the remaining part of the first divanadium(IV) solution is (1.5 - 6.0):(1.0 - 2.5), and the molar ratio of the sulfuric acid solution to the trivanadium(III) ions in the trivanadium(III) solution is (1.5 - 6.0):(1.0 - 2.5); the molar concentration of the sulfuric acid solution is 2.0 - 5.0 mol / L.

[0019] Further, the molar concentration ratio of the divanadium(IV) ions in the remaining part of the first divanadium(IV) solution to the trivanadium(III) ions in the trivanadium(III) solution is adjusted to 1:1 to obtain a 3.5-valent vanadium electrolyte.

[0020] Further, the vanadium-rich product obtained in step S1 is divided into two parts; among them, one part of the vanadium-rich product is mixed with the first ammonium salt and the first solvent to carry out reaction II to obtain a vanadium precipitation product; the remaining part of the vanadium-rich product is subjected to a roasting treatment;

[0021] Among them, the process of the roasting treatment includes: roasting the remaining part of the vanadium-rich product to obtain a roasted slag containing calcium oxide; mixing the roasted slag containing calcium oxide, the alkaline leaching agent and the vanadium slag in a mass ratio of (1.0 - 30):(2.5 - 80):(1.5 - 20) to carry out vanadium enrichment treatment to obtain a vanadium-enriched product, and the vanadium-enriched product is separated to obtain a calcium vanadate-enriched product.

[0022] Further, after mixing the calcium vanadate-enriched product and the vanadium-rich product, they are mixed with the first ammonium salt and the first solvent to carry out reaction II to obtain a vanadium precipitation product.

[0023] Further, the calcium vanadate-enriched product and the second reducing agent are mixed in a third solvent to carry out reaction V to obtain a second divanadium(IV) solution.

[0024] Further, between step S2 and step S3, there is also S23-1: mixing the first divanadium(IV) solution and the second divanadium(IV) solution to obtain a divanadium(IV)-containing mixed solution; reacting the divanadium(IV)-containing mixed solution under the condition of electrocatalytic reduction by a hydrogen evolution catalyst to obtain a trivanadium(III)-containing solution.

[0025] Further, the conditions of reaction V include: the reaction temperature is 60 - 120 °C, and the constant temperature reaction time is 3.0 - 5.0 h.

[0026] Further, the mass ratio of the calcium vanadate-enriched product to the second reducing agent is (1.0 - 3.5):(0.5 - 3.0).

[0027] Further, the second reducing agent is selected from at least one of oxalic acid, sodium oxalate, potassium oxalate, carboxylic acid, citric acid, tartaric acid and NH4HSO3.

[0028] Further, the third solvent is selected from at least one of H2SO4, HNO3, HCl, H2CO3, and H3PO4.

[0029] Further, the concentration of the third solvent is 0.1 - 5.0 mol / L.

[0030] Further, the pH value in the system of Reaction V is 1.0 - 7.0.

[0031] Further, the mixed solution containing vanadium(IV) obtained in Step S23-1 is divided into two parts; one part of the mixed solution containing vanadium(IV) is subjected to Reaction IV under the condition of electrocatalytic reduction by a hydrogen evolution catalyst to obtain a solution containing vanadium(III); the remaining part of the mixed solution containing vanadium(IV) undergoes the process of Step S4 after Step S3;

[0032] Among them, the process of Step S4 includes: mixing the remaining part of the mixed solution containing vanadium(IV) and the solution containing vanadium(III) and dissolving them in a sulfuric acid solution to obtain a vanadium electrolyte with a valence of 3 - 4.

[0033] Further, the molar ratio of the sulfuric acid solution to the vanadium(IV) ions in the remaining part of the mixed solution containing vanadium(IV) is (1.5 - 6.0):(1.0 - 2.5), and the molar ratio of the sulfuric acid solution to the vanadium(III) ions in the solution containing vanadium(III) is (1.5 - 6.0):(1.0 - 2.5); the molar concentration of the sulfuric acid solution is 2.0 - 5.0 mol / L.

[0034] Further, the molar concentration ratio of the vanadium(IV) ions in the remaining part of the mixed solution containing vanadium(IV) and the vanadium(III) ions in the solution containing vanadium(III) is adjusted to 1:1 to obtain a vanadium electrolyte with a valence of 3.5.

[0035] Further, the conditions of Reaction I in Step S1 include: the reaction temperature is 90 - 180 °C, and the constant-temperature reaction time is 3.0 - 5.0 h.

[0036] Further, the mass ratio of the alkaline leaching agent to the vanadium slag is (1.5 - 200):(2.5 - 50).

[0037] Further, the pH of the mixed system of the alkaline leaching agent and the vanadium slag is 10.5 - 12.0.

[0038] Further, the concentration of the alkaline leaching agent is 0.01 - 10.0 mol / L.

[0039] Further, the alkaline leaching agent is selected from at least one of an aqueous solution of NaOH, an aqueous solution of Na2CO3, an aqueous solution of NaHCO3, an aqueous solution of KOH, an aqueous solution of K2CO3, and an aqueous solution of KHCO3.

[0040] Further, the vanadium enrichment material, water, and organic solvent I in step S2 are first mixed to form a suspension of the vanadium enrichment material, and then the suspension of the vanadium enrichment material is mixed with the first ammonium salt and the first solvent to carry out Reaction II, obtaining a vanadium precipitation product.

[0041] Further, the volume ratio of water to organic solvent I is (50 - 1000):(5 - 500).

[0042] Further, organic solvent I is selected from at least one of the group consisting of methanol, ethanol, propanol, ethylene glycol, isopropanol, polyacrylic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide.

[0043] Further, the conditions for Reaction II in step S2 include: the reaction temperature is 10 - 90°C, and the constant-temperature reaction time is 1.0 - 5.0 h.

[0044] Further, the mass ratio of the vanadium enrichment material to the first ammonium salt is (2.0 - 30.0):(0.5 - 50.0).

[0045] Further, the first ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, polydopamine, hexamethylenetetramine, melamine, ammonium polyacrylate, and dopamine hydrochloride.

[0046] Further, the first ammonium salt and the first solvent are mixed to form a first ammonium salt solution; the concentration of the first ammonium salt solution is 0.01 - 5.0 mol / L.

[0047] Further, the first solvent is a mixture of water and inorganic acid.

[0048] Further, the volume ratio of water to inorganic acid is (50 - 1000):(5 - 500); wherein, the inorganic acid is selected from at least one of hydrochloric acid, nitric acid, boric acid, sulfuric acid, carbonic acid, and phosphoric acid; the concentration of the inorganic acid is 0.1 - 5.0 mol / L.

[0049] Further, the electrocatalytic reduction conditions in step S3 include: the electrolysis voltage is 0.5 - 5.0 V, the electrolysis temperature is 20 - 60°C, the electrolysis time is 0.5 - 3.0 h, and the distance between the electrode plates is 2.0 - 10.0 cm.

[0050] Further, the conditions for Reaction III in step S2 include: the reaction temperature is 60 - 120°C, and the constant-temperature reaction time is 3.0 - 5.0 h.

[0051] Further, the mass ratio of vanadium oxide to the first reducing agent is (1.0 - 3.5):(0.5 - 3.0).

[0052] Further, the first reducing agent is selected from at least one of SO2, NH4HSO3, H2O2, oxalic acid, sodium oxalate, potassium oxalate, ascorbic acid, sodium ascorbate, sodium sulfite, sodium metabisulfite, and carboxylic acid.

[0053] Further, the second solvent is selected from at least one of H2SO4, HCl, and phosphoric acid; the concentration of the second solvent is 0.1 - 5.0 mol / L.

[0054] Further, the process of the first calcination heat treatment sequentially includes: the first-stage heat treatment, the second-stage heat treatment, and the third-stage heat treatment; wherein, the process of the first-stage heat treatment includes: heating from 25°C to 150 - 200°C at a heating rate of 5 - 30°C / min and holding for 1.0 - 1.5 h; the process of the second-stage heat treatment includes: continuing to heat to 300 - 350°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h; the process of the third-stage heat treatment includes: continuing to heat to 450 - 500°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h.

[0055] Further, the process of the first calcination heat treatment is carried out in the first protective atmosphere; the first protective atmosphere is selected from at least one of argon, nitrogen, helium, and NH3.

[0056] Further, the gas flow rate of the first protective atmosphere is 1 - 10 L / min.

[0057] Further, the process of the first calcination heat treatment is carried out in a tubular vacuum furnace.

[0058] Further, after the first calcination heat treatment, an annealing treatment is also included, and its process includes: cooling to 300 - 400°C at a cooling rate of 5 - 10°C / min and introducing oxygen; the flow rate of oxygen is 1 - 10 L / min, and after holding for 1.0 - 3.0 h, it is naturally cooled to room temperature.

[0059] Further, between step S2 and step S3, step S23-2 is also included: dividing the vanadium-ammonium-containing sediment obtained in step S2 into two parts; performing the first calcination heat treatment in step S2 on one part of the vanadium-ammonium-containing sediment to obtain vanadium oxide; using the remaining part of the vanadium-ammonium-containing sediment to prepare a vanadium-based hydrogen evolution electrocatalyst;

[0060] Wherein, the preparation process of the vanadium-based hydrogen evolution electrocatalyst includes: mixing the remaining part of the vanadium-ammonium-containing sediment with a second ammonium salt, a noble metal compound, a chelating agent, a third reducing agent, and a fourth solvent to carry out reaction VI to obtain a reaction VI product; performing a second calcination heat treatment on the reaction VI product and a fourth reducing agent in a second protective atmosphere to obtain a noble metal-loaded vanadium-based hydrogen evolution electrocatalyst.

[0061] Further, the conditions for Reaction VI include: the reaction temperature is 0 to 160°C, and the isothermal reaction time is 1.0 to 5.0 h.

[0062] Further, the mass ratio of the ammonium vanadate deposit in the remaining part to the second ammonium salt is (1.0 to 20.0):(0.001 to 15.0).

[0063] Further, the mass ratio of the ammonium vanadate deposit in the remaining part to the noble metal compound is (1.0 to 20.0):(0.1 to 10.0).

[0064] Further, the mass ratio of the noble metal compound to the chelating agent is (0.1 to 10.0):(0.001 to 15.0).

[0065] Further, the molar ratio of the noble metal compound to the third reducing agent is (0.001 to 0.1):(0.005 to 0.5).

[0066] Further, the mass ratio of the fourth reducing agent to the product of Reaction VI is (0.1 to 0.5):1.

[0067] Further, the second ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, polydopamine, ethylenediaminetetraacetic acid, ethylenediamine, triethylenetetramine, triethanolamine, hexamethylenetetramine, melamine, ammonium polyacrylate, and dopamine hydrochloride.

[0068] Further, the noble metal compound is selected from at least one of chloroplatinic acid, chloroplatinate, [Pt(acac)2], diaminodinitroplatinum, and [Pt(CH3NH2)4][PtCl4].

[0069] Further, the chelating agent is selected from at least one of thiourea, sulfite, phosphoric acid, oxalic acid, arsonic acid, polyacrylic acid, and polyvinyl alcohol.

[0070] Further, the third reducing agent is selected from at least one of elemental sulfur, ascorbic acid, sodium ascorbate, sodium citrate, formaldehyde, acetaldehyde, formic acid, acetic acid, hydrazine hydrate, metal borohydride, sulfite, and oleylamine.

[0071] Further, the fourth reducing agent is elemental carbon.

[0072] Further, the fourth solvent is selected from water and / or ethylene glycol.

[0073] Further, the protective atmosphere for the second calcination heat treatment is selected from at least one of argon, nitrogen, helium, and NH3.

[0074] Further, the process of the second calcination heat treatment sequentially includes a first-stage heat treatment, a second-stage heat treatment, a third-stage heat treatment, and a fourth-stage heat treatment; wherein, the first-stage heat treatment includes: heating from 25°C to 300 - 350°C at a heating rate of 5 - 30°C / min and holding for 1.0 - 1.5 h; the second-stage heat treatment includes: continuing to heat to 600 - 650°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h; the third-stage heat treatment includes: continuing to heat to 800 - 900°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h; the fourth-stage heat treatment includes: continuing to heat to 1100 - 1200°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h, and after natural cooling to room temperature, a vanadium-based hydrogen evolution electrocatalyst loaded with noble metal is obtained.

[0075] Further, the vanadium-based hydrogen evolution electrocatalyst loaded with noble metal, Nafion solution, and organic solvent II are mixed to obtain a slurry; the slurry is coated on the surface of the membrane electrode to obtain a catalyst membrane electrode.

[0076] Further, the mixing ratio of the vanadium-based hydrogen evolution electrocatalyst loaded with noble metal, Nafion solution, and organic solvent II is (2.5 - 15.0) mg : (100.0 - 150.0) μL : (1.0 - 10.0) mL; the concentration of the Nafion solution is 0.5 - 10.0 wt%.

[0077] Further, the loading amount of the noble metal in the vanadium-based hydrogen evolution electrocatalyst is 20.0 - 40.0 wt%, the pore volume is 0.01 - 10.0 cm 3 / g, the average pore diameter is 5 - 50 nm, and the specific surface area is 450 - 600 m 2 / g.

[0078] According to the second aspect of the present application, a full vanadium electrolyte is provided, which is prepared by using the preparation method of the above full vanadium electrolyte.

[0079] According to the third aspect of the present application, a full vanadium flow battery is provided, including a full vanadium electrolyte, which is the above full vanadium electrolyte.

[0080] Applying the technical solution of the present application, a vanadium-containing electrolyte and a preparation method thereof are provided. The method uses vanadium slag as a raw material, adopts a hydrometallurgical process, and promotes the phase transformation of vanadium ion compounds according to the characteristics of different pH ranges in the solution system, so as to achieve efficient separation from impurity elements, thereby obtaining a vanadium-rich product. At the same time, valuable elements can be separated during this process, and vanadium extraction is realized through valence regulation. This method can not only efficiently extract vanadium-containing electrolytes with different valence states, but also make full use of vanadium slag solid waste, realizing the clean and high-value comprehensive utilization of vanadium slag resources. Furthermore, a closed-loop cycle system for green and low-carbon metallurgy of vanadium metal is constructed, reducing production, environmental costs and energy consumption. When preparing vanadium electrolyte by electrolysis, with the help of a hydrogen evolution electrocatalyst, vanadium electrolyte is prepared by electrocatalytic reduction at normal temperature and pressure to achieve the purpose of efficient catalytic reduction; through the electrocatalytic reduction reaction of the hydrogen evolution catalyst, the reduction reaction rate of vanadium ions is increased. Compared with the traditional electrolysis method, this method has a faster reaction rate and does not introduce other reducing agents, with better safety and economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] The accompanying drawings forming a part of this application are used to provide a further understanding of the application. The schematic embodiments and descriptions thereof of the application are used to explain the application and do not constitute an improper limitation of the application. In the drawings:

[0082] Figure 1 is a schematic flow chart for preparing vanadium electrolyte in an embodiment of the present application;

[0083] Figure 2 is a schematic structural diagram of the device for preparing vanadium electrolyte in an embodiment of the present application;

[0084] Figure 3 is an XRD phase diagram of the alkali leaching product of chromium vanadium slag in Example 1 of the present application;

[0085] Figure 4 is a transmission electron microscope image (TEM image) of the Pt / VN catalyst prepared in Example 3 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0086] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0087] In order to solve the problems of high cost, complex process and low reaction rate in the preparation of vanadium-containing electrolyte in the prior art, according to one aspect of the present application, a method for preparing vanadium-containing electrolyte is provided, including the following steps:

[0088] Step S1: Use an alkaline leaching agent to perform alkali leaching treatment on vanadium slag to carry out Reaction I, obtain an alkali leaching product, and after solid-liquid separation of the alkali leaching product, obtain a vanadium-rich product and a filtrate;

[0089] Step S2: Mix the vanadium enrichment, the first ammonium salt, and the first solvent to carry out Reaction II to obtain a vanadium precipitation product. After solid-liquid separation of the vanadium precipitation product, a sediment containing ammonium metavanadate is obtained; the sediment containing ammonium metavanadate is subjected to a first calcination heat treatment to obtain vanadium oxide; the vanadium oxide and the first reducing agent are mixed in the second solvent to carry out Reaction III to obtain a first solution containing tetravalent vanadium;

[0090] Step S3: React the first solution containing tetravalent vanadium under the electrocatalytic reduction conditions of a hydrogen evolution catalyst to obtain a solution containing trivalent vanadium.

[0091] By adopting the above-mentioned vanadium extraction method, the present application can not only efficiently extract vanadium electrolytes with different valence states, but also make full use of vanadium slag solid waste, realize the clean and high-value comprehensive utilization of vanadium slag resources, realize the green and low-carbon metallurgical closed-loop cycle of vanadium metal, thereby reducing production costs and energy consumption and reducing environmental protection risks; and in the process of preparing vanadium electrolyte, high-efficiency catalytic reduction can be realized with the help of a hydrogen evolution electrocatalyst. Through the electrocatalytic reduction reaction of the catalyst, the reduction reaction rate of vanadium ions is improved. Compared with the traditional electrolysis method, this method has a faster reaction rate and does not require the introduction of other reducing agents, and has better safety and economy.

[0092] In some embodiments, the first solution containing tetravalent vanadium obtained in Step S2 is divided into two parts; among them, a part of the first solution containing tetravalent vanadium is reacted under the electrocatalytic reduction conditions of a hydrogen evolution catalyst to generate a solution containing trivalent vanadium; the remaining part of the first solution containing tetravalent vanadium undergoes the process of Step S4 after Step S3; wherein, Step S4 is specifically: mixing the remaining part of the first solution containing tetravalent vanadium and the solution containing trivalent vanadium and dissolving them in a sulfuric acid solution to prepare a 3-4 valence vanadium electrolyte.

[0093] In some embodiments, the molar ratio of the sulfuric acid solution to the tetravalent vanadium ions in the remaining part of the first solution containing tetravalent vanadium is (1.5-6.0):(1.0-2.5), for example (2.0-5.0):(1.0-2.0); the molar ratio of the sulfuric acid solution to the trivalent vanadium ions in the solution containing trivalent vanadium is (1.5-6.0):(1.0-2.5), for example (2.0-5.0):(1.0-2.0); the concentration of the sulfuric acid solution is 2.0-5.0 mol / L, for example 2.0-4.0 mol / L. The above molar ratios of the sulfuric acid solution and vanadium ions in various valence states can be adjusted according to actual needs.

[0094] In some embodiments, the molar concentration ratio of the tetravalent vanadium ions in the remaining part of the first solution containing tetravalent vanadium and the trivalent vanadium ions in the solution containing trivalent vanadium is adjusted to 1:1 to obtain a 3.5 valence vanadium electrolyte.

[0095] In this application, the obtained tetravalent vanadium solution is divided into two parts. One part is directly used as the tetravalent vanadium solution to provide tetravalent vanadium; the other part is reduced to trivalent vanadium under electrocatalytic conditions to obtain a solution containing trivalent vanadium. By adjusting the molar concentration ratio of vanadium ions in these two solutions, a vanadium electrolyte with a valence state of 3 to 4 can be obtained; therefore, through the above method of this application, at least solutions of trivalent vanadium, 3.5-valent vanadium, and tetravalent vanadium ions can be obtained.

[0096] In some embodiments, the vanadium-rich product obtained in step S1 is divided into two parts; among them, one part of the vanadium-rich product is mixed with a first ammonium salt and a first solvent to carry out reaction II to obtain a vanadium precipitation product; the remaining part of the vanadium-rich product is subjected to a roasting treatment; among them, the process of the roasting treatment includes: roasting the remaining part of the vanadium-rich product to obtain a roasted slag containing calcium oxide; mixing the roasted slag containing calcium oxide, an alkaline leaching agent, and vanadium slag in a mass ratio of (1.0~30):(2.5~80):(1.5~20) to carry out vanadium enrichment treatment to obtain an enriched vanadium product, and the enriched vanadium product is separated to obtain a calcium vanadate enriched product. Through the above roasting treatment in this application, a new method for enriching vanadium can be realized, and the recycling of intermediate products of vanadium-rich products can be achieved.

[0097] In addition, in step S1 of this application, there is a situation where some calcium vanadate substances are converted into vanadate ions and enter the solution under the action of alkali solution, which may cause partial loss of vanadium ions. In this regard, the filtrate obtained from the solid-liquid separation can be sent back to the vanadium slag for alkali leaching treatment again; to solve the problem of possible loss of vanadium ions, a certain amount (which can be determined according to actual needs) of vanadium-rich product is roasted in this application to obtain calcium oxide (calcium oxide can also be obtained from the outside), so that it reacts with vanadate ions to generate calcium vanadate substances. After solid-liquid separation, the calcium vanadate substances are enriched to form a calcium vanadate enriched product; when calcium oxide is mixed and reacted with vanadium slag and alkali solution, a small amount of the vanadium-rich product prepared in step S1 can also be taken to further enhance the reaction activity of calcium oxide and improve the enrichment degree of vanadium; after obtaining the calcium vanadate enriched product, it can either be mixed with the vanadium-rich product for the subsequent process of the vanadium-rich product, or the pentavalent vanadium in the calcium vanadate enriched product can be reduced to tetravalent vanadium alone.

[0098] In some embodiments, the calcium vanadate enriched product and the vanadium-rich product are mixed, and then a first ammonium salt and a first solvent are added to carry out reaction II to obtain a vanadium precipitation product; or, the calcium vanadate enriched product and a second reducing agent are mixed in a third solvent to carry out reaction V to obtain a second tetravalent vanadium-containing solution.

[0099] Through the above treatment, the present application provides a brand-new method for preparing a second vanadium tetravalent solution. In this way, the reaction paths and treatment methods in the preparation process of the all-vanadium electrolyte are enriched, the ways to obtain different products are broadened, the entire preparation system becomes more flexible and diversified, which helps to better meet various requirements in actual preparation and further improve the preparation efficiency and quality of the all-vanadium electrolyte.

[0100] In some embodiments, the conditions for reaction V include: the reaction temperature is 60~120°C, such as 90~100°C; the constant-temperature reaction time is 3.0~5.0 h. Through the above reaction temperature and reaction time conditions, the present application can cause calcium vanadate to release vanadium ions and be converted into a vanadium tetravalent solution as much as possible.

[0101] In some embodiments, the mass ratio of the calcium vanadate concentrate to the second reducing agent is (1.0~3.5):(0.5~3.0); further (1.0~2.0):(1.5~3.0); the second reducing agent is selected from at least one of oxalic acid, sodium oxalate, potassium oxalate, carboxylic acid, citric acid, tartaric acid, and NH4HSO3; further, the concentration of the second reducing agent solution is 0.001~0.5 mol / L, such as 0.01~0.5 mol / L; the third solvent is selected from at least one of H2SO4, HNO3, HCl, H2CO3, and H3PO4; the concentration of the third solvent is 0.1~5.0 mol / L, and the pH value of the reaction V system is adjusted to 1.0~7.0, further 1.0~6.0, and still further 1.0~4.0.

[0102] By selecting the above suitable reducing agent, in an acid or mixed acid solvent reaction system, and limiting the acid concentration and the acidity of the reaction system to provide suitable reaction conditions, the present application can promote the efficient reduction of pentavalent vanadium in calcium vanadate to tetravalent vanadium, such as generating vanadyl sulfate.

[0103] In some embodiments, between step S2 and step S3, there is also S23-1: mixing the first vanadium tetravalent solution and the second vanadium tetravalent solution to obtain a vanadium tetravalent mixed solution; reacting the vanadium tetravalent mixed solution under the conditions of electrocatalytic reduction by a hydrogen evolution catalyst to obtain a vanadium trivalent solution.

[0104] The above method of the present application realizes a channel for obtaining a tetravalent vanadium solution in another new way, which not only makes full use of the intermediate product of vanadium enrichment, but also adds a new method for extracting vanadium. The present application mixes the tetravalent vanadium obtained by reaction V and the tetravalent vanadium obtained by reaction III, and can directly use them to perform step S3, and reduce all the tetravalent vanadium to trivalent vanadium through electrocatalysis. These tetravalent vanadium solutions can also be divided into two parts (the specific proportion can be determined according to actual needs), one part is used as tetravalent vanadium for standby, and the other part of the tetravalent vanadium is electrocatalytically reduced to trivalent vanadium. When the tetravalent vanadium and trivalent vanadium are mixed, a 3-4 valence vanadium solution can be obtained. Such operation can further expand the deployment and utilization methods of vanadium solutions of different valence states in the preparation of all-vanadium electrolyte, can better meet the diverse preparation needs, and further optimize the overall preparation effect of all-vanadium electrolyte.

[0105] In some embodiments, the mixed solution containing tetravalent vanadium obtained in step S23-1 is divided into two parts; one part of the mixed solution containing tetravalent vanadium is subjected to reaction IV under conditions of electrocatalytic reduction of a hydrogen evolution catalyst to obtain a solution containing trivalent vanadium; the remaining part of the mixed solution containing tetravalent vanadium is subjected to the process of step S4 after step S3; wherein the process of step S4 includes: mixing the remaining part of the mixed solution containing tetravalent vanadium and the solution containing trivalent vanadium and dissolving them in a sulfuric acid solution to obtain a 3~4-valent vanadium electrolyte.

[0106] In some embodiments, the molar ratio of the tetravalent vanadium ions in the sulfuric acid solution to the remaining tetravalent vanadium-containing mixed solution is (1.5-6.0): (1.0-2.5), for example (2.0-5.0): (1.0-2.0); the molar ratio of the sulfuric acid solution to the trivalent vanadium ions in the trivalent vanadium-containing solution is (1.5-6.0): (1.0-2.5), for example (2.0-5.0): (1.0-2.0); the concentration of the sulfuric acid solution is 2.0-5.0 mol / L, further 2.0-4.0 mol / L. The present application adopts the above sulfuric acid concentration and ratio to obtain a vanadium electrolyte that meets the use requirements.

[0107] In some embodiments, the molar concentration of tetravalent vanadium ions in the remaining portion of the mixed solution containing tetravalent vanadium and the molar concentration of trivalent vanadium ions in the solution containing trivalent vanadium are adjusted to be 1:1 to obtain a 3.5-valent vanadium electrolyte.

[0108] The main principles of the application for alkaline leaching, vanadium enrichment, vanadium precipitation and reduction of vanadium slag (or chrome vanadium slag) are as follows:

[0109] The vanadium slag involved in this application is specifically chrome-vanadium slag, which is a hazardous waste and urgently needs to be recycled. The main components of the slag are CaCrO4, Na2Cr2O7·2H2O, CaSO4·2H2O, Ca 1.5Na 3.5 (OH) 0.55 (SO4)3(H2O) 0.44 and components such as calcium vanadate (CaV2O6 (calcium metavanadate), Ca2V2O7 (calcium pyrophosphate), Ca3V2O8 (calcium orthovanadate)); in an alkaline system, Cr(VI) exists as CrO4 2- ions, and V(V) exists in the form of VO4 3- , V2O7 4- , VO3 - and other plasma forms. Therefore, through alkali leaching, taking advantage of the property differences of vanadium and chromium in the solution, that is, the solubility differences at different alkali concentrations and temperatures, chromium and vanadium can be effectively leached and separated. By adjusting the pH value of the vanadium-chromium solution, when pH > 11.0, that is, at a high pH value, hexavalent chromium in it exists as chromate (CrO4 2- ) in the solution, that is, CaCrO4 + HCO3 - +OH - =CrO4 2- + CaCO3↓+H2O. When the solution pH > 12.0, vanadate mainly exists in the form of VO4 3- , that is, vanadium is separated and enriched in the solution; after concentration, the vanadate in the leaching solution can be precipitated and crystallized to obtain a vanadium-enriched product (that is, Ca3(VO4)2); thus, vanadium and chromium are effectively separated. Therefore, in the alkaline leaching solution system of vanadium slag, through the difference in the solubility products of different ionic valences of valuable metal elements, valuable elements can be selectively separated.

[0110] To clearly describe the mechanism, in step S1, the reaction equations involved in adding alkaline leaching solution NaHCO3 and additive calcium oxide to vanadium slag are:

[0111] Ca2V2O7 +2HCO3 - +2OH - = V2O7 4- + 2CaCO3↓+2H2O;

[0112] Ca3V2O8+ 3HCO3 - +3OH - = 2VO4 3- + 3CaCO3↓+3H2O;

[0113] V2O7 4- +2CaO+2H2O = Ca2V2O7↓+ 4OH - ;

[0114] 2VO3 - +CaO+H2O=Ca(VO3)2↓+2OH- ;

[0115] 2VO4 3- +3CaO + 3H2O = Ca3(VO4)2↓ + 6OH - 。

[0116] Meanwhile, the vanadium-rich material contains calcium carbonate. By roasting at 1000 °C for 3.0 h, CaCO3 can be thermally decomposed to prepare CaO, which has good reaction activity; it shows that the lime obtained from the leaching residue and roasting heat treatment can be used as a vanadium precipitation aid in the vanadium slag and vanadate solution system.

[0117] Through the calcium salt and CaO in the process flow, the enrichment of vanadium in the vanadate leaching solution system is carried out to obtain a calcium vanadate intermediate product. Under acidic and / or reducing agent (the acidic environment is provided by sulfuric acid, hydrochloric acid, carbonic acid, etc., and the reducing agent is provided by oxalic acid, citric acid, tartaric acid, etc.) conditions, the phase transformation of vanadate ions (i.e., reaction V) occurs, realizing the recovery, purification, and extraction of vanadium elements from low-concentration vanadium slag, and preparing a V 4+ ionic solution. The specific reaction equations are as follows:

[0118] Ca(VO3)2 + CO3 2- = 2VO3 - + CaCO3↓;

[0119] Ca3(VO4)2 + 3CO3 2- = 3CaCO3↓ + 2VO4 3- ;

[0120] VO4 3- + H2O = VO3 - + 2OH - ;

[0121] 2VO3 - + 2HCl + 3H2SO4 = 2VOSO4 + Cl2 + 4H2O + SO4 2- 。

[0122] Meanwhile, in step S2, the vanadate in the vanadium-rich material reacts with the first ammonium salt to obtain ammonium metavanadate (NH4VO3) (i.e., reaction II). At the same time, the excess vanadate that does not participate in the precipitation reaction undergoes hydration to obtain [VO2(H2O)3] + , and after deprotonation, VO(OH)3 is formed, obtaining a vanadium precipitation product containing NH4VO3 and a small amount of VO(OH)3; the following reactions occur during this process:

[0123] VO3 - + NH4Cl + H2O → HVO3 + Cl - + NH4OH;

[0124] Since the solubility of NH4VO3 in water is very small, the following metathesis reaction occurs:

[0125] HVO3 + NH4OH → NH4VO3↓ + H2O;

[0126] VO3 - + NH4Cl → NH4VO3↓ + Cl - .

[0127] In some embodiments, the alkali leaching treatment process in step S1 specifically includes: mixing vanadium slag with an alkaline leaching agent to obtain a mixed system, and then adjusting the pH value of the mixed system to 10.5 - 12.0 with an alkali solution, such as pH being 10.5, 11.0, 11.5, 12.0; for another example, pH being 10.5 - 11.5. Subsequently, vanadium is extracted from the vanadium slag suspension solution to obtain a vanadium-enriched product. Based on the difference in the solubility products of impurity elements, by controlling the pH value of the mixed system within the above range in this application, it is beneficial to achieve the purification and impurity removal of the vanadium-containing leaching solution (especially removing chromium ions), improve the purity of vanadate, and finally obtain a vanadate-enriched product.

[0128] In some embodiments, the alkaline leaching agent (alkali solution or leaching solution) is selected from at least one of an aqueous solution of NaOH, an aqueous solution of Na2CO3, an aqueous solution of NaHCO3, an aqueous solution of KOH, an aqueous solution of K2CO3, and an aqueous solution of KHCO3; further, the concentration of the alkaline leaching agent is 0.01 - 10.0 mol / L, further 1.0 - 10.0 mol / L; still further 1.0 - 5.0 mol / L; for example, any value among 1.0 mol / L, 2.0 mol / L, 3.0 mol / L, 4.0 mol / L, 5.0 mol / L, 6.0 mol / L, 7.0 mol / L, 8.0 mol / L, 9.0 mol / L, 10.0 mol / L or the range value between any two of them.

[0129] In some embodiments, the conditions for Reaction I in step S1 include: the reaction temperature is 90~180°C, and the isothermal reaction time is 3.0~5.0 h; the mass ratio of the alkaline leaching agent to the vanadium slag is (1.5~200):(2.5~50), further (1.5~20):(2.5~50), and still further (1.0~15):(1.0~3.0); for example, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1; the above reaction temperature is further 90~150°C, and still further 90~120°C, and the reaction time is 3.0 h, 4.0 h, 5.0 h. In order to further improve the solubility and dispersibility of the vanadium slag raw material in the mixed system, stirring is carried out during the mixing process of the vanadium slag and the alkaline solution, and the stirring speed is 300~500 rpm; adopting the above mixing temperature and mixing ratio is beneficial to improving the solubility and dispersibility of the vanadium slag, the raw material containing vanadate, in the mixed system, thus facilitating subsequent treatment.

[0130] In this application, the above-mentioned alkaline solution is selected as the leaching agent, and the concentration of the alkaline solution is limited to provide a suitable acid-base reaction environment for the alkaline leaching treatment. Under this reaction environment, the vanadium ions in the vanadium slag are fully extracted and enriched, and are efficiently separated from the chromium ions. The main components of the separated enriched product are vanadium-rich substances and part of calcium carbonate, and the calcium carbonate can be used for subsequent operations; the extracted vanadium-rich substances include calcium vanadate, calcium pyrovanadate, calcium metavanadate, etc.

[0131] In some embodiments, the conditions for Reaction II in step S2 include: the reaction temperature is 10~90°C, and the isothermal reaction time is 1.0~5.0 h; for example, the reaction temperature is 20~60°C, and the reaction time is 1.0~3.0 h. Limiting it within the above range in this application is beneficial to improving the yield of NH4VO3, facilitating subsequent utilization, or being conducive to the preparation of vanadium oxides, or being conducive to the preparation of catalyst precursors.

[0132] In some embodiments, the mass ratio of the vanadium-rich substance to the first ammonium salt is (2.0~30.0):(0.5~50.0); further (2.0~20.0):(0.5~40.0); still further (2.0~10.0):(0.5~30.0), for example (2.0~10.0):(0.5~10.0) or (2.0~10.0):(0.5~5.0), and still further (2.0~10.0):(0.5~3.0); by limiting the above ratio in this application, it is beneficial to improve the raw material utilization rate, increase the formation rate of ammonium metavanadate, and is conducive to subsequent processes.

[0133] In this application, the above ammonium salt and vanadium-rich concentrate are selected to undergo a metathesis reaction of vanadate at the above-defined temperature, and ammonium metavanadate (NH4VO3) precipitate is obtained through solid-liquid separation. The obtained ammonium metavanadate in this application is divided into two parts for use. One part is used as a precursor of a hydrogen evolution electrocatalyst support for subsequent processes, and the other part reduces pentavalent vanadium to tetravalent vanadium through pyrolysis and reduction.

[0134] In some embodiments, to facilitate the reaction rate, the vanadium-rich concentrate can be first formulated into its suspension and then participate in the reaction. For example, the vanadium-rich concentrate, water, and organic solvent I in step S2 are first mixed to form a suspension of the vanadium-rich concentrate, and then the suspension of the vanadium-rich concentrate is mixed with the first ammonium salt and the first solvent to carry out Reaction II to obtain a vanadium precipitation product. The component ratios of the above suspension are not limited and can be adjusted according to actual needs. For example, the volume ratio of water to organic solvent I can be (50~1000):(5~500); for another example, the mixing ratio of the vanadium-rich concentrate, water, and organic solvent I is (1.0~10.0) g:(10.0~100.0) mL:(10.0~100.0) mL; wherein, the organic solvent I can be selected from at least one of the group consisting of methanol, ethanol, propanol, ethylene glycol, isopropanol, polyacrylic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide; further, the weight ratio of the suspension solution of the vanadium-rich concentrate, the first ammonium salt, and the first solvent is (50.0~1500.0):(0.5~50.0):(100.0~1500.0).

[0135] In some embodiments, the first ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, polydopamine, hexamethylenetetramine, melamine, ammonium polyacrylate, and dopamine hydrochloride. By selecting the above ammonium salt in this application, the vanadium precipitation reaction with the vanadium-rich concentrate can be fully carried out to obtain an ammonium vanadate deposit, and this product can form vanadium oxide through the first calcination pyrolysis.

[0136] In some embodiments, a first ammonium salt and a first solvent are mixed to form a first ammonium salt solution; the concentration of the first ammonium salt solution is 0.01 - 5.0 mol / L; further 0.1 - 1.0 mol / L; for example, any value among 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L or a range value between any two of them; wherein, the first solvent is a mixture of water and an inorganic acid; the volume ratio of water to the inorganic acid is (50 - 1000):(5 - 500), for example (1 - 10):1; the inorganic acid is selected from at least one of hydrochloric acid, nitric acid, boric acid, sulfuric acid, carbonic acid and phosphoric acid, and the concentration of the inorganic acid is 0.1 - 5.0 mol / L, further 2.0 - 5.0 mol / L. The above-mentioned inorganic acid selected in the present application is beneficial to improving the dispersibility of vanadate and ammonium salt, and facilitating the metathesis reaction of the two.

[0137] The present application controls the concentration of ammonium salt, which is beneficial to the raw material utilization rate of ammonium salt, improves the reaction efficiency of the metathesis reaction, and increases the yield of reaction products (including NH4VO3 and a small amount of VO(OH)3). Mixing the vanadium-enriched material obtained after purification and impurity removal with the ammonium salt solution and carrying out the metathesis reaction is beneficial to improving the quality of vanadium electrolyte and the purity of the precursor for preparing the catalyst support used subsequently. Mixing the vanadium-enriched material with the ammonium salt solution and carrying out the metathesis reaction to obtain an ammonium vanadate product. Moreover, the above method adsorbs free water molecules on the surface of the subsequently prepared support precursor, which is beneficial to improving its wettability, facilitating subsequent treatment and the impregnation, adsorption and loading of noble metals on the surface of the support, thus omitting the support pretreatment process, making the process more concise, efficient, with lower energy consumption and more significant cost advantages.

[0138] In order to better control the metathesis reaction rate, the ammonium salt solution is dropped into the suspension solution of the vanadium-enriched material. In order to further control the reaction rate and make the formed NH4VO3 structure more stable, the dropping rate is controlled at 1 - 30 mL / min, for example 5 - 15 mL / min.

[0139] In some embodiments, step S2 further includes: successively performing solid-liquid separation and drying on the vanadium precipitation product to obtain a sediment of ammonium vanadate, which is more beneficial for the first calcination heat treatment and the preparation of the precursor of the catalyst support.

[0140] The solid-liquid separation of the vanadium precipitation product system in this application is conducive to separating the ammonium vanadate deposit and the first solvent to obtain a solid-phase product. After drying the solid-phase product, it is conducive to removing the residual solvent entrained in the solid-phase product, facilitating subsequent processing. To improve the separation efficiency of the ammonium vanadate deposit and the solvent, the solid-liquid separation includes but is not limited to centrifugal separation. To improve the removal rate of the residual solvent entrained in the solid-phase product, the drying temperature is 50-90 °C and the time is 6.0-24.0 h.

[0141] In some embodiments, the conditions for electrolysis in step S3 include: the electrolysis voltage is 0.5-5.0 V, such as 1.0-3.0 V, the electrolysis temperature is 20-60 °C, such as 20-40 °C, the electrolysis time is 0.5-3.0 h, such as 0.5-1.5 h; the distance between the electrode plates is 2.0-10.0 cm. By setting the above electrolysis conditions in this application, the vanadium-based catalyst can be used for efficient electrocatalytic reduction to improve the kinetic reaction rate of the reduction of tetravalent vanadium to trivalent vanadium.

[0142] In some embodiments, the conditions for reaction III in step S2 include: the reaction temperature is 60-120 °C, further 80-100 °C, and the constant-temperature reaction time is 3.0-5.0 h; the mass ratio of vanadium oxide to the first reducing agent is (1.0-3.5):(0.5-3.0), such as (1.0-3.5):1. By adopting the above reaction ratio, reaction temperature, etc. in this application, pentavalent vanadium oxide can be fully reduced to tetravalent vanadium.

[0143] In some embodiments, the first reducing agent is selected from at least one of SO2, NH4HSO3, H2O2, oxalic acid, sodium oxalate, potassium oxalate, ascorbic acid, sodium ascorbate, sodium sulfite, sodium metabisulfite, and carboxylic acid; further, the concentration of the first reducing agent solution is 0.001-5.0 mol / L, further 0.5-5.0 mol / L, and further 1.0-5.0 mol / L; the second solvent is selected from at least one of H2SO4, HCl, and phosphoric acid; the concentration of the second solvent is 0.1-5.0 mol / L, further 1.0-5.0 mol / L.

[0144] By setting appropriate reaction temperatures, reaction times, and reaction ratios for vanadium pentoxide and the reducing agent, acid, or mixed acid in this application, pentavalent vanadium can be fully reduced to tetravalent vanadium (i.e., reaction III). For example, in a sulfuric acid solution, under the action of the reducing agent H2O2, pentavalent vanadium can be reduced to tetravalent vanadyl sulfate. The solubility of V2O5 in the water / acid system is small at room temperature. Under the action of the reducing agent, V2O5 is reduced to soluble VOSO4, promoting the dissolution of V2O5. In a strong acid system with pH ≤ 1, V 5+ mainly exists as VO2 +exists in the form, and the related reaction formulas involved are as follows:

[0145] V2O5 + H2O → 2HVO3;

[0146] HVO3 + H + → VO2 + + H2O;

[0147] 2VO2 + + H2O2 + 2H + → 2VO 2+ + 2H2O + O2;

[0148] V2O5 + H2O2 + 2H2SO4 → 2VOSO4 + 3H2O + O2.

[0149] In some embodiments, the process of the first calcination heat treatment sequentially includes: the first-stage heat treatment, the second-stage heat treatment, and the third-stage heat treatment; wherein, the process of the first-stage heat treatment includes: heating from 25°C to 150 - 200°C at a heating rate of 5 - 30°C / min and holding for 1.0 - 1.5 h; the process of the second-stage heat treatment includes: continuing to heat to 300 - 350°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h; the process of the third-stage heat treatment includes: continuing to heat to 450 - 500°C at a heating rate of 5 - 10°C / min and holding for 1.0 - 3.0 h. Through the above multi-stage pyrolysis process in this application, the ammonium vanadate-containing deposit can be fully calcined and pyrolyzed, thereby generating pentavalent vanadium oxide.

[0150] In some embodiments, the process of the first calcination heat treatment is carried out in a first protective atmosphere; the first protective atmosphere is selected from at least one of argon, nitrogen, helium, and NH3; the gas flow rate of the first protective atmosphere is 1 - 10 L / min; the process of the first calcination heat treatment is carried out in a tubular vacuum furnace. The above-mentioned calcination heat treatment conditions selected in this application are more conducive to the progress of the pyrolysis process.

[0151] In some embodiments, an annealing treatment is further included after the first calcination heat treatment, and the process includes: cooling to 300 - 400°C at a cooling rate of 5 - 10°C / min and introducing oxygen; the flow rate of oxygen is 1 - 10 L / min, and after holding at a constant temperature for 1.0 - 3.0 h, it is naturally cooled to room temperature.

[0152] By setting the above step-by-step heating, holding, and pyrolysis process in this application, ammonium metavanadate (NH4VO3) can be fully pyrolyzed into pentavalent vanadium oxide (such as vanadium pentoxide and ammonia), thereby improving the extraction efficiency.

[0153] In some embodiments, between step S2 and step S3, there is also step S23-2: divide the vanadium-ammonium-oxide-containing deposit obtained in step S2 into two parts, subject one part of the vanadium-ammonium-oxide-containing deposit to the first calcination heat treatment in step S2 to obtain vanadium oxide; use the remaining part of the vanadium-ammonium-oxide-containing deposit to prepare a vanadium-based hydrogen evolution electrocatalyst;

[0154] Among them, the preparation process of the vanadium-based hydrogen evolution electrocatalyst includes: mixing the remaining part of the vanadium-ammonium-oxide-containing deposit (as a carrier precursor), a second ammonium salt, a noble metal compound, a chelating agent, a third reducing agent, and a fourth solvent to carry out reaction VI to obtain a reaction VI product; subjecting the reaction VI product and a fourth reducing agent to a second calcination heat treatment in a second protective atmosphere to obtain a noble metal-loaded vanadium-based hydrogen evolution electrocatalyst.

[0155] The hydrogen evolution electrocatalyst of the present application can be selected from the prior art or prepared by itself. In the above self-preparation process, the vanadium-ammonium-oxide deposit is fully utilized as the precursor of the catalyst carrier, and a suitable noble metal compound and other additives are selected. After the reaction, a catalyst precursor is obtained. By subjecting the precursor to reduction and a second calcination and annealing treatment, a supported catalyst can be obtained, such as noble metal supported on vanadium nitride. The noble metal has good dispersion, good durability, and good activity on the carrier; the raw materials for preparing the above hydrogen evolution electrocatalyst of the present application directly use the intermediate product in the vanadium extraction process, and its electrocatalytic reaction activity effect is better, which can improve the reaction rate of reducing tetravalent vanadium to trivalent vanadium.

[0156] The main principle of preparing the above noble metal-loaded vanadium-based electrocatalyst in the present application is as follows:

[0157] For example, when the second ammonium salt is ammonium chloride and the noble metal compound-containing is chloroplatinic acid:

[0158] H2PtCl6 + NH4Cl → (NH4)2PtCl6↓ + HCl;

[0159] In the method for preparing the catalyst in the present application, introducing a chelating agent can form a noble metal chelate with the noble metal, so that the noble metal is more evenly dispersed under the steric hindrance of the chelating agent in the fourth solvent. That is, through the noble metal cation chelate effect, the concentration of noble metal ions can be regulated. Therefore, the growth rate of nanocrystalline nuclei slows down, and the nanometer Pt grains are in-situ deposited with a smaller volume, so that the reaction in the system can proceed more stably, improving the distribution uniformity of the noble metal in the carrier precursor.

[0160] When the chelating agent is oxalic acid, a phase transformation of the platinum salt (NH4)2PtCl6 occurs, generating ammonium chloroplatinate (NH4)2PtCl4 (ammonium chloroplatinate is highly soluble in water), that is, the following reaction occurs:

[0161] (NH4)2PtCl6 + H2C2O4 → (NH4)2PtCl4 + HCl + CO2↑;

[0162] And under the action of reducing agents hydrogen peroxide and / or hydrazine hydrate, the following reactions occur:

[0163] (NH4)2PtCl4 + HCl + H2O2 → (NH4)2PtCl6↓ + H2O;

[0164] (NH4)2PtCl6 + NH2NH2 → (NH4)2PtCl4 + N2↑ + H2↑ + HCl.

[0165] In this application, under a protective atmosphere, the V and N atoms derived from NH4VO3 are in-situ converted into VN small particles, while consuming the carbon in the noble metal chelate framework and the added fourth reducing agent (elemental carbon) to promote the reduction of elemental Pt. Since the atoms in the polymer act as dispersants and protectants, it is ensured that while phase separation occurs, the particles do not agglomerate and grow. At the same time, in step S23-2, ammonium metavanadate undergoes thermal decomposition to obtain vanadium pentoxide (V2O5) and ammonia (NH3), that is, the following reactions occur: NH4VO3 → V2O5 + NH3↑ + H2O, V2O5 + NH3 → VN + N2 + H2O, V2O5 + C + N2 → VN + CO2 + CO; at the same time, VO(OH)3 dehydrates to form V2O5, and V2O5 is reduced by the fourth reducing agent to form vanadium nitride (VN), thereby forming a supported catalyst with vanadium nitride as the carrier to support noble metals (i.e., reaction VI).

[0166] The following reactions occur during this process:

[0167] (NH4)2PtCl6 → Pt + HCl + NH4Cl + N2↑;

[0168] (NH4)2PtCl6 → Pt + NH4Cl + Cl2↑.

[0169] When the pH value of the solution is between 2 and 14, the hydrogen evolution potential is lower than the potential to reduce V 4+ to V 3+ , that is, thermodynamically, V in the solution can be reduced 4+ to the lower-valent V 3+ . However, experiments prove that it is very difficult to occur in terms of reaction kinetics and the reaction rate is slow. This means that a high activation energy is required for the reduction reaction to occur in the solution system. In addition, the reduction process involving vanadium ions is rate-controlled by the interfacial reaction. Therefore, by introducing a hydrogen evolution electrocatalyst (such as a Pt / C catalyst), the reaction rate can be accelerated, and it is easier to reduce V 4+ to V 3+ at normal temperature and pressure. That is, V in the electrolytic cell 4+Ions gain electrons on the surface of the membrane electrode and undergo a reduction reaction: V 4+ +e − →V 3+ ; A hydrogen evolution electrocatalyst (such as a Pt / C catalyst) is used to accelerate the rate of this reduction reaction. The catalyst accelerates the reaction by providing a lower-energy activation energy path.

[0170] In step S3, the vanadium-based catalyst (Pt / VN) obtained in step S23-2 is used as an electrocatalyst to prepare a membrane electrode. The VOSO4 prepared in step S2 is dissolved in sulfuric acid, and the concentration of vanadium ions is adjusted to 1.0 - 2.5 mol / L, and the concentration of SO4 2- is 2.0 - 5.0 mol / L. Under the action of the electrocatalyst (Pt / VN), V 4+ can be electrocatalytically reduced to V 3+ , which can improve the reaction rate of vanadium ion reduction. At the same time, compared with the traditional electrolysis method, no other reducing agents are introduced, and it has better safety and economy.

[0171] The vanadium ion solution prepared in steps S2 and S3 is fully mixed with concentrated sulfuric acid and pure water, heated and dissolved. After the reaction is completed, the vanadium electrolyte is obtained by filtration. The involved reaction equations are:

[0172] V2O3 + 3H2SO4 = V2(SO4 )3 + 3H2O;

[0173] V2O5 + 2V 3+ + 4H2SO4 = 4VOSO4 + H2O + 6H + ;

[0174] V2O5 + H2SO4 = (VO)2SO4 + H2O;

[0175] V2O5 + V2O3 + 4H2SO4 = 4VOSO4 + 4H2O.

[0176] Different from the traditional deposition reduction method in which vanadium nitride (VN) support is first prepared and then impregnated in a platinum salt compound solution to load elemental platinum, the present application enables the in-situ deposition of noble metals on the surface of the support precursor during the reduction of noble metal-containing compounds. This not only improves the utilization rate of the active component (i.e., noble metal Pt) in the supported catalyst, but also enhances the uniformity of the distribution of noble metals on the support surface. Moreover, the noble metals in the prepared supported catalyst are not prone to migration and aggregation, and can regulate the interaction between the noble metal and the support, enhancing the force between the two, thereby effectively anchoring the noble metal and improving the durability of the supported catalyst. At the same time, the preparation method of the present application requires less noble metal consumption and has higher atomic utilization rate. In addition, compared with the traditional impregnation deposition reduction method, the preparation method of the present application can better retain its morphology while changing the composition and structure of the supported catalyst, exposing more active sites.

[0177] When the catalyst precursor is calcined and annealed, the release of NH3 gas helps to form a porous structure, achieve pore expansion and increase the specific surface area of the support, enabling the supported catalyst to expose more catalytically active sites. When applied in a vanadium redox flow battery, it can significantly accelerate the transport rate of reactants and products during the reduction reaction, enhancing the mass transfer effect; on the other hand, under the influence of the osmotic pressure of the local microjet airflow in the capillary pores at the support interface, the interaction between noble metal particles is weakened, thereby effectively inhibiting the aggregation and migration of noble metal particles.

[0178] In addition, the supported catalyst prepared by the phase separation method assisted by noble metal chelates has the following advantages: (1) Due to the good interaction between Pt and VN, it can regulate the electronic structure at the coupled interface; (2) Reducing the particle size can provide more active sites; (3) The unique porous network of the VN support can not only enhance the structural integrity and electrochemical durability, but also provide ultra-fast electron / ion transfer pore channels.

[0179] In some embodiments, the conditions for reaction VI include: the reaction temperature is 0~160 °C, such as 10~150 °C or 10~100 °C or 10~60 °C; the isothermal reaction time is 1.0~5.0 h.

[0180] In some embodiments, the mass ratio of the remaining ammonium vanadate deposit to the second ammonium salt is (1.0~20.0):(0.001~15.0); further (1.0~20.0):(0.1~10.0), still further (1.0~20.0):(0.5~1.0), and for example (10.0~20.0):1. The present application adopts this reaction ratio to improve the effect of the phase transformation of the platinum salt to form noble metal chelates.

[0181] In some embodiments, the mass ratio of the remaining ammonium metavanadate deposit to the noble metal compound is (1.0 to 20.0):(0.1 to 10.0), further (1.0 to 20.0):(1 to 10.0), and for example (10.0 to 20.0):1. Limiting it within the above range in the present application is beneficial to improving the utilization rate of noble metals and at the same time beneficial to reducing the amount of noble metals used.

[0182] In some embodiments, the mass ratio of the noble metal compound to the chelating agent is (0.1 to 10.0):(0.001 to 15.0), further (0.1 to 10.0):(0.01 to 10.0), such as (1.0 to 10.0):1, and for example (1.0 to 5.0):1; further, the concentration of the noble metal compound solution is 0.1 to 10.0 mol / L or 1.0 to 10.0 mol / L, and for example 1.0 to 3.0 mol / L; the concentration of the chelating agent solution is 0.001 to 15.0 mol / L, such as 0.1 to 3.0 mol / L. In the present application, the weight ratio of the noble metal compound-containing to the chelating agent includes but is not limited to the above range, but limiting it within the above range is beneficial to improving the dispersibility of the noble metal compound-containing, and then improving the uniformity of the distribution of the noble metal in the catalyst precursor on the surface of the support precursor, facilitating the subsequent suppression of the migration and agglomeration of the noble metal in the supported catalyst.

[0183] In some embodiments, in order to improve the reaction efficiency, the molar ratio of the noble metal compound to the third reducing agent is (0.001 to 0.1):(0.005 to 0.5); such as (0.01 to 0.1):(0.05 to 0.5), and for example (0.05 to 0.1):0.5; when in use, the concentration of the noble metal compound solution is 0.001 to 0.1 mol / L or 0.01 to 0.1 mol / L; the concentration of the third reducing agent solution is 0.005 to 0.5 mol / L, further 0.005 to 0.1 mol / L, and for example 0.01 to 0.1 mol / L.

[0184] In some embodiments, in order to improve the reduction efficiency in the second calcination heat treatment and thus improve the yield of the VN support, the weight ratio of the fourth reducing agent to the reaction VI product is (0.1 to 0.5):1.

[0185] In order to make the amount of noble metal in the subsequently prepared supported catalyst more appropriate and improve the catalytic activity of the supported catalyst, the weight ratio of the support precursor, the noble metal compound-containing to the fourth solvent can be (1 to 20):(0.1 to 10):(100 to 5000).

[0186] In some embodiments, to improve the effect of phase transformation of platinum salts and generate noble metal chelates, the second ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, polydopamine, ethylenediaminetetraacetic acid, ethylenediamine, triethylenetetramine, triethanolamine, hexamethylenetetramine, melamine, ammonium polyacrylate, and dopamine hydrochloride. The types of the first ammonium salt and the second ammonium salt in the present application may be the same or different.

[0187] In some embodiments, the noble metal compound is selected from at least one of chloroplatinic acid, chloroplatinate, [Pt(acac)2], platinum diamine dinitrite, and [Pt(CH3NH2)4][PtCl4].

[0188] In some embodiments, the chelating agent is selected from at least one of thiourea, sulfite, phosphoric acid, oxalic acid, arsonic acid, polyacrylic acid, and polyvinyl alcohol.

[0189] In some embodiments, the third reducing agent is selected from at least one of elemental sulfur, ascorbic acid, sodium ascorbate, sodium citrate, formaldehyde, acetaldehyde, formic acid, acetic acid, hydrazine hydrate, metal borohydride salts, sulfites, and oleylamine; compared with other types, using the reducing agent of the above types is beneficial to improving the reaction efficiency.

[0190] In some embodiments, the fourth reducing agent is elemental carbon; elemental carbon can reduce V2O5 to VN, thereby obtaining a VN support. To better exert the reducing effect of the fourth reducing agent and thus improve the yield of the VN support, the average particle size of the fourth reducing agent is 50 - 1000 nm.

[0191] In some embodiments, to improve the dispersion degree of each raw material in the fourth solvent, the fourth solvent includes, but is not limited to, water and / or ethylene glycol. When the fourth solvent includes the above types of organic solvents, the functional groups of the organic solvents replace the non-bridging hydroxyl groups on the surface of the support precursor and play a certain steric hindrance effect to inhibit the agglomeration of the support precursor, and at the same time are beneficial to controlling the noble metal nucleation rate and noble metal particle size; in addition, the organic solvent has a lower surface tension than water, which is beneficial to subsequent calcination and annealing treatment.

[0192] In some embodiments, in step S23-2, in order to more specifically dissolve each raw material in a suitable solvent to improve the solubility of the raw materials, different raw materials can be dissolved in different solvents; for example, step S23-2-1: Dissolve the carrier precursor (deposit containing ammonium vanadate,), the second ammonium salt, the noble metal compound-containing compound, and the chelating agent in solvent A to form a first mixed solution; step S23-2-2: Mix the third reducing agent, the dispersant, and solvent B to form a second mixed solution; step S23-2-3: Then mix the first mixed solution with the second mixed solution and carry out a reaction to obtain a reaction VI product system. Among them, solvent A is water and / or ethylene glycol, and solvent B includes at least one of water, methanol, ethanol, propanol, ethylene glycol, isopropanol, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide.

[0193] In the above steps of the present application, a mixed solution of solvent A, water and / or ethylene glycol, is used in the first mixed solution, which is beneficial to improving the wettability of the carrier precursor, and further improving the loading effect of noble metal particles. In addition, using the above process to prepare the reaction VI product system is beneficial to improving the dispersibility of each reaction raw material in the solvent, beneficial to improving the utilization rate of each reaction raw material, and at the same time beneficial to the reaction proceeding more smoothly, thereby improving the yield of the catalyst precursor and enhancing the uniformity of the distribution of the noble metal in the catalyst precursor; when solvent B includes the above types of organic solvents, the functional groups of the organic solvents replace the non-bridging hydroxyl groups on the surface of the carrier precursor and play a certain steric hindrance effect to inhibit the agglomeration of the carrier precursor, and are beneficial to controlling the nucleation rate of the noble metal and the noble metal particle size; moreover, the organic solvent has a lower surface tension than water, which is convenient for subsequent annealing treatment.

[0194] In some embodiments, in step S23-2-1, the mixing temperature is 0~160°C and the time is 1.0~5.0 h. The mixing temperature and time include but are not limited to the above range. Limiting them within the above range is beneficial to improving the dispersibility of the carrier precursor, the noble metal compound-containing compound, and the chelating agent in the fourth solvent, and facilitating subsequent reactions.

[0195] In some embodiments, in step S23-2-2, the molar concentration of the third reducing agent in the second mixed solution is 0.005~0.5 mol / L, such as 0.01~0.5 mol / L, and again such as 0.01~0.1 mol / L. The molar concentration of the third reducing agent includes but is not limited to the above range. Limiting it within the above range is beneficial to improving the reaction efficiency, thereby being beneficial to improving the yield of zero-valent noble metal and the catalytic activity of the supported catalyst.

[0196] In some embodiments, in step S23-2-3, the first mixture is dropped into the second mixture at a rate of 1 to 10 mL / min. Mixing the first mixture and the second mixture in the above manner is beneficial to improving the reaction efficiency and thus beneficial to increasing the yield of zero-valent noble metals.

[0197] In order to further improve the reaction efficiency and further increase the yield of zero-valent noble metals, the mixing in step S23-2-3 is carried out under stirring conditions, and the stirring speed is 300 to 500 rpm; the reaction time is 10 to 60 min.

[0198] In order to further improve the dispersibility and compatibility of each raw material during the reaction, in the second mixture, the mass concentration of the dispersant is 1.0 to 5.0 g / L; the dispersant includes, but is not limited to, one or more of the group consisting of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, methylcellulose, polyacrylic acid, sulfosuccinate, sodium dodecyl sulfate, sodium citrate, and quaternary ammonium salts.

[0199] In some embodiments, the above preparation method provided by the present application further includes: sequentially performing solid-liquid separation and freeze-drying treatment on the reaction VI product system to obtain a catalyst precursor. Performing solid-liquid separation on the reaction VI product system can separate the catalyst precursor from the solvent, thereby obtaining a solid-phase product; performing freeze-drying treatment on the solid-phase product can remove the residual solvent in the solid-phase product, and compared with other drying treatments, freeze-drying treatment helps to inhibit the particle aggregation phenomenon caused by the capillary adsorption effect.

[0200] In order to improve the separation efficiency of the catalyst precursor from water and the solvent, the solid-liquid separation includes, but is not limited to, centrifugal separation treatment. In order to further improve the removal efficiency of the residual water and the fourth solvent in the solid-phase product and further inhibit particle aggregation, the temperature of the freeze-drying treatment is -10 to -60 °C, and the time is 6.0 to 24.0 h.

[0201] In some embodiments, the second protective atmosphere is selected from at least one of argon, nitrogen, helium, and NH3.

[0202] In some embodiments, the process of the second calcination heat treatment sequentially includes a first-stage heat treatment, a second-stage heat treatment, a third-stage heat treatment, and a fourth-stage heat treatment. Compared with heat treatment under a single temperature condition, using staged heating treatment is beneficial to improving the annealing treatment effect, beneficial to increasing the yield of VN, and beneficial to providing more active sites.

[0203] In some embodiments, the first-stage heat treatment includes: heating from 25°C to 300 - 350°C at a heating rate of 5 - 30°C / min and holding for 1.0 - 1.5 h; the second-stage heat treatment includes: continuing to heat at a heating rate of 5 - 10°C / min to 600 - 650°C and holding for 1.0 - 3.0 h; the third-stage heat treatment includes: continuing to heat at a heating rate of 5 - 10°C / min to 800 - 900°C and holding for 1.0 - 3.0 h; the fourth-stage heat treatment includes: continuing to heat at a heating rate of 5 - 10°C / min to 1100 - 1200°C and holding for 1.0 - 3.0 h. After natural cooling to room temperature, a supported noble metal vanadium-based hydrogen evolution electrocatalyst is obtained.

[0204] During the first-stage heat treatment of this application, free water evaporates upon heating. Meanwhile, the carrier precursor begins to thermally decompose; during the second-stage heat treatment, a pre-reaction of V2O5 occurs. During this process, the ammonium salt decomposes sufficiently, and the released NH3 can act as a reducing agent at high temperatures. Moreover, under the erosion and dilution of the gas, it is beneficial to increase the specific surface area and porosity of the carrier; during the third-stage heat treatment, the thermally decomposed platinum salt decomposes into elemental Pt; during the fourth-stage heat treatment, the electronic structure between Pt and VN is adjusted, and the elemental Pt changes from a disordered state to an ordered structure, thereby enhancing the electrocatalytic activity and stability of the supported catalyst.

[0205] In some embodiments, the second calcination heat treatment is carried out in a tubular vacuum furnace, and the volumetric flow rate of the protective gas is 1 - 10 L / min. The introduction rate of the protective gas includes but is not limited to the above range. Limiting it within the above range is beneficial to improving the effect of the calcination annealing treatment, thereby being beneficial to enhancing the catalytic activity and stability of the supported catalyst.

[0206] In some embodiments, the above preparation method provided by this application further includes: after the second calcination heat treatment, grinding the thermally reduced product to obtain a supported catalyst. Compared with not grinding, the supported catalyst obtained after grinding has a more uniform particle size and is beneficial for application in a vanadium redox flow battery.

[0207] In some embodiments, in order to improve the catalytic activity of the supported catalyst, the loading amount of the noble metal in the supported catalyst is 20.0 - 40.0 wt%.

[0208] The supported catalyst provided by this application has a relatively large specific surface area and can load more noble metals. For example, the pore volume of the supported catalyst is 0.01 - 10.0 cm 3 / g, the average pore diameter is 5 - 50 nm, and the specific surface area is 450 - 600 m 2 / g.

[0209] In some embodiments, a vanadium-based catalyst loaded with noble metal, a Nafion solution, and an organic solvent II are mixed to obtain a slurry; the slurry is attached to the surface of a membrane electrode to obtain a catalyst membrane electrode; wherein, the organic solvent II is ethanol and / or acetone.

[0210] In some embodiments, the mixing ratio of the vanadium-based catalyst loaded with noble metal, the Nafion solution, and the organic solvent II is (2.5 - 15.0) mg : (100.0 - 150.0) μL : (1.0 - 10.0) mL; wherein, the mass concentration of the Nafion solution is 0.5 - 10.0 wt%.

[0211] In some embodiments, the electrocatalytic reduction process of the hydrogen evolution electrocatalyst in step S3 is that the catalyst membrane electrode reacts under the condition of direct current to obtain a solution containing trivalent vanadium.

[0212] Specifically, 12.0 mg of the catalyst prepared in S23-2 is weighed and mixed with 5.0 mL of ethanol and 120.0 μL of Nafion solution (5.0 wt%), and ultrasonic dispersion is carried out for 15 - 45 min. The catalyst coated membrane method is used to prepare a membrane electrode with an effective area of 2×2 cm 2 , and the electrolysis conditions are: electrolysis voltage 0.5 - 5.0 V, electrolysis temperature 20 - 60 °C, electrolysis time 0.5 - 3.0 h, and the distance between the electrode plates is 2.0 - 10.0 cm; vanadium ion modulation, the vanadium ion-containing acid solution (the first solution containing tetravalent vanadium) in step S2 and the electrolyzed vanadium ion solution (the solution containing trivalent vanadium) in step S3 are fully mixed, and the molar concentration ratio of vanadium ions is 1:1 to obtain the electrolyte for a vanadium redox flow battery.

[0213] According to the second aspect of the present application, a vanadium redox electrolyte is provided, which is prepared by using the preparation method of the above vanadium redox electrolyte.

[0214] According to the third aspect of the present application, a vanadium redox flow battery is provided, including a vanadium redox electrolyte, which is the above vanadium redox electrolyte.

[0215] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0216] Example 1

[0217] Step S1: Weigh 10.0 g of vanadium slag. After crushing and grinding, add it to a NaHCO3 solution with a concentration of 1.0 mol / L (liquid-solid mass ratio is 4:1), and stir well to make it disperse evenly. Subsequently, use a 1.5 mol / L NaOH aqueous solution to adjust the pH value of the reaction system to 11.5 ± 0.5. Place this system in a magnetic stirring environment at 90 °C and 500 rpm for a 3.0 h constant-temperature reaction (Reaction I). After Reaction I ends, cool it naturally to room temperature, and then perform filtration and washing in sequence, and collect the filtrate and vanadium-enriched product respectively. The XRD of the vanadium-enriched product is as Figure 3 shown.

[0218] Step S2: At the same time, dissolve 0.0175 mol of ammonium carbonate in a mixed solvent composed of 50 mL of water and 10 mL of carbonic acid with a concentration of 5.0 mol / L, and stir for 60 min under the condition of 300 rpm to obtain a first ammonium salt solution with a concentration of 0.292 mol / L. Prepare a 40 mL vanadium-enriched product suspension according to the ratio of 5.0 g of vanadium-enriched product, 20 mL of water, and 20 mL of ethanol. Drop the above 60 mL ammonium salt solution into the 40 mL vanadium-enriched product suspension at a rate of 10 mL / min, and carry out Reaction II at 25 °C for a 60 min constant-temperature reaction. After the reaction ends, centrifuge the product in sequence, wash it three times with absolute ethanol, and dry it at 60 °C for 12.0 h to obtain ammonium metavanadate.

[0219] Grind the ammonium metavanadate sample and place it in a tube furnace, heat it at a rate of 15 °C / min, first heat it to 150 °C and keep it warm for 1.0 h; then heat it to 350 °C at a rate of 10 °C / min and keep it warm for 1.0 h; then heat it to 500 °C at a rate of 5 °C / min and keep it warm for 1.0 h. When the furnace temperature drops to 350 °C, introduce oxygen (gas flow rate 1 L / min), keep it at a constant temperature for 3.0 h, and then let the furnace cool down to room temperature to obtain vanadium oxide. Mix the obtained vanadium oxide with the first reducing agent oxalic acid according to a mass ratio of 1.75:1, add it to an acid reaction system with a sulfuric acid concentration of 0.1 mol / L, and carry out Reaction III at a reaction temperature of 90 °C for a 3.0 h constant-temperature reaction. After the reaction system cools down to room temperature, filter it to obtain a first tetravalent vanadium solution (VOSO4), which is used as the electrolyte to be reduced.

[0220] Step S3: Add H2SO4 solution with a molar concentration of 4.5 mol / L to the positive electrode reaction cell of the electrolytic cell, and prepare a mixed solution containing 1.5 mol / L VOSO4 (the tetravalent vanadium ion solution prepared in Step S2) and 3.0 mol / L H2SO4 in the negative electrode electrolytic cell; The hydrogen evolution electrocatalyst uses a commercial Pt / C catalyst (Macklin P822267, platinum-carbon catalyst), and the Pt loading in this catalyst is 20.0 wt%; The Pt / C catalyst is used to prepare membrane electrodes on both poles of the electrolytic cell by the CCM method; The preparation process is as follows: Weigh 12.0 mg of Pt / C catalyst respectively, mix it with 5 ml of ethanol and 120 μL of Nafion solution (5.0 wt%, a solution of perfluorosulfonic acid resin polymer dissolved in a solvent) to make a slurry, and ultrasonically disperse it for 30 min. Then spray the slurry on both sides of the Nafion 115 membrane, and the effective area is 2×2 cm 2 ; Divide the electrolytic cell, adopt a constant voltage electrolysis method, adjust the plate spacing to 3.0 cm, control the cell voltage to 1.8 V, set the electrolysis temperature to 30 °C, and react for 0.5 h to completely discharge the vanadium ion solution in the negative electrode electrolytic cell. During this process, the vanadium ion solution obtains electrons on the surface of the negative electrode membrane electrode and undergoes an electrocatalytic reduction reaction, that is, V in the solution 4+ undergoes reduction transformation to V 3+ (V 4+ + e - → V 3+ ). During the reaction, the valence state of vanadium ions is analyzed and measured by redox titration method. When V in the solution 4+ is completely converted to V 3+ , mix it with the V 4+ ion solution prepared in Step S2 according to the molar ratio of n (V3+) : n (V4+) of 1:1 and stir well; After full mixing, filter to obtain a 3.5-valent vanadium electrolyte for all-vanadium redox flow batteries; As Figure 1 and Figure 2 shown.

[0221] Example 2

[0222] The difference between Example 2 and Example 1 is that a certain amount of 5.0 g of the vanadium enrichment obtained in Step S1 (which contains calcium carbonate) is calcined at a temperature of 1000 °C for 3.0 h to obtain a calcined slag containing calcium oxide (its composition contains calcium oxide) to utilize calcium oxide to enrich vanadium ions in the vanadium slag lye.

[0223] The specific steps are as follows: Weigh 5.0 g of the vanadium-enriched product obtained in step S1, add 62.5 mL of 1.5 mol / L NaHCO3 alkaline leaching solution, adjust the vanadium slag concentration to 80.0 g / L (calculated as V2O5, and the vanadium-enriched product is added to promote the reaction between calcium oxide and vanadium ions), then add 1.5 g of calcined slag (containing CaO), adjust the pH value of the solution system to 11.5 ± 0.5 with 1.5 mol / L NaOH solution, and perform the vanadium enrichment operation; react at 95 °C for 180 min. After the reaction is completed, obtain the vanadium-enriched product, filter and wash this product to obtain a solid powder containing calcium vanadate, then add 200 mL of deionized water, and at the same time add a mixed acid of 100 mL of sulfuric acid with a mass fraction of 98% and hydrochloric acid with a mass fraction of 20%, adjust the pH value of the system to 1.5, perform ultrasonic dispersion for 15 min. After uniform dispersion, add the second reducing agent sodium oxalate according to the mass ratio m(calcium vanadate):m(second reducing agent) = 1:1, and stir at 95 °C for 3.0 h to carry out reaction V. After the reaction is completed, perform a filtration operation to obtain a white precipitate (mainly calcium carbonate) and a blue solution of vanadyl sulfate (VOSO4) in the tetravalent state, that is, the second tetravalent vanadium-containing solution; mix the first tetravalent vanadium-containing solution prepared in step S2 with the above-mentioned second tetravalent vanadium-containing solution to use this as a mixed solution of tetravalent vanadium and as the electrolyte to be reduced (VOSO4), and then continue with step S3; as Figure 1 and Figure 2 shown.

[0224] Example 3

[0225] The difference between Example 3 and Example 1 is that in step S3, a self-made hydrogen evolution catalyst is used. The specific steps are as follows: Divide the ammonium metavanadate prepared in step S2 into two parts. One part is used to pyrolyze into vanadium oxide and then reduced to tetravalent vanadium, and this process is the same as that in Example 1; the other part is used to prepare the hydrogen evolution catalyst.

[0226] The preparation process of the hydrogen evolution catalyst is as follows: Take an appropriate amount of ammonium metavanadate in step S2 as the carrier precursor, that is, mix 5.75 g of this carrier precursor, 0.3 g of ammonium chloride, 0.42 g of chloroplatinic acid, 0.02 g of oxalic acid, 150 mL of water and 50 mL of ethylene glycol at 10 °C for 60 min to form a first mixed solution; Mix 1.76 g of the third reducing agent ascorbic acid with 0.2 g of polyacrylic acid and 100 mL of water to prepare a 100 mL, 0.1 mol / L ascorbic acid solution (i.e., the second mixed solution); Under stirring at 300 rpm, add 200 mL of the first mixed solution to 100 mL of the second mixed solution at a dropping rate of 10 mL / min to carry out the phase transformation metathesis reaction VI of the Pt salt. After reacting at 30 °C for 60 min, a VI product system containing the catalyst precursor is obtained. It is successively centrifuged, washed three times with absolute ethanol, and freeze-dried at -50 °C for 12.0 h to obtain the catalyst precursor.

[0227] Transfer the above-prepared catalyst precursor and carbon powder with an average particle size of 100 nm to a tubular vacuum furnace for calcination and annealing treatment; Introduce nitrogen, and its flow rate is controlled at 1 L / min. First, heat from 25 °C to 350 °C at a heating rate of 15 °C / min and hold for 1.0 h. Then continue to heat to 650 °C at a rate of 10 °C / min and hold for 1.0 h. Subsequently, continue to heat to 900 °C at a rate of 5 °C / min and hold for 1.0 h. Then increase the temperature to 1200 °C at a heating rate of 5 °C / min and hold for 3.0 h. After naturally cooling to room temperature, a supported catalyst is obtained. This catalyst uses VN as the carrier and noble metal Pt as the active component. The loading amount of the noble metal is 20.0 wt%. The characterization of the prepared catalyst is as Figure 4 shown.

[0228] Use the CCM method to apply the above-obtained Pt / VN catalyst to prepare a membrane electrode on both electrodes of an electrolytic cell; The preparation steps of the membrane electrode are as follows: Weigh 12.0 mg of the catalyst, 5.0 ml of ethanol and 120.0 μL of a 5 wt% Nafion solution (a solution of perfluorosulfonic acid resin polymer dissolved in a solvent) respectively, mix them to form a slurry, and then ultrasonically disperse it for 30 min. Spray the slurry on both the front and back sides of the Nafion 115 membrane, divide the electrolytic cell according to the specification with an effective area of 2×2 cm 2 , adopt a constant voltage electrolysis method, control the cell voltage at 1.8 V, completely discharge the vanadium ion solution in the negative electrode electrolytic cell, and an electrocatalytic reduction reaction occurs on the surface of the negative electrode membrane electrode, that is, V in the solution 4+ is reduced to V 3+ . During the reaction, the valence state of vanadium ions is analyzed and measured by redox titration. When it is completely converted to V in the solution 3+ , mix it with the V 4+ ion solution prepared in step S2 according to n(V3+) : n (V4+) Mix in a 1:1 ratio, stir thoroughly, filter after thorough mixing to obtain a 3.5-valent vanadium electrolyte for a vanadium redox flow battery; as Figure 1 and Figure 2 shown.

[0229] Example 4

[0230] The difference between Example 4 and Example 1 is that the alkaline leaching agent is an aqueous solution of KHCO3, the temperature of the alkali leaching reaction I is 180 °C, and the reaction time of reaction I is 3.0 h.

[0231] Example 5

[0232] The difference between Example 5 and Example 1 is that the first ammonium salt is ammonium sulfate, the temperature of the vanadium precipitation reaction II is 90 °C, and the reaction time is 2.0 h.

[0233] Example 6

[0234] The difference between Example 6 and Example 1 is that in the reduction reaction III, the first reducing agent is potassium oxalate, its solution concentration is 5.0 mol / L, the temperature of reaction III is 120 °C, and the reaction time is 3.0 h.

[0235] Example 7

[0236] The difference between Example 7 and Example 3 is that in step S2, the first ammonium salt is ammonium bisulfate, and its solution concentration is 5.0 mol / L.

[0237] Example 8

[0238] The difference between Example 8 and Example 3 is that the third reducing agent is acetic acid; the molar ratio of chloroplatinic acid to the third reducing agent acetic acid is 0.001:0.005.

[0239] Example 9

[0240] The difference between Example 9 and Example 3 is that 0.85 g of sodium chloroplatinate is used to replace chloroplatinic acid to obtain the first mixed solution.

[0241] Example 10

[0242] The difference between Example 10 and Example 3 is that 0.4 g of [Pt(acac)2] is used to replace chloroplatinic acid to obtain the first mixed solution.

[0243] Example 11

[0244] The difference between Example 11 and Example 3 is that 0.65 g of platinum diammine dinitrite is used to replace chloroplatinic acid to obtain the first mixed solution.

[0245] Comparative Example 1

[0246] Comparative Example 1 provides a method for preparing an electrolyte for a vanadium redox flow battery, including:

[0247] (1) Mix vanadium pentoxide and concentrated sulfuric acid at a molar ratio of 1:2.25. The concentration of the concentrated sulfuric acid is 18.4 mol / L. Stir magnetically at 500 rpm, with a temperature of 25 °C and a stirring time of 0.5 h to obtain a mixture;

[0248] (2) Activate the mixture obtained in step (1) at an activation temperature of 150 °C and an activation time of 2.0 h to obtain an activator (VO2)2SO4, and cool it to room temperature;

[0249] (3) Stir and dissolve the activator (VO2)2SO4 obtained in step (2) at a dissolution temperature of 40 °C. The solvent is dilute sulfuric acid with a concentration of 3.0 mol / L. The liquid-solid ratio of the dilute sulfuric acid to the activator (VO2)2SO4 is 6 mL / g, and the dissolution stirring time is 1.0 h to obtain a solution;

[0250] (4) Add a Pd / C catalyst to the solution obtained in step (3), and introduce hydrogen at a flow rate of 450 mL / min to carry out a reduction reaction to obtain an electrolyte for a vanadium redox flow battery. The hydrogen partial pressure of the reduction reaction is 0.1 MPa, the temperature of the reduction reaction is 120 °C, and the reduction reaction time is 6.0 h.

[0251] For the vanadium electrolytes prepared in Examples 1 to 11 and Comparative Example 1, the valence states of vanadium ions in the electrolyte were tested and analyzed according to GB / T 37204 2018. At the same time, vanadium redox flow batteries were assembled to test the constant current charge-discharge performance of the vanadium electrolyte. Constant current charge-discharge test conditions: use current densities of 100, 80, 60, and 40 mA / cm 2 Current density; Cycle stability test conditions: use a current density of 100 mA / cm 2 Current density; The charge-discharge cut-off voltages are 1.65 V and 0.8 V respectively. The coulombic efficiency (%), voltage efficiency (%), and energy efficiency (%) of the test battery were measured, and the results are shown in Table 1.

[0252] Table 1

[0253]

[0254] The detection data in Table 1 show that: The charge-discharge performance of the vanadium electrolytes prepared in Examples 1 to 11 of this application is all good.

[0255] In Example 1 of this application, the initial raw material for preparing vanadium electrolyte is solid waste vanadium slag. First, it is subjected to alkali leaching treatment and ammonium salt vanadium precipitation treatment to obtain pure tetravalent vanadium; then, electrocatalytic method is adopted to reduce tetravalent vanadium to trivalent vanadium; finally, the ratio of trivalent vanadium to tetravalent vanadium is adjusted and mixed to finally prepare a 3.5-valent vanadium electrolyte.

[0256] In Example 2 of this application, the filter residue generated in the preparation of vanadium electrolyte is further utilized: First, active calcium oxide is obtained through roasting; then, calcium metavanadate with pentavalent vanadium is obtained through vanadium enrichment treatment; then, reduction is carried out in an acid / mixed acid system using a reducing agent to obtain a tetravalent vanadium solution; finally, trivalent vanadium is prepared through electrocatalytic reduction. In this method, not only the waste calcium carbonate roasting slag is fully utilized, but also the purity of the vanadium enrichment is improved, and a new way to prepare tetravalent vanadium is realized, further improving the vanadium extraction efficiency.

[0257] In Example 3 of this application, the intermediate product ammonium metavanadate deposit generated during the preparation of vanadium electrolyte is used to prepare a new type of electrocatalyst. Specifically, by selecting a suitable noble metal (such as Pt) as the active metal, and at the same time selecting suitable reducing agents, solvents, ammonium salts, etc., the noble metal Pt is in-situ deposited on the surface of the carrier, and after in-situ reduction by heat treatment, a new type of vanadium-based catalyst loaded with platinum metal is obtained; then, this catalyst is applied to the membrane electrode of the flow battery to catalyze the electrolytic reduction reaction to improve the efficiency of reducing tetravalent vanadium electrolyte to trivalent vanadium electrolyte.

[0258] In Comparative Example 1, vanadium pentoxide is used as the raw material, and after reduction and high-temperature activation treatment, pentavalent vanadium is reduced to a valence state between 3 and 5 under the action of an electrocatalyst. The electrochemical performance of the electrolyte in Comparative Example 1 is lower than that of the electrolyte prepared in the examples of this application, and the process in Comparative Example 1 can only obtain an electrolyte of one valence state and cannot achieve the goal of preparing a full vanadium electrolyte with more than three valence states using a single process. In addition, the raw material of Comparative Example 1 is vanadium oxide, while this application uses solid waste vanadium slag as the raw material, realizing the full utilization of waste resources and greatly reducing the cost.

[0259] The method for preparing all-vanadium electrolytes provided by the embodiments of the present application can make full use of solid waste vanadium slag. From the initial raw materials to the intermediate products, this method is used to prepare high-value-added materials, and such materials can also be applied to the preparation process of vanadium electrolytes. This method can not only obtain vanadium electrolytes with different valence states, but also realize the clean and high-value comprehensive utilization of vanadium slag resources. At the same time, it realizes the closed-loop cycle of green and low-carbon metallurgy of vanadium metal, greatly reducing the production cost and the production risks brought by the traditional hydrogen reduction process. The prepared vanadium-based hydrogen evolution electrocatalyst has high reduction activity, effectively improving the reduction rate of vanadium ions; compared with the traditional electrolyte preparation method, this method has a faster reaction rate, lower risks, and does not introduce other reducing agents, with better safety and economy.

[0260] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects and do not necessarily need to describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented in an order other than those described herein.

[0261] The above are only the preferred embodiments of the present application and are not used to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing an all-vanadium electrolyte, characterized in that: The preparation method comprises the following steps: Step S1: using an alkaline leaching agent to perform an alkali leaching treatment on the vanadium slag to perform reaction I, to obtain an alkali leaching product, and the alkali leaching product is subjected to solid-liquid separation to obtain a vanadium enriched product and a filtrate; Step S2: mixing the vanadium-enriched material, the first ammonium salt and the first solvent to carry out reaction II to obtain a vanadium precipitation product, and the vanadium precipitation product is subjected to solid-liquid separation to obtain a sediment containing ammonium vanadate; dividing the sediment containing ammonium vanadate into two parts, and performing a first calcination heat treatment on one part of the sediment containing ammonium vanadate to obtain vanadium oxide; mixing the vanadium oxide and the first reducing agent in a second solvent to carry out reaction III to obtain a first tetravalent vanadium-containing solution; and using the remaining part of the sediment containing ammonium vanadate to prepare a hydrogen evolution electrocatalyst; The preparation process of the hydrogen evolution electrocatalyst comprises: mixing the remaining part of the ammonium vanadate-containing sediment, the second ammonium salt, the noble metal compound, the chelating agent, the third reducing agent and the fourth solvent to carry out reaction VI to obtain a reaction VI product; performing a second calcination heat treatment on the reaction VI product and the fourth reducing agent in a second protective atmosphere to obtain a vanadium-based hydrogen evolution electrocatalyst loaded with noble metals; the noble metal compound is selected from at least one of chloroplatinic acid, chloroplatinate, [Pt(acac)2], diammine dinitrite platinum and [Pt(CH3NH2)4][PtCl4]; Step S3: subjecting the first tetravalent vanadium-containing solution to reaction IV under the electrocatalytic reduction conditions of the hydrogen evolution electrocatalyst to obtain a trivalent vanadium-containing solution.

2. The method for preparing the all-vanadium electrolyte according to claim 1, characterized in that: The first tetravalent vanadium-containing solution obtained in step S2 is divided into two parts; a part of the first tetravalent vanadium-containing solution is subjected to reaction IV under the conditions of electrocatalytic reduction of the hydrogen evolution electrocatalyst to obtain the trivalent vanadium-containing solution; The remaining portion of the first tetravalent vanadium-containing solution is subjected to the process of step S4 after step S3; The process of step S4 includes: mixing the remaining portion of the first tetravalent vanadium-containing solution and the trivalent vanadium-containing solution and dissolving them in a sulfuric acid solution to obtain a trivalent-to-quadrivalent vanadium electrolyte.

3. The method for preparing the all-vanadium electrolyte according to claim 2, characterized in that: The molar ratio of sulfate ions in the sulfuric acid solution to tetravalent vanadium ions in the remaining portion of the first tetravalent vanadium-containing solution is (1.5-6.0):(1.0-2.5), and the molar ratio of sulfate ions in the sulfuric acid solution to trivalent vanadium ions in the trivalent vanadium-containing solution is (1.5-6.0):(1.0-2.5); the concentration of the sulfuric acid solution is 2.0-5.0 mol / L; And / or, the molar concentration of tetravalent vanadium ions in the first tetravalent vanadium-containing solution of the remaining part and the molar concentration of trivalent vanadium ions in the trivalent vanadium-containing solution are adjusted to be 1:1 to obtain a 3.5-valent vanadium electrolyte.

4. The method for preparing the all-vanadium electrolyte according to claim 1, characterized in that: The vanadium-enriched material obtained in step S1 is divided into two parts; wherein a part of the vanadium-enriched material is mixed with the first ammonium salt and the first solvent to carry out the reaction II to obtain the vanadium precipitation product; and the remaining part of the vanadium-enriched material is subjected to roasting treatment; The roasting process comprises: roasting the remaining part of the vanadium enriched material to obtain a roasted slag containing calcium oxide; mixing the roasted slag containing calcium oxide, the alkaline leaching agent and the vanadium slag in a mass ratio of (1.0-30): (2.5-80): (1.5-20) to perform vanadium enrichment treatment to obtain a vanadium enriched product, and separating the vanadium enriched product to obtain a calcium vanadate enriched material; The calcium vanadate enriched material and the vanadium enriched material are mixed, and then mixed with the first ammonium salt and the first solvent to carry out the reaction II to obtain the vanadium precipitation product; Alternatively, the calcium vanadate enriched material and the second reducing agent are mixed in a third solvent to carry out reaction V to obtain a second tetravalent vanadium-containing solution; and between step S2 and step S3, step S23-1 is also included: the first tetravalent vanadium-containing solution and the second tetravalent vanadium-containing solution are mixed to obtain a mixed solution containing tetravalent vanadium; the tetravalent vanadium-containing mixed solution is subjected to reaction IV under the conditions of electrocatalytic reduction of the hydrogen evolution electrocatalyst to obtain the solution containing trivalent vanadium.

5. The method for preparing the all-vanadium electrolyte according to claim 4, characterized in that: The conditions of the reaction V include: a reaction temperature of 60 to 120° C. and a constant temperature reaction time of 3.0 to 5.0 h; And / or, the mass ratio of the calcium vanadate enrichment to the second reducing agent is (1.0-3.5): (0.5-3.0); and / or, the second reducing agent is selected from at least one of oxalic acid, sodium oxalate, potassium oxalate, citric acid, tartaric acid and NH4HSO3; And / or, the third solvent is selected from at least one of H2SO4 solution, HNO3 solution, HCl solution, H2CO3 solution and H3PO4 solution; the concentration of the third solvent is 0.1-5.0 mol / L; And / or, the pH value of the system of reaction V is 1.0-7.0; And / or, the tetravalent vanadium-containing mixed solution obtained in step S23-1 is divided into two parts; a part of the tetravalent vanadium-containing mixed solution is subjected to reaction IV under the conditions of electrocatalytic reduction of the hydrogen evolution electrocatalyst to obtain the trivalent vanadium-containing solution; the remaining part of the tetravalent vanadium-containing mixed solution is subjected to the process of step S4 after step S3; wherein the process of step S4 comprises: mixing the remaining part of the tetravalent vanadium-containing mixed solution and the trivalent vanadium-containing solution and dissolving them in a sulfuric acid solution to obtain a 3-4-valent vanadium electrolyte; The molar ratio of sulfate ions in the sulfuric acid solution to tetravalent vanadium ions in the remaining part of the mixed solution containing tetravalent vanadium is (1.5~6.0):(1.0~2.5), and the molar ratio of sulfate ions in the sulfuric acid solution to trivalent vanadium ions in the trivalent vanadium-containing solution is (1.5~6.0):(1.0~2.5); the concentration of the sulfuric acid solution is 2.0~5.0 mol / L.

6. The method for preparing the all-vanadium electrolyte according to any one of claims 1 to 3, characterized in that: The conditions of the reaction I in step S1 include: a reaction temperature of 90 to 180° C. and a constant temperature reaction time of 3.0 to 5.0 h; And / or, the mass ratio of the alkaline leaching agent to the vanadium slag is (1.5-200): (2.5-50); and / or, the pH of the mixed system of the alkaline leaching agent and the vanadium slag is 10.5-12.0; And / or, the concentration of the alkaline leaching agent is 0.01-10.0 mol / L; and / or, the alkaline leaching agent is selected from at least one of a NaOH aqueous solution, a Na2CO3 aqueous solution, a NaHCO3 aqueous solution, a KOH aqueous solution, a K2CO3 aqueous solution and a KHCO3 aqueous solution; And / or, the vanadium enriched material, water and organic solvent I in step S2 are first mixed to form a suspension of the vanadium enriched material, and the suspension of the vanadium enriched material is then mixed with the first ammonium salt and the first solvent to carry out the reaction II to obtain the vanadium precipitation product; wherein the volume ratio of the water to the organic solvent I is (50-1000): (5-500); the organic solvent I is selected from at least one of methanol, ethanol, propanol, ethylene glycol, isopropanol, N,N-dimethylformamide, N,N-dimethylacetamide and diethylformamide; And / or, the conditions of the reaction II in the step S2 include: the reaction temperature is 10-90° C., and the constant temperature reaction time is 1.0-5.0 h; And / or, the mass ratio of the vanadium enriched material to the first ammonium salt is (2.0-30.0): (0.5-50.0); and / or, the first ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, ammonium polyacrylate and dopamine hydrochloride; and / or, the first ammonium salt and the first solvent are mixed to form a first ammonium salt solution; the concentration of the first ammonium salt solution is 0.01-5.0 mol / L; And / or, the first solvent is a mixture of water and an inorganic acid; the volume ratio of the water to the inorganic acid is (50-1000): (5-500); wherein the inorganic acid is selected from at least one of hydrochloric acid, nitric acid, boric acid, sulfuric acid, carbonic acid and phosphoric acid; the concentration of the inorganic acid is 0.1-5.0 mol / L; And / or, the electrocatalytic reduction conditions in step S3 include: electrolysis voltage of 0.5-5.0 V, electrolysis temperature of 20-60° C., electrolysis time of 0.5-3.0 h, and a distance between electrode plates of 2.0-10.0 cm.

7. The method for preparing the all-vanadium electrolyte according to any one of claims 1 to 5, characterized in that: The conditions of the reaction III in step S2 include: a reaction temperature of 60-120° C. and a constant temperature reaction time of 3.0-5.0 h; And / or, the mass ratio of the vanadium oxide to the first reducing agent is (1.0-3.5): (0.5-3.0); and / or, the first reducing agent is selected from at least one of SO2, NH4HSO3, H2O2, oxalic acid, sodium oxalate, potassium oxalate, ascorbic acid, sodium ascorbate, sodium sulfite and sodium pyrosulfite; and / or, the second solvent is selected from at least one of H2SO4 solution, HCl solution and phosphoric acid solution; the concentration of the second solvent is 0.1-5.0 mol / L; And / or, the process of the first calcination heat treatment includes: a first stage heat treatment, a second stage heat treatment, and a third stage heat treatment in sequence; wherein the process of the first stage heat treatment includes: heating from 25°C to 150-200°C at a heating rate of 5-30°C / min, and keeping the temperature for 1.0-1.5h; the process of the second stage heat treatment includes: continuing to heat to 300-350°C at a heating rate of 5-10°C / min, and keeping the temperature for 1.0-3.0h; the process of the third stage heat treatment includes: continuing to heat to 450-500°C at a heating rate of 5-10°C / min, and keeping the temperature for 1.0-3.0h; And / or, the first calcination heat treatment process is carried out in a first protective atmosphere; the first protective atmosphere is selected from at least one of argon, nitrogen, helium and NH3; the gas flow rate of the first protective atmosphere is 1-10 L / min; the first calcination heat treatment process is carried out in a tubular vacuum furnace; And / or, the first calcination heat treatment also includes an annealing treatment, the process of which includes: cooling to 300-400°C at a cooling rate of 5-10°C / min, introducing oxygen; the flow rate of the oxygen is 1-10 L / min, the temperature is kept constant for 1.0-3.0 hours, and then the temperature is naturally cooled to room temperature.

8. The method for preparing the all-vanadium electrolyte according to claim 7, characterized in that: The conditions of reaction VI include: reaction temperature of 0-160° C., constant temperature reaction time of 1.0-5.0 h; And / or, the mass ratio of the remaining part of the ammonium vanadate-containing sediment to the second ammonium salt is (1.0-20.0): (0.001-15.0); And / or, the mass ratio of the remaining part of the ammonium vanadate-containing sediment to the precious metal compound is (1.0-20.0): (0.1-10.0); And / or, the mass ratio of the noble metal compound to the chelating agent is (0.1-10.0): (0.001-15.0); and / or, the molar ratio of the noble metal compound to the third reducing agent is (0.001-0.1):(0.005-0.5); and / or, the mass ratio of the fourth reducing agent to the product of reaction VI is (0.1-0.5):1; And / or, the second ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonium bisulfate, ammonium bicarbonate, ammonium carbonate, ammonium polyacrylate and dopamine hydrochloride; And / or, the chelating agent is selected from at least one of thiourea, sulfite, phosphoric acid, oxalic acid, arsenic acid, polyacrylic acid and polyvinyl alcohol; and / or, the third reducing agent is selected from at least one of elemental sulfur, ascorbic acid, sodium ascorbate, sodium citrate, formaldehyde, acetaldehyde, formic acid, acetic acid, hydrazine hydrate, metal borohydride salt, sulfite and oleylamine; And / or, the fourth reducing agent is elemental carbon; and / or, the fourth solvent is selected from water and / or ethylene glycol; And / or, the second protective atmosphere is selected from at least one of argon, nitrogen, helium and NH3; And / or, the process of the second calcination heat treatment includes a first stage heat treatment, a second stage heat treatment, a third stage heat treatment and a fourth stage heat treatment in sequence; wherein, the first stage heat treatment includes: heating from 25°C to 300-350°C at a heating rate of 5-30°C / min, and keeping warm for 1.0-1.5h; the second stage heat treatment includes: continuing to heat to 600-650°C at a heating rate of 5-10°C / min, and keeping warm for 1.0-3.0h; the third stage heat treatment includes: continuing to heat to 800-900°C at a heating rate of 5-10°C / min, and keeping warm for 1.0-3.0h; the fourth stage heat treatment includes: continuing to heat to 1100-1200°C at a heating rate of 5-10°C / min, and keeping warm for 1.0-3.0h, and naturally cooling to room temperature to obtain the vanadium-based hydrogen evolution electrocatalyst loaded with precious metals; And / or, the noble metal-loaded vanadium-based hydrogen evolution electrocatalyst, Nafion solution and organic solvent II are mixed to obtain slurry; the slurry is coated on the surface of a membrane electrode to obtain a catalyst membrane electrode; Wherein, the mixing ratio of the noble metal-loaded vanadium-based hydrogen evolution electrocatalyst, the Nafion solution and the organic solvent II is (2.5-15.0) mg: (100.0-150.0) μL: (1.0-10.0) mL; the concentration of the Nafion solution is 0.5-10.0 wt%; And / or, the loading amount of the noble metal in the vanadium-based hydrogen evolution electrocatalyst is 20.0-40.0 wt % and the pore volume is 0.01-10.0 cm 3 / g, average pore size of 5~50nm, specific surface area of ​​450~600m 2 / g.

9. An all-vanadium electrolyte, characterized in that: The all-vanadium electrolyte is prepared by the method for preparing the all-vanadium electrolyte according to any one of claims 1 to 8.

10. An all-vanadium liquid flow battery, comprising an all-vanadium electrolyte; characterized in that: The all-vanadium electrolyte is the all-vanadium electrolyte according to claim 9.

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