JP6589592B2 - Treatment method of ion exchange membrane for ionic liquid - Google Patents

Description

The present invention relates to a process how ionic liquids for ion-exchange membrane, in particular, suitable for use as a separator for a redox flow battery, ionic for the ionic liquid to allow use of an ion exchange membrane with respect to the ionic liquid about the process how the exchange membrane.

  Secondary batteries are attracting attention as energy storage sources with a low environmental load that can repeatedly charge and discharge electricity. Known secondary batteries for industrial use include lead storage batteries, sodium sulfur (NAS) batteries, lithium ion batteries, redox flow batteries, and the like. Among these, a redox flow battery that stores an active material solution in an external tank or the like, and supplies and charges and discharges it to a flow-through electrolytic cell by a pump or the like operates at room temperature, and can be stored in an external tank as a liquid active material. Therefore, it is easy to increase the size, and further has advantages such as easy reproduction and long life compared with the electrolyte solution of other secondary batteries.

  As this redox flow battery, a vanadium redox flow battery using vanadium as a positive electrode electrolyte (hereinafter simply referred to as a positive electrode solution) and a negative electrode electrolyte (hereinafter simply referred to as a negative electrode solution) has been put into practical use (patent). References 1 and 2). As illustrated in FIG. 1, this includes a flow-through electrolytic cell 10, and a positive electrode (hereinafter simply referred to as a positive electrode) 14 made of, for example, carbon felt and graphite, and a negative electrode (hereinafter referred to as a positive electrode) inserted into the electrolytic cell 10. 24, and a separator 12 made of a cation exchange membrane such as Nafion provided to separate the positive electrode 14 side and the negative electrode 24 side in the electrolytic cell 10, and the electrolytic cell 10 The positive electrode liquid tank 16 and the pump 18 for supplying and circulating the positive electrode liquid 17 to the positive electrode 14 side, and the negative electrode liquid tank 26 for supplying and circulating the negative electrode liquid 27 to the negative electrode 24 side of the electrolysis cell 10 A pump 28 is provided, and electricity charged in the electrolytic cell 10 is discharged to an external load 30.

Here, in the vanadium redox flow battery, in general, a pentavalent vanadium sulfuric acid aqueous solution as the positive electrode solution 17, a divalent vanadium sulfuric acid aqueous solution as the negative electrode solution 27, a pentavalent vanadium ion V ⁺⁵ as a positive active material 16A, a negative As the active material 26A, divalent vanadium ions V ^{+2 are used} , the positive electrolyte 26B is hydrogen ions H ⁺ , the negative electrolyte 16B is sulfate ions SO ₄ ²⁻ , and the electrolyte solution is water H ₂ O.

In this vanadium redox flow battery, charging and discharging are performed by changing the valence of vanadium ions when the vanadium ion solution circulates in the electrolytic cell 10. The chemical reaction by charging / discharging is as shown in the following equation. In the positive electrode 14, the charging / discharging reaction of formula (1) occurs, and in the negative electrode 24, charging / discharging reaction of formula (2) occurs.

However, since the vanadium redox flow battery uses an aqueous solution as an electrolytic solution, it can cope with only an electromotive force of about 1.8 V or less in order to prevent generation of hydrogen due to electrolysis of water. Further, the reduction potential V _R on the positive electrode 14 side according to the above formula (1) is 1.00 V, the oxidation potential V _O on the negative electrode 24 side is 0.26 V, and the electromotive force of the positive and negative active material pair is V _R + V _O = 1. It is only 26V. Also, the ion concentration of the electrolytic solution is about 1.7 mol / L, and the energy density is very low at a theoretical capacity of 29 Wh / L. For example, in the case of a NAS battery, the theoretical capacity is 180 Wh / L. Therefore, it is necessary to make the electrolytic cell 10 and the tanks 16 and 26 large, and the equipment cost becomes high.

  In addition, vanadium, which is a rare metal metal, is used as an active material, which is expensive because it is produced by electrolytic reduction. Furthermore, when a cation exchange membrane is used as the separator 12, vanadium ions and hydrogen ions have the same polarity and are mixed, and crossover occurs, so self-discharge occurs, and charge and discharge efficiency decreases. There is a problem that the balance of charge and liquid concentration is lost and the charge / discharge capacity is reduced.

  On the other hand, Patent Documents 3 to 7 describe using an ionic liquid as a solvent for an electrolytic solution.

  Patent Documents 8 to 10 describe a method for treating an ion exchange membrane for a redox flow battery, and Patent Document 11 describes a method for producing an anion exchange membrane for a fuel cell.

JP 2014-157793 A JP 2006-147374 A Special table 2012-518247 gazette JP 2014-127358 A JP 2010-086935 A JP 2010-170782 A JP 2010-244972 A JP 2001-160409 A JP 2008-027627 A International Publication WO2013 / 027758 JP 2010-092660 A

  However, conventional ion exchange membranes are premised on use in aqueous solutions, and have the problem that they cannot be used in solutions containing ionic liquids.

  As in the case of the aqueous solution, it may be possible to simply immerse in an ionic liquid, or perform boiling treatment at 100 ° C. or lower as in the case of removing impurities with a Nafion film. There was a problem that resistance was high and it could not be used.

  FIG. 2 shows a comparison result in the case of the membrane resistance measurement result (◯ mark) and the sulfuric acid aqueous solution (Δ mark) of the conventional treatment using various ion exchange membrane selemions (trade names) manufactured by AGC Engineering. In the case of the lowest resistance Selemion DSV, it can be seen that the membrane resistance of the ion exchange membrane is about 500 times that of the sulfuric acid aqueous solution.

  The conventional treatment is a heat treatment performed at 40 ° C. in a vacuum to remove moisture, and then taken out into the atmosphere and immersed in an ionic liquid for one day.

  The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide an ion exchange membrane for an ionic liquid that has a low membrane resistance and can be used for an ionic liquid.

  In the present invention, a base ion exchange membrane is boiled with a solution of ionic liquid anion at a concentration of 0.1M or more to replace counter ions in the ion exchange membrane and heated in vacuum to remove moisture. After that, the above-mentioned problem is solved by impregnating the ionic liquid into the ion exchange membrane by an impregnation treatment at a temperature exceeding 100 ° C. to 160 ° C. in vacuum.

  Here, the impregnation of the ionic liquid into the ion exchange membrane can be performed in a state where the viscosity of the ionic liquid is reduced by an organic solvent that does not damage the ion exchange membrane.

  According to the present invention, the ion exchange membrane as a base material is boiled with a solution having a concentration of ionic liquid anion of 0.1 M or more to replace the counter ions in the ion exchange membrane and heated in a vacuum to form moisture. After removing the ionic liquid, it is possible to drastically reduce the membrane resistance by impregnating the ionic liquid into the ion exchange membrane by impregnation treatment in a vacuum above 100 ° C. to 160 ° C. Can be used as

The figure which shows the principle structure of the conventional vanadium redox flow battery The figure which compares the result of membrane resistance measurement of the ion exchange membrane which processed with the conventional example, and the result in sulfuric acid aqueous solution The figure which shows the state at the time of charge operation for demonstrating the principle of the redox flow battery using an ionic liquid The figure which similarly shows the state at the time of discharge operation Figure showing reaction formula at positive and negative electrodes Sectional drawing for demonstrating the processing method of an anion exchange membrane similarly Similarly, a flowchart showing the processing procedure of the anion exchange membrane The figure which shows the example of the relationship between the vacuum impregnation process temperature of an ionic liquid and ion-exchange membrane resistance for demonstrating the principle of this invention The figure which similarly shows the influence of the anion concentration in the anion substitution treatment Figure showing the effect of adding ion exchange membrane species and ethanol

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by the content described in the following embodiment and an Example. In addition, the constituent elements in the embodiments and examples described below include those that can be easily assumed by those skilled in the art, those that are substantially the same, and those in the so-called equivalent range. Furthermore, the constituent elements disclosed in the embodiments and examples described below may be appropriately combined or may be appropriately selected and used.

  The redox flow battery using an ionic liquid to which the present invention is applied has the same basic structure as the conventional vanadium redox flow battery shown in FIG. As shown in Fig. 5 and Fig. 5 (showing the reaction formula), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) is used as a positive and negative active material. An ionic liquid is used as the solvent and electrolyte of the active material TEMPO.

  As shown in FIG. 5, in the reaction at the positive electrode 14, a nitroxide radical (referred to as a TEMPO radical) releases electrons to become an oxoammonium cation (TEMPO cation or TEMPO cation). On the other hand, in the negative electrode 24, the TEMPO radical receives an electron and becomes an aminooxy anion (TEMPO anion or TEMPO anion).

From FIG. 3 showing the state during the charging operation, electrons e ⁻ are supplied from the charging DC power source 32 to the negative electrode 24, and the TEMPO radical (TEMPO ^* ) 34 ^{* in the} negative electrode liquid 27 that has received and received the electrons e ⁻ is TEMPO negative. Ion (TEMPO ⁻ ) 34 ⁻ . On the other hand, the TEMPO radical 34 ^* in the positive electrode solution 17 releases electrons e ⁻ to the positive electrode 14 to become TEMPO cations (TEMPO ⁺ ) 34 ⁺ and supplies the electrons e ⁻ to the DC power source 32. As a result, the potential on the negative electrode 24 side (referred to as the negative electrode liquid potential) V ₋ decreases and the potential on the positive electrode 14 side (referred to as the positive electrode liquid potential) V ₊ rises, so that a potential difference ΔV ₁ is generated. The DCA anion 36 ^−, which is a counter ion, moves from the negative electrode solution 27 to the positive electrode solution 17 by the electric field of the membrane 12 ′, and this potential difference ΔV ₁ is eliminated.

Conversely, during discharging operation as shown in FIG. 4, TEMPO anion 34 ^- it has been supplied, TEMPO anions 34 in the negative electrode solution 27 ^{- -} to the negative electrode 24 electrons e from the back to the TEMPO radical 34 ^*. The electron e ^{− of the} negative electrode 24 moves to the positive electrode 14 through the load 30, and the TEMPO cation 34 ^{+ in} the positive electrode solution 17 receives the electron e ⁻ and returns to the TEMPO radical 34 ^* . As a result, the potential V ₋ on the negative electrode 24 side increases and the potential V ₊ on the positive electrode 14 side decreases, so that a potential difference ΔV _{2 in the} opposite direction to that during the charging operation is generated. The DCA anion 36 ^−, which is an ion, moves from the positive electrode solution 17 to the negative electrode solution 27 and eliminates this potential difference ΔV ₂ .

Any ionic liquid can be used as long as it does not react with TEMPO, but one example is 1-butyl 1-methylimidazolium dicyanamide [Bmim] [DCA]. In this case, the positive electrolyte is 1-butyl 1-methylimidazolium cation (Bmim ⁺ ) 38 ⁺ , the negative electrolyte and the counter ion are dicyanamide anions (DCA ⁻ ) 36 ⁻ , and the positive and negative active material TEMPO is used as a separator. the bypassing of the counter ion DCA ^- ions only pass, is obtained using a small anion exchange membrane 12 'of the hole diameter.

  TEMPO is an organic radical species that is a solid at normal temperature (melting point: about 40 ° C.), but is weakly attracted by solvation when mixed with an ionic liquid and is in a liquid state at normal temperature.

As the anion exchange membrane 12 ′, for example, a low resistance exchange membrane base such as an anion exchange membrane DSV manufactured by AGC Engineering Co., Ltd., as illustrated in FIGS. 6 and 7,
(1) DCA is a high concentration (1M) solution anion SO ₄ ^2-a counterion ion exchange the membrane in boiling treatment with (NaDCA solution) of ionic liquid anions ^- which was completely replaced, the washed by rinsing Removing the cationic Na ⁺ (step 100 in FIG. 7);
(2) After vacuuming, for example, by heating the inside of the glove box to remove moisture by vacuum heating (Step 110 in FIG. 7),
(3) For example, by heating the inside of the vacuuming glove box, while suppressing the oxidation and alteration of the ion exchange membrane and the liquid, an organic solvent (for example, ethanol or the like) that prevents the membrane from being damaged, if necessary, is used. In this case, the material is infiltrated in a state of low viscosity with an alcohol-based solvent) and impregnated into the ion exchange membrane (step 120 in FIG. 7).

  The examination result of the vacuum impregnation treatment temperature of the ionic liquid is shown in FIG. 8 that the vacuum impregnation temperature is more than 100 ° C. to 160 ° C., preferably 120 ° C. to 160 ° C. That is, the film resistance is large at 100 ° C. or lower, and when the temperature exceeds 160 ° C., the color changes and deteriorates.

  Similarly, an example of the relationship between NaDCA concentration (M) and membrane resistance is shown in FIG. It can be seen that as the NaDCA concentration increases, the membrane resistance decreases particularly at a concentration of 1 M or more, and the membrane resistance increases at a concentration of less than 0.1 M.

  Similarly, FIG. 10 shows the effect of reducing the viscosity of an ionic liquid by adding an ion exchange membrane species (AFB manufactured by Astom) and ethanol. It can be seen that the ion exchange membrane resistance can be further reduced by the addition of ethanol. In addition to ethanol, examples of the organic solvent that exhibits the same effect include alcohol solvents such as methanol, propanol, and isopropyl alcohol, ether solvents such as ethylene glycol and dimethyl glycol, acetonitrile, and dimethylformamide.

  When TEMPO is used as the organic radical on the positive electrode 14 side and the negative electrode 24 side, the anion exchange membrane 12 ′ according to the present invention has a low viscosity of 20 cP of 50 cP or less and a voltage of 1.5 V equivalent to the vanadium redox flow battery. Two times or more high capacity, positive electrode side 4.5M (charge), negative electrode side 3.4M (charge) could be realized.

In the above embodiment, 1-butyl 1-methylimidazolium dicyanamide is used as the ionic liquid and TEMPO is used as the positive and negative active material. However, the types of the ionic liquid and the positive and negative active material are limited to this. However, other organic radical species of solid at normal temperature can also be used, for example, as positive and negative active materials. The ionic liquid may be one having a small cation or anion relative to the organic radical species. As the anion, in addition to the dicyanamide anion (DCA ⁻ ), a tetrafluoroborate anion (BF ₄ ⁻ ), a hexafluorophosphate anion (PF) ₆ ⁻ ), bisfluorosulfonylamide anion (FSA ⁻ ), bistrifluoromethylsulfonylamide anion (TFSA ⁻ ) are possible. The anion exchange membrane 12 ′ is not limited to those based on the DSV or AFB shown in the embodiment, and a cation exchange membrane is also possible when a cation is used as a counter ion. The electrodes 14 and 24 are not limited to carbon felt or graphite.

  Ion exchange membranes are not limited to redox flow batteries, but are used for various types of batteries such as various flow batteries, lithium ion batteries, and fuel cells, as well as separation / purification processes using ionic liquids, chemical synthesis / catalytic processes, etc. it can.

DESCRIPTION OF SYMBOLS 10 ... Electrolytic cell 12 '... Anion exchange membrane 14 ... Positive electrode (electrode)
16 ... Positive electrode (electrolytic) solution tank 17 ... Positive electrode (electrolytic) solution 24 ... Negative electrode (electrode)
26 ... Negative electrode (electrolytic) solution tank 27 ... Negative electrode (electrolytic) solution 30 ... Load 32 ... DC power supply for charging 34 ^* ... TEMPO radical 34 ⁺ ... TEMPO cation (active material of positive electrode)
34 ^- ... TEMPO anion (active material of the negative electrode)
38 ⁺ ... Bmim ⁺ (electrolyte positive ion)
36 ^- ... DCA ^- (negative ions in the electrolyte)

Claims (2)

The ion exchange membrane as a substrate is boiled with a solution having a concentration of ionic liquid anion of 0.1 M or more to replace the counter ion in the ion exchange membrane,
After removing moisture by heating in vacuum,
A method for treating an ion exchange membrane for an ionic liquid, comprising impregnating an ionic liquid into an ion exchange membrane by an impregnation treatment in a vacuum at a temperature exceeding 100 ° C to 160 ° C.   2. The ion exchange membrane for ionic liquid according to claim 1, wherein the ionic liquid is impregnated into the ion exchange membrane in a state where the viscosity of the ionic liquid is reduced by an organic solvent that does not damage the ion exchange membrane. Processing method.