JP2010244972A - Redox flow battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a redox flow battery having high electromotive force with substantially no degradation of its battery performance by inhibiting deposition of an active material. <P>SOLUTION: The redox flow battery includes a positive electrode cell 100 and a negative electrode cell 110, an ion-exchange membrane 120 for separating the cells from one another, a first tank 103 for storing a positive electrode active material for the positive electrode cell, a second tank 113 for storing an electrolyte solution containing a negative electrode active material and a solvent for the negative electrode cell, first pipelines 104 and 105 connected with each other to allow the positive active material to circulate between the first tank and the positive electrode cell, second pipelines 114 and 115 connected with each other to allow the electrolyte solution to circulate between the second tank and the negative electrode cell, a first pump 107 provided in the middle of the first pipeline for circulating the positive electrode active material, and a second pump 116 provided in the middle of the second pipeline for circulating the electrolyte solution. The solvent for the negative electrode cell is an ionic liquid. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a redox flow battery. More specifically, the present invention relates to a redox flow battery using an ionic liquid as a solvent for a negative electrode cell and using water and air or oxygen as a positive electrode active material.

Renewable energies such as solar power, wind power and hydropower are expected to become major energy sources for this century. However, these energy sources have the disadvantage of large output fluctuations. For this reason, when these energy sources are connected to the power system, it has been studied to temporarily store power for output smoothing. The use of batteries for the storage of electric power is being studied, and among the batteries, redox flow batteries are one of the leading options from the viewpoint of good adaptability to time fluctuations.
Currently, a vanadium redox flow battery, which is a kind of redox flow battery, is in a practical stage (see Non-Patent Document 1).

The vanadium redox flow battery has a problem that the active material is precipitated due to the difference in stability of vanadium ions, and the battery capacity is reduced. However, there have been reported means for preventing the precipitation of the active material by using additives or stabilizers in the electrolytic solution, limiting the concentration of the solvent and the active material, controlling the temperature of the electrolytic solution, the operation method, and the like (Patent Document 1). reference).
In addition, a uranium redox flow battery using an aprotic organic solvent and U ⁴⁺ / U ^{3+ for} the negative electrode reaction and UO ₂ ⁺ / UO ₂ ²⁺ for the positive electrode reaction in order to obtain a high electromotive force has been reported. (See Patent Document 2).
Further, in order to increase the output and size of the battery, a redox battery using a divalent or trivalent vanadium solution dissolved in a polar solvent for the negative electrode reaction and oxygen or air for the positive electrode reaction has been reported (patent) Reference 3).

JP-A-8-64223 JP 2005-209525 A Japanese Patent No. 3163370

Electronic Technology Research Institute Vocabulary Vol. 63, No. 4, 5

The redox flow battery described in Non-Patent Document 1 and Patent Documents 2 and 3 uses a volatile solvent for the electrolytic solution. Moreover, the redox flow battery described in Patent Document 3 uses a volatile solvent for the electrolyte solution on the negative electrode side.
When an electrolytic solution using a volatile solvent is used for a long period of time, the electrolytic solution is gradually vaporized, the electrolytic solution concentration is increased, and the active material is deposited. Since the battery performance is reduced by the precipitation of the active material, there is a problem that the battery performance is lowered by repeating charging and discharging for a long time.
Furthermore, the redox flow battery described in Patent Document 3 uses oxygen or air, which is a gas, as the positive electrode active material for the purpose of increasing the output and reducing the size of the battery. The solvent on the negative electrode side may be vaporized from the positive electrode side through the ion exchange membrane between the positive electrode and the negative electrode. Therefore, there has been a problem that the concentration of the electrolyte on the negative electrode side is increased and the active material is precipitated, resulting in a decrease in battery performance.

As a result of intensive studies in view of the above problems, the inventors of the present invention use an ionic liquid as the solvent of the negative electrode electrolyte solution to suppress the precipitation of the active material due to the evaporation of the solvent, thereby reducing the battery performance. The present invention has been reached by finding that there is almost no redox flow battery having a high electromotive force.
That is, according to the present invention, a positive electrode cell and a negative electrode cell, an ion exchange membrane separating the two cells, a first tank in which a positive electrode active material for the positive electrode cell is stored, and a negative electrode active for the negative electrode cell A second tank in which an electrolyte containing a substance and a solvent is stored; a first pipe connected so that the positive electrode active material can circulate between the first tank and the positive electrode cell; and the second tank. A second pipe connected so that the electrolytic solution can circulate between the first and the negative electrode cells, a first pump for positive electrode active material circulation provided in the middle of the first pipe, and a second pipe A second pump for circulating the electrolyte provided in the middle,
A redox flow battery is provided in which the positive electrode active material is water during charging, air or oxygen during discharging, and the solvent for the negative electrode cell is an ionic liquid.

  According to the present invention, by using a non-volatile ionic liquid as a solvent for the negative electrode electrolyte, it is possible to suppress deposition of the negative electrode active material caused by vaporization of the solvent. As a result, it is possible to realize a redox flow battery having a high electromotive force with little deterioration in battery performance.

It is a schematic block diagram which shows typically an example of the redox flow battery of this invention. It is a figure which shows the charging / discharging curve in 25 degreeC of the redox flow battery of this invention. It is a figure which shows the charging / discharging curve in 40 degreeC of the redox flow battery of this invention. It is a figure which shows the charging / discharging curve in 80 degreeC of the redox flow battery of this invention. It is a figure which shows the discharge time with respect to the cycle number of the redox flow battery of this invention.

(Configuration of redox flow battery)
The redox flow battery according to the present invention includes a positive electrode cell and a negative electrode cell, an ion exchange membrane separating both cells, a first tank in which a positive electrode active material for the positive electrode cell is stored, a negative electrode active material and a solvent for the negative electrode cell. Between the second tank and the negative electrode cell, the first pipe connected so that the positive electrode active material can circulate between the first tank and the positive electrode cell, and the second tank and the negative electrode cell. The second pipe connected so that the electrolyte can circulate, the first pump for circulating the positive electrode active material provided in the middle of the first pipe, and the electrolyte circulation provided in the middle of the second pipe The second pump.

Furthermore, the first tank stores water and air or oxygen simultaneously as a water phase and a gas phase,
A first pipe A is connected to be able to transport water from the first tank to the positive electrode cell, and a first pipe is connected to be able to transport air or oxygen from the first tank to the positive electrode cell. With B,
1st piping A and B merge, it becomes one piping, and the switching cock of water, air, or oxygen is provided in the joined part,
The first pump includes a first pump A for positive electrode active material circulation provided in the middle of the first pipe A and a first pump B for positive electrode active material circulation provided in the middle of the first pipe B. ,
The switching cock is preferably positioned between the positive electrode cell and the first pump A and the first pump B.

Moreover, in the said preferable structure, although the 1st piping A and B merge and become one piping and the merged piping is connected to the positive electrode cell, However, each may be directly connected to the positive electrode cell (this configuration is referred to as a non-merging configuration).
By providing such a configuration, when switching between charge and discharge, water can be effectively supplied to the positive electrode cell during charging, and air or oxygen can be effectively supplied to the positive electrode cell during discharge, so that the desired reaction is efficiently generated in the positive electrode cell. It becomes possible. As a result, the charge / discharge efficiency can be further improved.
In the case of a non-merging configuration, the position of the positive electrode cell to which the first pipes A and B are connected is as far as possible from the position where the positive electrode active material is discharged from the positive electrode cell. It is preferable from a viewpoint of generating efficiently.

Hereinafter, an embodiment of a redox flow battery will be described with reference to FIG. Hereinafter, the positive electrode and the negative electrode may be collectively referred to as an electrode.
FIG. 1 is a schematic configuration diagram of a redox flow battery of the present invention. The redox flow battery shown in FIG. 1 includes a positive electrode cell 100 and a negative electrode cell 110.
The positive electrode cell 100 has a configuration capable of holding a positive electrode 101 and a positive electrode active material (water at the time of charging, air or oxygen at the time of discharging) 102. Similarly, the negative electrode cell 110 has a configuration capable of holding the negative electrode 111 and the negative electrode electrolyte solution 112. Here, the positive electrode cell 100 and the negative electrode cell 110 are separated by an ion exchange membrane 120.

The positive electrode active material 102 is interposed between the first tank (positive electrode active material tank) 103 holding the positive electrode active material 102 and the positive electrode cell 100 via the first pipes 104 and 105 by the first pump (first pump A) 107. It is circulating. The first pipe 104 is used for transporting water and is also referred to as a first pipe A. The first pipes 104 and 105 can be provided at appropriate positions in the positive electrode cell 100 as long as there is no problem in supplying the active material to the positive electrode 101.
Further, a first pipe B for transporting air or oxygen may be provided between the first tank 103 and the positive electrode cell 100. Further, the first pipe B may be provided with a first pump B108 for circulating the positive electrode active material (air or oxygen). Further, the first pipe B may be connected to the first pipe A via the switching cock 130. By having these configurations, the positive electrode active material can be circulated through the first pipe B106, a part of the first pipe A104 (between the switching cock 130 and the positive electrode cell 100) and the first pipe 105.
The connecting portion between the first pipe B106 and the first tank 103 is preferably located at a location higher than the liquid surface of the positive electrode active material (water) 102 of the first tank 103.

With the above configuration, the positive electrode active material (water) 102 is circulated through the first pipe A 104 and the first pipe 105 by the first pump A 107 during charging. At the time of discharge, the positive electrode active material (air or oxygen) is passed through the first pipe B106, a part of the first pipe A (between the switching cock 130 and the positive electrode cell 100) and the first pipe 105 by the first pump B108. Circulate. Switching between the two circulations is performed by operating the switching cock 130.
As described above, by providing the first pipe B106, the first pump B108, and the switching cock 130, it is possible to supply the target reactant to the positive electrode cell 100 during charging and discharging, and to improve the efficiency of the entire battery. It becomes.

The negative electrode electrolyte 112 is circulated between the second tank (negative electrode electrolyte tank) 113 holding the negative electrode electrolyte 112 and the negative electrode cell 110 via the second pipes 114 and 115 by the second pump 116. The second pipes 114 and 115 can be provided at appropriate positions in the negative electrode cell 110 as long as there is no problem in supplying the active material to the negative electrode 111.
The positive electrode active material 102 and the negative electrode electrolyte solution 112 are supplied between the positive electrode cell 100 and the ion exchange membrane 120 and between the negative electrode cell 110 and the ion exchange membrane 120, respectively.
The positive electrode 101 and / or the negative electrode 111 may be made of a porous material. In this case, in order to efficiently supply the active material to the inside of the positive electrode 101 and / or the negative electrode 111, as long as the active material and the electrolyte can be directly supplied to the inside of the positive electrode 101 and the negative electrode 111, each pipe is connected to the positive electrode cell 100. And / or it can be provided at any position of the negative electrode cell 110.

The positive electrode active material 102 and the negative electrode electrolyte solution 112 supplied into the positive electrode cell 100 and the negative electrode cell 110 are subjected to a predetermined redox reaction in each cell. For example, in a redox flow battery in which vanadium salt is dissolved in an ionic liquid as a negative electrode active material, an oxidation reaction of 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ occurs during charging in the positive electrode cell 100. Further, during discharge, a reduction reaction of O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O occurs. On the other hand, in the negative electrode 110, a reduction reaction of V ³⁺ + e ⁻ → V ²⁺ occurs during charging, and an oxidation reaction of V ²⁺ → V ³⁺ + e ⁻ occurs during discharging. Therefore, an electromotive force of about 1.5 V can be obtained with charging / discharging based on the above-described redox reaction in the battery.

  As shown in FIG. 1, the first tank 103 may include an introduction pipe 132 for introducing a positive electrode active material (water and air or oxygen). The introduction pipe 132 includes a cock 131 that opens and closes the introduction pipe. When the first tank 103 includes the introduction pipe 132 and the cock 131, the positive electrode active material and / or the positive electrode solvent can be introduced as necessary. In addition, although not shown in the figure, maintenance-free operation can be achieved by connecting the introduction pipe and the water pipe and / or the rainwater storage tank through the pure water production apparatus, thereby eliminating the trouble of introducing the positive electrode solvent. Can do. Furthermore, a switching cock is provided between the introduction pipe and the pure water production equipment, and a device that can take in outside air, such as a pump, can be used to save time and labor for air introduction. Is possible.

Two positive electrodes in the positive electrode cell 110 may be provided (not shown). By providing two positive electrodes, the positive electrode can be properly used for charging and discharging. Specifically, 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ oxidation reaction during charging and O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O reduction reaction during discharge can be caused in separate positive electrodes. Thereby, while being able to reduce each reaction overvoltage in a positive electrode, the voltage loss of a redox flow battery can be suppressed effectively. Furthermore, by providing two positive electrodes, a positive electrode material suitable for each reaction can be used. Therefore, the life of each positive electrode can be extended and the durability of the redox flow battery can be improved.

The components of the redox flow battery according to the present invention will be described in further detail below.
(Positive electrode active material)
As the positive electrode active material, water is used during charging, and air or oxygen is used during discharging. From the viewpoint of easy availability, oxygen in the atmosphere may be used.
By using water and air or oxygen as the positive electrode active material, the amount of expensive vanadyl sulfate used can be reduced, so that a cheaper redox flow battery can be provided.
The concentration of air or oxygen is not particularly limited as long as sufficient oxygen is supplied to the positive electrode and the electrode reaction of the positive electrode is not diffusion-controlled by the concentration gradient.

Water is usually used in large volumes. Further, the water may be general tap water supplied from a water pipe and / or rainwater stored in a water storage tank. In order to prevent impurities from inhibiting the electrode reaction at the positive electrode, it is more preferable to use pure water or ultrapure water. Therefore, when using general tap water or rainwater, it is desirable to use them after pre-treatment with a pure water production apparatus.
The water may contain other components. Examples of other components include a supporting electrolyte. The supporting electrolyte can be used when the solution resistance of water is high and the electrode reaction hardly occurs.
The type and amount of the supporting electrolyte are not particularly limited as long as the water can be made conductive by being dissolved in water.

The supporting electrolyte acts as an ion in water. Therefore, the supporting electrolyte is preferably a salt, acid or alkali combined with an anion and a cation that are easily ionized.
Examples of the cation include hydrogen ions, alkali metal ions such as Li ⁺ , Na ⁺ , K ⁺ , Rb ⁺ , and Cs ⁺ , ammonium ions, tetramethylammonium ions, tetraethylammonium ions, and tetra (n-propyl) ammonium. Examples thereof include quaternary alkyl ammonium ions such as ions, tetra (i-propyl) ammonium ions, tetra (n-butyl) ammonium ions, and tetra (n-hexyl) ammonium ions.

Examples of the anion include halogen ions such as Cl ⁻ , Br ⁻ and I ⁻ , hydroxyl ions, acetate ions, sulfate ions, nitrate ions, perchlorate ions, BF ₄ ⁻ , PF ₆ ⁻ , bis (trifluoromethanesulfonyl). ) Imide, bis (1,1,2,2,3,3,3-heptafluoro-1-propanesulfonyl) imide, bis (1,1,2,2,3,3,4,4,4-nonafluoro) -1-butanesulfonyl) imide, various sulfonate ions, and the like.

(Negative electrolyte)
The negative electrode electrolyte contains at least a negative electrode active material, and may further contain a negative electrode solvent and other components.
(1) Negative electrode active material A negative electrode active material is a substance containing an ionic species that undergoes redox reaction.
Examples of the negative electrode active material include transition metals such as vanadium, uranium, iron, and chromium, alkali metals such as zinc, sodium, and potassium, alkaline earth metals such as magnesium, calcium, and strontium, chlorine, bromine, sulfur, and the like. Those compounds are mentioned. Of these, it is more preferable to use vanadium, iron, sodium, bromine, sulfur, and compounds thereof from the viewpoint of reducing environmental burden.
The negative electrode active material is preferably present in the negative electrode electrolyte in the form of a chloride salt, sulfide salt, sulfate, nitrate, trihalomethanesulfonate, bis (trifluoromethanesulfonyl) imide salt, or the like.

  The addition amount of the negative electrode active material is preferably in the range of 0.001 to 10 mol / liter with respect to the whole negative electrode electrolyte. If it is less than 0.001 mol / liter, the battery may have a low energy density. On the other hand, when the amount is more than 10 mol / liter, the negative electrode active material may be deposited and may not function as an energy storage medium. A more preferable addition amount is in the range of 0.1 to 5 mol / liter.

(2) Negative electrode solvent An ionic liquid is used for a negative electrode solvent.
In the present specification, the “ionic liquid” refers to a liquid that is in a liquid state at room temperature despite being composed only of ions. The “ionic liquid” may be referred to as “ionic liquid”, “room temperature molten salt”, or “room temperature molten salt”.
The ionic liquid is composed of a combination of a cation such as imidazolium and an appropriate anion.
Since the ionic liquid is flame retardant and non-volatile, when using the ionic liquid as the negative electrode solvent, the redox flow battery is superior in durability and safety compared to the case of using the water solvent and / or the organic solvent. Can be realized.

By using an ionic liquid as the negative electrode solvent, the negative electrode solvent is not reduced by volatilization, so that precipitation of the negative electrode active material can be prevented. Thereby, the fall of battery capacity can be prevented effectively and the reliability of a redox flow battery can be improved. In addition, since it is almost unnecessary to replenish the negative electrode solvent, the maintenance of the redox flow battery can be reduced, leading to a reduction in maintenance costs.
Furthermore, since there is no risk of vaporization of the negative electrode solvent, the redox flow battery can be operated in a wide temperature range, so that the characteristics and utility value of the redox flow battery can be enhanced.

Any ionic liquid may be used as long as it does not interfere with the electrochemical reaction of the redox flow battery. Preferred ionic liquids are those having a potential window, viscosity and / or ionic conductivity in the following ranges.
The potential window of the ionic liquid is −4.0 to 2.0 Vvs. Ag / Ag ⁺ is preferred. When the potential on the low potential side is higher than −4.0 V, it becomes difficult to use an alkali metal such as sodium or potassium, or an alkaline earth metal such as magnesium, calcium, or strontium as an active material. When the potential on the high potential side is lower than 2.0 V, it becomes difficult to use materials such as uranium and sulfur as the active material. In addition, the ionic liquid itself may be decomposed outside the range of the potential window of the ionic liquid, and an abrupt oxidation current or reduction current may be observed. A more preferred potential window is -3.5 to 1.5 Vvs. It is a range of Ag / Ag ⁺ . If it is this range, the battery of higher electromotive force can be comprised.
Here, the potential window means a value obtained by performing cyclic voltammetry and measuring a potential at which an oxidation current and a reduction current are rapidly detected.

The viscosity of the ionic liquid is preferably in the range of 1 to 500 mPa · s at 20 ° C. If it is lower than 1 mPa · s, the stability of the ionic liquid may be lowered. If it is higher than 500 mPa · s, the load on the pump for circulating the ionic liquid may become too high. A more preferable viscosity is in the range of 10 to 150 mPa · s, and if it is within this range, the penetration of the ionic liquid into the negative electrode can be made better.
In addition, the viscosity of an ionic liquid shows the value measured by AR2000 by TA Instruments.

The ionic conductivity of the ionic liquid is preferably in the range of 0.05 to 25 mS / cm at 25 ° C. If it is lower than 0.05 mS / cm, the electric resistance of the battery becomes too high, and the energy efficiency of charge / discharge may be lowered. If it is higher than 25 mS / cm, the leakage current increases and the energy storage property may be lowered. More preferable ionic conductivity is in the range of 1 to 15 mS / cm. If it is this range, the charge / discharge reaction of a redox flow battery can be made more favorable.
In addition, ion conductivity shows the value which measured the alternating current impedance of 1000 Hz using the Solartron 1280Z type | mold electrochemical measurement system.

Examples of the ionic liquid include imidazolium cation and borofluoride anion (BF ₄ ⁻ ), hexafluorophosphate anion (PF ₆ ⁻ ), trifluoromethanesulfonate anion (CF ₃ SO ₃ ⁻ ) (TF), bis (Trifluoromethanesulfonyl) imide anion (N (CF ₃ SO ₂ ) ₂ ⁻ ) (TFSI) or iodide ion (I ⁻ ), molten salt, aliphatic quaternary ammonium cation and BF ₄ ⁻ , PF ₆ ⁻ , Examples thereof include molten salts with TF, TFSI, or I ⁻ .
Examples of imidazolium-based cations include 1-ethyl-3-methylimidazolium (EMI) ion, 1-butyl-3-methylimidazolium (BMI) ion, 1-hexyl-3-methylimidazolium (HMI) ion, 1 -Propyl-3-methylimidazolium (PMI) ion, 1,2-dimethyl-3-propylimidazolium (DMPI) ion and the like can be preferably used.

As the aliphatic quaternary ammonium cation, tetraethylammonium (TEA) ion, triethylmethylammonium (TEMA) ion, trimethylpropylammonium (TMPA) ion or the like can be preferably used.
As other cationic species, methylpropylpiperidinium (MPPi) ion, butylmethylpiperidinium (BMPi) ion, methylpropylpyrrolidinium (MPPy) ion, butylmethylpyrrolidinium (BMPy) ion, etc. are preferably used. it can.
Among these ionic liquids, TMPA-TFSI, MPPy-TFSI, EMI-TFSI, and EMI-TF are preferred because of their wide potential window for redox. In particular, EMI-TF is preferred as the ionic liquid because of its low viscosity at room temperature, high solubility of metal ions, and high permeability to electrodes.

(3) Other components Examples of other components include a non-aqueous solvent and / or a supporting electrolyte.
(A) Nonaqueous solvent A nonaqueous solvent may be added to the negative electrode electrolyte. The non-aqueous solvent serves to improve the dissolution of the negative electrode active material into the ionic liquid and to make the negative electrode electrolyte more easily penetrate into the negative electrode.
Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate; dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, dipropyl carbonate, and the like. Chain carbonates, γ-butyrolactone (hereinafter sometimes abbreviated as GBL), lactones such as γ-valerolactone, furans such as tetrahydrofuran and 2-methyltetrahydrofuran, diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, dioxane, ethers such as triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, dimethyl sulfoxide, sulfolane, methyl sulfate Examples include holan, acetonitrile, methyl formate, and methyl acetate.

Among these, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol have high compatibility with ionic liquids and high coordination ability to ions that are negative electrode active materials. Glymes such as dimethyl ether are preferred. In particular, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether are preferable because they have a very low vapor pressure, are highly safe even when added to an ionic liquid, and do not impair the advantages of no liquid evaporation.
The addition amount of the non-aqueous solvent is preferably 0.5 to 50% by weight with respect to the total weight of the ionic liquid from the viewpoint that evaporation of the non-aqueous solvent can be suppressed, and more preferably 1 to 30% by weight.

(B) Support electrolyte In order to improve the ionic conductivity of the negative electrode electrolyte and to construct a redox flow battery having high output characteristics, a support electrolyte may be added to the negative electrode electrolyte.
Examples of the supporting electrolyte include proton acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, formic acid, hydrofluoric acid, perchloric acid, trihalomethanesulfonic acid, lithium perchlorate, lithium borofluoride (LiBF ₄ ), and phosphorus hexafluoride. Lithium such as lithium acid (LiPF ₆ ), lithium trifluoroacetate (LiCF ₃ COO), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium bis (trifluoromethanesulfonyl) imide (LiN (CF ₃ SO ₂ ) ₂ ) Salt can be used. Further, at least one selected from cations of sodium, potassium, rubidium, cesium and tetramethylammonium, a borofluoride anion (BF ₄ ⁻ ), a hexafluorophosphate anion (PF ₆ ⁻ ), a trifluoromethanesulfonate anion (CF ₃ SO ₃ ⁻ ) (TF), bis (trifluoromethanesulfonyl) imide anion (N (CF ₃ SO ₂ ) ₂ ⁻ ) (TFSI), and an anion of iodide ion (I ⁻ ). Salt can also be used.
The amount of supporting electrolyte added is preferably in the range of 0.01 to 2 mol / liter with respect to the entire electrode solution. In particular, in order to construct a redox flow battery using an ionic liquid having high output characteristics, a range of 0.1 to 1 mol / liter is more preferable.

(Positive electrode and negative electrode)
The positive electrode cell 100 includes a positive electrode 101, and the negative electrode cell 110 includes a negative electrode 111. The electrode may be provided with a circuit for discharging by flowing an electric current to the outside and for charging by taking in an electric current from the outside (not shown).
Examples of the material for the electrode include metal materials, carbonaceous materials, and conductive inorganic materials.
As the metal material, a material having electronic conductivity and resistance to corrosion in an acidic atmosphere is preferable. Specifically, noble metals such as Au, Pt, Pd, metals such as Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu, Zn, Su, Si, and nitrides and carbides of these metals, Examples of the alloy include stainless steel, Cu—Cr, Ni—Cr, and Ti—Pt. From the viewpoint that there are few other chemical side reactions, it is more preferable that the metal material contains at least one element selected from the group consisting of Pt, Ti, Au, Ag, Cu, Ni, and W. Further, in the positive electrode, from the viewpoint of causing an oxidation reaction of 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ during charging, the metal material of the positive electrode is more preferably a metal material that causes the reaction. These metal materials have a small specific resistance and can suppress a decrease in voltage even when a current is extracted in the surface direction.

As the carbonaceous material, a chemically stable and conductive material is preferable. Examples thereof include carbon powders and carbon fibers such as acetylene black, vulcan, ketjen black, furnace black, VGCF, carbon nanotube, carbon nanohorn, and fullerene.
Examples of the inorganic material having conductivity include tin oxide, indium tin oxide (ITO), and antimony oxide-doped tin oxide.
Furthermore, when using a metal material having poor corrosion resistance under an acidic atmosphere such as Cu, Ag, Zn, etc., noble metals and metals having corrosion resistance such as Au, Pt, Pd, carbon, graphite, glassy carbon, The surface of the metal having poor corrosion resistance may be coated with a conductive polymer, a conductive nitride, a conductive carbide, a conductive oxide, or the like.

Examples of the conductive polymer include polyacetylene, polythiophene, polyaniline, polypyrrole, polyparaphenylene, polyparaphenylene vinylene, and the like.
Examples of the conductive nitride include carbon nitride, silicon nitride, gallium nitride, indium nitride, germanium nitride, titanium nitride, zirconium nitride, and thallium nitride.
Examples of the conductive carbide include tantalum carbide, silicon carbide, zirconium carbide, titanium carbide, molybdenum carbide, niobium carbide, iron carbide, nickel carbide, hafnium carbide, tungsten carbide, vanadium carbide, chromium carbide, and the like.
Examples of the conductive oxide include tin oxide, indium tin oxide (ITO), and antimony oxide-doped tin oxide.

As the electrode structure, a plate shape, a foil shape, a rod shape, a mesh shape, a lath plate shape, a porous plate shape, a porous rod shape, a woven fabric shape, a nonwoven fabric shape, a fiber shape, and a felt shape can be preferably used. Further, a grooved electrode in which the surface of the felt-like electrode is pressure-bonded in a groove shape is preferable because electric resistance and flow resistance of the electrode liquid can be reduced.
In particular, as the structure of the negative electrode, a porous plate shape, a porous rod shape, a woven fabric shape, a nonwoven fabric shape, a fiber shape, and a felt shape having a fine structure can be suitably used. By using an electrode material having a fine structure inside the negative electrode, the negative electrode electrolyte can be efficiently supplied to the surface and inside of the negative electrode. As a result, the rate of material supply can be effectively prevented, and the entire surface area of the electrode can be used for the electrode reaction.

When two positive electrodes are provided as described above, the material of one of the positive electrodes (first positive electrode) can cause an oxidation reaction of 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ at the time of charging, and in an acidic atmosphere Any material may be used as long as it has corrosion resistance, and is not particularly limited. In particular, it is preferable to use an oxygen generating electrode used for ordinary electrolysis.
For example, metals and alloys such as iridium, platinum, ruthenium, rhodium, palladium, nickel, tin, titanium, gold, tungsten, niobium, cadmium, manganese, thallium, lead, mercury, their oxides, glassy carbon, spectral analysis grade Carbon materials such as graphite, pyrolytic graphite, and carbon cloth, and those obtained by dispersing these powders in xylene wax, epoxy resin, silicone rubber, nujol and the like can be used. These materials may be used alone or in combination of two or more for the first positive electrode.

In addition, when the metal is deposited by vapor deposition or sputtering, a thin film such as chromium or titanium may be deposited as a base in order to improve adhesion to the substrate. Carbon may be coated with a paste in which carbon powder is dispersed in an adhesive material on the surface of the metal electrode. Similarly, a hydrocarbon compound may be applied to the surface of the metal electrode and thermally decomposed under reduced pressure to obtain pyrolytic graphite.
A technique such as metal plating may be applied to the surface of the first positive electrode so as to widen the electrode surface, and platinum black or palladium black having particularly high catalytic activity may be adhered.

The material of the other positive electrode (second positive electrode) is not particularly limited as long as it can cause a reduction reaction of O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O during discharge. In particular, it is preferable to use a cathode electrode material in a polymer electrolyte fuel cell (PEFC) and a direct methanol fuel cell (DMFC).
For example, the cathode electrode is composed of a catalyst layer, a gas diffusion layer, and a current collecting layer, and the catalyst layer further includes a support carrying a catalyst and an electrolyte.
Examples of the catalyst include noble metals such as Pt, Ru, Au, Ag, Rh, Pd, Os, and Ir, and base metals such as Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W, and Zr. , These precious metals, base metal oxides, carbides, carbonitrides, and carbon. These materials can be used alone or in combination of two or more as a catalyst. The catalyst is not necessarily limited to the same type, and different materials may be used.

  The carrier used for the catalyst layer is preferably a carbon-based material having high electrical conductivity. Specific examples include acetylene black, ketjen black (registered trademark), amorphous carbon, carbon nanotube, and carbon nanohorn. In addition to carbon-based materials, noble metals such as Pt, Ru, Au, Ag, Rh, Pd, Os, Ir, Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W, Zr, etc. Base metals, oxides, carbides, nitrides, carbonitrides of these noble metals and base metals may be used. These materials can be used as a carrier alone or in combination of two or more. In addition, a material that imparts proton conductivity to the carrier, specifically, sulfated zirconia, zirconium phosphate, or the like may be used.

The electrolyte used for the catalyst layer is not particularly limited as long as it is a material having proton conductivity and electrical insulation, but is preferably a solid or a gel. Specifically, organic polymers having strong acid groups such as sulfonic acid and phosphoric acid groups and weak acid groups such as carboxyl groups are preferred. Such organic polymers include perfluorocarbon (NAFION (registered trademark): manufactured by DuPont), carboxyl group-containing perfluorocarbon (Flemion (registered trademark): manufactured by Asahi Kasei)), polystyrene sulfonic acid copolymer, polyvinyl Examples include sulfonic acid copolymers, ionic liquids (room temperature molten salts), sulfonated imides, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
Further, when using a carrier imparted with proton conductivity, the carrier does not necessarily require an electrolyte because the carrier conducts proton conduction.

  The thickness of the catalyst layer is preferably 0.5 mm or less in order to reduce the proton conduction resistance and the electron conduction resistance and to reduce the oxygen diffusion resistance. Moreover, since it is necessary to carry | support sufficient catalyst in order to improve the output as a battery, it is preferable that it is at least 0.1 micrometer or more, respectively.

The gas diffusion layer is preferably made of a conductive porous material. For example, carbon paper, carbon cloth, metal foam, metal sintered body, metal fiber nonwoven fabric, and the like can be used.
The porosity as the definition of the porosity of the gas diffusion layer is preferably 30% or more in order to reduce the diffusion resistance of oxygen. Moreover, 95% or less is preferable in order to reduce electrical resistance. More preferably, it is 50 to 85%. The porosity means a value measured by a mercury intrusion method.
The thickness of the gas diffusion layer is preferably 10 μm or more in order to reduce the diffusion resistance of oxygen in the direction perpendicular to the stacking direction. In order to reduce the diffusion resistance of oxygen in the stacking direction of the gas diffusion layer, it is preferably 1 mm or less. More preferably, it is 100-500 micrometers.

Examples of the material for the current collecting layer include carbon materials, conductive polymers, noble metals such as Au, Pt, and Pd, metals such as Ti, Ta, W, Nb, and Cr, and nitrides, carbides, and the like of these metals, and It is preferable to use an alloy of stainless steel, Cu—Cr, Ni—Cr, Ti—Pt, or the like. In addition, when using metals with poor corrosion resistance in an acidic atmosphere such as Cu, Ag, Zn, Ni, etc., noble metals and metal materials having resistance to corrosion, conductive polymers, conductive oxides, conductive Conductive carbonitrides such as nitrides and conductive carbides can be used as the surface coating.
The shape of the current collecting layer is not particularly limited as long as oxygen in the atmosphere can be taken into the gas diffusion layer.

The structure of the first positive electrode and the second positive electrode is a plate-like, foil-like, rod-like, mesh-like, lath-plate-like, porous plate-like, porous rod-like, woven fabric-like, non-woven-like, fiber-like, or felt-like structure. Can be suitably used. Further, a grooved electrode in which the surface of an electrode having a felt-like structure is pressure-bonded in a groove shape is preferable because electric resistance and flow resistance of the electrode liquid can be reduced.
The positions of the first positive electrode and the second positive electrode in the positive electrode cell 100 may be appropriately determined as long as they do not affect the electrochemical reaction in each positive electrode.

(Ion exchange membrane)
The ion exchange membrane may be formed by a known method. Specifically, a proton conductive membrane, a cation exchange membrane, an anion exchange membrane, etc. are mentioned.
(1) Proton Conducting Membrane The material of the proton conducting membrane is not particularly limited as long as it is a material having proton conductivity and electrical insulation. For example, a polymer film, an inorganic film, or a composite film can be used.
Examples of the polymer membrane include perfluorosulfonic acid electrolyte membranes such as Nafion (manufactured by DuPont), Aciplex (manufactured by Asahi Kasei), Flemion (manufactured by Asahi Glass), polystyrene sulfonic acid, Examples include hydrocarbon electrolyte membranes such as ether ether ketone.

Examples of the inorganic film include films made of phosphate glass, cesium hydrogen sulfate, polytungstophosphoric acid, ammonium polyphosphate, and the like.
Examples of the composite film include a film made of a composite of a sulfonated polyimide polymer, an inorganic material such as tungstic acid, and an organic material such as polyimide. Specific examples include a Gore Select membrane (manufactured by Gore) and a pore filling electrolyte membrane.

Furthermore, when the battery is used in a high temperature environment (for example, 100 ° C. or more), the ion exchange membrane is sulfonated polyimide, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), sulfonated polybenzimidazole, phosphone. Polybenzimidazole, cesium hydrogen sulfate, ammonium polyphosphate and the like are used. The ion exchange membrane preferably has a proton conductivity of 10 ⁻⁵ S / cm or more. More preferably, a polymer electrolyte membrane having a proton conductivity of 10 ⁻³ S / cm or more such as a perfluorosulfonic acid polymer or a hydrocarbon polymer is more preferably used.
For example, a film of Nafion (manufactured by DuPont), Aciplex (manufactured by Asahi Kasei Co., Ltd.), Flemion (manufactured by Asahi Glass Co., Ltd.), or the like can be used. In order to impart water repellency, PTFE or PVDF may be added. In order to impart hydrophilicity, silica particles, hygroscopic resin, or the like may be added.

(2) Cation Exchange Membrane The cation exchange membrane is not particularly limited as long as it is a solid polymer electrolyte that can move cations. Specifically, fluorine ion exchange membranes such as perfluorocarbon sulfonic acid membranes and perfluorocarbon carboxylic acid membranes, polybenzimidazole membranes impregnated with phosphoric acid, polystyrene sulfonic acid membranes, sulfonated styrene / vinylbenzene copolymers Examples include membranes.

(3) Anion exchange membrane When the anion transport number of the supporting electrolyte solution is high, an anion exchange membrane can be used. As the anion exchange membrane, a solid polymer electrolyte capable of transferring anions can be used. Specifically, a polyorthophenylenediamine film, a fluorine ion exchange film having an ammonium salt derivative group, a vinylbenzene polymer film having an ammonium salt derivative group, and a chloromethylstyrene / vinylbenzene copolymer on the positive and negative electrode plates The film | membrane etc. which were made into are mentioned.

(4) Ew value It is preferable that the ion exchange membrane has Ew value of the range of 400-2000. In particular, in the case of an ion exchange membrane made of Nafion, the Ew value is preferably in the range of 800 to 1200. If the Ew value is low, the resistance of the battery may increase. If the Ew value is high, a battery using a fluid such as a redox flow battery has a problem that the film strength is low.
The Ew value is a value defined by the following formula.
Ew = dry weight of ion exchange membrane per equivalent of functional group = (dry weight of ion exchange membrane) / (number of functional groups having ion exchange ability)
The dry weight of the ion exchange membrane was determined by weighing the ion exchange membrane after vacuum drying at 60 ° C. for 72 hours. The number of functional groups having ion exchange capacity was determined by a sodium chloride titration method. Specifically, sodium chloride is added, the pH value is measured, and the active functional group is quantified.

(4) Production method of ion exchange membrane The ion exchange membrane may be formed by a known method. For example, a method of coating the electrode plate by an electrolytic polymerization method, a plasma polymerization method, a liquid phase polymerization method, a solid phase polymerization method, or the like can be given.
These methods can be appropriately selected according to the selection of the monomer for film production. Furthermore, the electrode plate can be directly immersed in the polymer solution constituting the ion exchange membrane and adhered to the surface. In general, the coating amount is preferably at least 1 mg / cm ² or more or 2 mg / cm ² or more.

(tank)
Two tanks are used, a first tank for the positive electrode active material and a second tank for the negative electrode electrolyte. The shape of the tank may be appropriately determined according to the use of the battery, the place of use, and the like. The capacity of the tank may be appropriately determined according to the desired capacity of the battery. The material of the tank is not particularly limited as long as it can hold the electrolytic solution.

(Piping)
As for the piping, the first piping 104 and 105 connected so that the positive electrode active material 102 can circulate between the first tank 103 and the positive electrode cell 100, and the negative electrode electrolysis between the second tank 113 and the negative electrode cell 110. The second pipes 114 and 115 connected so that the liquid 112 can circulate are used. Further, the first pipe B106 is used to connect the first tank 103 and the middle of the first pipe 104 (a predetermined position between the first tank 103 and the positive electrode cell 100) via the switching cock 130. May be.
Moreover, you may provide the introductory piping 132 connected so that a positive electrode active material and / or a positive electrode solvent may be introduce | transduced into the 1st tank 103. FIG.
The shape of the pipe may be appropriately determined according to the use of the battery, the place of use, and the like. Moreover, the material which comprises piping is not specifically limited if it can hold | maintain electrolyte solution.

(pump)
The pump is provided with a first pump (first pump A) 107 and a second pump 116 in the middle of the first pipe 104 and the second pipe 114, respectively, and circulates the positive electrode active material 102 and the negative electrode electrolyte 112, respectively. Used for. In addition, a first pump B108 is installed in the middle of the first pipe B, and is used for circulating the positive electrode active material.
As long as it has a function of moving a substance in the pipe, the configuration and type of the pump are not limited. A pump having a function capable of discharging the electrolytic solution at a flow rate of 0.1 ml / min or more is more preferable.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to an Example.
Example 1
A redox flow battery using the ionic liquid having the configuration shown in FIG. 1 was produced. The first tank 103 and the second tank 113 have a capacity of 1.2 L, and the positive electrode active material 102 obtained by dissolving 50 g of 18M sulfuric acid (manufactured by Wako Pure Chemical Industries, Ltd.) in 1.0 L of ultrapure water is contained in the first tank. A negative electrode electrolyte solution 112 in which 16 g of vanadium chloride is dissolved in 1 L of EMI-TF is held in the tank. For the positive electrode 101 and the negative electrode 111, porous carbon electrodes (manufactured by BAS) (5 cm width × 5 cm length × 5 mm thickness) were used. As the ion exchange membrane 120, a Nafion membrane (manufactured by DuPont, thickness 170 μm) was used. Between the first tank 103 and the positive electrode cell 100, the first pipes 104 and 105, between the first tank 103 and the first pipe A 104, the first pipe B 106, and between the second tank 113 and the negative electrode cell 110. The second pipes 114 and 115 are provided. In addition, the first pump A 107 and the first pump B 108 were used to circulate the positive electrode active material 102 and oxygen, respectively, and the second pump 116 (manufactured by Ato Corporation) was used to circulate the negative electrode electrolyte solution 112. A switching cock 130 is provided at the connection between the first pipe A and the first pipe B, and the cock is switched so that water is supplied to the positive electrode cell during charging and oxygen is supplied during discharging. The first tank 103 was provided with an introduction pipe 132 having a cock 131, and the cock was opened and closed as necessary to introduce a positive electrode active material (water and oxygen). The flow rate of the positive electrode active material (water and oxygen) and the negative electrode electrolyte was 200 mL / min.

  The temperature of the positive electrode active material 102 and the negative electrode electrolyte solution 112 was 25 ° C., the battery was charged at a constant current of 200 mA for 13 hours, and completely discharged at a constant current of 50 mA. The open circuit voltage before the start of charging was 1.41V. The charge / discharge curve at this time is shown in FIG. This charging / discharging was made into 1 cycle, and charging / discharging of 10 cycles in total was performed. The time from the start of the discharge reaction to the battery voltage reaching 0.8 V was taken as the discharge time, and the discharge time in each cycle is shown in FIG. Since the discharge time showed a substantially constant value, it was shown that the negative electrode solvent was not vaporized, the electric capacity was not reduced by the precipitation of the negative electrode active material, and the battery performance was maintained. Moreover, when the inside of the 2nd tank and the negative electrode cell was confirmed visually for every cycle, precipitation of the negative electrode active material was not recognized.

Example 2
A redox flow battery using an ionic liquid having the same configuration as in Example 1 except that the temperature of the positive electrode active material 102 and the negative electrode electrolyte solution 112 is set to 40 ° C. is charged with a constant current of 200 mA for 13 hours. The battery was completely discharged at a constant current of 50 mA. The open circuit voltage before the start of charging was 1.42V. The charge / discharge curve at this time is shown in FIG. Further, as shown in FIG. 5, the same result as in Example 1 was obtained. Furthermore, when the inside of the second tank and the negative electrode cell was visually confirmed for each cycle, no deposition of the negative electrode active material was observed.

Example 3
A redox flow battery using an ionic liquid having the same structure as in Example 1 except that the temperature of the positive electrode active material and the negative electrode electrolyte is 80 ° C. is prepared, charged at a constant current of 200 mA for 13 hours, and 50 mA. The battery was completely discharged at a constant current. The open circuit voltage before the start of charging was 1.45V. The charge / discharge curve at this time is shown in FIG. Further, as shown in FIG. 5, the same results as in Example 1 and Example 2 were obtained. Furthermore, when the inside of the second tank and the negative electrode cell was visually confirmed for each cycle, no deposition of the negative electrode active material was observed.

Comparative Example 1
A redox flow battery having the same configuration as in Example 1 was prepared except that 3M sulfuric acid was used as the negative electrode solvent, and the battery was charged with a constant current of 200 mA for 13 hours and completely discharged with a constant current of 50 mA. The open circuit voltage before the start of charging was 1.38V. The charge / discharge curve at this time is shown in FIG. Compared to Example 1, similar results were obtained, but as shown in FIG. 5, the discharge time decreased as the number of cycles increased. Precipitation occurred, indicating that the battery capacity was reduced. Moreover, when the inside of the 2nd tank and the negative electrode cell was confirmed visually for every cycle, precipitation of the negative electrode active material was recognized from the 5th cycle.

Comparative Example 2
A redox flow battery having the same configuration as in Example 2 was prepared except that 3M sulfuric acid was used as the negative electrode solvent, and the battery was charged with a constant current of 200 mA for 13 hours and completely discharged with a constant current of 50 mA. The open circuit voltage before the start of charging was 1.38V. The charge / discharge curve at this time is shown in FIG. Since the discharge time decreased, it was shown that the battery capacity decreased due to the deposition of the negative electrode active material due to the vaporization of the negative electrode solvent. Further, as shown in FIG. 5, it was confirmed that the discharge time decreased as the number of cycles increased, and further, the deposition of the negative electrode active material was observed from the second cycle.

Comparative Example 3
A redox flow battery having the same configuration as in Example 3 was prepared except that 3M sulfuric acid was used as the negative electrode solvent, and the battery was charged with a constant current of 200 mA for 13 hours and completely discharged with a constant current of 50 mA. The open circuit voltage before the start of charging was 1.40V. The charge / discharge curve at this time is shown in FIG. Since the discharge time was greatly reduced, it was shown that the battery capacity was reduced due to deposition of the negative electrode active material due to vaporization of the negative electrode solvent. In addition, the deposition of the negative electrode active material was confirmed in the first cycle, and as shown in FIG. 5, the decrease in the discharge time was confirmed as the number of cycles increased. Therefore, the vaporization of the negative electrode solvent greatly affected the battery performance. It became clear that.
It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

100 positive electrode cell 101 positive electrode 102 positive electrode active material 103 first tank 104 first pipe (first pipe A)
105 First piping 106 First piping B
107 1st pump (1st pump A)
108 First pump B
110 Negative electrode 111 Negative electrode 112 Negative electrode electrolyte 113 Second tank 114, 115 Second pipe 116 Second pump 120 Ion exchange membrane 130 Switching cock 131 Cock 132 Introducing pipe

Claims (5)

Electrolyte containing a positive electrode cell and a negative electrode cell, an ion exchange membrane separating the two cells, a first tank storing a positive electrode active material for the positive electrode cell, and a negative electrode active material and a solvent for the negative electrode cell Between the second tank and the negative electrode cell, the first pipe connected so that the positive electrode active material can circulate between the first tank and the positive electrode cell, and the second tank and the negative electrode cell. A second pipe connected so that the electrolyte can circulate, a first pump for positive electrode active material circulation provided in the middle of the first pipe, and an electrolyte circulation provided in the middle of the second pipe And a second pump for
The redox flow battery, wherein the positive electrode active material is water during charging, air or oxygen during discharging, and the solvent for the negative electrode cell is an ionic liquid. The first tank stores water and air or oxygen simultaneously as a water phase and a gas phase,
The first piping is connected so that the water can be transported from the first tank to the positive electrode cell, and the air or oxygen can be transported from the first tank to the positive electrode cell. A first pipe B,
The first pipes A and B merge to form a single pipe, and a switching cock between water and air or oxygen is provided at the merged site,
The first pump is a first pump A for positive electrode active material circulation provided in the middle of the first pipe A, and a first pump B for positive electrode active material circulation provided in the middle of the first pipe B. And consist of
The redox flow battery according to claim 1, wherein the switching cock is located between the positive electrode cell and the first pump A and the first pump B.   The redox flow battery according to claim 1, wherein the negative electrode cell includes a porous negative electrode.   The redox flow battery according to any one of claims 1 to 3, wherein the ionic liquid includes at least 1-ethyl-3-methylimidazolium-trifluoromethanesulfonic acid.   The redox flow battery according to any one of claims 1 to 4, wherein the positive electrode cell has two positive electrodes that are selectively used for charging and discharging.

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