Electrolyte for flow battery, preparation method of electrolyte and flow battery

Abstract

The invention belongs to the technical field of flow batteries, and particularly relates to an electrolyte for a flow battery, a preparation method of the electrolyte and the flow battery. The electrolyte comprises an alkali solution, a zinc salt, a phenazine additive and a solvent, wherein the structural general formula of the phenazine additive is shown as a formula I or a formula II: (formula I) and (formula II); in the formula I and the formula II, R1 is an electron donor functional group, and R2 is a hydrophilic functional group. According to the electrolyte disclosed by the invention, the redox medium function of the phenazine additive is utilized, dead zinc losing electric contact can be reconverted into zinc with electrochemical activity, and meanwhile, hydrogen evolution side reaction is inhibited, so that the cycle life of the flow battery is remarkably prolonged. The preparation method of the electrolyte is simple and convenient, the formula can be flexibly adjusted according to actual requirements, the cost is low, and the electrolyte required by the flow battery can be rapidly prepared.

Description

Technical Field

[0001] This invention belongs to the field of flow battery technology, specifically relating to an electrolyte for flow batteries, a method for preparing the electrolyte, and the flow battery itself. Background Technology

[0002] With the growing global demand for renewable energy and environmental protection technologies, among many energy storage technologies, flow batteries have attracted widespread attention due to their low material cost, high safety, and large theoretical specific capacity.

[0003] However, under alkaline conditions, the zinc anode is prone to forming dead zinc, which loses electrical contact, during electrochemical cycling, reducing the reversible utilization rate of zinc. Furthermore, the high reactivity of water molecules in alkaline electrolytes often leads to severe hydrogen evolution side reactions during charging, resulting in decreased coulombic efficiency and an imbalance with the electrolyte's alkaline concentration. Therefore, problems such as dead zinc accumulation and hydrogen evolution side reactions severely restrict the long-term reversible operation of flow batteries. Summary of the Invention

[0004] In view of this, the present invention provides an electrolyte for a flow battery, a method for preparing the same, and a flow battery. The electrolyte can both convert dead zinc that has lost electrical contact back into electrochemically active zinc and suppress hydrogen evolution side reactions, thereby improving capacity and extending the cycle life of the zinc-based flow battery. To achieve the above-mentioned objectives, the technical solution of the present invention is as follows: A first aspect of the present invention provides an electrolyte for a flow battery, the electrolyte comprising an alkaline solution, a zinc salt, a phenazine additive, and a solvent; wherein the phenazine additive has a general structural formula as shown in Formula I or Formula II: (Formula I) (Formula II); In Formulas I and II, R1 is an electron donor functional group and R2 is a hydrophilic functional group.

[0005] As can be seen from the above technical solutions, the electrolyte for flow batteries proposed in the first aspect of the present invention utilizes the redox mediator function of phenazine additives to convert dead zinc that has lost electrical contact back into electrochemically active zinc, while suppressing the hydrogen evolution side reaction. That is, by utilizing the redox reaction between phenazine additives and dead zinc, dead zinc in the electrolyte can be reused and lost capacity can be obtained, thereby significantly extending the cycle life of the flow battery.

[0006] A second aspect of this invention provides a method for preparing the electrolyte of the above embodiments, comprising the following steps: mixing an alkaline solution, a zinc salt, and a solvent to obtain a mixed solution; adding a phenazine additive to the mixed solution and stirring thoroughly to obtain an electrolyte; wherein the phenazine additive has the general structural formula shown in Formula I or Formula II: (Formula I) (Formula II); In Formulas I and II, R1 is an electron donor functional group and R2 is a hydrophilic functional group.

[0007] As can be seen from the above technical solution, the preparation method of electrolyte for flow battery proposed in the second aspect of the present invention involves first mixing an alkaline solution, zinc salt and solvent to obtain a mixed solution, then adding a phenazine additive to the mixed solution and stirring thoroughly to obtain the electrolyte. This preparation method is simple and convenient, the formula can be flexibly adjusted according to actual needs, and the cost is low, enabling the rapid preparation of the electrolyte required for flow battery.

[0008] A third aspect of the present invention provides a flow battery, the flow battery comprising a battery stack, a positive electrode storage tank and a negative electrode storage tank; wherein the electrolyte in the negative electrode storage tank is the electrolyte of the above embodiments or the electrolyte prepared by the preparation method of the above embodiments.

[0009] As can be seen from the above technical solutions, the flow battery proposed in the third aspect of the present invention, by using the electrolyte of the above embodiments or the electrolyte prepared by the above embodiments as the negative electrode electrolyte, has the advantages brought by the electrolyte. It can not only convert dead zinc that has lost electrical contact back into electrochemically active zinc, but also suppress hydrogen evolution side reaction, thereby improving capacity and extending the cycle life of zinc-based flow batteries. Attached Figure Description To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0010] Figure 1 This is a schematic flowchart of a method for preparing an electrolyte for a flow battery according to some embodiments of the present invention; Figure 2 This is a schematic flowchart of a method for preparing an electrolyte for a flow battery according to other embodiments of the present invention; Figure 3 This is the UV-Vis absorption spectrum of 2-amino-3-hydroxyphenazine obtained in Example 5 of the present invention after being diluted 100 times, at a wavelength of 220 nm to 550 nm. Figure 4 This is a multicycle voltammetry curve of 2-amino-3-hydroxyphenazine prepared in Example 5 of this invention; Figure 5This is a cyclic voltammetry curve of the electrolyte prepared in Example 5 of the present invention; Figure 6 The electrolyte prepared in Example 5 and the electrolyte prepared in Comparative Example 1 are compared at 20 mA cm⁻¹. -2 Current density and 20 mAh cm -2 Charge-discharge curves under areal capacity; Figure 7 The electrolyte prepared in Example 5 and the electrolyte prepared in Comparative Example 1 are compared at 20 mA cm⁻¹. -2 Current density and 20 mAh cm -2 A comparison of areal capacity and coulombic efficiency during a long-cycle process under areal capacity. Detailed Implementation

[0011] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0012] The flowchart shown in the attached diagram is for illustrative purposes only and does not necessarily include all content and operations / steps, nor does it necessarily have to be performed in the order described. For example, some operations / steps can be broken down, combined, or partially merged, so the actual execution order may change depending on the actual situation.

[0013] It should be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0014] It should also be understood that the term “and / or” as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0015] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0016] With the growing global demand for renewable energy and environmental protection technologies, among many energy storage technologies, flow batteries have attracted widespread attention due to their low material cost, high safety, and large theoretical specific capacity.

[0017] However, under alkaline conditions, the zinc anode is prone to forming dead zinc, which loses electrical contact, during electrochemical cycling, reducing the reversible utilization rate of zinc. Furthermore, the high reactivity of water molecules in alkaline electrolytes often leads to severe hydrogen evolution side reactions during charging, resulting in decreased coulombic efficiency and an imbalance with the electrolyte's alkaline concentration. Therefore, problems such as dead zinc accumulation and hydrogen evolution side reactions severely restrict the long-term reversible operation of flow batteries.

[0018] Currently, the relevant technologies mainly employ electrolyte additives, negative electrode modification, and ion regulation. Among them, electrolyte additives introduce organic amines, quinones, or surface-active molecules to regulate deposition morphology or improve interfacial conductivity; negative electrode modification uses carbon paper / carbon felt to support metal catalysts to improve electrode interfacial reactions; and ion regulation utilizes multiple anions or complexing agents to improve the solvation structure of zinc ions and enhance desolvation kinetics.

[0019] Nevertheless, most of the related technologies are based on electrochemical kinetics or interfacial film formation, which makes it difficult to reversibly repair the already formed dead zinc and effectively suppress the hydrogen evolution side reaction.

[0020] In view of this, the present invention provides an electrolyte for a flow battery, comprising an alkaline solution, a zinc salt, a phenazine additive, and a solvent; wherein the phenazine additive has a general structural formula as shown in Formula I or Formula II: (Formula I) (Formula II); In Formulas I and II, R1 is an electron donor functional group and R2 is a hydrophilic functional group.

[0021] The phenazine additive of this invention contains both electron-donating and hydrophilic functional groups in its monomer chain structure. The electron-donating functional group enhances the electron mobility of the phenazine molecule, allowing it to act as an "electron bridge" near the electrode, converting dead zinc (which has lost electrical contact) back into electrochemically active zinc, thereby reducing the accumulation of dead zinc. The hydrophilic functional group improves the water solubility of the phenazine molecule, giving it a certain degree of solubility in aqueous solution to fully utilize its function as a redox medium.

[0022] In some embodiments, R1 is selected from any one of hydroxyl and amino groups, and R2 is selected from any one of amino, carboxyl, or sulfonic acid groups. The oxygen atom of the hydroxyl group and the nitrogen atom of the amino group have lone pairs of electrons, which resonate with the π-conjugated system of the phenazine-pyridine core, increasing the electron cloud density of the carbon-nitrogen bond in the core. This allows the phenazine molecule to act as a redox medium, promoting the re-conversion of dead zinc (which has lost electrical contact) into electrochemically active zinc, thereby reducing the accumulation of dead zinc. The amino, carboxyl, or sulfonic acid groups are all polar groups that form hydrogen bonds with water molecules, increasing the solubility of the phenazine molecule in aqueous solution and preventing its aggregation and precipitation.

[0023] In some embodiments, the phenazine additive is selected from any of the compounds shown in the following structural formulas: , , .

[0024] All the compounds in these examples have a planar conjugated rigid framework constructed from a phenazine-pyridine core. In the first two examples, the electron donor functional group and the hydrophilic functional group are distributed at different sites on the same benzene ring. Due to the spatial constraints of the planar conjugated rigid framework, the functional group has a relatively lower degree of conformational freedom, which makes the phenazine molecule more rigid and has better thermal stability. In the last example, the electron donor functional group and the hydrophilic functional group are distributed at different sites on different benzene rings, which has a relatively higher degree of conformational freedom, thereby improving the dispersibility of the phenazine molecule in the electrolyte.

[0025] In some embodiments, the phenazine additive is selected from at least one of 2-amino-3-hydroxyphenazine, 2,3-diaminophenazine, and 6-aminophenazine-1-carboxylic acid. These substances, using hydroxyl or amino groups as electron-donating functional groups, possess redox mediator functions, enabling them to not only revert "dead zinc" to active zinc but also suppress hydrogen evolution side reactions. Simultaneously, the use of amino or carboxyl groups as hydrophilic functional groups ensures high solubility of these substances in the electrolyte. Furthermore, these substances have low synthesis costs, simple reactions, and high yields, which is beneficial for preparing electrolytes to improve the capacity and cycle life of flow batteries.

[0026] In some embodiments, the alkaline solution is selected from at least one of sodium hydroxide solution and potassium hydroxide solution. These substances are all strongly alkaline and can dissociate to release OH-. - In order to maintain the charge balance of the electrochemical reaction.

[0027] In some embodiments, the zinc salt is selected from at least one of zinc oxide, zinc chloride, and zinc bromide. These substances can all provide zinc ions, serving as the active material for the zinc anode, and achieving energy storage and release through the reversible electrochemical reaction of zinc ions.

[0028] In some examples, the solvent is deionized water. Deionized water undergoes purification treatment to achieve extremely low impurity ion content, ensuring high purity of the electrolyte and preventing impurity ions from interfering with the reversible electrochemical reactions of the active substances.

[0029] In some embodiments, the concentration of the alkaline solution is 3.0 mol / L to 4.0 mol / L. For example, the concentration of the alkaline solution can be 3.0 mol / L, 3.1 mol / L, 3.3 mol / L, 3.5 mol / L, 3.7 mol / L, 3.9 mol / L, or 4.0 mol / L, etc. Controlling the concentration of the alkaline solution within the above range, or within any two of these specific values, allows for the reaction of zinc salt and OH-. - The formation of stable zincates provides sufficient OH groups. - To prevent zinc salt crystallization; if the concentration of the alkaline solution is below 3.0 mol / L, OH... - Insufficient OH- will hinder the normal reaction at the negative electrode and exacerbate the growth of zinc dendrites; if the concentration of the alkaline solution is higher than 4.0 mol / L, excess OH- will... - This may lead to zincate supersaturation, which in turn triggers zincate crystallization.

[0030] In some embodiments, the concentration of zinc salt is 0.1 mol / L to 0.3 mol / L. For example, the concentration of zinc salt can be 0.10 mol / L, 0.12 mol / L, 0.15 mol / L, 0.18 mol / L, 0.20 mol / L, 0.23 mol / L, 0.27 mol / L, or 0.30 mol / L, etc. Controlling the concentration of zinc salt within the above range, or within any two of these specific values, provides sufficient active material for the reversible electrochemical reaction of the negative electrode, avoiding the problem of zinc dendrite growth caused by excessive concentration. If the concentration of zinc salt is lower than 0.1 mol / L, insufficient content of negative electrode active material will aggravate reaction polarization and reduce charge and discharge efficiency. If the concentration of zinc salt is higher than 0.3 mol / L, excessive content of negative electrode active material is prone to uncontrolled zinc dendrite growth, leading to battery short circuit.

[0031] In some embodiments, the concentration of the phenazine additive is 1.0 mmol / L to 5.0 mmol / L. For example, the concentration of phenazine additive can be 1.0 mmol / L, 1.5 mmol / L, 1.8 mmol / L, 2.0 mmol / L, 2.3 mmol / L, 2.7 mmol / L, 3.1 mmol / L, 3.4 mmol / L, 3.6 mmol / L, 4.0 mmol / L, 4.5 mmol / L, or 5.0 mmol / L. Controlling the concentration of phenazine additive within the above ranges, or within any two of these specific values, allows the phenazine additive to fully exert its redox mediator function, effectively reactivating dead zinc and inhibiting the hydrogen evolution side reaction. If the concentration of phenazine additive is below 1.0 mmol / L, the concentration is insufficient, leading to a decrease in its activation effect on dead zinc and its inhibition effect on the hydrogen evolution reaction. If the concentration of phenazine additive is above 5.0 mmol / L, the excessively high concentration of phenazine additive will increase the cost of the electrolyte and may also trigger side reactions between phenazine additives.

[0032] Please see Figure 1 , Figure 1 This is a method for preparing an electrolyte for a flow battery provided in some embodiments of the present invention, including steps S101 to S102.

[0033] Step S101: Mix the alkaline solution, zinc salt and solvent to obtain a mixed solution.

[0034] By first mixing the alkaline solution, zinc salt, and solvent, the OH- provided by the alkaline solution can be utilized. - It reacts with zinc salt to form soluble zincate, ensuring that the zinc salt is fully dissolved and forms a homogeneous and stable mixed solution.

[0035] Step S102: Add the phenazine additive to the mixed solution and stir thoroughly to obtain the electrolyte.

[0036] After obtaining the mixed solution, add the phenazine additive and stir thoroughly to avoid the phenazine additive reacting with undissolved zinc salts or excess OH-. - The aggregation and degradation caused by direct action ensure that the phenazine additive is uniformly dispersed in the electrolyte, thereby fully exerting its function as a redox medium.

[0037] The general structural formula of the phenazine additive is shown in Formula I or Formula II: (Formula I) (Formula II); In Formulas I and II, R1 is an electron donor functional group and R2 is a hydrophilic functional group.

[0038] Provided that the technical solutions are not contradictory, the types and functions of electron donor functional groups and hydrophilic functional groups, as well as the concentration range of phenazine additives, can be referred to in the previous text and will not be repeated here.

[0039] Please see Figure 2 , Figure 2 This invention provides a method for preparing an electrolyte for a flow battery, comprising steps S100 to S102, the specific steps of which are the same as those described in other embodiments of the invention. Figure 1 The steps are largely the same, except that this embodiment also includes step S100: S100, Preparation of phenazine additives.

[0040] In other embodiments, a method for preparing phenazine additives is provided, with 2-amino-3-hydroxyphenazine as an example for detailed explanation.

[0041] First, o-phenylenediamine and 3-aminocatechol are dissolved in a solvent, and ferric chloride ethanol solution is added dropwise while stirring to obtain a mixed solution.

[0042] The molar ratio of o-phenylenediamine to 3-aminocatechol is (0.8~1.5):(0.8~1.5), for example, the molar ratio of o-phenylenediamine to 3-aminocatechol can be 0.8:1, 0.8:1.5, 1:0.8, 1:1, 1.5:0.8, or 1.5:1, etc.; the solvent is selected from any one of ethanol, methanol, dichloromethane, and ethyl acetate; the molar ratio of ferric chloride to o-phenylenediamine is (0.5~2):1, for example, the molar ratio of ferric chloride to o-phenylenediamine can be 0.5:1, 0.6:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.7:1, or 2:1, etc.; the ferric chloride ethanol solution is prepared by dissolving ferric chloride in 3 mL~5 mL of ethanol, for example, the volume of ethanol can be 3.0 mL, 3.5 mL, 4.0 mL, 4.5 mL, or 5.0 mL. mL equivalent; the dropping time of the ferric chloride ethanol solution is 5 min to 60 min, for example, the dropping time of the ferric chloride ethanol solution can be 5 min, 8 min, 12 min, 15 min, 18 min, 22 min, 27 min, 35 min, 40 min, 46 min, 53 min or 60 min equivalent; in some other more specific embodiments, the dropping time of the ferric chloride ethanol solution is 10 min to 20 min.

[0043] By controlling the molar ratio of o-phenylenediamine to 3-aminocatechol, the molar ratio of ferric chloride to o-phenylenediamine, and the dropping time of the ferric chloride ethanol solution, it is possible to ensure that the reactants are fully dissolved and uniformly dispersed, thus avoiding reactant aggregation due to excessively high local concentrations.

[0044] Next, the mixed solution is heated to reflux to carry out the reaction.

[0045] The reflux reaction temperature is 35℃~80℃, for example, the reflux reaction temperature can be 35℃, 38℃, 40℃, 45℃, 50℃, 52℃, 56℃, 60℃, 73℃ or 80℃, etc.; the reflux reaction time is 0.5 h~24 h, for example, the reflux reaction time can be 0.5 h, 1 h, 1.2 h, 2 h, 3 h, 3.5 h, 4 h, 5 h, 11 h, 15 h, 18 h or 24 h, etc.; in some more specific embodiments, the reflux reaction temperature is 40℃~60℃; the reflux reaction time is 1 h~4 h.

[0046] By controlling the temperature and time of the reflux reaction, the reaction can be ensured to proceed fully, avoiding incomplete reaction due to excessively low temperature or insufficient time, or side reactions caused by excessively high temperature or excessively long time, which would lead to low yield.

[0047] Next, after the reflux reaction is complete, the reaction system is cooled to room temperature and filtered to obtain the crude product.

[0048] Finally, the crude product was extracted with water / ethyl acetate, dried under reduced pressure, and purified by chromatographic column to obtain 2-amino-3-hydroxyphenazine.

[0049] The present invention also provides a flow battery, comprising: a battery stack, a positive electrode reservoir, and a negative electrode reservoir.

[0050] The battery stack has a conventional structure, including a positive electrode, a negative electrode, and a separator. The positive electrode reservoir contains the positive electrolyte, and the negative electrode reservoir contains the negative electrolyte. The electrolytes are delivered into the battery stack for reaction via a pump and pipeline. The electrolyte in the negative electrode reservoir is either the electrolyte described in the above embodiment or the electrolyte prepared by the method described in the above embodiment; further details will not be provided here.

[0051] In some examples, flow batteries include, but are not limited to, alkaline zinc-iron flow batteries, alkaline zinc-nickel flow batteries, and alkaline zinc-manganese flow batteries.

[0052] The flow batteries in the above examples all suffer from dead zinc accumulation and hydrogen evolution side reactions. To address these issues, the electrolyte provided in this embodiment of the invention, or the electrolyte prepared by the method provided in this embodiment of the invention, can convert dead zinc that has lost electrical contact back into electrochemically active zinc and suppress hydrogen evolution side reactions, thereby improving capacity and extending the cycle life of the flow battery.

[0053] The electrolyte for flow batteries provided by the present invention, its preparation method, and the flow battery itself are described in detail below through specific embodiments and experimental data.

[0054] Example 1 The electrolyte used in this embodiment for the flow battery includes sodium hydroxide solution, zinc oxide, phenazine additive, and deionized water; The structural formula of the phenazine additive is: That is, 2-amino-3-hydroxyphenazine.

[0055] Please see Figure 1 This embodiment prepares an electrolyte for a flow battery, including the following steps: Sodium hydroxide solution, zinc oxide, and deionized water were mixed to obtain a mixed solution; wherein the concentration of sodium hydroxide solution was 3.0 mol / L; the concentration of zinc oxide was 0.1 mol / L; and the amount of deionized water was 15 mL. 2-Amino-3-hydroxyphenazine was added to the mixed solution and stirred thoroughly to obtain the electrolyte. The concentration of 2-amino-3-hydroxyphenazine was 1.0 mmol / L.

[0056] Example 2 The electrolyte used in this embodiment for the flow battery includes sodium hydroxide solution, zinc oxide, phenazine additive, and deionized water; The structural formula of the phenazine additive is: That is, 6-aminophenazine-1-carboxylic acid.

[0057] Please see Figure 1 This embodiment prepares an electrolyte for a flow battery, including the following steps: Sodium hydroxide solution, zinc oxide, and deionized water were mixed to obtain a mixed solution; wherein the concentration of sodium hydroxide solution was 4.0 mol / L; the concentration of zinc oxide was 0.1 mol / L; and the amount of deionized water was 15 mL. 6-Aminophenazine-1-carboxylic acid was added to the mixed solution and stirred thoroughly to obtain the electrolyte. The concentration of 6-aminophenazine-1-carboxylic acid was 1.0 mmol / L.

[0058] Example 3 The electrolyte used in this embodiment for the flow battery includes potassium hydroxide solution, zinc oxide, phenazine additive, and deionized water; The structural formula of the phenazine additive is: That is, 2-amino-3-hydroxyphenazine.

[0059] Please see Figure 1 This embodiment prepares an electrolyte for a flow battery, including the following steps: A mixed solution was prepared by mixing potassium hydroxide solution, zinc oxide, and deionized water; wherein the concentration of potassium hydroxide solution was 4.0 mol / L, the concentration of zinc oxide was 0.1 mol / L, and the amount of deionized water was 15 mL. 2-Amino-3-hydroxyphenazine was added to the mixed solution and stirred thoroughly to obtain the electrolyte. The concentration of 2-amino-3-hydroxyphenazine was 5.0 mmol / L.

[0060] Example 4 The electrolyte used in this embodiment for the flow battery includes sodium hydroxide solution, zinc bromide, phenazine additive, and deionized water; The structural formula of the phenazine additive is: That is, 2-amino-3-hydroxyphenazine.

[0061] Please see Figure 1 This embodiment prepares an electrolyte for a flow battery, including the following steps: Sodium hydroxide solution, zinc bromide, and deionized water were mixed to obtain a mixed solution; wherein the concentration of sodium hydroxide solution was 4.0 mol / L; the concentration of zinc bromide was 0.1 mol / L; and the amount of deionized water was 15 mL. 2-Amino-3-hydroxyphenazine was added to the mixed solution and stirred thoroughly to obtain the electrolyte. The concentration of 2-amino-3-hydroxyphenazine was 5.0 mmol / L.

[0062] Example 5 The electrolyte used in this embodiment for the flow battery includes sodium hydroxide solution, zinc oxide, phenazine additive, and deionized water; The structural formula of the phenazine additive is: That is, 2-amino-3-hydroxyphenazine.

[0063] Please see Figure 2 This embodiment first prepares 2-amino-3-hydroxyphenazine, specifically including the following steps: o-phenylenediamine and 3-aminocatechol are dissolved in 50 mL of ethanol at a molar ratio of 1:1. Ferric chloride ethanol solution is added dropwise over 10 min, and stirring is continued for 2 h after the addition is complete to obtain a mixed solution. Ferric chloride is dissolved in 3 mL of ethanol at a molar ratio of 1:1 to o-phenylenediamine to obtain a ferric chloride ethanol solution. The mixed solution is heated to reflux for reaction at 60 °C for 4 h. After the reflux reaction is complete, the reaction system is cooled to room temperature and filtered to obtain the crude product. The crude product is extracted with water / ethyl acetate, dried under reduced pressure, and purified by column chromatography to obtain 2-amino-3-hydroxyphenazine.

[0064] After completing the preparation of 2-amino-3-hydroxyphenazine, this embodiment continues to prepare an electrolyte for flow batteries, including the following steps: Sodium hydroxide solution, zinc oxide, and deionized water were mixed to obtain a mixed solution; wherein the concentration of sodium hydroxide solution was 4.0 mol / L; the concentration of zinc oxide was 0.1 mol / L; and the amount of deionized water was 15 mL. 2-Amino-3-hydroxyphenazine was added to the mixed solution and stirred thoroughly to obtain the electrolyte. The concentration of 2-amino-3-hydroxyphenazine was 5.0 mmol / L.

[0065] Comparative Example 1 The electrolyte for flow batteries was prepared in a comparative manner, including the following steps: Sodium hydroxide solution, zinc oxide, and deionized water were thoroughly mixed to obtain the electrolyte; wherein the concentration of sodium hydroxide solution was 4.0 mol / L, the concentration of zinc oxide was 0.1 mol / L, and the amount of deionized water was 15 mL.

[0066] Experimental data results The 2-amino-3-hydroxyphenazine obtained in Example 5 of this invention was dissolved in sodium hydroxide solution, wherein the concentration of 2-amino-3-hydroxyphenazine was 2.0 mmol / L and the concentration of sodium hydroxide solution was 0.1 mol / L. After 100-fold dilution, UV-Vis absorption spectroscopy was performed at a wavelength of 220 nm to 550 nm to obtain the following results: Figure 3 The UV-Vis absorption spectrum shown is a graph where the horizontal axis represents wavelength (nm) and the vertical axis represents absorbance (au). Figure 3 It can be seen that the absorption intensity at 422.5 nm is 1.0661, which is the absorption intensity of the carbon-nitrogen double bond in the 2-amino-3-hydroxyphenazine prepared in Example 5. This indicates that 2-amino-3-hydroxyphenazine was successfully generated by the reaction of o-phenylenediamine and 3-aminocatechol.

[0067] The 2-amino-3-hydroxyphenazine prepared in Example 5 of this invention was dissolved in sodium hydroxide solution, wherein the concentration of 2-amino-3-hydroxyphenazine was 1.0 mmol / L and the concentration of sodium hydroxide solution was 0.1 mol / L. Subsequently, multiple cycles of cyclic voltammetry (CV) were performed at a scan rate of 0.1 V / s to obtain the following results: Figure 4 The multi-cycle voltammetry plot shown depicts potential on the horizontal axis (V vs. Hg / HgO) and current on the vertical axis (mA). Figure 4It can be seen that the oxidation peak at -0.81 V and the reduction peak at -0.92 V still maintain a good degree of overlap with the first curve (1st) after 500 cycles (500th), which indicates that the 2-amino-3-hydroxyphenazine prepared in Example 5 has excellent cycling stability and structural stability in an alkaline environment.

[0068] The electrolyte prepared in Example 5 of this invention was subjected to cyclic voltammetry scanning at a scan rate of 0.01 V / s to obtain the following results: Figure 5 The cyclic voltammetry curve shown is illustrated, where the horizontal axis represents electric potential (unit: V vs. Hg / HgO) and the vertical axis represents current (unit: mA). Figure 5 It is known that the equilibrium potential of the 2-amino-3-hydroxyphenazine prepared in Example 5 is 0.89 V, and the equilibrium potential of the zincate / zinc couple is 1.4 V, with a potential difference of 0.51 V. This indicates that the potential difference can drive the 2-amino-3-hydroxyphenazine prepared in Example 5 to carry out a spontaneous redox process on elemental zinc. Therefore, the 2-amino-3-hydroxyphenazine prepared in Example 5 is suitable as a redox medium for flow batteries and can realize the reactivation of dead zinc.

[0069] like Figure 6 As shown, the horizontal axis represents the areal capacity (unit: mAh cm⁻¹). -2 The vertical axis represents voltage (unit: V). Figure 6 It can be seen that in the electrolyte prepared in Comparative Example 1 (i.e., w / o PZ), when dead zinc accumulates to a certain extent, the negative electrode electrolyte undergoes an irreversible water electrolysis and hydrogen evolution reaction, leading to rapid electrolyte deterioration and decreased battery cycle stability. However, the electrolyte prepared in Example 5 (i.e., w PZ), after introducing the synthesized 2-amino-3-hydroxyphenazine as a redox medium, showed a new potential plateau in its charge-discharge curve. This indicates that 2-amino-3-hydroxyphenazine participates in the redox reaction. The light-colored area in the figure corresponds to the main plateau region of zincate ion deposition / dissolution, with a median potential of approximately 1.96 V during charging and approximately 1.73 V during discharging. The dark-colored area in the figure corresponds to the plateau region of 2-amino-3-hydroxyphenazine, with a narrow charging plateau in the range of 1.5 V to 1.8 V and a smaller capacity; the discharging plateau is in the range of 0.5 V to 1.2 V. In the V range, the capacity is significantly greater than that during the charging process. This indicates that 2-amino-3-hydroxyphenazine gains additional capacity by dissolving dead zinc during the discharge process, thus exhibiting a higher capacity than during the charging process, effectively achieving the recovery of dead zinc.

[0070] like Figure 7 As shown, the horizontal axis represents the number of cycles (unit: times), and the left vertical axis represents the areal capacity (unit: mAh cm³). -2The right-hand vertical axis represents the coulomb efficiency (in %). Figure 7 It can be seen that the flow battery assembled with the electrolyte prepared in Comparative Example 1 (i.e., w / o PZ) experienced a decrease in discharge capacity (i.e., areal capacity) to 17.23 mAh cm⁻¹ during 250 cycles due to the slow accumulation of dead zinc. -2 The coulombic efficiency remained at a relatively low level of 86.14%. However, the flow battery assembled with the electrolyte prepared in Example 5 (i.e., wPZ) maintained a stable charge-discharge process, effectively mitigating dead zinc accumulation, and maintaining a discharge capacity (i.e., areal capacity) of 19.4 mAh cm⁻¹. -2 Around 97.01%, the coulombic efficiency also remained at a relatively high level, and the reversibility of the cycle was significantly improved.

[0071] Table 1

[0072] As shown in Table 1, compared with Comparative Example 1, Examples 1 to 5 of this application all showed better coulombic efficiency after 250 cycles. This indicates that the phenazine additive can improve coulombic efficiency and ensure the long-cycle stability of the battery through its redox mediator function.

[0073] In summary, the electrolyte prepared in this application embodiment utilizes the redox mediator function of the phenazine additive to convert dead zinc that has lost electrical contact back into electrochemically active zinc, while suppressing the hydrogen evolution side reaction during water electrolysis, thereby significantly extending the cycle life of the flow battery.

Claims

1. An electrolyte for a flow battery, characterized in that, The electrolyte comprises an alkali solution, a zinc salt, a phenazine additive and a solvent; The general structural formula of the phenazine additive is shown in Formula I or Formula II: (Formula I) (Formula II) In Formula I and Formula II, R1 is an electron donor functional group, and R2 is a hydrophilic functional group.

2. The electrolyte for a flow battery according to claim 1, characterized by, R1 is selected from any one of a hydroxyl group and an amino group, and R2 is selected from any one of an amino group, a carboxyl group and a sulfonic acid group.

3. The electrolyte for a flow battery according to claim 1, wherein, The phenazine additive is selected from any one of the compounds shown in the following structural formulas: 、 、 。 4. The electrolyte for a flow battery according to claim 1, wherein The phenazine additive is selected from at least one of 2-amino-3-hydroxyphenazine, 2,3-diaminophenazine and 6-amino-phenazine-1-carboxylic acid.

5. The electrolyte for a flow battery according to claim 1, wherein The alkali solution is selected from at least one of a sodium hydroxide solution and a potassium hydroxide solution.

6. The electrolyte for a flow battery according to claim 1, wherein The zinc salt is selected from at least one of zinc oxide, zinc chloride and zinc bromide.

7. The electrolyte for a flow battery according to claim 1, wherein The phenazine additive is selected from at least one of 2-amino-3-hydroxyphenazine, 2,3-diaminophenazine and 6-amino-phenazine-1-carboxylic acid; the alkali solution is selected from at least one of a sodium hydroxide solution and a potassium hydroxide solution; and the zinc salt is selected from at least one of zinc oxide, zinc chloride and zinc bromide.

8. The electrolyte for a flow battery according to claim 1, characterized by, The concentration of the alkali solution is 3.0 mol / L to 4.0 mol / L; And / or, the concentration of the zinc salt is 0.1 mol / L to 0.3 mol / L; And / or, the concentration of the phenazine additive is 1.0 mmol / L to 5.0 mmol / L.

9. A method of preparing an electrolyte for a flow battery, characterized in that, The method comprises the following steps: Mixing an alkali solution, a zinc salt and a solvent to obtain a mixed solution; Adding a phenazine additive to the mixed solution, and fully stirring and mixing to obtain an electrolyte; The general structural formula of the phenazine additive is shown in Formula I or Formula II: (Formula I) (Formula II) In Formula I and Formula II, R1 is an electron donor functional group, and R2 is a hydrophilic functional group.

10. A flow battery, characterized in that, The liquid flow battery comprises a battery stack, a positive electrode liquid storage tank and a negative electrode liquid storage tank, wherein the electrolyte in the negative electrode liquid storage tank is the electrolyte according to any one of claims 1 to 8 or the electrolyte prepared by the preparation method of claim 9.

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