WO2015147582A1

WO2015147582A1 - Redox flow battery

Info

Publication number: WO2015147582A1
Application number: PCT/KR2015/003010
Authority: WO
Inventors: Byung-Man Kang; Soo-Whan Kim; Ok-Seon KIM
Original assignee: Oci Company Ltd.
Priority date: 2014-03-26
Filing date: 2015-03-26
Publication date: 2015-10-01
Also published as: KR101436779B1

Abstract

The present invention provides a redox flow battery which is capable of delivering charge/discharge cycles while supplying a positive electrolyte solution and a negative electrolyte solution to a battery cell, in which the battery cell includes an ion exchange membrane, each of the positive electrolyte solution and the negative electrolyte solution contains an active material that includes anions, including chloride ions, and vanadium ions, and the positive electrolyte solution contains an anionic surfactant containing both a hydrophilic group and a hydrophobic group.

Description

REDOX FLOW BATTERY

The present invention relates to a redox flow battery.

In flow batteries that use halogen as an active material, a large amount of halogen gas is generated, resulting in an increase in pressure and gas leakage, thus causing safety problems.

In the case of batteries (H₂/Br₂, Zn/Br₂, or V/Br₂) that use bromine as an active material where typically a high concentration (>1.0 M) of bromine is used, a toxic bromine gas is generated, leading to an increase in pressure in the battery cell and the electrolyte tank, resulting in gas leakage, thus causing a very serious safety problem. A method for solving this problem is to employ various additives into electrolytes to thereby inhibit the gasification of bromine.

In the case of a Zn/Cl₂ battery that uses chlorine as an active material, it was recently reported to solve the safety problem by a method of performing redox reactions while liquefying chlorine at low temperature. However, a method of adding additives to electrolytes, like the case of the battery that uses bromine, has not yet been proposed. This is why it is difficult to capture chlorine by an additive.

The reason that it is difficult to capture high-concentration chlorine may be due to different physical and chemical properties of chlorine from those of bromine. Specifically, the water solubility, boiling point, density and the like of chlorine are very lower than those of bromine, which makes it difficult to steadily capture a large amount of chlorine in aqueous solution. On the other hand, an additive material used to capture bromine the most efficiently has a very strong binding force for bromine, resulting in a stable complex formation with bromine, such as a solid precipitate, even when it is used in an equivalent amount. This characteristic enables the battery employing bromine to be safely handled, while in the case of chlorine, a material that forms a complex at room temperature is rare and the binding force of this chlorine complex for chlorine is very weaker than the binding force of the above-described bromine complex for bromine. Thus, to capture chlorine effectively in the battery using chlorine, the larger amount of additive than the active materials is necessary, which makes this approach impractical unlike the bromine-capturing additive.

Given this fact, in the case of a redox flow battery that uses a high concentration (> 1.0 M) of chlorine, it is difficult to obtain the chlorine capture effect by the use of an additive, unlike the case of bromine capture. Thus, it is known that the best method for safely controlling an excessive amount of chlorine is to liquefy chlorine at low temperature.

Various embodiments of the present invention are intended to provide a redox flow battery that uses strong acid electrolytes containing chloride ions, in which safety problems resulting not only from the increase in pressure in battery cells and electrolyte tanks by the generation of chlorine gas, but also from the leakage of chlorine gas, are prevented by capturing chlorine (Cl₂), which is generated by side reactions in a positive electrolyte solution during the operation of the redox flow battery, and efficiently inhibiting the gasification of chlorine.

In accordance with an embodiment of the present invention, there is provided a redox flow battery which is capable of delivering charge/discharge cycling while supplying a positive electrolyte solution and a negative electrolyte solution to a battery cell, in which the battery cell includes an ion exchange membrane, each of the positive electrolyte solution and the negative electrolyte solution contains an active material that includes anions, including chloride ions, and vanadium ions, and the positive electrolyte solution contains an anionic surfactant containing both a hydrophilic group and a hydrophobic group.

The anionic surfactant may contain the hydrophobic group of a C4-C15 straight or branched chain alkyl group unsubstituted or substituted where at least one hydrogen atom is substituted with fluorine, and the hydrophilic group of at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, and combinations thereof, or a salt thereof.

For example, the anionic surfactant may include at least one selected from the group consisting of a fatty acid represented by RCOO^-M⁺, a monoalkylsulfonic acid represented by RSO₃ ^-M⁺, a monoalkylphosphonic acid represented by RPO(OH)O^-M⁺, and combinations thereof, wherein R is a fluorocarbon group obtained by substituting all hydrogen atoms in a C6-C12 straight or branched chain alkyl group with fluorine, and M is H, Na or K.

The concentration of the anionic surfactant in the positive electrolyte solution may range from 1 mM to 30 mM.

The anionic surfactant may form a micelle in the positive electrolyte solution such that the hydrophobic group is oriented toward the inside of the micelle, and the hydrophilic group is oriented toward the outside of the micelle, and the micelle may capture chlorine therein.

The redox flow battery may be fully charged by supplying a voltage of greater than 1.25 V but not greater than 1.85 V to a positive electrode to which the positive electrolyte solution is supplied.

The ion exchange membrane may be a cation exchange membrane or an anion exchange membrane.

The positive electrolyte solution may contain an acid as an electrolyte, and the acid may include an aqueous hydrochloric acid solution alone or a mixture of an aqueous hydrochloric acid solution with an aqueous strong acid solution different from the aqueous hydrochloric acid solution.

Each of the positive electrolyte solution and the negative electrolyte solution may further contain sulfate ions.

The concentration of chlorine in the positive electrolyte solution may range from 0.1 mM to 100 mM.

In the redox flow battery according to the present invention, chlorine generated by side reactions in the positive electrolyte solution is effectively captured, thereby solving safety problems resulting from an increase in the battery cell and the electrolyte tank and the leakage of chlorine gas.

FIG. 1 is a schematic view of a redox flow battery according to the present invention.

FIG. 2 shows the results of measuring the pressure in an electrolyte tank as a function of time after the completion of charge for redox flow batteries of Examples 1 and 2.

FIG. 3 is a graph showing the efficiency of a redox flow battery of Example 3 during charge and discharge cycles.

FIG. 4 shows the results of measuring the pressure in an electrolyte tank as a function of time during charge and discharge for a redox flow battery of Example 3.

Hereinafter, exemplary embodiments of the present invention will be described in detail. It is to be understood, however, that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention as defined in the appended claims.

In an embodiment, the present invention provides a redox flow battery which is capable of delivering charge/discharge cycles while supplying a positive electrolyte solution and a negative electrolyte solution to a battery cell, in which the battery cell includes an ion exchange membrane, each of the positive electrolyte solution and the negative electrolyte solution contains an active material that includes anions, including chloride ions, and vanadium ions, and the positive electrolyte solution contains an anionic surfactant containing both a hydrophilic group and a hydrophobic group.

The redox flow battery includes vanadium ions as an active material, and produces electrical energy by redox reactions in the positive and negative electrolyte solutions. The positive and negative electrolyte solutions may contain ions dissociated from the active material and the electrolytes.

In the negative electrolyte solution, a redox reaction as shown in the following reaction equation 1 occurs during charge and discharge, and in the positive electrolyte solution, a redox reaction as shown in the following reaction equation 2 occurs during charge and discharge:

Reaction equation 1

V³⁺ + e- ↔ V²⁺, E⁰ = -0.25V

Reaction equation 2

VO₂ ⁺ + 2H⁺ + e^- ↔ VO²⁺ + H₂O, E⁰ = 1.00V

The redox flow battery is configured such that the negative electrolyte solution and the positive electrolyte solution are stored in different storage tanks and connected to electrode cells, which are separated from each other by an ion exchange membrane, so that the negative electrolyte solution and the positive electrolyte solution will be circulated through the cells.

The storage tanks that store the negative electrolyte solution and the positive electrolyte solution are connected to a negative electrode cell and a positive electrode cell, respectively, so that the negative electrolyte solution and the positive electrolyte solution can be circulated through the cells by fluid communication. Herein, the redox flow battery may include a pump for circulating the negative electrolyte solution or a pump for circulating the positive electrolyte solution.

The redox flow battery according to the present invention will now be described in further detail with reference to FIG. 1.

FIG. 1 is a schematic view showing the configuration of a redox flow battery which is controlled according to the present invention.

Referring to FIG. 1, a positive electrolyte solution (catholyte) is stored in a positive electrolyte solution storage tank 110, and a negative electrolyte solution (anolyte) is stored in a negative electrolyte solution storage tank 112.

The negative electrolyte solution may include divalent vanadium ions (V²⁺) or trivalent vanadium ions (V³⁺), while the positive electrolyte solution may include tetravalent vanadium ions (V⁴⁺) or pentavalent vanadium ions (V⁵⁺).

The positive electrolyte solution and negative electrolyte solution stored in a positive electrolyte solution storage tank 110 and a negative electrolyte solution storage tank 112 are introduced into the positive electrode cell 102A and negative electrode cell 102B of a cell 102, respectively, by

pumps

114 and 116. In the positive electrode cell 102A, the transfer of electrons through an electrode 106 occurs as a result of the operation of a power source/load 118, and thus a redox reaction of V⁵⁺↔ V⁴⁺occurs. Similarly, in the negative electrode cell 102B, the transfer of electrons through an electrode 108 occurs as a result of the operation of the power source/load 118, and thus a redox reaction of V²⁺↔ V³⁺occurs. After the redox reactions, the positive electrolyte solution and the negative electrolyte solution are recycled to the positive electrolyte solution storage tank 110 and the negative electrolyte solution storage tank 112, respectively.

Meanwhile, the positive electrode cell 102A and the negative electrode cell 102B are separated from each other by an ion exchange membrane 104 through which ions can pass. Thus, the transfer of ions between the positive electrode cell 102A and the negative electrode cell 102B, that is, crossover, may occur. In other words, during the charge and discharge of the redox flow battery, positive electrolyte solution ions (V⁵⁺ and V⁴⁺) in the positive electrode cell 102A may be transferred to the negative electrode cell 102B, and negative electrolyte solution ions (V²⁺ and V³⁺) in the negative electrode cell 102B may be transferred to the positive electrode cell 102A.

The redox flow battery may use an aqueous hydrochloric acid solution as a positive electrolyte solution, and thus the positive and the negative electrolyte solutions may include chloride ions.

When the positive and the negative electrolyte solutions include an aqueous hydrochloric acid solution as an electrolyte, a neutral pentavalent vanadium compound (VO₂Cl) will be produced by complexation of pentavalent vanadium ions (VO₂ ⁺) with the chloride ions derived from hydrochloric acid. The pentavalent vanadium compound (VO₂Cl) does not easily precipitate as V₂O₅even at high temperature, for example, about 40 to 60℃, and thus the redox flow battery that uses hydrochloric acid as an electrolyte can be stably operated with high efficiency even in the presence of a high concentration (e.g., 2.5 M or higher) of vanadium without having to use an additional heat exchanger. Accordingly, the operating conditions of the redox flow battery that uses hydrochloric acid as an electrolyte, that is, an electrolyte including chloride ions, make it possible to simplify the design of the overall system and minimize energy loss to thereby reduce the system cost.

In an embodiment, each of the positive electrolyte solution and the negative electrolyte solution may include chloride ions alone as anions. Specifically, an aqueous hydrochloric acid solution may be used as an electrolyte, and a vanadium chloride (VCl₂, VCl₃, VOCl₂, or VO₂Cl) may be used as an active material.

In another embodiment, the positive electrolyte solution and the positive electrolyte solution may further include other anions in addition to chloride ions. The additional anions may be those derived from an aqueous strong acid solution, and for example, the positive electrolyte solution and the negative electrolyte solution may further include sulfate ions as anions in addition to chloride ions. Specifically, an aqueous sulfuric acid solution and a hydrochloric acid solution may be used as an electrolyte, and vanadium sulfate may be used as an active material.

The positive electrolyte solution may include an acid as an electrolyte, and this acid may include, for example, either an aqueous hydrochloric acid solution or a mixture of an aqueous hydrochloric acid solution and an aqueous strong acid solution different from the aqueous hydrochloric acid solution.

In an embodiment, each of the positive electrolyte solution and the positive electrolyte solution may further include sulfate ions. If a mixture of an aqueous sulfuric acid solution and an aqueous hydrochloric acid solution is used as an electrolyte, the mixing ratio of the solutions may be determined such that the concentration of sulfate ions (SO₄ ^2-) is lower than the concentration of chloride ions (Cl^-).

If the state of charge (SOC) (%) of the redox flow battery is driven from 0% to 100%, the charge/discharge capacity of the redox flow battery can be maximally used. However, if the SOC of the redox flow battery is more than about 90% while the positive and negative electrolyte solutions include chloride ions, chlorine gas can be generated in the positive electrolyte solution at the end of charge of the redox flow battery. The tendency of electrochemical oxidation of chloride ions to chlorine gas is similar to the tendency of oxidation of tetravalent vanadium ions to pentavalent vanadium ions in the positive electrolyte solution, and thus the possibility of generation of chlorine gas can increase at the end of charge of the redox flow battery.

In addition, if the redox flow battery is fully charged by applying an overvoltage higher than the full charge voltage, for example, a voltage of greater than 1.25 V but not greater than 1.85 V, the possibility of generation of chlorine gas in the positive electrolyte solution will increase.

Meanwhile, when the redox flow battery in which the positive and negative electrolyte solutions include chloride ions is maintained in the fully charged state without being discharged, pentavalent vanadium ions in the positive electrolyte solution can act as an oxidizing agent to oxidize chloride ions, thereby producing chlorine gas. Particularly, under external high-temperature conditions, for example, at 40℃ or higher, the generation of chlorine gas by pentavalent vanadium ions can increase rapidly in proportion to an increase in the temperature.

When chlorine gas is generated, the pressure in the battery cells, particularly the positive electrode cell, can occur to cause safety problems such as tank failure and gas leakage. To eliminate such problems, the positive electrolyte solution may include an anionic surfactant containing both a hydrophilic group and a hydrophobic group, and the anionic surfactant can serve to capture chlorine that is produced in the positive electrolyte solution as described above.

According to the present invention, the anionic surfactant is contained in the positive electrolyte solution so that it will capture chlorine, and thus the accumulation of chlorine gas in the positive electrode cell can be prevented. Accordingly, the leakage of chlorine gas due to tank failure caused by an increase in the internal pressure of the positive electrode cell can be fundamentally prevented, thereby ensuring the stability of the redox flow battery.

Because the stability of the redox flow battery is ensured by the anionic surfactant, the battery can be stably operated even when an overvoltage greater than about 1.25 V is applied thereto for full charge. The overvoltage may be greater than about 1.25 V, but not greater than about 1.85 V.

The anionic surfactant may be compound containing both a hydrophilic group and a hydrophilic group, and may serve to form a micelle in the positive electrolyte solution such that the hydrophilic group is oriented toward the inside of the micelle, and the hydrophobic group is oriented toward the outside of the micelle. The micelle has an empty space in which chlorine can be trapped.

The chlorine capture property of the anionic surfactant contributes to only a decrease in gas pressure in the battery cell without reducing the charge/discharge efficiency of the battery.

Specifically, the hydrophobic group in the anionic surfactant may include a C4-C15 straight or branched chain alkyl group unsubstituted or having at least one hydrogen atom substituted with fluorine; and the hydrophilic group in the anionic surfactant may include at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, and combinations thereof, or a salt thereof.

For example, the anionic surfactant may contain at least one selected from the group consisting of a fatty acid represented by RCOO^-M⁺, a monoalkylsulfonic acid represented by RSO₃ ^-M⁺, a monoalkylphosphonic acid represented by RPO(OH)O^-M⁺, and combinations thereof, wherein R represents the above-described alkyl group that is a C4-C15 straight or branched chain alkyl group unsubstituted or having at least one hydrogen atom substituted with fluorine, and M may be H, Na or K.

When the alkyl group has 4 to 15 carbon atoms, it can be rendered hydrophobic. Specifically, the alkyl group may have 6 to 12 carbon atoms, and thus the anionic surfactant may have a molecular weight of about 88-890 g/mole.

When the alkyl group is substituted with fluorine, the resistance thereof to strong acid can further be increased. Specifically, the alkyl group may be a fluorocarbon group obtained by substituting all the hydrogen atoms of the alkyl group with fluorine.

In one embodiment, the anionic surfactant may contain at least one selected from the group consisting of a fatty acid represented by RCOO^-M⁺, a monoalkylsulfonic acid represented by RSO₃ ^-M⁺, a monoalkylphosphonic acid represented by RPO(OH)O^-M⁺, and combinations thereof, wherein R is a fluorocarbon group obtained by substituting all hydrogen atoms in a C6-C12 straight or branched chain alkyl group with fluorine, and M is H, Na or K. This fluorocarbon group has excellent resistance to strong acid, and thus is advantageous in terms of its long-term stability in the positive electrolyte solution.

The anionic surfactant does not reduce pentavalent vanadium ions in the positive electrolyte solution, because the oxidation stability of pentavalent vanadium ions in a pentavalent vanadium ion solution. For this reason, the content of pentavalent vanadium ions in the positive electrolyte solution is not influenced by the anionic surfactant, and thus the battery efficiency is not reduced by the anionic surfactant.

In addition, the anionic surfactant does not promote the precipitation of vanadium, and does not cause side reactions such as its reaction with the ion exchange membrane, which increase the internal resistance of the battery. Further, the anionic surfactant can be easily mixed with the positive electrolyte solution, because the hydrophilic group is oriented toward the outside of the micelle structure.

The anionic surfactant shows excellent reaction stability for both a cation exchange membrane and an anion exchange membrane. Thus, the ion exchange membrane may be a cation exchange membrane or an anion exchange membrane.

The concentration of the anionic surfactant in the positive electrolyte solution may be from 1 mM to 30 mM. Specifically, it may be from 5 mM to 25 mM. When the positive electrolyte solution contains the anionic surfactant in the above concentration range, the capture of chlorine therein can be suitably maintained in an unsaturated state, and thus an increase in pressure in the positive electrolyte solution can be effectively inhibited.

The absolute amount of chlorine that is generated in the positive electrolyte solution is very smaller than the total concentration of vanadium ions serving as an active material. The vanadium redox flow battery uses vanadium ions only as an active material, and chlorine is generated in the positive electrolyte solution when the battery is operated in the near full charge state at a high temperature of 40 to 60℃ or an overvoltage is applied thereto. Thus, the chlorine can be easily captured when the anionic surfactant is added to the positive electrolyte solution in an amount smaller than the amount of vanadium ions serving an active material.

For example, the concentration of chlorine in the positive electrolyte solution may be from about 0.1 mM to about 100 mM. Specifically, chlorine can be generated at a concentration of about 0.1-50 mM depending on the operating temperature when 2.0 M of vanadium ions are used.

Because the concentration of chlorine is very low as described above, the vanadium redox flow battery can be driven while the micelles formed of the anionic surfactant are maintained in a chlorine-unsaturated state, even if the micelles formed of the anionic surfactant are in a state in which they somewhat capture chlorine. Because the micelles formed of the anionic surfactant are in a chlorine-unsaturated state, chlorine captured by the micelles are not detached from the micelles and can be easily maintained in the captured state.

As described above, in the redox flow battery according to the present invention, the anionic surfactant added to the positive electrolyte solution can form a micelle that can capture chlorine, and thus the pressure in the tank storing the positive electrolyte solution can be prevented from being increased by chlorine gas.

Hereinafter, the present invention will be described in detail with reference to examples. It is to be understood, however, that these examples are for illustrative purposes and are not intended to limit the scope of the present invention.

Examples

Example 1

50 mL of an aqueous solution of tetravalent vanadium (2M VOSO₄ and 5M HCl) was introduced into a 100 mL measuring cylinder, which was then connected to a negative electrode of a small-sized single cell (electrode area: 25 cm²) provided with a cation exchange membrane (DuPont, N115) by a Viton tube. Unlike the negative electrolyte solution, a positive electrolyte solution contained perfluorooctanesulfonic acid (PFOS) as an anionic surfactant at a concentration of 24 mM together with 100 mL of an aqueous solution of tetravalent vanadium ions (2M VOSO₄, and 5M HCl), and was connected to a pressure sensor. The cell and the measuring cylinders containing the electrolytes were placed in an oven maintaining at 50℃, and then the positive electrolyte solution and the negative electrolyte solution were circulated using peristaltic pumps, thereby operating the manufactured redox flow battery.

Example 2

A redox flow battery was manufactured and operated in the same manner as described in Example 1, except that perfluorononanoic acid (PFNA) in place of perfluorooctanesulfonic acid (PFOS) as the anionic surfactant was added to the positive electrolyte solution.

Example 3

A 100 measuring cylinder filled with 50 mL of an aqueous solution of trivalent vanadium ions (2M V³⁺, and 5M HCl) was connected to a negative electrode of a small-sized single cell (electrode area: 25 cm²) by a tube. Unlike the negative electrolyte solution, a positive electrolyte solution contained the anionic surfactant PFOS at a concentration of 16 mM together with 50 mL of an aqueous solution of tetravalent vanadium ions (2M VOSO₄, and 5M HCl), and was connected to a pressure sensor. As an ion exchange membrane, the same cation exchange membrane (DuPont, N115) as used in Example 1 was used. The cell and the measuring cylinders containing the electrolytes were placed in an oven maintaining at 50℃, and then the positive electrolyte solution and the negative electrolyte solution were circulated using peristaltic pumps, thereby operating the manufactured redox flow battery.

Example 4

A redox flow battery was manufactured and operated in the same manner as described in Example 1, except that octylphosphonic acid (OPA) in place of perfluorooctanesulfonic acid (PFOS) as the anionic surfactant was added to the positive electrolyte solution.

Example 5

A redox flow battery was manufactured and operated in the same manner as described in Example 1, except that octylphosphonic acid (OPA) in place of perfluorooctanesulfonic acid (PFOS) as the anionic surfactant was added to the positive electrolyte solution at a concentration of 16 mM and that the cation exchange membrane F-940RF (Fumatech) was used instead of the cation exchange membrane N115 (DuPont).

Example 6

A redox flow battery was manufactured and operated in the same manner as described in Example 1, except that perfluorooctanesulfonic acid (PFOS) as the anionic surfactant was added to the positive electrolyte solution at a concentration of 16 mM and that the anion exchange membrane FAP-450 (Fumatech) was used instead of the cation exchange membrane N115 (DuPont).

Comparative Example 1

A redox flow battery was manufactured and operated in the same manner as described in Example 1, except that perfluorooctanesulfonic acid (PFOS) as the anionic surfactant was not added to the positive electrolyte solution.

Comparative Example 2

A redox flow battery was manufactured and operated in the same manner as described in Comparative Example 1, except that the cation exchange membrane F-940RF (Fumatech) was used instead of the cation exchange membrane N115 (DuPont).

Comparative Example 3

A redox flow battery was manufactured and operated in the same manner as described in Comparative Example 1, except that the anion exchange membrane FAP-450 (Fumatech) was used instead of the cation exchange membrane N115 (DuPont).

Test Example 1

Each of the redox flow batteries manufactured in Examples 1 and 2 and Comparative Example 1 was charged with a constant current at a current density of 150 mA/cm²until the state of charge (SOC) of vanadium ions reached 100%. After completion of the charge, the electrolytes solution (the positive electrolyte solution and the negative electrolyte solution) in the cell were completely drawn into the measuring cylinders, and then the operation of the pumps was stopped, and the pressure of gas in the positive electrolyte solution-containing measuring cylinder at 50℃ was measured by the pressure sensor for 15 hours (the void volume of the positive electrolyte solution-containing measuring cylinder was about 35 mL).

FIG. 2 is a graph showing the results of the measurement.

From the results in FIG. 2, it could be seen that, in the case of Example 1 in which the redox flow battery is full charged with 24 mM of the anionic surfactant PFOS being added to 100 mL of the positive electrolyte solution having a void volume of 35 mL, the internal pressure of the measuring cylinder decreased by 2 times at an external temperature of 50℃ compared to the case in which the anionic surfactant was not added.

It could be observed that the redox flow batteries of Examples 1 and 2 showed a normal cell resistance of 35 mΩ or less and the concentration of pentavalent vanadium ions therein was maintained at the same level as that in the redox flow battery of Comparative Example 1 even when these batteries were allowed to stand for 1 month or more.

Test Example 2

For the redox flow battery manufactured in Example 3, the continuous charge and discharge cycles of the battery were performed in a constant current mode at a current density of 80 mA/cm², at a full cell voltage of 0.8-1.7 V and at a temperature of 50℃. The battery efficiencies (coulombic efficiency, voltage efficiency and energy efficiency) during the charge and discharge cycles were calculated based on the data recorded by an Arbin charge/discharge unit (model: BT-2000, manufactured by Arbin Instruments), and the results of the efficiencies as a function of the cycle number are graphically shown in FIG. 3. In addition, a change in the internal pressure of the measuring cylinder was measured by the pressure sensor, and the results of the measurement are graphically shown in FIG. 4. The void volume of the measuring cylinder containing the positive electrolyte solution was about 85 mL.

As can be seen in FIG. 3, the redox flow battery of Example 1 showed excellent battery efficiencies (coulombic efficiency: 95%, voltage efficiency: 89%, and energy efficiency: 84%) during the charge and discharge cycles. The observation of this normal coulombic efficiency demonstrates that the serious loss of electrical energy by the external leakage of chlorine gas did not occur, and may suggest that the anionic surfactant added did not influence the oxidation/reduction and precipitation of vanadium ions.

In addition, as can be seen from the results in FIG. 4, a large amount of chlorine was simultaneously generated very rapidly at the end of 1^st charging, and for this reason, the anionic surfactant did not sufficiently capture chlorine for a short time, and thus the pressure increased to about 40 kPa. However, with the passage of time, equilibrium for chlorine capture was reached, and the SOC slightly decreased due to the generation of chlorine, and thus the generation of chlorine gas decreased compared to that in the initial stage. It was observed that the gas pressure that appeared during charge and discharge cycles was maintained at a level similar to the pressure (10 kPa) of the aqueous hydrochloric acid solution itself, suggesting that the conventional safety problem of the system by the generation of chlorine gas was significantly alleviated.

Test Example 3

Each of the redox flow batteries manufactured in Examples 1, 2 and 4 to 6 and Comparative Examples 1 to 3 was charged with a constant current at a current density of 150 mA/cm²until the state of charge (SOC) of vanadium ions reached 100%. After completion of the charge, the electrolyte solution in the cell were completely drawn into the measuring cylinders, and then the operation of the pumps was stopped, and the pressure of gas in the positive electrolyte solution-containing measuring cylinder at 50℃ was measured by the pressure sensor for 15 hours, and the maximum gas pressure in the initial stage within 2.5 hours after the start of the charge and the gas pressure in the equilibrium state after 10 hours are recorded in Table 1 below. The void volume of the measuring cylinder containing the positive electrolyte solution was about 35 mL.

Table 1

	Surfactant	Ion exchange membrane	Maximum gas pressure in initial stage after start of charge (kPa, within 2.5 hours, 50℃)	Gas pressure in equilibrium state (kPa, after 10 hours, 50℃)
Comparative Example 1	Not added	N115	105	58
Example 1	PFOS	N115	57	30
Example 2	PFNA	N115	76	65
Example 4	OPA	N115	78	63

Comparative Example 2	Not added	F-940RF	101	73
Example 5	OPA	F-940RF	91	79

Comparative Example 3	Not added	FAP-450	118	140
Example 6	PFOS	FAP-450	83	121

As can be seen from the results in Table 1 above, the gas pressures in Examples 1, 2 and 4 were significantly lower than that in Comparative Example 1 including the cation exchange membrane. In addition, the gas pressure was significantly lower in Example 5 and Example 6 than in Comparative Example 2 and in Comparative Example 3, respectively being compared to the case using the same type of the cation or the anion exchange membrane. This suggests that various anionic surfactants effectively acted to capture chlorine, and were also effectively applied to both the cation exchange membrane and the anion exchange membrane.

Although the preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

A redox flow battery which is capable of delivering charge/discharge cycles while supplying a positive electrolyte solution and a negative electrolyte solution to a battery cell,

wherein the battery cell includes an ion exchange membrane,

each of the positive electrolyte solution and the negative electrolyte solution contains an active material that includes anions, including chloride ions, and vanadium ions, and

the positive electrolyte solution contains an anionic surfactant containing both a hydrophilic group and a hydrophobic group.
The redox flow battery of claim 1, wherein the anionic surfactant contains the hydrophobic group of a C4-C15 straight or branched chain alkyl group, unsubstituted or substituted where at least one hydrogen atom is substituted with fluorine, and the hydrophilic group of at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, and combinations thereof, or a salt thereof.
The redox flow battery of claim 1, wherein the anionic surfactant includes at least one selected from the group consisting of a fatty acid represented by RCOO^-M⁺, a monoalkylsulfonic acid represented by RSO₃ ^-M⁺, a monoalkylphosphonic acid represented by RPO(OH)O^-M⁺, and combinations thereof, wherein R is a fluorocarbon group obtained by substituting all hydrogen atoms in a C6-C12 straight or branched chain alkyl group with fluorines, and M is H, Na or K.
The redox flow battery of claim 1, wherein a concentration of the anionic surfactant in the positive electrolyte solution ranges from 1 mM to 30 mM.
The redox flow battery of claim 1, wherein the anionic surfactant forms a micelle in the positive electrolyte solution such that the hydrophobic group is oriented toward an inside of the micelle, and the hydrophilic group is oriented toward an outside of the micelle, and the micelle captures chlorine therein.
The redox flow battery of claim 1, which is fully charged by supplying a voltage of greater than 1.25 V but not greater than 1.85 V to a positive electrode to which the positive electrolyte solution is supplied.
The redox flow battery of claim 1, wherein the ion exchange membrane is a cation exchange membrane or an anion exchange membrane.
The redox flow battery of claim 1, wherein the positive electrolyte solution contains an acid as an electrolyte, and the acid includes an aqueous hydrochloric acid solution alone or a mixture of an aqueous hydrochloric acid solution and an aqueous strong acid solution different from the aqueous hydrochloric acid solution.
The redox flow battery of claim 1, wherein each of the positive electrolyte solution and the negative electrolyte solution further contains sulfate ions.
The redox flow battery of claim 1, wherein a concentration of chlorine in the positive electrolyte solution ranges from 0.1 mM to 100 mM.