WO2019078146A1 - Liquid electrolyte and redox flow cell - Google Patents

Abstract

Provided is a liquid electrolyte comprising an aqueous solution that includes tetrafluoroborate ions and vanadium ions.

Description

Electrolyte and redox flow battery

The present invention relates to an electrolyte, and more particularly to an electrolyte for a redox flow battery and a redox flow battery comprising the electrolyte.
Priority is claimed on Japanese Patent Application No. 2017-201097, filed Oct. 17, 2017, the content of which is incorporated herein by reference.

A redox flow battery is known as a large capacity storage battery. Generally, a redox flow battery is composed of a positive electrode chamber provided with a positive electrode, an electrode chamber provided with a negative electrode, and a diaphragm made of an ion exchange membrane sandwiched between these two electrode chambers Supply and charge / discharge. It is common to use an aqueous electrolyte solution containing metal ions whose valence changes by oxidation-reduction as the 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 negative electrode electrolyte containing titanium ions And manganese-titanium-based (Mn-Ti) redox flow batteries; and all vanadium-based (VV) redox flow batteries using positive and negative electrode electrolytes containing vanadium ions. Among them, in particular, development of all-vanadium-based (V-V) redox flow batteries is widely promoted all over the world.

In addition, in the redox flow battery using the electrolyte solution containing vanadium ion, the reaction of vanadium ion is made as follows at the time of charging / discharging.
Positive electrode: VO ₂ ⁺ + H ₂ O → VO ₂ ⁺ + 2H ⁺ + e ⁻ (charge)
VO ²⁺ + H ₂ O ← VO ₂ ⁺ + 2H ⁺ + e ⁻ (discharge)
The negative ^{electrode: V 3+ + e - → V} 2+ ( charging)
^{V 3+ + e - ← V 2+} ( discharge)

However, in the redox flow battery, when charge and discharge are repeated, a phenomenon called active crossover (especially metal ions) and a solvent move through the membrane occurs. The electrical capacity is significantly reduced due to the mixing of the active materials in the positive and negative electrode electrolytes and the imbalance between the concentration of the active materials and the amount of the electrolyte due to the crossover. At present, various devices have been tried because it is difficult to prevent crossover with only an ion exchange membrane widely used as a diaphragm.

According to Patent Document 1, the redox flow battery described in the same document includes an organic radical positive / negative active material having a higher electromotive force than vanadium ions and an ion exchange membrane having a pore diameter which does not pass through the positive / negative active material. It is assumed that the occurrence of over has been suppressed. However, this method is not suitable for redox flow batteries in which metal ions are used as an active material.

In Patent Document 2, “a positive electrode cell and a negative electrode cell separated by a diaphragm, a positive electrode and a negative electrode built in each cell, a positive electrode tank for introducing and discharging an electrolytic solution for the positive electrode to the positive electrode cell, and a negative electrode cell An electrolyte flow type battery comprising a negative electrode tank for introducing and discharging a negative electrode electrolyte, provided in a communicating pipe connecting both tanks at a position lower than the liquid level of the electrolytic solution in each tank, and a communicating pipe Based on the detected valve, the means for detecting the state of charge, and the detection result by means for detecting the state of charge, the valve is opened when the state of charge of the battery is lower than the specified state, and the amount of And a valve opening and closing mechanism. Although this electrolytic solution flow type battery can rebalance the amount of electrolytic solution, the equipment becomes complicated and the cost increases.

JP, 2017-117752, A Japanese Patent Application Laid-Open No. 11-204124

The object of the present invention is made in view of the above-mentioned problems, and is to provide an electrolytic solution and a redox flow battery which can suppress crossover.

As a result of intensive investigations, the inventors of the present invention suppress the reduction of the redox flow battery capacity caused by the crossover of vanadium ions by using an electrolytic solution consisting of an aqueous solution containing tetrafluoroborate ions and vanadium ions. As a result, it has been found that the cycle life of the battery is improved, and the present invention has been completed.

The present invention includes the inventions of the following [1] to [12].
[1] An electrolytic solution comprising an aqueous solution containing tetrafluoroborate ion and vanadium ion.
[2] The electrolytic solution according to the above [1], wherein the concentration of tetrafluoroborate ion is 3 M or less.
[3] The electrolytic solution according to the above [2], wherein a concentration of the tetrafluoroborate ion is 0.1 to 2 M.
[4] The electrolytic solution according to any one of the above [1] to [3], wherein the total concentration of the vanadium ion is 1 M to 4 M.
[5] The electrolyte according to any one of the above [1] to [4], which contains a sulfate ion.
[6] The electrolyte according to [5], wherein the concentration of the sulfate ion is 1 M to 10 M.
[7] Furthermore, it contains anion A,
The electrolytic solution according to any one of the above items [1] to [6], wherein the anion A is at least one selected from the group consisting of a fluorine ion, a chloride ion, a bromide ion, and a phosphate ion.
[8] The electrolytic solution according to the above [7], wherein the concentration of the anion A is 0.01 M to 2 M.
[9] The electrolytic solution according to any one of the above [1] to [8], further containing molybdenum ions.
[10] The electrolytic solution according to the above [9], wherein the concentration of the molybdenum ion is 0.01 M to 3 M.
[11] The electrolyte according to any one of the above-mentioned [1] to [10] for a redox flow battery.
[12] A redox flow battery comprising the electrolytic solution according to any one of the preceding items [1] to [10].

ADVANTAGE OF THE INVENTION According to this invention, the electrolyte solution for redox flow batteries which can suppress crossover, and a redox flow battery can be provided.

FIG. 6 is a view showing the relationship between the tetrafluoroborate ion concentration and the amount of vanadium ion transfer in Examples 1 to 6 and Comparative Example 1. FIG. 6 is a graph showing the relationship between the tetrafluoroborate ion concentration and the coulombic efficiency in Examples 1 to 6 and Comparative Example 1. FIG. 6 is a view showing the relationship between the tetrafluoroborate ion concentration and the discharge capacity reduction rate in Examples 1 to 6 and Comparative Example 1. It is a schematic diagram of the redox flow battery used by the example and the comparative example.

Hereinafter, modes for carrying out the present invention will be described in detail. In addition, this invention is not limited to the following embodiment, It can change and implement within the range of the summary.

[Electrolyte solution]
The electrolytic solution according to the present embodiment is an aqueous solution containing at least tetrafluoroborate ions and vanadium ions.

[Tetrafluoroborate ion]
The concentration of tetrafluoroborate ion (BF ₄ ⁻ ) in the electrolytic solution is preferably 3 M (M is “mol / L”) from both the viewpoint of suppressing crossover and suppressing the increase in cell resistance. And the same applies to the following.), More preferably 0.1 M to 2 M, still more preferably 0.1 M to 1.5 M, and most preferably 0.5 to 1.5 M. As a raw material for generating tetrafluoroborate ion by being dissolved in the electrolytic solution, any material can be selected, but tetrafluoroboric acid, potassium tetrafluoroborate, sodium tetrafluoroborate, tetrafluoroboronate Preferred are lithium acid and the like. Among these, tetrafluoroboric acid is preferable in that metal ions other than the active material are not increased in the electrolytic solution.

[Vanadium ion]
The vanadium ions in the electrolyte solution are divalent vanadium ions (V 2 ⁺ ), trivalent vanadium ions (V ^{3 +} ), tetravalent vanadium ions (VO ₂ ⁺ ), and pentavalent vanadium ions (VO ₂ ⁺ ). At least one kind of. As a raw material which dissolve | melts in the said electrolyte solution and produces | generates such a vanadium ion, although it can select arbitrarily, the vanadium salt which can be melt | dissolved in water or acidic aqueous solution is preferable. Although the kind of vanadium salt can be selected arbitrarily, vanadium oxide sulfate is more preferable from the viewpoint of high solubility in water. The total concentration of vanadium ions in the electrolytic solution is preferably 1 M to 4 M. Within this range, the occurrence of the precipitation of the vanadium compound is suppressed while securing the energy density. More preferably, it is 1.0 M to 3 M, particularly preferably 1.0 M to 2.5 M.
In the electrolytic solution, the ratio of the concentrations of tetrafluoroborate ion and vanadium ion can be arbitrarily selected as required. For example, the ratio represented by tetrafluoroborate ion concentration / vanadium ion concentration is preferably 0.1 to 1.0, and more preferably 0.2 to 0.5. As another example, for example, the lower limit of the range of the ratio of tetrafluoroborate ion concentration / vanadium ion concentration may be 0.001, 0.01, 0.1, 1 or the like. The upper limit of the ratio may be 100, 50, 10, 5, 3, 3 or the like. It can be set arbitrarily according to the required characteristics.

[Sulfate ion]
It is preferable that the electrolytic solution in the present embodiment contains a sulfate ion (SO ₄ ²⁻ ). The presence of sulfate ions tends to dissolve vanadium ions more stably. The concentration of the sulfate ion can be arbitrarily selected, but is preferably 1 M to 10 M, more preferably 1 M to 8 M, still more preferably 2 M to 6 M. As a raw material which dissolves in the electrolytic solution to generate a sulfate ion, it can be selected arbitrarily, and sulfuric acid or a sulfate of vanadium is mentioned, and sulfuric acid is preferable in that the electrolytic solution is kept acidic.
The ratio of the concentration of tetrafluoroborate ion to that of sulfate ion can be arbitrarily selected as required. For example, the ratio represented by tetrafluoroborate ion concentration / sulfate ion concentration is included in the range of 0.01 to 1.0, 0.05 to 0.5, 0.05 to 0.2, etc. Is also preferred. However, it is not limited only to these ranges.
For example, the lower limit of the range of the ratio of tetrafluoroborate ion concentration / sulfate ion concentration may be 0.0001, 0.001, 0.1 or the like. The upper limit of the ratio may be 50, 10, 5, 3, 3 or the like. It can be set arbitrarily according to the required characteristics.

[Anion A]
The electrolytic solution according to the present embodiment is at least one ion selected from the group consisting of fluorine ion (F ⁻ ), chloride ion (Cl ⁻ ), bromine ion (Br ⁻ ), and phosphate ion (PO ₄ ^3- ). It is preferable to further contain (hereinafter sometimes referred to as "anion A"). Among these, chloride ion - and more preferably contains ^(Cl). The inclusion of the anion A is considered to increase the ion conductivity of the electrolytic solution and the reactivity of the metal ions, and hence the internal resistance of the battery decreases. Furthermore, the improvement of the solubility of the vanadium ion in electrolyte solution and the molybdenum ion mentioned later is obtained. The total concentration of the anion A can be arbitrarily selected, but is preferably 0.01 M to 2 M, more preferably 0.1 M to 1.5 M, still more preferably 0.1 M to 1 M. As a raw material which melt | dissolves in the said electrolyte solution and produces the anion A, the acid containing the anion A, a vanadium salt, and a molybdenum salt are preferable.
The ratio of the concentration of tetrafluoroborate ion and anion A can be optionally selected as required. For example, the ratio represented by the concentration of tetrafluoroborate ion / concentration of anion A is preferably in the range of 1 to 1000, 5 to 100, and the like. However, it is not limited only to these ranges.
For example, the lower limit of the range of the ratio of the concentration of tetrafluoroborate ion to the concentration of anion A may be 0.01, 0.01, 0.1, 1 or the like. The upper limit of the above ratio may be 500, 100, 30, 10, 5, 5, 3, or the like. It can be set arbitrarily according to the required characteristics.

[Molybdenum ion]
It is preferable that the electrolyte solution which concerns on this embodiment further contains a molybdenum ion from a viewpoint of improving energy density. During charge and discharge, both vanadium ions and molybdenum ions are oxidized and reduced as active materials. The concentration of molybdenum ion is preferably 0.01 M to 3 M, more preferably 0.01 M to 2 M, and still more preferably 0.1 M to 2 M. Within this concentration range, the solubility of vanadium ions does not decrease, and water decomposition as a side reaction hardly occurs, and as a result, the energy density can be improved. The reaction equation when the molybdenum ion acts as an active material is estimated as follows.
The positive electrode: Mo (V) → Mo ( VI) + e - ( charging)
Mo (V) ← Mo (VI ) + e - ( discharge)
^{Negative: Mo (V) + 2e -} → Mo (III) ( charging)
^{Mo (V) + 2e - ←} Mo (III) ( discharge)

The raw material which melt | dissolves in the said electrolyte solution and produces a molybdenum ion can be selected arbitrarily, and if it is a molybdenum salt, it will not be specifically limited. However, salts containing trivalent to hexavalent molybdenum ions, or molybdenum oxides are preferred. Specifically, halogen salts such as MoCl ₃ , MoCl ₅ , MoO ₂ Cl, MoO ₂ OHCl, MoBr ₃ , MoO ₂ Br, MoO ₂ OHBr, etc .; Mo (SO ₄ ) _3/2 , MoO ₂ HSO ₄ , MoO ₂ Sulfates such as (SO ₄ ) _1/2 , MoO ₂ ClSO ₄ , MoO ₂ OH (SO ₄ ) _1/2 , phosphates such as MoPO ₄ , nitrates such as MoO ₂ NO ₃ , and molybdenum oxide Be Among these, from the viewpoint of solubility, the above-mentioned hydrochloride and molybdenum oxide are more preferable from the viewpoint of not increasing the anion.
The ratio of the concentration of tetrafluoroborate ion to that of molybdenum ion can be arbitrarily selected as required. For example, the ratio represented by tetrafluoroborate ion concentration / molybdenum ion concentration is preferably in the range of 0.1 to 100, 0.3 to 50, 0.5 to 20, and the like. However, it is not limited only to these ranges.
For example, the lower limit of the range of the ratio of tetrafluoroborate ion concentration / molybdenum ion concentration may be 0.01, 0.01, 0.1, 1 or the like. The upper limit of the above ratio may be 500, 100, 30, 10, 5, 5, 3, or the like. It can be set arbitrarily according to the required characteristics.
The electrolytic solution of the present invention can be preferably formed by mixing the above-mentioned material with water such as pure water.
The electrolytic solution of the present invention comprises (a) a raw material for producing tetrafluoroborate ions, (b) pure water, (c) a raw material for producing vanadium ions, and (d) a raw material for producing sulfate ions, an anion It is preferable to include at least one selected from the group consisting of a raw material for producing A and a raw material for producing molybdenum ions. Moreover, it is also preferable that the electrolyte solution of this invention consists only of said (a) to (c), or consists only of said (a) to (d).

[Redox flow battery]
The redox flow battery according to the present invention is characterized by containing the electrolyte. The redox flow battery of the present invention can adopt a known configuration.

EXAMPLES The present invention will be more specifically described below based on examples, but the present invention is not limited to these examples.

Example 1
(Preparation of electrolyte)
A mixed aqueous solution was prepared by mixing 100 ml of a sulfuric acid aqueous solution containing 4 M (mol / L) of sulfuric acid (H ₂ SO ₄ ) and 0.4 ml of an aqueous tetrafluoroboric acid solution containing 5 M of tetrafluoroboric acid (HBF ₄ ). Obtained. 0.12 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ) and 0.12 mol of vanadium oxide sulfate (VOSO ₄ ) are dissolved in the mixed aqueous solution so that the volume of the solution is 200 ml. Was added to make 200 ml of an electrolytic solution. The respective ion concentrations in the electrolytic solution are described in Table 2.

(Measurement of charge and discharge characteristics)
The schematic diagram of the redox flow battery used for experiment is shown in FIG. The battery cell 2 used a carbon felt (AAF 304 ZS) manufactured by Toyobo Co., Ltd. with an area of 50 cm ² (5 cm × 10 cm) as the positive electrode 10 and the negative electrode 20, and Nafion (trademark) 212 as an ion exchange membrane. 50 ml of each of the prepared electrolytes was prepared as a positive electrode electrolyte and a negative electrode electrolyte. While circulating the electrolytic solution at a flow rate of 50 ml / min to the positive electrode cell 11 and the negative electrode cell 21, charging / discharging was performed at a current of 10 A (current density 0.2 A / cm ² ). First, charging was performed, charging was stopped when the voltage reached 1.75 V, and then discharging was performed, and discharging was finished when the voltage reached 1.0 V. This charge / discharge was repeated for 7 more cycles (8 cycles in total). The charge time (h), discharge time (h), and cell voltage (V) during charge and discharge of each cycle were measured. The cell voltage at the half of the charging time is V ₁ , and the cell voltage at the half of the discharging time is V ₂ . Then, the charge capacity (Ah), discharge capacity (Ah), coulombic efficiency (%), cell resistance (Ω · cm ² ) of the fourth cycle (cyc 4), discharge capacity (Ah) of the eighth cycle, and The discharge capacity reduction rates of the fourth cycle (cyc4) and the eighth cycle (cyc8) were determined as follows. The determined values are described in Table 2.
· Charge capacity (Ah) = charge current × charge 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 (cyc8)] / [discharge capacity (cyc4))) × 100

(Measurement of vanadium ion transfer amount)
In the measurement of the vanadium ion transfer amount, the same cell as the charge and discharge experiment was used. 50 ml each of the prepared electrolytic solution and a sulfuric acid aqueous solution (3.2 M) were prepared. The electrolytic solution was circulated to the positive electrode cell 11 and the sulfuric acid aqueous solution was circulated to the negative electrode cell 21 at a flow rate of 50 ml / min for 2 h. After circulating for 2 h, the number of vanadium ions transferred from the positive electrode cell 11 through the ion exchange membrane to the negative electrode cell 21 was determined using a UV-vis spectrophotometer (UV-1700 manufactured by SHIMADZU). The vanadium ion concentration (hereinafter referred to as "vanadium ion transfer amount") in the aqueous solution was measured. The determined values are described in Table 2.

[Examples 2 to 6]
An electrolyte was prepared in the same manner as in Example 1 except that the amount of the tetrafluoroboric acid aqueous solution was as shown in Table 1, and the measurement of charge / discharge characteristics and the measurement of vanadium ion transfer amount as in Example 1 Carried out.

[Example 7]
Mix 100ml of sulfuric acid aqueous solution with 4M sulfuric acid (H ₂ SO ₄ ) concentration, 40ml aqueous solution of tetrafluoroboric acid with tetrafluoroboric acid (HBF ₄ ) concentration of 5M and 8.3ml of 12M hydrochloric acid aqueous solution The mixed aqueous solution was obtained. 0.12 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ) and 0.12 mol of vanadium oxide sulfate (VOSO ₄ ) are dissolved in the mixed aqueous solution so that the volume of the solution is 200 ml. Was added to make an electrolyte. Measurement of charge and discharge characteristics and measurement of vanadium ion transfer amount were performed in the same manner as in Example 1 except for the above.

[Example 8]
An electrolyte is prepared in the same manner as in Example 7 except that 10 ml of a phosphoric acid aqueous solution having a phosphoric acid concentration of 2 M is used instead of a 12 M hydrochloric acid aqueous solution, and measurement of charge / discharge characteristics and vanadium ion transfer amount Measurement was conducted.

[Example 9]
A mixed aqueous solution was obtained by mixing 100 ml of a sulfuric acid aqueous solution having a sulfuric acid (H ₂ SO ₄ ) concentration of 4 M and ₄₀ ml of an aqueous tetrafluoroboric acid solution having a tetrafluoroboric acid (HBF ₄ ) concentration of 5 M. To this mixed aqueous solution, 0.03 mol of molybdenum oxide (MoO ₃ ) was added, and stirred at room temperature using a stirrer for 48 hours to obtain a solution containing molybdenum ions (Mo ⁶⁺ ). Thereafter, 0.12 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ) and 0.12 mol of vanadium oxide sulfate (VOSO ₄ ) are dissolved in the solution containing the molybdenum ion to make the solution volume 200 ml. As such, pure water was added to prepare an electrolyte. Measurement of charge and discharge characteristics was carried out in the same manner as in Example 1 except for the above.

Comparative Example 1
Dissolve 0.12 mol of vanadium sulfate (V ₂ (SO ₄ ) ₃ ) and 0.12 mol of vanadium oxide sulfate (VOSO ₄ ) in 100 ml of an aqueous solution of sulfuric acid (H ₂ SO ₄ ) having a concentration of 4 M Water was added to make 200 ml of an electrolyte. Measurement of charge and discharge characteristics and measurement of vanadium ion transfer amount were performed in the same manner as in Example 1 except for the above.

Table 1 summarizes the amounts of raw materials used to make the electrolyte in the above Examples and Comparative Examples. The concentration of each ion contained in the electrolytic solution of each example and comparative example, the measurement result of vanadium ion transfer amount, and the measurement result of charge and discharge characteristics are shown in Table 2.

As shown in Comparative Example 1 and Examples 1 to 6 of Table 2 and FIGS. 1 to 3, when the electrolyte solution contains a slight amount of tetrafluoroborate ion (BF ₄ ⁻ ), the following results are obtained. Was found to be That is, the vanadium ion transfer amount and the discharge capacity reduction rate rapidly decrease, and the concentration of tetrafluoroborate ions in the electrolytic solution increases, and the vanadium ion transfer amount and the discharge capacity reduction rate further decrease, and The coulomb efficiency was found to be high. Therefore, it was confirmed that the crossover of electrolyte solution can be suppressed by containing tetrafluoroborate ion. It is to be noted that the cell resistance increased in Examples 2 to 6 (1% in Examples 2 to 4, 3% in Example 5, and 6% in Example 6) relative to Comparative Example 1. However, the coulombic efficiency and discharge capacity of all the examples were nevertheless improved. Therefore, if the concentration of tetrafluoroborate ion is 3 M or less, it is considered that the cell performance does not deteriorate.

In Examples 7 to 8, it is presumed that the ion conductivity of the electrolytic solution and the reactivity of the vanadium ion are improved by further including the anion A in addition to the tetrafluoroborate ion (BF ₄ ⁻ ). Therefore, as compared with Comparative Example 1, the crossover of vanadium ions was suppressed, and the cell resistance could be reduced. Such an electrolytic solution can be suitably used particularly for high current density (charge / discharge current density is 100 mA / cm ² or more) redox flow battery.

In Example 9, the discharge capacity at the fourth cycle was 1.30 Ah. The discharge capacity was improved by 10% and 7%, respectively, relative to Comparative Example 1 and Example 4. It was confirmed that the battery capacity was improved by containing molybdenum ions.

It is possible to provide an electrolyte and a redox flow battery capable of suppressing crossover. In the redox flow battery of the present invention, crossover of vanadium ions in the electrolytic solution can be suppressed by using an electrolytic solution comprising an aqueous solution containing at least tetrafluoroborate ions and vanadium ions.

Reference Signs List 1 redox flow battery 2 battery cell 3 AC / DC converter 4 AC power supply 5 load power supply 10 positive electrode 11 positive electrode cell 12 positive electrode electrolyte tank 13 positive outgoing pipe 14 positive return pipe 15 pump 20 negative electrode 21 negative electrode 22 negative electrolyte Tank 23 negative going forward piping 24 negative return piping 25 pump 30 diaphragm

Claims (12)

An electrolytic solution comprising an aqueous solution containing tetrafluoroborate ions and vanadium ions. The electrolytic solution according to claim 1, wherein a concentration of the tetrafluoroborate ion is 3 M or less. The electrolytic solution according to claim 2, wherein a concentration of the tetrafluoroborate ion is 0.1 to 2M. The electrolytic solution according to any one of claims 1 to 3, wherein the total concentration of the vanadium ion is 1M to 4M. The electrolytic solution according to any one of claims 1 to 4, which contains a sulfate ion. The electrolytic solution according to claim 5, wherein the concentration of the sulfate ion is 1 M to 10 M. Furthermore, it contains anion A,
The electrolytic solution according to any one of claims 1 to 6, wherein the anion A is at least one selected from the group consisting of a fluorine ion, a chlorine ion, a bromine ion, and a phosphate ion. The electrolytic solution according to claim 7, wherein the concentration of the anion A is 0.01M to 2M. The electrolytic solution according to any one of claims 1 to 8, further comprising molybdenum ions. The electrolytic solution according to claim 9, wherein the concentration of the molybdenum ion is 0.01 M to 3 M. The electrolyte according to any one of claims 1 to 10 for a redox flow battery. A redox flow battery comprising the electrolytic solution according to any one of claims 1 to 10.

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