TW201931658A - Electrolytic solution and redox flow battery - Google Patents

Abstract

An electrolytic solution which contains alkali metal ions and vanadium ions, and the total concentration of the alkali metal ions is 0.3 M to 2.0 M.

Description

Electrolyte and redox flow battery

The present invention relates to an electrolytic solution and a redox flow battery including the same. The present application claims priority from Japanese Patent Application No. 2017-242312, filed on Jan.

As a large-capacity storage battery, a redox flow battery is known. In general, a redox flow battery includes a positive electrode chamber including a positive electrode, a negative electrode chamber including a negative electrode, and a separator composed of an ion exchange membrane sandwiched between the two electrode chambers. Generally, in a redox flow battery, an electrolyte solution is supplied to the two electrode chambers to charge and discharge. Hereinafter, in the present specification, the electrolytic solution supplied to the positive electrode chamber is referred to as a positive electrode electrolyte, and the electrolyte supplied to the negative electrode chamber is referred to as a negative electrode electrolyte. In the redox flow battery, an aqueous solution-based electrolytic solution containing a metal ion having a valence change by redox is generally used as an active material. For example, an iron-chromium (Fe-Cr) redox flow battery using a positive electrode electrolyte containing iron ions and a negative electrode electrolyte containing chromium ions, a positive electrode electrolyte containing manganese ions, and a titanium ion containing titanium ion may be used. A manganese-titanium (Mn-Ti) redox flow battery of a negative electrode electrolyte, a vanadium-based (VV) redox flow battery using a positive electrode electrolyte containing vanadium ions and a negative electrode electrolyte. In particular, the development of an all-vanadium-based (V-V) redox flow battery has been widely carried out in the world.

Further, in the all-vanadium redox flow battery, the following reaction occurs in the positive electrode chamber (positive electrode) and the negative electrode chamber (negative electrode) during charge and discharge.
Positive electrode: VO ²⁺ + H ₂ O → VO ₂ ⁺ + 2H ⁺ +e ^- (charge)
VO ²⁺ +H ₂ O←VO ₂ ⁺ +2H ⁺ +e ^- (discharge)
Negative electrode: V ³⁺ +e ^- →V ²⁺ (charging)
V ³⁺ +e ^- ←V ²⁺ (discharge)

However, in the case of a redox flow battery, when it is repeatedly charged and discharged, a phenomenon of crossover occurs. The span refers to a phenomenon in which an active material (particularly a metal ion) and a solvent move through the separator to the positive electrode chamber and the negative electrode chamber. The active material in the positive electrode chamber is mixed with the active material in the negative electrode chamber, and the concentration of the active material and the amount of the electrolytic solution become unbalanced between the positive electrode chamber and the negative electrode chamber. That is, the capacitance of the redox flow battery is significantly reduced due to the span. In the current situation, it is difficult to prevent the crossing by only the ion exchange membrane widely used as a separator, so various attempts have been made.

According to Patent Document 1, the redox flow battery described in the document includes an organic radical positive/negative active material having a higher electric power than vanadium ions, and an ion exchange membrane that does not pass through the pore size of the positive and negative active material. And suppress the occurrence of the leap. However, this method is not applicable to a redox flow battery using metal ions as an active material.

Patent Document 2 discloses "an electrolyte-distributing battery comprising: a positive electrode and a negative electrode separated by a separator, a positive electrode and a negative electrode contained in each cell, and a positive electrode for introducing and discharging a positive electrode electrolyte into a positive electrode; a tank, a negative electrode tank for introducing and discharging a negative electrode electrolyte solution into a negative electrode cell; and an electrolyte flow-through type battery comprising: a connecting pipe connecting the two grooves at a position lower than an electrolyte liquid level in each tank; The valve of the communication pipe, the means for detecting the state of charge, and the means for detecting the state of charge are known as the valve opening and closing mechanism that opens the valve when the state of charge of the battery is lower than the predetermined state, and makes the amount of the electrolyte in both tanks equal. "." This electrolyte flow-through type battery implies that the amount of the electrolyte is rebalanced, but the equipment is complicated and the cost is increased.
[Previous Technical Literature]
[Patent Literature]

[Patent Document 1] Japanese Laid-Open Patent Publication No. 2017-117752
[Patent Document 2] Japanese Patent Laid-Open No. Hei 11-204124

[Problems to be solved by the invention]

In view of the above problems, an object of the present invention is to provide an electrolytic solution capable of suppressing a decrease in capacitance due to crossover and a redox flow battery having the same.

[Means for solving the problem]

The inventors of the present invention repeatedly conducted a keen review in order to solve the above problems. As a result, it has been found that the use of an electrolyte containing an alkali metal ion and a vanadium ion in a redox flow battery, suppressing a decrease in the capacity of the redox flow battery caused by the crossing of the vanadium ion, and improving the cycle life of the battery, thereby completing the present invention. .

The present invention includes the following inventions [1] to [8].
[1] The electrolytic solution according to the first aspect of the present invention contains an alkali metal ion and a vanadium ion, and the total concentration of the alkali metal ion is 0.3 M to 2.0 M.
[2] The electrolytic solution according to [1] above, wherein the alkali metal ion may be at least one selected from the group consisting of sodium ions and potassium ions.
[3] The electrolyte of the above [1] or [2], wherein the concentration of the vanadium ion may be 1.0 M to 4.0 M.
[4] The electrolyte solution of the above [1] to [3] may further contain a sulfate ion.
[5] The electrolyte solution according to the above [1] to [4], wherein the concentration of the sulfate ion may be 1.0 M to 10.0 M.
[6] The electrolyte solution according to the above [1] to [5] may further contain at least one anion selected from the group consisting of fluoride ions, chloride ions, bromide ions, and phosphate ions.
[7] The electrolyte of the above [1] to [6], wherein the anion concentration may be 0.01 M to 2.0 M.
[8] A redox flow battery according to a second aspect of the present invention, comprising the electrolyte solution of the above [1] to [7].

[Effects of the Invention]

According to the present invention, it is possible to provide an electrolytic solution capable of suppressing a decrease in capacitance when used in a redox flow battery, and a redox flow battery having the same.

Hereinafter, preferred embodiments for carrying out the embodiment of the present invention will be described in detail. In addition, the present invention is not limited to the following embodiments, and can be appropriately modified and implemented within the scope of achieving the effects. For example, the dimensions, numerical values, quantities, ratios, characteristics, and the like of the materials may be omitted or added or changed within the scope of the gist of the invention.

[electrolyte]
The electrolytic solution according to this embodiment contains an alkali metal ion and a vanadium ion, and the total concentration of the alkali metal ion is 0.3 M to 2.0 M. Here, M expressed as a unit of concentration is a volume molar concentration, that is, Mohr/L (mol/L). The following representations are also the same. For example, in the present specification, 0.3 M represents 0.3 mol/L. Further, the electrolytic solution relating to the present embodiment can be preferably used as an electrolytic solution for a redox flow battery as described later.

[alkali metal ion]
The alkali metal ions in the electrolytic solution are not limited thereto, and may be, for example, lithium ions (Li ⁺ ), sodium ions (Na ⁺ ), potassium ions (K ⁺ ), cesium ions (Rb ⁺ ), and cesium ions. (Cs ⁺ ) at least one of the choices. Among them, sodium ions (Na ⁺ ) and potassium ions (K ⁺ ) are preferred from the viewpoint of suppressing cost, and sodium ions (Na ⁺ ) are more preferable. When such an electrolytic solution is used for a redox flow battery, it is possible to suppress a decrease in battery capacity caused by a span of vanadium ions in the electrolytic solution. Further, from the viewpoint of suppressing an increase in cell resistance, the total concentration of alkali metal ions in the electrolytic solution is from 0.3 M to 2.0 M, preferably from 0.3 M to 1.5 M, more preferably from 0.3 M to 1.0 M, and furthermore Good is 0.4M ~ 0.8M. The raw material which causes the alkali metal ion to be dissolved in the electrolytic solution is not particularly limited, and it is preferable to use a salt containing the alkali metal ion safely. From the viewpoint of stability of the electrolytic solution and improvement of ion conductivity, a halogen salt, a sulfate salt, or a mixture of a halogen salt and a sulfate salt of the above metal ion is particularly preferable.

[Vanadium ion]
The vanadium ion in the electrolyte solution is at least a divalent vanadium ion (V ²⁺ ), a trivalent vanadium ion (V ³⁺ ), a tetravalent vanadium ion (VO ²⁺ ), and a pentavalent vanadium ion (VO ₂ ⁺ ). One. That is, for example, a combination of a trivalent vanadium ion (V ³⁺ ) and a tetravalent vanadium ion (VO ²⁺ ) may be used. As the raw material which is dissolved in the electrolytic solution to produce such vanadium ions, a vanadium salt which can be dissolved in water or an acidic aqueous solution is preferred. From the viewpoint of high solubility in water, vanadium oxysulfate (VOSO ₄ ) is more preferable. The total concentration of vanadium ions in the electrolytic solution is preferably from 1.0 M to 4.0 M. Within this range, the energy density is ensured and the precipitation of vanadium is suppressed. More preferably, it is 1.0 M to 3.0 M, and particularly preferably 1.0 M to 2.5 M. For example, the total concentration of vanadium ions may be in the range of 1.0 M to 1.5 M, 1.5 M to 2.0 M, 1.0 M to 2.0 M, or 2.0 M to 3.0 M, as needed. As a specific example, it can also be 1.8M.

[sulfate ion]
The electrolyte of this embodiment preferably contains a sulfate ion (SO ₄ ^2- ). There is a moderate amount of sulfate ions, and vanadium ions tend to dissolve more stably. The concentration of the sulfate ion for the stabilization of the vanadium ion is preferably 1.0 M to 10.0 M, more preferably 1.0 M to 8.0 M, and still more preferably 2.0 M to 6.0 M. Further, for example, it may be 3.0 M to 6.0 M, 4.0 M to 6.0 M, 4.0 M to 5.5 M, or 4.3 M to 5.3 M. As a raw material which melt|dissolved in electrolyte solution and generate|occur|produces a sulfate ion, the sulfate of a sulfuric acid or a vana

[anion other than sulfate ion]
The electrolyte according to the present embodiment further contains a group consisting of fluoride ions (F ^- ), chloride ions (Cl ^- ), bromide ions (Br ^- ), and phosphate ions (PO ₄ ^3- ) At least one anion selected is preferred. Among them, it is more preferable to contain a chloride ion (Cl ^- ). It should be related to the electrolytic solution of the present embodiment, and by further containing an appropriate amount of the anion, the ionic conductivity of the electrolytic solution or the reactivity of the metal ion becomes high. Therefore, the redox flow battery having the electrolytic solution containing the anion described above has a small internal resistance of the battery. Further, the effect of improving the solubility of vanadium ions in the electrolytic solution was also obtained. The total anion concentration is preferably from 0.01 M to 2.0 M, more preferably from 0.1 M to 1.5 M, still more preferably from 0.1 M to 1.0 M. The raw material which is dissolved in the electrolytic solution to produce the anion described above is preferably an acid or a vanadium salt containing the anion. Further, the electrolytic solution according to the present embodiment may not necessarily have an anion composition. That is, the total anion concentration may be 0.00M.

[redox flow battery]
A redox flow battery according to this embodiment is characterized by containing the aforementioned electrolyte. The redox flow battery of the present invention may have a known configuration. For example, a redox flow battery as in the preferred embodiment shown in FIG. 4 may be used. Fig. 4 is a schematic view showing a redox flow battery used in an experiment relating to an embodiment to be described later, and is an example of a redox flow battery according to the present embodiment. The redox flow battery shown in FIG. 4 includes a battery cell 2, a positive electrode electrolyte tank 12, a positive electrode outward pipe 13, a positive electrode return pipe 14, a negative electrode electrolyte tank 22, a negative electrode outward pipe 23, and a negative electrode return pipe 24, And pumps 15 and 25. The positive electrode solution tank 12, the positive electrode outflow pipe 13, the positive electrode return pipe 14, the negative electrode electrolyte tank 22, the negative electrode outward pipe 23, the negative electrode return pipe 24, and the pumps 15 and 25 can be suitably applied to a known redox flow. Battery. The battery cell 2 has a positive electrode cell 11, a negative electrode cell 21, and a separator separating the positive electrode cell 11 and the negative electrode cell 21. The positive electrode cell 11 and the negative electrode cell 21 have a positive electrode 10 and a negative electrode 20, respectively. Further, inside the positive electrode cell 11 and the negative electrode cell 21, the electrolytic solution related to the above embodiment is circulated. As the positive electrode 10 and the negative electrode 20, an electrode applied to a known redox flow battery can be used.

The electrolytic solution is accumulated in the positive electrode solution tank 12, and is supplied to the positive electrode cell 11 through the positive electrode outward pipe 13 by the pump 15. The flow rate of the electrolytic solution supplied to the positive electrode cell 11 can be appropriately selected so as to sufficiently supply the active material in an amount necessary to obtain a desired output. Further, after the electrolyte solution is charged and discharged in the positive electrode cell 11, it is returned to the positive electrode solution tank 12 through the positive electrode outward pipe 14, and is again circulated in the same path. The amount of discharge of the electrolytic solution from the positive electrode cell 11 may be substantially the same as the amount of supply into the positive electrode cell 11.
The electrolyte solution is also subjected to the same cycle as the circulation of the positive electrode cells 11 for the negative electrode cells 21. That is, the electrolytic solution is accumulated in the negative electrode electrolyte tank 22, and is supplied to the negative electrode cell 21 through the negative electrode outward pipe 23 by the pump 25. The flow rate of the electrolytic solution supplied to the negative electrode cell 21 can be appropriately selected so as to sufficiently supply the active material in an amount necessary to obtain a desired output. Further, after the electrolyte solution is charged and discharged in the negative electrode cell 21, it is returned to the negative electrode electrolyte tank 22 through the negative electrode outward pipe 24, and is again circulated in the same path.

The electrolyte supplied to the battery cells 2 contributes to the reaction when the positive electrode 10 and the negative electrode 20 are charged and discharged. Charging and discharging utilizes electric power from an external power generation unit (not shown). The electric power is supplied from the power generation unit, and is supplied to the positive electrode 11 and the negative electrode 21 through an external AC/DC converter (not shown).
A well-known ion exchange membrane can be used to separate the separator 30 of the positive electrode cell 11 and the negative electrode cell 21.

According to the redox flow battery of the present embodiment, with this configuration, it is possible to suppress a decrease in capacitance due to the straddle, and it is possible to improve the cycle life.

[Examples]
Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[Example 1]
(modulation of electrolyte)
In 100 ml of sulfuric acid (H ₂ SO ₄ ) concentration 4.0 M sulfuric acid aqueous solution, 0.03 mol of sodium sulfate (Na ₂ SO ₄ ), 0.09 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ), 0.18 mol of vanadium oxysulfate were added. (VOSO ₄ ), 200 ml of an electrolytic solution was prepared by adding pure water and stirring so that the volume of the solution became 200 ml.

(Measurement of charge and discharge characteristics)
Figure 4 is a schematic view of a redox flow battery used in the experiment. As the battery cell 2, a carbon fiber felt (AAF304ZS) manufactured by Toyobo Co., Ltd. having an area of 50 cm ² (5 cm × 10 cm) was used as the positive electrode 10 and the negative electrode 20, and Nafion (trade name) 212 was used as an ion exchange membrane. 50 ml of the electrolytic solution prepared above was prepared as a positive electrode electrolyte solution and a negative electrode electrolyte solution, and the electrolyte solution was circulated at a flow rate of 50 ml/min while a current of 10 A was applied to the positive electrode cell 11 and the negative electrode cell 21 (current density: 0.2 A/cm ^{2 ).} ) Charge and discharge. First, charging was performed. When the voltage was 1.75 V, the charging was stopped, and then discharging was performed. When the voltage was 1.0 V, the discharge was terminated. Further, this charge and discharge was repeated for 49 cycles (total of 50 cycles), and the charging time (h), the discharge time (h), and the cell voltage (V) during charge and discharge were measured for each cycle. The cell voltage at the time point of half of the charging time is V ₁ , and the cell voltage at the time point of half of the discharging time is V ₂ . Next, as shown in the following formula, the Coulomb efficiency (%), the cell resistance (Ω·cm ² ) of the 10th cycle (cyc10), and the discharge capacity of the 10th cycle (cyc10) and the 50th cycle (cyc50) were determined. Reduction rate (hereinafter referred to as discharge capacity reduction rate).
・Charging capacity (Ah) = charging current × charging time / discharge capacity (Ah) = discharge current × discharge time / Coulomb efficiency (%) = ([discharge capacity] / [charge capacity]) × 100
・Cell resistance (Ω·cm ² )=(V ₁ -V ₂ )/(2× current density)
・Discharge capacity reduction rate (%) = (1 - [discharge capacity (cyc50)] / [discharge capacity (cyc10)]) × 100

[Examples 2 to 5, Comparative Examples 1, 2]
A 200 ml electrolytic solution was prepared in the same manner as in Example 1 except that the amount of sodium sulfate added was as shown in Table 1, and the charge and discharge characteristics were measured.

[Embodiment 6]
100 ml of a sulfuric acid aqueous solution having a sulfuric acid (H ₂ SO ₄ ) concentration of 4.0 M was mixed with 10 ml of a hydrochloric acid aqueous solution having a hydrochloric acid concentration of 10.0 M to obtain a mixed aqueous solution. To the above mixed aqueous solution, 0.06 mol of sodium sulfate (Na ₂ SO ₄ ), 0.09 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ), and 0.18 mol of vanadium oxysulfate (VOSO ₄ ) were added to make the solution volume 200 ml. The method was carried out by adding pure water and stirring to prepare 200 ml of an electrolytic solution. Next, the measurement of the charge and discharge characteristics was carried out in the same manner as in Example 1.

In the above examples and comparative examples, the amounts of the raw materials used to prepare the electrolytic solution were summarized in Table 1. The measurement results of the composition and charge and discharge characteristics of the electrolytic solutions of the respective examples and comparative examples are shown in Table 2.

As is clear from Table 2 and Figs. 1 and 2, when the sodium ion-containing sodium ion is 0.3 M in the electrolytic solution, the discharge capacity reduction rate is sufficiently low, and when the sodium ion concentration in the electrolytic solution is increased, the rate of decrease in discharge capacity is changed. Lower, in addition to Coulomb efficiency. Thus, it was confirmed that when an electrolytic solution containing vanadium ions is further contained and an appropriate amount of alkali metal ions is contained, crossover of the electrolytic solution containing vanadium ions can be suppressed. On the other hand, as shown in FIG. 3, it is understood that the amount of sodium ions in the electrolytic solution increases, and the increase in the cell resistance is also observed, so that the power consumption due to the cell resistance becomes high. It is understood that the concentration of sodium ions is from 0.3 M to 2.0 M, particularly preferably from 0.3 M to 1.0 M, from the viewpoint of both suppression of suppression and suppression of increase in cell resistance.
Comparative Examples 1 and 2, which did not contain an alkali metal ion or an amount of alkali metal ions, were inferior in characteristics as compared with the examples.

In Example 6, it was estimated that the anion other than the sulfate ion and the chloride ion were added, and the ionic conductivity of the electrolytic solution and the reactivity of the vanadium ion were improved. Therefore, as compared with Example 2, it is possible to suppress a decrease in discharge capacity due to the crossing of vanadium ions, and at the same time, to lower the cell resistance. Such an electrolytic solution can be suitably used in a redox flow battery having a high current density (charge and discharge current density of 100 mA/cm ² or more).

[Industry use possibility]

The redox flow battery of the present invention may be used for power storage such as a power generation station or a substation, and may be used for a reduction in electricity cost or an instantaneous pressure drop.

1‧‧‧Redox flow battery

2‧‧‧ battery cell

10‧‧‧ positive electrode

11‧‧‧ positive electrode

12‧‧‧ positive electrolyte tank

13‧‧‧Positive forward piping

14‧‧‧Actual return pipe

15‧‧‧ pump

20‧‧‧Negative electrode

21‧‧‧negative cell

22‧‧‧Negative electrolyte tank

23‧‧‧Negative negative travel piping

24‧‧‧Negative return pipe

25‧‧‧ pump

30‧‧‧Separator

Fig. 1 shows the relationship between the sodium ion concentration and the discharge capacity reduction rate in Examples 1 to 5 and Comparative Examples.

2 shows the relationship between the sodium ion concentration and the coulombic efficiency of Examples 1 to 5 and Comparative Examples.

Fig. 3 shows the relationship between the sodium ion concentration and the cell resistance of Examples 1 to 5 and Comparative Examples.

Fig. 4 is a schematic view showing a redox flow battery used in the examples and comparative examples.

Claims (8)

An electrolyte characterized by containing an alkali metal ion and a vanadium ion, and the total concentration of the alkali metal ion is 0.3 M to 2.0 M. The electrolyte of claim 1, wherein the alkali metal ion is at least one selected from the group consisting of sodium ions and potassium ions. The electrolyte according to claim 1 or 2, wherein the concentration of the vanadium ion is 1.0 M to 4.0 M. The electrolyte of any one of claims 1 to 3, which comprises a sulfate ion. The electrolyte of claim 4, wherein the concentration of the sulfate ion is 1.0 M to 10.0 M. The electrolyte solution according to any one of claims 1 to 5, further comprising at least one anion selected from the group consisting of fluoride ions, chloride ions, bromide ions, and phosphate ions. The electrolyte of claim 6, wherein the total anion concentration is 0.01 M to 2.0 M. A redox flow battery comprising the electrolyte of any one of claims 1 to 7.