KR102408365B1 - Manufacturing method of electrolyte for vanadium redox flow battery - Google Patents

Abstract

The present invention relates to a method for preparing an electrolyte for a vanadium redox flow battery, comprising: a) preparing a reaction solution by adding vanadium pentoxide (V ₂ O ₅ ), a reducing agent, and a catalyst to a first acidic solution; and b) preparing a vanadium 3.5 electrolytic solution by catalyzing the reaction solution, wherein a second acidic solution satisfying Equation 1 below is added before or after step b).
[Relational Expression 1]
3A< W < 8A
In Relation 1, A is the number of moles (mole) of the vanadium pentoxide, and W is the number of moles (mole) of hydrogen ions (H ⁺ ) in the second acidic solution.

Description

Manufacturing method of electrolyte for vanadium redox flow battery {MANUFACTURING METHOD OF ELECTROLYTE FOR VANADIUM REDOX FLOW BATTERY}

The present invention relates to a method for preparing an electrolyte for a vanadium redox flow battery, and more particularly, to a method for preparing a vanadium 3.5-valent electrolyte using a catalytic reactor and using vanadium pentoxide (V ₂ O ₅ ).

Since the output of renewable energy varies greatly depending on the given environment, large-capacity energy storage technology (Energy Storage System, ESS) is required to respond to the generated electric energy according to the time of demand. One of the currently commercialized technologies, the Vanadium Redox Flow Battery (VRFB), is safer than other technologies because there is no risk of ignition, harmful gas generation, or explosion. is attracting attention. In VRFB, the energy density is determined by the amount of vanadium ions dissolved in the electrolyte, and this electrolyte has the highest price share in the system. Therefore, in order for VRFB to secure commercial competitiveness in the ESS field, it is essential to reduce the price of electrolyte.

Vanadium electrolytes currently used commercially are generally in the form of vanadium 3.5 dissolved in sulfuric acid solution. The vanadium 3.5-valent electrolyte is a mixture of trivalent vanadium ions and tetravalent vanadium ions in a 1:1 concentration. The reason is that storage in the form of 3.5 is more stable for long-term storage. The prepared vanadium 3.5 electrolyte is used for the positive and negative electrodes of VRFB to perform charging and discharging.

Vanadium precursors used for preparing the electrolyte of VRFB include V ₂ O ₅ , VOSO ₄ , and V ₂ O ₃ . In a general method for preparing a vanadium 3.5-valent electrolyte, V ₂ O ₅ , which is the cheapest among the vanadium precursors, is first reduced to a tetravalent vanadium electrolyte using a reducing agent such as oxalic acid. The reason why oxalic acid is used as a tetravalent vanadium reducing agent is that when oxalic acid is oxidized, only CO ₂ and hydrogen ions are produced, leaving no impurities in the electrolyte. However, since the reducing power of oxalic acid is not large enough to reduce vanadium tetravalent to trivalent, an electrolysis method and a metal reducing agent are applied as an additional process for this purpose. In the electrolysis method, when a vanadium tetravalent electrolyte is put into the anode and cathode of the VRFB system and charged, vanadium pentavalent ions are generated at the anode and vanadium trivalent ions are generated at the cathode. A vanadium 3.5-valent electrolyte solution may be prepared by mixing the prepared vanadium trivalent solution and the tetravalent solution prepared by oxalic acid reduction. However, the disadvantages of this method are that unwanted extra vanadium pentaelectrolyte is generated at the anode during the electrolysis process, electricity must be used for electrolysis, and pure vanadium trivalent electrolyte can be obtained only at a very slow charging rate. Therefore, the efficiency of the production process is lowered, and the manufacturing cost of the electrolyte is increased. Alternatively, a vanadium tetravalent solution prepared by dissolving a VOSO ₄ precursor in sulfuric acid may be electrolyzed in the same manner to obtain a vanadium 3.5 valence electrolyte. However, there is a disadvantage that the price of the VOSO ₄ precursor is 5 times higher than that of V ₂ O ₅ .

On the other hand, the metal reduction method using a metal such as Zn as a reducing agent has the advantage of not producing a surplus 5-valent electrolyte and not using additional electrical energy, but impurities such as Zn ions remain in the electrolyte, which reduces the durability of the VRFB. . In addition, there is a method of preparing a trivalent electrolyte using a V ₂ O ₃ salt, which is a trivalent vanadium precursor, and mixing it with a tetravalent electrolyte to prepare a vanadium 3.5 valence electrolyte, but V ₂ O ₃ is very expensive and It is not commonly used due to its low solubility limitation.

An object of the present invention is to provide a method for preparing a vanadium 3.5-valent electrolyte solution in a one-step method by applying vanadium pentoxide, which is a low-cost precursor of vanadium, to a catalytic reactor.

Another object of the present invention is to minimize the crossover of vanadium ions during the charging and discharging process, thereby reducing the energy density and current efficiency.

A method for preparing an electrolyte for a vanadium redox flow battery according to an aspect of the present invention comprises: a) preparing a reaction solution by adding vanadium pentoxide (V ₂ O ₅ ), a reducing agent, and a catalyst to a first acidic solution; and b) preparing a vanadium 3.5 electrolyte solution by catalyzing the prepared reaction solution, wherein a second acidic solution satisfying the following relation 1 is added before or after step b).

[Relational Expression 1]

3A< W < 8A

In Relation 1, A is the number of moles (mole) of the vanadium pentoxide, and W is the number of moles (mole) of hydrogen ions (H ⁺ ) in the second acidic solution.

In the method of manufacturing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the number of moles of hydrogen ions (H ⁺ ) in the second acidic solution in the above-mentioned relation 1 may be 5A or more and 6A or less.

In the method for preparing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the first and second acidic solutions are acidic solutions containing at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid. can be

In the method of manufacturing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the above-described first and second acidic solutions may be the same material.

In the method for preparing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the step of preparing the electrolyte for vanadium 3.5 through a catalytic reaction may be performed at 50 to 100°C.

In the method for preparing the electrolyte solution for a vanadium redox flow battery according to an embodiment of the present invention, the molar ratio of vanadium pentoxide to the reducing agent in the reaction solution may be 1: 1.5 to 1: 1.65.

In the method for producing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the concentration of vanadium pentoxide in the reaction solution is 0.5 to 1.0 M, and the hydrogen ion concentration contained in the first acidic solution may be 4 to 8M. have.

In the method for producing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the reducing agent input to the first acidic solution in the step of preparing the reaction solution may be formic acid.

In the method for preparing an electrolyte for a vanadium redox flow battery according to an embodiment of the present invention, the catalyst input to the first acidic solution in the step of preparing the reaction solution may include a noble metal catalyst.

The manufacturing method of the electrolyte for a vanadium redox flow battery according to the present invention is a one-step method by applying vanadium pentoxide (V ₂ O ₅ ), which is 80% cheaper than the existing vanadium precursor VOSO ₄ , to a catalytic reactor. Therefore, it is possible to significantly lower the manufacturing cost of the electrolyte.

In addition, the method for preparing an electrolyte for a vanadium redox flow battery according to the present invention can prepare a high-purity vanadium 3.5 electrolyte, and thus has the advantage of exhibiting stable energy density and current efficiency in the charging/discharging process.

1 is a schematic diagram of the preparation of a vanadium 3.5-valent electrolyte using V ₂ O ₅ .
FIG. 2 shows the transition of the conversion rate when vanadium pentavalent ions are converted to 3.5-valent ions during the catalytic reduction reaction according to Example 2 and Comparative Example 4 of the present invention.
3 shows the results of Ultraviolet Visible Spectrometry (UV) of the vanadium 3.5-valent electrolyte prepared according to Examples and Comparative Examples of the present invention.
4 is a comparison of the results of 20 cycles of charging/discharging at an applied current of 50 mA/cm ² using the vanadium 3.5-valent electrolyte prepared according to Comparative Examples 1 and 3 of the present invention.
5 is a comparison of the results of 20 cycles of charging/discharging at an applied current of 50 mA/cm ² using the vanadium 3.5 solution prepared according to Example 2 and Comparative Example 3 of the present invention.
6 is a comparison of battery performance when charging/discharging for 20 cycles at an applied current of 50 mA/cm ² using the vanadium 3.5-valent electrolyte prepared according to Examples 1 and 2 and Comparative Examples of the present invention.
7 is a graph comparing the results of 10 cycles of charging/discharging at an applied current of 50 mA/cm ² using the vanadium 3.5 solution prepared according to Example 3 and Comparative Example 3 of the present invention.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention belongs It is provided to fully inform the possessor of the scope of the invention, and the present invention is only defined by the scope of the claims. With reference to the accompanying drawings will be described in detail for the implementation of the present invention. Irrespective of the drawings, like reference numbers refer to like elements, and "and/or" includes each and every combination of one or more of the recited items.

Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used with meanings commonly understood by those of ordinary skill in the art to which the present invention belongs. In the entire specification, when a part "includes" a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated. The singular also includes the plural, unless the phrase specifically states otherwise.

In this specification, when a part of a layer, film, region, plate, etc. is “on” or “on” another part, this includes not only the case where it is “directly on” the other part, but also the case where there is another part in between. do.

The vanadium 3.5-valent electrolyte refers to an electrolyte in which vanadium trivalent ions (V ³⁺ ) and vanadium tetravalent ions (V ⁴⁺ ) are mixed in a 1:1 ratio. The vanadium 3.5-valent electrolyte can be used in common for both the positive electrode and the negative electrolyte, so it is easy to maintain and has excellent stability during long-term storage.

In the past, studies have been conducted to prepare a vanadium 3.5 electrolyte solution using VOSO ₄ as a vanadium precursor, but VOSO ₄ has the disadvantage of being expensive. There are difficulties in manufacturing.

The present invention comprises the steps of: a) preparing a reaction solution by adding vanadium pentoxide (V ₂ O ₅ ), a reducing agent, and a catalyst to a first acidic solution; and b) catalyzing the reaction solution to prepare a vanadium 3.5 electrolyte solution, wherein a second acidic solution satisfying the following relation 1 is added before or after step b). A method for preparing an electrolyte for a battery is provided.

[Relational Expression 1]

3A< W < 8A

(In Relation 1, A is the number of moles (mole) of the vanadium pentoxide, and W is the number of moles (mole) of hydrogen ions (H ⁺ ) in the second acidic solution.)

In step a), a reaction solution containing vanadium pentoxide (V ₂ O ₅ ), a reducing agent, and a catalyst is prepared.

Specifically, the reaction solution may be prepared by adding a reducing agent and a catalyst to the first acidic solution and finally adding vanadium pentoxide. In this case, the order of input of the reducing agent and the catalyst may not be significantly limited, but as an example, vanadium pentoxide may be added in the presence of a reducing agent.

The reducing agent may be used without limitation as long as it is a reducing agent used in a method for preparing an electrolyte for a vanadium redox flow battery using a reduction method, but preferably formic acid. The process of reducing the vanadium pentoxide to a vanadium 3.5-valent electrolyte using formic acid as a reducing agent can be expressed by the following Reaction Formula 1.

[Scheme 1]

Referring to Scheme 1, in order to make a final 3.5-valent vanadium ion from 1 mole of vanadium pentoxide, theoretically, 1.5 moles of formic acid is required. In practice, however, an excess of formic acid is required than in theory to reach 3.5.

Accordingly, in order to finally produce a 3.5-valent vanadium ion, the molar ratio of vanadium pentoxide (V ₂ O ₅ ): reducing agent is 1: 1.50 to 1.65, advantageously 1: 1.50 to 1.60, more advantageously 1: 1.54 to 1.58 days can

Here, the aforementioned molar ratio of vanadium pentoxide (V ₂ O ₅ ): reducing agent may refer to a molar ratio of vanadium pentoxide (V ₂ O ₅ ) and 100% purity of the reducing agent.

Since an excess of formic acid was added as a reducing agent, excess formic acid remains in the electrolyte after vanadium reaches 3.5 valency. However, a platinum catalyst is required for formic acid to act as a reducing agent. In actual VRFB, since carbon felt is used as an electrode, formic acid does not act as a reducing agent, so it does not affect the performance of VRFB.

On the other hand, when the molar ratio of the vanadium pentoxide (V ₂ O ₅ ) to the reducing agent is less than 1: 1.5, the reduction of pentavalent vanadium does not proceed sufficiently because the amount of the reducing agent is not sufficient, ^and as a result, vanadium 3.6+ or vanadium 3.6+ or Like vanadium ^3.7+ , the valence increases. As described above, when the vanadium electrolyte deviating from 3.5 ⁺ is used for the positive electrode and the negative electrode at the same time, the composition of the electrolyte may be out of balance, causing a significant decrease in battery capacity and/or performance.

In addition, when the molar ratio of the vanadium pentoxide (V ₂ O ₅ ): reducing agent exceeds 1: 1.65, the reduction process occurs excessively to produce a vanadium electrolyte close to 3 ⁺ valence, and as described above, the battery capacity reduction problem occurs. can occur Therefore, it is very important for battery performance to obtain 3.5 ⁺ vanadium electrolyte by accurately controlling the concentration of the reducing agent.

The hydrogen ion concentration of the first acidic solution to which the vanadium pentoxide (V ₂ O ₅ ) and the reducing agent are added in the above-described molar ratio in the reaction solution preparation step may be 4 to 8M, preferably 5 to 7M.

As the concentration of hydrogen ions in the first acidic solution increases, the solubility of vanadium decreases and the concentration of vanadium ions decreases. As a result, the discharge energy density of the battery is lowered. Conversely, when the concentration of hydrogen ions decreases, the solubility of vanadium increases, but the viscosity of the electrolyte decreases, and the permeability of vanadium ions through the electrolyte membrane increases, resulting in lower energy efficiency. Therefore, since the concentration of vanadium and the concentration of hydrogen ions are not independent and affect each other, the hydrogen ion concentration of the first acidic solution preferably satisfies the above-mentioned range.

In one embodiment, the concentration of vanadium pentoxide may be 0.5 to 1.0 M, advantageously 0.75 to 0.9 M. 1 mole of vanadium pentoxide produces 2 moles of vanadium, and it is advantageous to increase the concentration of vanadium as much as possible because it determines the energy density of the battery. However, it has limited solubility and can easily precipitate when the electrolyte temperature is increased.

Therefore, in the aspect of ensuring the stability of each vanadium ion in the vanadium electrolyte during the charging and discharging process and expressing an excellent capacity, the hydrogen ion concentration and the concentration of vanadium pentoxide in the first acidic solution in the reaction solution preparation step are within the above-mentioned ranges. it is preferable

The first acidic solution may be an acidic solution containing one or more acids selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid.

The catalyst may include a noble metal catalyst, and specifically, the catalyst is platinum (Pt), ruthenium (Ru), palladium (Pd), iridium (Ir), rhodium (Rh), silver (Ag), nickel (Ni) and any one or more selected from alloys thereof, but is not limited thereto, as long as it is a catalyst capable of simultaneously increasing the rate of the above-described vanadium reduction and oxidation reaction of the reducing agent.

The content of the catalyst in the reaction solution may be 0.001 to 0.05% by weight, preferably 0.001 to 0.04% by weight, but the content of the catalyst included in the reaction solution can be adjusted in consideration of economic aspects, so it is not limited thereto.

In step b), the prepared reaction solution is catalyzed to prepare a vanadium 3.5-valent electrolyte. At this time, the catalytic reaction may be carried out in an inert gas atmosphere such as nitrogen or carbon dioxide and 50 to 100 ℃, preferably 60 to 100 ℃, more preferably 70 to 90 ℃ conditions.

According to an embodiment of the present invention, when a vanadium 3.5 electrolyte solution is prepared using vanadium pentoxide (V ₂ O ₅ ) and formic acid, it can be specifically expressed by the following Reaction Scheme 2.

[Scheme 2]

Referring to Scheme 2, when 1 mole of formic acid is oxidized, 2 moles of electrons and hydrogen ions are each generated. 2 moles of electrons produced by formic acid oxidation reduce the vanadium pentavalent ions generated by the equilibrium reaction of V ₂ O ₅ to tetravalent ions and consume 4 moles of hydrogen ions. Here, V ₂ O ₅ is converted into a pentavalent ion through an equilibrium reaction with 2 moles of hydrogen ions. As the pentavalent ions are consumed as they are reduced to tetravalent, the equilibrium reaction that creates new pentavalent ions may continue. In order to prepare vanadium 3.5 valence, only one of the two generated vanadium tetravalent ions must be reduced to trivalence. Thus, 1 mole of electrons and 2 moles of hydrogen ions are consumed. As a result, when the equations are integrated and rearranged, Scheme 1 is obtained. According to this equation, 5 moles of hydrogen ions are consumed per 1 mole of V ₂ O ₅ . Therefore, the pH of the reaction solution using vanadium pentoxide increases during the catalytic reduction reaction. In a catalytic reactor using VOSO ₄ as a precursor, the pH change is relatively small because 0.5 moles of hydrogen ions are consumed per 1 mole of VOSO ₄ in the process of making an electrolyte at 3.5, but when V ₂ O ₅ is used as a precursor, 5 moles of The change in pH is large because hydrogen ions are consumed.

A change in the pH of the electrolyte solution for a vanadium redox flow battery greatly affects the performance of the battery. During charging and discharging, hydrogen ions move between the anode and the cathode to balance the charge. In the cathodic reaction (vanadium divalent/trivalent), the hydrogen ion does not participate in the reaction, but in the anode reaction (vanadium pentavalent/4valent), it participates directly. Therefore, when the concentration of hydrogen ions is low, the anode reaction is negatively affected. In addition, when the concentration of hydrogen ions in the electrolyte decreases, the viscosity of the electrolyte decreases, and competition between hydrogen ions and vanadium ions is reduced, so that the vanadium ions can more easily penetrate the electrolyte membrane. The permeation rate of vanadium ions depends on the oxidation number of the vanadium ions, and since the permeation rate of vanadium divalent ions is the fastest, a large amount of vanadium divalent ions permeate from the cathode to the anode. As a result, the vanadium concentration of the anode electrolyte increases to increase the osmotic pressure, and as water from the anode moves to the anode, the volume of the electrolyte at the cathode decreases and the volume of the electrolyte at the anode increases. This phenomenon continues to accumulate as the charge/discharge cycle progresses, and the imbalance between the positive and negative electrolytes deepens, resulting in a sharp decrease in energy density and current efficiency. A decrease in current efficiency eventually leads to a decrease in energy efficiency. Accordingly, this problem is further exacerbated when a 3.5-valent vanadium electrolyte is prepared using vanadium pentoxide.

In order to solve the above-described problem, the method for preparing an electrolyte for a vanadium redox flow battery according to the present invention is characterized in that the second acidic solution satisfying the following relation 1 is added before or after step b).

[Relational Expression 1]

3A< W < 8A

(In Relation 1, A is the number of moles (mole) of the vanadium pentoxide, and W is the number of moles (mole) of hydrogen ions (H ⁺ ) in the second acidic solution.)

As an advantageous example, Relation 1 may satisfy 4A<W≤7A, and advantageously satisfy 5A≤W≤6A.

By supplementing hydrogen ions consumed in the process of reducing vanadium pentavalent ions to 3.5-valent ions through addition of a second acidic solution satisfying Relational Equation 1, it is possible to maintain the same pH as the initially set pH of the electrolyte, It is possible to suppress the imbalance of the volume and the concentration of vanadium ions of the positive and negative electrodes of the vanadium redox flow battery to which this is applied, thereby exhibiting stable energy density and current efficiency.

At this time, when the number of moles of hydrogen ions in the second acidic solution is 8A or more, since it affects the solubility of vanadium and causes precipitation according to temperature change, which can lead to a decrease in stability, the second acidic solution to be added is the relational formula in the above-mentioned range 1 is preferably satisfied.

The second acidic solution may be an acidic solution containing at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid, and preferably the same as the first acidic solution.

According to one embodiment of the present invention, after adding the second acidic solution satisfying Relational Equation 1 to the reaction solution prepared in step a), the catalytic reaction according to step b) is performed to prepare a vanadium 3.5-valent electrolyte can do.

According to another embodiment of the present invention, the second acidic solution satisfying the above relation 1 may be added at the time when the catalytic reaction according to step b) is finished. In this case, as the second acidic solution is added after the catalytic reduction reaction, the volume of the vanadium 3.5 electrolyte solution may increase, and thus the concentration of vanadium ions may be decreased. Accordingly, when the second acidic solution is added after step b), the concentration of vanadium pentoxide and the reducing agent must be adjusted in step a) in consideration of the concentration of vanadium ions that decrease according to the volume of the second acid solution. do.

After step b), the method may further include separating the catalyst to recover the vanadium 3.5 electrolyte solution. At this time, since the catalyst can be used repeatedly for the next catalytic reduction reaction, it can be recovered through centrifugation or a filter after the reaction is completed.

Hereinafter, the present invention will be described in detail with reference to Examples, but these are for describing the present invention in more detail, and the scope of the present invention is not limited by the Examples below.

Example

(Example 1)

To prepare 50 ml of 3M sulfuric acid, 15.89 g of 95% pure sulfuric acid is prepared. In addition, 3.6 ml (2.09 g) of sulfuric acid is prepared to supplement hydrogen ions consumed during the catalytic reaction. A total of 50 ml of sulfuric acid solution is prepared by mixing the prepared sulfuric acid and distilled water. In 50 mL of the sulfuric acid solution, platinum catalyst (15 mg based on metal), formic acid 3.2 g (5% excess based on the theoretical amount required for conversion of vanadium pentavalent to 3.5 valence, purity 85% formic acid), V ₂ O ₅ 6.85 g (purity 99.5 %) and reacted at a constant temperature of 70° C. while purging nitrogen gas using the experimental apparatus shown in FIG. 1 . When the vanadium electrolyte reaches 3.5 valence, the catalyst is separated from the electrolyte to obtain a final 1.5 M vanadium 3.5 valence electrolyte. V ₂ O ₅ in the reaction solution: The actual molar ratio considering the purity of formic acid is 1: 1.575.

(Example 2)

The same procedure was carried out in Example 1, except that 5.3 ml of sulfuric acid was additionally added instead of 3.6 ml of sulfuric acid to prepare a 1.5 M electrolyte solution containing 3.5 M of vanadium. Here, the reason for adding 5.3 ml of sulfuric acid is as follows.

[Scheme 1]

Referring to Scheme 1, 5 moles of hydrogen ions are consumed per 1 mole of V ₂ O ₅ . In Example 2, 0.0375 moles of V ₂ O ₅ are required to make an electrolyte of 1.5 M vanadium 3.5 based on 50 mL of 50 mL, and hydrogen ions consumed are 0.0375 × 5 = 0.1875 mol. Therefore, the amount of sulfuric acid to be added corresponding to the consumed hydrogen ions is as shown in Equation 1 below.

[Formula 1]

(Example 3)

To prepare 50 mL of 3M sulfuric acid, a total of 50 mL of sulfuric acid solution is prepared by mixing 15.89 g of 95% pure sulfuric acid and distilled water. In 50 mL of the sulfuric acid solution, a platinum catalyst (15 mg based on metal), 3.58 g of formic acid (5% excess based on the theoretical amount required for the conversion of vanadium pentavalent to 3.5 valence, purity 85% formic acid), V ₂ O ₅ 7.72 g (purity 99.5 %), and reacted at a constant temperature of 70° C. while purging nitrogen gas using the experimental apparatus shown in FIG. 1 . When the vanadium electrolyte reaches 3.5 valence, the electrolyte is separated from the catalyst. Next, in order to supplement the hydrogen ions consumed during the catalytic reaction, 5.8 mL of 95% pure sulfuric acid was additionally added to the vanadium 3.5 solution and mixed to obtain a final 1.5 M vanadium 3.5 solution. The actual molar ratio of V ₂ O ₅ in the reaction solution considering the purity of formic acid was 1: 1.575.

(Example 4)

In Example 1, the same procedure was performed except that 2.0 g of formic acid was added instead of 3.2 g. At this time, the actual molar ratio in consideration of the purity of V ₂ O ₅ : formic acid in the reaction solution was about 1:1.

(Example 5)

In Example 1, the same procedure was performed except that 4.0 g of formic acid was added instead of 3.2 g. At this time, the actual molar ratio of V ₂ O ₅ in the reaction solution considering the purity of formic acid was about 1:2.

(Comparative Example 1)

Mix 15.89 g of 95% pure sulfuric acid and distilled water to prepare 50 ml sulfuric acid 3M solution. In 50 mL of the sulfuric acid solution, platinum catalyst (15 mg based on metal), formic acid 3.2 g (theoretical amount required for vanadium pentavalent to 3.5 valence conversion + 5% excess, purity 85% formic acid), V ₂ O ₅ 6.85 g (purity 99.5 % V ₂ O ₅ ) and react at a constant temperature of 70 ℃ while purging nitrogen gas. When the vanadium electrolyte reaches 3.5 valence, the catalyst is separated from the electrolyte to obtain a final 1.5 M vanadium 3.5 valence electrolyte.

(Comparative Example 2)

The same procedure was performed except that in Example 1, an electrolyte solution containing 1.5 M vanadium 3.5 was prepared by additionally adding 2.4 ml instead of 3.6 ml of sulfuric acid.

(Comparative Example 3)

A) VOSO ₄ is dissolved in 3.0 M sulfuric acid solution to prepare 1.5 M vanadium tetravalent electrolyte. In order to prepare a vanadium trivalent electrolyte, the same volume of the vanadium quadrivalent electrolyte is put into the negative/positive electrolyte tank of the redox flow battery unit cell, and charged under the conditions of a cut-off voltage of 1.6 V and an applied current density of 4 mAcm ^-2 only to obtain a pure vanadium trivalent electrolyte from the cathode. A vanadium 3.5-valent electrolyte is prepared by mixing the vanadium tetravalent prepared in step A) and the trivalent electrolyte obtained by electrolysis in a 1:1 ratio. This is called standard vanadium 3.5.

(Comparative Example 4)

The same procedure was carried out in Example 1, except that an electrolyte solution containing 1.5 M vanadium 3.5 was prepared without using a catalyst.

Test Example 1: UV-Vis　Spectrophotometer analysis

The conversion of vanadium pentavalent to 3.5 valence according to Example 2 and Comparative Example 4 was analyzed by UV-vis, and the results are shown in FIG. 2 .

As can be seen in FIG. 2 , in the case of Example 2, it can be confirmed that pentavalent vanadium is converted to 3.5 valence within 60 minutes after the start of the reaction, and in Comparative Example 4, as the reaction proceeds without a catalyst, vanadium pentavalent to 3.5 valence at all You can see that there is no conversion. On the other hand, if the amount of formic acid is too large, the reduction reaction may occur excessively and vanadium may be lower than 3.5. If formic acid is added as much as the theoretical amount, it is preferable to add about 3 to 5% excess of formic acid because the vanadium 3.5 value cannot be reached.

Next, the vanadium 3.5 prepared according to Examples 1 to 5 and Comparative Examples 1 to 3 was analyzed by UV-vis for the electrolyte, and the results are shown in FIG. 3 .

As can be seen from FIG. 3 , it can be confirmed that an electrolyte solution containing the same concentration of vanadium 3.5 was prepared according to Examples 1 to 3 and Comparative Examples 1 to 3 . That is, it shows that, even if the concentration of sulfuric acid is different, when the concentration of formic acid is constant, the electrolyte solution of vanadium 3.5 can be prepared by catalytic reduction using V ₂ O ₅ . On the other hand, as in Examples 4 and 5, when the concentration of formic acid is too much or less than the theoretical amount, it can be seen that the vanadium 3.5 value cannot be reached.

Test Example 2: Measurement of pH and Viscosity of Vanadium 3.5-valent Electrolyte

The pH and viscosity of the electrolytes prepared according to Examples 1 to 2 and Comparative Examples 1 to 3 were measured and shown in Table 1 below.

[Table 1]

In Table 1, the method of calculating the theoretical pH considering hydrogen ions consumed after the catalytic reduction reaction is shown in Equation 2.

[Formula 2]

As shown in Table 1, it can be seen that the experimentally measured pH of the electrolyte prepared by the catalytic reduction method and the theoretically calculated pH are almost the same. This means that the pH of the electrolyte changes by the amount of hydrogen ions consumed. In Comparative Example 1 in which sulfuric acid was not supplemented, the pH was increased and the viscosity was decreased due to the consumed hydrogen ions. When the consumed hydrogen ions were supplemented with sulfuric acid, the pH decreased and the viscosity increased as the amount of sulfuric acid increased, showing similar values to the standard solution.

Test Example 3: Evaluation of charging and discharging performance

A charge-discharge experiment was performed using the vanadium 3.5-valent electrolyte solution prepared according to Examples 1 to 5 and Comparative Examples 1 to 3. Specifically, the electrode (3 x 3 cm ² ) used Toyobo XF-30A, the current density 50 mA/cm ² , the flow rate 20 cm ³ min ^-1 , and the volume of the negative/positive electrolyte solution was 20 mL each. and the results are shown in FIGS. 4 to 7, and the performance at 20 cycles is shown in Table 2 below.

[Table 2]

Referring to FIG. 4 and Table 2, the energy capacity of Comparative Example 1 decreased as charging and discharging proceeded, and compared with Comparative Example 3 (standard vanadium 3.5), the discharge energy density, current, and energy efficiency were significantly reduced. In Comparative Example 2, the discharge energy density, current, and energy efficiency were recovered due to the sulfuric acid supplementation effect, but it was still lower than the values of Comparative Example 3 (standard vanadium 3.5). On the other hand, Examples 1 and 2 showed similar charging and discharging performance to Comparative Example 3. Referring to FIG. 5 , it can be seen that the charge/discharge graph of Example 2 is the same as that of Comparative Example 3. On the other hand, in the case of Example 4 in which the concentration of formic acid was lower than the theory, tetravalent vanadium was obtained after the reaction. Therefore, when tetravalent vanadium is applied to the anode and cathode, charging and discharging do not occur. Conversely, when the concentration of formic acid is higher than the theoretical concentration, trivalent vanadium can be obtained, and even in this case, charging and discharging are measured to be very low. Discharge energy density, current/voltage/energy efficiency, volume increase of anode electrolyte, and change in the amount of vanadium in anode/cathode electrolyte after 20 cycles of charging and discharging are shown.

Specifically, in Comparative Example 1 prepared without supplementation of sulfuric acid, the discharge energy density was greatly reduced during the cycle. However, as the amount of supplementary sulfuric acid was increased, the decrease was reduced, and in Examples 1 and 2, the discharge energy density did not decrease in almost the same way as Comparative Example 3, which is a standard electrolyte. These characteristics were also shown in current efficiency and energy efficiency. As the amount of supplementary sulfuric acid was increased, the current efficiency was improved and the energy efficiency was the same as that of the standard 3.5 electrolyte solution. In addition, the change in the electrolyte volume of the anode decreased as the amount of added sulfuric acid was increased. The reason is that, as shown in FIG. 6(d), the amount of vanadium permeated from the cathode to the anode is reduced, and thus the osmotic pressure is reduced. During the catalytic reduction reaction, the pH increases as hydrogen ions are consumed. According to Table 1, the higher the pH, the lower the viscosity, and the amount of vanadium permeation from the cathode to the anode electrolyte increases. When the amount of vanadium permeation increases, the electrolyte from the anode moves to the anode due to the osmotic effect, and the volume of the anode electrolyte increases. Therefore, the asymmetry due to the difference in the electrolyte volume occurs and the energy density and current density are lowered. These problems can be solved as the pH change of the electrolyte decreases by supplementing the consumed hydrogen ions with sulfuric acid. Examples 1-2 demonstrated this by showing the same normal behavior as the standard 3.5 as the electrolyte.

7 shows that Example 3 and Comparative Example 3 have the same 10 cycle charge/discharge performance at an applied current of 50 mAcm ^-2 . Example 3 is a case in which sulfuric acid was added after the catalytic reduction reaction was completed in order to supplement the amount of hydrogen ions consumed. Therefore, the timing of adding sulfuric acid does not affect the electrolyte performance. However, when sulfuric acid is added after the reaction, the total volume of the electrolyte increases and the concentration of vanadium may be diluted. Therefore, it is necessary to calculate this and add more vanadium and formic acid during the catalytic reduction reaction.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and may be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present invention pertains will appreciate the technical spirit of the present invention. However, it will be understood that the invention may be embodied in other specific forms without changing essential features. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (9)

a) preparing a reaction solution by adding vanadium pentoxide (V ₂ O ₅ ), a reducing agent, and a catalyst to a first acidic solution; and
b) catalyzing the reaction solution to prepare a vanadium 3.5 electrolyte solution;
A second acidic solution satisfying the following relation 1 is added before or after step b), wherein the molar ratio of vanadium pentoxide: reducing agent in the reaction solution is 1: 1.5 to 1: 1.65 for a vanadium redox flow battery electrolyte Manufacturing method:
[Relational Expression 1]
3A< W < 8A
In Relation 1, A is the number of moles (mole) of the vanadium pentoxide, and W is the number of moles (mole) of hydrogen ions (H ⁺ ) in the second acidic solution. According to claim 1,
In the above relation 1, the number of moles of hydrogen ions (H ⁺ ) in the second acidic solution is 5A or more and 6A or less. According to claim 1,
The first and second acidic solutions are acidic solutions containing at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid, a method for producing an electrolyte for a vanadium redox flow battery. 4. The method of claim 3,
The first and second acid solutions are the same material, a method for producing an electrolyte for a vanadium redox flow battery. According to claim 1,
The step b) is performed at 50 to 100 ℃, a method for producing an electrolyte for a vanadium redox flow battery. delete According to claim 1,
The concentration of vanadium pentoxide in the reaction solution is 0.5 to 1.0 M, and the hydrogen ion concentration included in the first acid solution is 4 to 8 M, a method for producing an electrolyte for a vanadium redox flow battery. According to claim 1,
The reducing agent is formic acid, a method for producing an electrolyte for a vanadium redox flow battery. According to claim 1,
The catalyst is a method for producing an electrolyte for a vanadium redox flow battery, including a noble metal catalyst.

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