JP2013168365A - Electrolyte membrane for redox flow secondary battery, and redox flow secondary battery including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte membrane for a redox flow secondary battery offering excellent selective ion permeability to inhibit the penetration of ions of an active material without impairing proton (H) permeability, and having low electric resistance, excellent current efficiency, and oxidation degradation resistance (hydroxy radical resistance), and a redox flow secondary battery including the same.SOLUTION: The electrolyte membrane for a redox flow secondary battery contains a fluorine-based polyelectrolyte polymer with a specific structure. The electrolyte membrane has a multilayer structure including three or more layers, and the equilibrium moisture content of the outer layers adjacent to a positive electrode or a negative electrode is higher than the equilibrium moisture content of the intermediate layer that is not adjacent to the positive electrode or the negative electrode.

Description

  The present invention relates to an electrolyte membrane for a redox flow secondary battery and a redox flow secondary battery using the same.

  The redox flow secondary battery stores and discharges electricity, and belongs to a large stationary battery used for leveling the amount of electricity used. A redox flow secondary battery is composed of an electrolyte containing a positive electrode and a positive electrode active material (positive electrode cell), and a negative electrode electrolyte containing a negative electrode and a negative electrode active material (negative electrode cell) separated by a diaphragm, and oxidizing both active materials. Charging and discharging is performed using a reduction reaction, and an electrolytic solution containing both the active materials is circulated from a storage tank to an electrolytic cell to be used.

  As the active material contained in the electrolytic solution, for example, iron-chromium-based, chromium-bromine-based, zinc-bromine-based, vanadium-based utilizing a difference in charge, or the like is used.

  In particular, since vanadium batteries have advantages such as high electromotive force, fast electrode reaction of vanadium ions, a small amount of hydrogen generation as a side reaction, and high output, they are being developed in earnest.

In addition, the diaphragm is devised so as not to mix the electrolyte containing the active material of both electrodes. However, the conventional diaphragm has problems such as being easily oxidized and having a sufficiently low electric resistance. In order to increase the current efficiency, the permeation of each active material ion contained in each cell electrolyte (contamination of the electrolyte in the bipolar electrolyte) is prevented as much as possible, and protons (H ⁺ ) that carry charges are sufficient. An ion exchange membrane that is easy to permeate and excellent in ion selective permeability is required.

In this vanadium secondary battery, vanadium divalent (V ²⁺ ) / 3 valent (V ³⁺ ) in the negative electrode cell and vanadium tetravalent (V ⁴⁺ ) / 5 valent (V ⁵⁺ ) in the positive electrode cell. ) Redox reaction. Therefore, since the electrolyte solution of the positive electrode cell and the negative electrode cell is the same kind of metal ion species, even if the electrolyte solution is mixed through the diaphragm, it is regenerated normally by charging, so compared to other types of metal species Hard to be a big problem. However, since active materials that are wasted increase and current efficiency decreases, active material ions should not permeate freely as much as possible.

  Conventionally, there are batteries that use various types of diaphragms (sometimes simply abbreviated as “membranes” in this specification). For example, the ionic differential pressure and osmotic pressure of an electrolyte can be freely used as a driving force. A battery using a porous film to pass through has been reported. For example, Patent Document 1 discloses a polytetrafluoroethylene (PTFE) porous film, a polyolefin (PO) porous film, a PO non-woven fabric, and the like as such a porous film.

  Patent Document 2 discloses a composite membrane in which a porous membrane and a hydrous polymer are combined so that both electrolytes do not move due to a pressure difference between cells.

  Patent Document 3 discloses that a cellulose or ethylene-vinyl alcohol copolymer film is used as a nonporous hydrophilic polymer film having a hydrophilic hydroxyl group.

  Patent Document 4 describes that by using a polysulfone membrane (anion exchange membrane) as a hydrocarbon ion exchange resin, the current efficiency is 80% to 88.5%, and the radical oxidation resistance is also excellent. ing.

  Patent Document 5 discloses a method in which a fluorine-based or polysulfone-based ion exchange membrane is used as a diaphragm, and in order to increase current efficiency, expensive platinum is supported on positive electrode porous carbon to increase reaction efficiency.

Patent Document 6 discloses an iron-chromium redox flow battery in which a hydrophilic resin is applied to pores of a porous film such as polypropylene (PP). In the example of this document, there is an example of a membrane in which both surfaces of a 100 μm-thick PP porous membrane are coated with a fluorine-based ion exchange resin (manufactured by DuPont, trademark Nafion) with a thickness of several μm. Here, Nafion, - a repeating unit represented _{by, - - (CF 2 -CF 2} ) (CF 2 -CF (-O- (CF 2 CFXO) n - (CF 2) m -SO 3 H) )-, A copolymer containing X = CF ₃ , n = 1, m = 2.

  Patent Document 7 discloses an example of a vanadium type battery that uses a two-layer liquid-permeable porous carbon electrode having a specific surface lattice and reduces the cell electrical resistance as much as possible by increasing the efficiency by devising from the electrode side. It is disclosed.

Patent Document 8 discloses a cross-linked polymer having a low resistance and excellent proton permeability, copolymerized with an anion exchange type styrenic group and divinylbenzene having a pyridinium group (using cation N ⁺ ). An example of a vanadium redox flow battery used as a diaphragm is disclosed.

  Patent Document 9 uses a membrane having a structure in which a cation exchange membrane (fluorine polymer or other hydrocarbon polymer) and an anion exchange membrane (polysulfone polymer) are alternately stacked, and uses positive electrode electrolysis. An example in which ion selective permeability is improved by a method in which the cation exchange membrane side is in contact with the liquid side is disclosed.

Patent Document 10 discloses that a vinyl heterocyclic compound having two or more hydrophilic groups on a porous substrate made of a porous PTFE resin as a membrane having excellent chemical resistance, low resistance and excellent ion selective permeability. An example of use of an anion exchange membrane formed by combining a crosslinked polymer having a repeating unit (such as vinylpyrrolidone having an amino group) is disclosed. As for the principle, when a metal cation having a large ionic diameter and charge amount is subjected to a potential difference, the cation on the surface of the diaphragm is electrically repelled and the metal cation is inhibited from passing through the membrane, but the ionic diameter is also small. It is described that the monovalent proton (H ⁺ ) can easily diffuse and permeate the diaphragm having a cation, and therefore has a low electric resistance.

JP 2005-158383 A Japanese Patent Publication No. 6-105615 JP-A-62-226580 JP-A-6-188005 JP-A-5-242905 JP-A-6-260183 JP-A-9-92321 JP-A-10-208767 Japanese Patent Laid-Open No. 11-260390 JP 2000-235849 A

However, the porous membrane disclosed in Patent Document 1 does not have sufficient electrical resistance and selective ion permeability, and current efficiency and durability of a battery obtained using the porous membrane are insufficient.
The composite membrane disclosed in Patent Document 2 has a high electrical resistance, and there is a problem that each ion is diffused freely although not as much as a porous membrane. The film disclosed in Patent Document 3 also has the same problem as described above and is inferior in oxidation resistance.
The battery disclosed in Patent Document 4 still has insufficient current efficiency, and is inferior in oxidation resistance deterioration resistance in a sulfuric acid electrolyte over a long period of time. Moreover, it is described in the comparative example of the document that the current efficiency as a Teflon (registered trademark) -based ion exchange membrane is 64.8 to 78.6%, which has a problem in performance.
Also in Patent Document 5, the same problem as described above cannot be solved, and there is a problem that a large facility is expensive in terms of price.
The film disclosed in Patent Document 6 describes that the internal resistance increases unless the thickness of the coating film is extremely thin (several μm). Further, there is no description of any device for improving ion selective permeability.
Since the battery disclosed in Patent Document 7 uses a polysulfone-based diaphragm, the ion selective permeability and oxidation degradation resistance of the diaphragm are not sufficient, and the electric resistance, current efficiency, and durability of the battery are not sufficient.
The battery disclosed in Patent Document 8 has insufficient current efficiency and also has problems with long-term use because of oxidative degradation.
The film disclosed in Patent Document 9 has a problem that electric resistance is increased.
According to the results shown in the examples of Patent Document 10, it cannot be said that the internal resistance (electrical resistance) of the film is sufficiently low, and oxidation resistance deterioration becomes a problem in long-term use.

Conventional vanadium-based redox flow battery electrolyte (separation) membranes consist of a cell in which the majority of vanadium ion ions, which are the active material of the electrolyte solution of both electrodes, and a high-valence ion group. In each of the majority of cells (negative electrode side, positive electrode side), diffusion / transmission to the counter electrode (cell) is suppressed, and proton (H ⁺ ) is selectively selected in accordance with the intended charge / discharge operation. It is used for the purpose of making it permeate. However, at present, the performance is not sufficient.
As membrane base materials mainly composed of hydrocarbon-based resins, porous membranes that are not simply ion-selective by simply isolating the electrolyte containing the electrolyte that plays the main role in both cells, and those that are not ion-selective (non-porous) A hydrophilic membrane substrate, a porous membrane in which a hydrophilic membrane substrate is embedded or coated, and the like are used. Further, a so-called cation exchange membrane having various anion groups, or a composite membrane in which a cation exchange resin is coated or embedded in pores of a porous membrane substrate, similarly, an anion exchange membrane in which the membrane itself has a cation group, Similarly, a composite membrane in which an anion exchange resin is coated or embedded on a porous membrane base material, a laminated type of the two, and the like are used as a diaphragm, and researches that make use of the respective characteristics are being conducted.
As the diaphragm, an ion-exchange resin diaphragm that sufficiently satisfies the two contradictory properties of electrical resistance (mainly dependent on proton permeability) and metal ion (polyvalent cation) permeability blocking, which is the main active material. Has not been developed so far. Fluorine ion exchange resins are also excellent in proton (H ⁺ ) permeability and have not been fully studied for contradictory properties of suppressing the permeation of active material ions. An electrolyte membrane for a redox flow battery that sufficiently satisfies oxidation resistance (hydroxy radical resistance) has not been developed.

In view of the above circumstances, the present invention has excellent ion selective permeability that can suppress the ion permeability of the active material without deteriorating proton (H ⁺ ) permeability, has low electrical resistance, An object of the present invention is to provide an electrolyte membrane for a redox flow secondary battery that is excellent in efficiency and also has oxidation resistance (hydroxy radical resistance) and a redox flow secondary battery using the same.

  As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention include a fluorine-based polymer electrolyte polymer having a specific structure, and the electrolyte membrane has a laminated structure of three or more layers. By making the equilibrium moisture content of the outer layer adjacent to the higher than the equilibrium moisture content of the intermediate layer that is not adjacent to either the positive electrode or the negative electrode, it has excellent ion selective permeability, low electrical resistance, and high current efficiency. In addition, the present inventors have found that an electrolyte membrane for a redox flow secondary battery and a redox flow secondary battery using the same can be achieved that have excellent oxidation resistance and resistance to oxidation degradation (hydroxy radical resistance).

That is, the present invention is as follows.
[1]
A positive electrode cell chamber including a positive electrode made of a carbon electrode;
A negative electrode cell chamber including a negative electrode made of a carbon electrode;
An electrolyte membrane as a diaphragm for separating and separating the positive electrode cell chamber and the negative electrode cell chamber;
Having an electrolytic cell containing
The positive electrode cell chamber contains a positive electrode electrolyte containing an active material, and the negative electrode cell chamber contains a negative electrode electrolyte containing an active material,
A redox flow secondary battery that charges and discharges based on a valence change of an active material in the electrolyte solution,
The electrolyte membrane includes an ion exchange resin composition mainly composed of a fluorine-based polymer electrolyte polymer having a structure represented by the following formula (1):
The redox flow secondary battery in which the electrolyte membrane has a laminated structure of three or more layers, and the equilibrium water content of the outer layer adjacent to the positive electrode and the negative electrode is higher than the equilibrium water content of the intermediate layer not adjacent to the positive electrode or the negative electrode .
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, Selected from the group consisting of C1-C10 perfluoroalkyl groups and fluorochloroalkyl groups A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1. D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time).
[2]
The redox flow secondary battery according to the above [1], wherein the redox flow secondary battery is a vanadium redox flow secondary battery using a sulfuric acid electrolyte containing vanadium as a positive electrode and a negative electrode electrolyte.
[3]
The redox flow secondary battery according to [1] or [2], wherein the fluorine-based polymer electrolyte polymer is a perfluorocarbon sulfonic acid resin (PFSA) having a structure represented by the following formula (2).
_{- [CF 2 CF 2] a} - [CF 2 -CF ((- O- (CF 2) m -X 4)] g - (2)
(In the formula (2), a and g represents a number satisfying 0 ≦ a <1,0 <g ≦ 1, a + g = 1, m represents an integer of 1 to 6, the X ⁴ is SO ₃ H Show.)
[4]
Equivalent mass EW (dry mass in grams per equivalent of ion-exchange groups) of the fluorine-based polymer electrolyte polymer constituting the outer layer is 300 to 900 g / eq, equivalent mass of the fluorine-based polymer electrolyte polymer constituting the intermediate layer Any of the above [1] to [3], wherein the equilibrium water content of the outer layer is 15 to 150% by mass and the equilibrium water content of the intermediate layer is 0.1 to 100% by mass. Or a redox flow secondary battery.
[5]
An ion exchange resin composition mainly comprising a fluorine-based polymer electrolyte polymer having a structure represented by the following formula (1);
An electrolyte membrane for a redox flow secondary battery having a laminated structure of three or more layers, wherein an equilibrium water content of an outer layer adjacent to the positive electrode and the negative electrode is higher than an equilibrium water content of an intermediate layer not adjacent to the positive electrode or the negative electrode.
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, From the group consisting of a C 1-10 perfluoroalkyl group and a fluorochloroalkyl group A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1 D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time).
[6]
The electrolyte membrane for a redox flow secondary battery according to the above [5], wherein the fluoropolymer electrolyte polymer is a perfluorocarbon sulfonic acid resin (PFSA) having a structure represented by the following formula (2).
_{- [CF 2 CF 2] a} - [CF 2 -CF ((- O- (CF 2) m -X 4)] g - (2)
(In the formula (2), a and g represents a number satisfying 0 ≦ a <1,0 <g ≦ 1, a + g = 1, m represents an integer of 1 to 6, the X ⁴ is SO ₃ H Show.)
[7]
Equivalent mass EW (dry mass in grams per equivalent of ion-exchange groups) of the fluorine-based polymer electrolyte polymer constituting the outer layer is 300 to 900 g / eq, equivalent mass of the fluorine-based polymer electrolyte polymer constituting the intermediate layer The above-mentioned [5] or [6], wherein is 600 to 1300 g / eq, the equilibrium water content of the outer electrolyte membrane is 15 to 150% by mass, and the equilibrium water content of the intermediate layer is 0.1 to 100% by mass. ] The electrolyte membrane for redox flow secondary batteries of description.
[8]
The electrolyte membrane for a redox flow secondary battery according to any one of [5] to [7], wherein the electrolyte membrane is heat-treated at 130 to 200 ° C. for 1 to 60 minutes.

  The electrolyte membrane for a redox flow secondary battery of the present invention has excellent ion selective permeability. Therefore, it has high proton (hydrogen ion) permeability, low electrical resistance, and can suppress the permeation of active material ions in the electrolytic solution. Furthermore, in order to demonstrate high current efficiency and to exhibit a high oxidative degradation prevention effect for a long time against hydroxy radicals generated in the electrolyte solution cell in the system, a normal hydrocarbon electrolyte was used. It is possible to suppress the detachment of ionic groups and the collapse phenomenon of the polymer electrolyte.

An example of the schematic diagram of the redox flow secondary battery in this embodiment is shown.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following this embodiment.

The redox flow secondary battery in this embodiment is
A positive electrode cell chamber including a positive electrode made of a carbon electrode;
A negative electrode cell chamber including a negative electrode made of a carbon electrode;
An electrolyte membrane as a diaphragm for separating and separating the positive electrode cell chamber and the negative electrode cell chamber;
Having an electrolytic cell containing
The positive electrode cell chamber contains a positive electrode electrolyte containing an active material, and the negative electrode cell chamber contains a negative electrode electrolyte containing an active material,
A redox flow secondary battery that charges and discharges based on a valence change of an active material in the electrolyte solution,
The electrolyte membrane includes an ion exchange resin composition mainly composed of a fluorine-based polymer electrolyte polymer having a structure represented by the following formula (1):
The redox flow secondary battery in which the electrolyte membrane has a laminated structure of three or more layers, and the equilibrium water content of the outer layer adjacent to the positive electrode and the negative electrode is higher than the equilibrium water content of the intermediate layer not adjacent to the positive electrode or the negative electrode It is.
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, Selected from the group consisting of C1-C10 perfluoroalkyl groups and fluorochloroalkyl groups A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1. D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time).

  FIG. 1 shows an example of a schematic diagram of a redox flow secondary battery in the present embodiment. The redox flow secondary battery 10 in this embodiment includes a positive electrode cell chamber 2 including a positive electrode 1 made of a carbon electrode, a negative electrode cell chamber 4 including a negative electrode 3 made of a carbon electrode, the positive electrode cell chamber 2, and the negative electrode cell. An electrolytic cell 6 including an electrolyte membrane 5 as a diaphragm that separates and separates the chamber 4 from the chamber 4, the positive electrode cell chamber 2 containing a positive electrode electrolyte containing an active material, and the negative electrode cell chamber 4 containing an active material. Includes negative electrode electrolyte. The positive electrode electrolyte and the negative electrode electrolyte containing the active material are stored, for example, by the positive electrode electrolyte tank 7 and the negative electrode electrolyte tank 8, and are supplied to each cell chamber by a pump or the like. In addition, the current generated by the Redox flow secondary battery may be converted from direct current to alternating current via the AC / DC converter 9.

The redox flow secondary battery according to the present embodiment has a liquid-permeable porous collector electrode (for negative electrode and positive electrode) arranged on both sides of the diaphragm, sandwiched by pressing, and partitioned by the diaphragm. Is a positive electrode cell chamber and the other is a negative electrode cell chamber, and the thickness of both cell chambers is secured by a spacer.
In the case of a vanadium-based redox flow secondary battery, a positive electrode electrolyte composed of a sulfuric acid electrolyte containing vanadium tetravalent (V ⁴⁺ ) and pentavalent (V ⁵⁺ ) in the positive electrode cell chamber, The battery is charged and discharged by circulating a negative electrode electrolyte containing vanadium trivalent (V ³⁺ ) and divalent (V ²⁺ ). At this time, at the time of charging, V ⁴⁺ is oxidized to V ⁵⁺ because vanadium ions emit electrons in the positive electrode cell chamber, and V ³⁺ is changed to V ² by electrons returning through the outer path in the negative cell chamber. Reduced to ⁺ . In this oxidation-reduction reaction, protons (H ⁺ ) are excessive in the positive electrode cell chamber, while protons (H ⁺ ) are insufficient in the negative electrode cell chamber. The diaphragm selectively moves excess protons in the positive electrode cell chamber to the negative electrode chamber, thereby maintaining electrical neutrality. The reverse reaction proceeds during discharge. The current efficiency (%) at this time is expressed as a ratio (%) obtained by dividing the discharge power amount by the charge power amount. Both power amounts depend on the internal resistance of the battery cell, the ion selectivity of the diaphragm, and other current losses. To do. A decrease in internal resistance improves voltage efficiency, and an improvement in ion selectivity and other reductions in current loss improve current efficiency, and thus are important indicators for redox flow secondary batteries.

  The electrolyte membrane for a redox flow secondary battery in the present embodiment includes an ion exchange resin composition mainly composed of a fluorine-based polymer electrolyte polymer having a specific structure, and has a laminated structure of three or more layers, and a positive electrode And the equilibrium moisture content of the outer layer adjacent to the negative electrode is higher than the equilibrium moisture content of the intermediate layer not adjacent to the positive electrode or the negative electrode.

  In this embodiment, “mainly” means that the corresponding component is contained in the resin composition in an amount of preferably 50 to 100% by mass, more preferably 80 to 100% by mass, and still more preferably 90 to 100% by mass. Say.

The fluorine-based polymer electrolyte polymer in the present embodiment has a structure represented by the following formula (1).
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, Selected from the group consisting of C1-C10 perfluoroalkyl groups and fluorochloroalkyl groups A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1. D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time).

X ¹ , X ² and X ³ each independently represent one or more selected from the group consisting of a halogen atom and a C 1-3 perfluoroalkyl group. Here, examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. X ¹ , X ² and X ³ are preferably a fluorine atom or a C 1 to C 3 perfluoroalkyl group from the viewpoint of chemical stability of the polymer.

X ⁴ represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ. In the present specification, X ⁴ is also referred to as an ion exchange group. Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ). Here, it does not specifically limit as an alkali metal atom, A lithium atom, a sodium atom, a potassium atom, etc. are mentioned. Moreover, it does not specifically limit as an alkaline-earth metal atom, A calcium atom, a magnesium atom, etc. are mentioned. R ₁ , R ₂ , R ₃ and R ₄ each independently represent one or more selected from the group consisting of an alkyl group and an arene group. Here, when X ⁴ is PO ₃ Z ₂ , Z may be the same or different. X ⁴ is preferably SO ₃ Z from the viewpoint of chemical stability of the polymer.

R ¹ and R ² each independently represent one or more selected from the group consisting of a halogen atom, a C 1-10 perfluoroalkyl group and a fluorochloroalkyl group. Here, examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

  a and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1. b shows the integer of 0-8. c represents 0 or 1; d, e, and f each independently represent an integer of 0-6. However, d, e, and f are not 0 at the same time.

  The fluoropolymer electrolyte polymer in the present embodiment is preferably a perfluorocarbon sulfonic acid resin (PFSA resin) because the effects of the present invention tend to become more prominent. The PFSA resin in this embodiment has a main chain composed of a Teflon (registered trademark) skeleton chain, a perfluorocarbon as a side chain, and one to two or more sulfonic acid groups in each side chain (some of which are partially salted). It may be in the form of

The PFSA resin in the present embodiment may contain a repeating unit represented by-(CF ₂ -CF ₂ )-and a repeating unit derived from a compound represented by the following formula (3) or (4). Preferably, it further comprises a repeating unit represented by — (CF ₂ —CF ₂ ) — and a repeating unit derived from the compound represented by Formula (3) or Formula (4).
Equation _{(3): CF 2 = CF} -O- (CF 2 CFXO) n - in [A] (wherein, X represents F or a perfluoroalkyl group having 1 to 3 carbon atoms, n represents 0-5 an integer. [a] is (CF _{2) m} -W (an integer of m 0-6. Here, n and m are not 0 at the same time .W shows the SO ₃ H.),
Or formula _{(4): CF 2 = CF} -O- (CF 2) P -CFX (-O- (CF 2) K -W) or _{CF 2 = CF-O- (CF} 2) P -CFX (- ( _{CF 2) L -O- (CF 2} ) m -W) ( wherein, X represents a perfluoroalkyl group having 1 to 3 carbon atoms, P is an integer of 0 to 12, k is 1 to 5 L represents an integer of 1 to 5, m represents an integer of 0 to 6. However, k and L may be the same or different, and P, K, and L are simultaneously 0. Must not.)

PFSA resin, - and a repeating unit represented _{by, - - (CF 2 -CF 2} ) (CF 2 -CF (-O- (CF 2 CFXO) n - (CF 2) m -SO 3 H)) - (Wherein X represents F or CF ₃ , n represents an integer of 0 to 5, and m represents an integer of 0 to 12. However, n and m are not 0 at the same time. )) Is more preferable. When the PFSA resin is a copolymer having the above structure and has a predetermined equivalent mass EW, the obtained electrolyte membrane has sufficient hydrophilicity and has high resistance to radical species generated by oxidative degradation. There is a tendency.

Further, _{n in the} repeating unit of the PFSA resin represented by — (CF ₂ —CF (—O— (CF ₂ CFXO) _n — (CF ₂ ) _m —SO ₃ H)) — is 0, m those There is an integer from 1 to 6, or _{CF 2 = CF-O- (CF} 2) represented by the formula _{(4) P -CFX (-O- (} CF 2) K -W) and CF ₂ = CF In the case where both repeating units of —O— (CF ₂ ) _P —CFX (— (CF ₂ ) _L —O— (CF ₂ ) _m —W) are contained, the equivalent mass EW is lowered, and the hydrophilicity of the obtained electrolyte membrane Tend to be higher.

The fluoropolymer electrolyte polymer in the present embodiment is a perfluorocarbon sulfonic acid resin (PFSA) having a structure represented by the following formula (2) because the effects of the present invention tend to become more prominent. Is preferred.
_{- [CF 2 CF 2] a} - [CF 2 -CF ((- O- (CF 2) m -X 4)] g - (2)
(In the formula (2), a and g represents a number satisfying 0 ≦ a <1,0 <g ≦ 1, a + g = 1, m represents an integer of 1 to 6, the X ⁴ is SO ₃ H Show.)

The fluorine-based polymer electrolyte polymer in the present embodiment can be obtained, for example, by producing a polymer electrolyte polymer precursor (hereinafter also referred to as “resin precursor”) and then hydrolyzing it.
In the case of a PFSA resin, for example, a PFSA resin precursor composed of a copolymer of a fluorinated vinyl ether compound represented by the following general formula (5) and a fluorinated olefin monomer represented by the following general formula (6) is added with water. It is obtained by decomposing.

_{CF 2 = CF-O- (CF} 2 CFXO) n - [A] (5)
(In the formula, X represents F or a C 1 to C 3 perfluoroalkyl group, n represents an integer of 0 to 5, and A represents (CF ₂ ) _m —W or CF ₂ ═CF—O—. _{(CF 2) P -CFX (-O-} (CF 2) K -W) or _{CF 2 = CF-O- (CF} 2) P -CFX (- (CF 2) L -O- (CF 2) m - W), p represents an integer of 0 to 12, m represents an integer of 0 to 6 (provided that n and m are not 0 simultaneously), k represents an integer of 1 to 5, and L represents indicates an integer of 1 to 5 (where, n and L or K is not 0 at the same time.), W represents a functional group which can be converted to SO ₃ H by hydrolysis.)
CF ₂ = CFZ (6)
(In the formula, Z represents H, Cl, F, a perfluoroalkyl group having 1 to 3 carbon atoms, or a cyclic perfluoroalkyl group that may contain oxygen.)

The W shown a functional group capable of conversion to SO ₃ H by hydrolysis in the above formula (5) is not particularly _{limited, SO 2 F, SO 2 Cl} , SO 2 Br are preferred. In the above formula, it is more preferable that X = CF ₃ , W = SO ₂ F, and Z = F. Among them, n = 0, m = 1 to 6 (however, n and m are not 0 at the same time), and X = CF ₃ , W = SO ₂ F, and Z = F are highly hydrophilic. It is particularly preferable because it tends to obtain a solution with high resin concentration.

  The fluorine-based polymer electrolyte polymer precursor in the present embodiment can be synthesized by a known means. For example, in a polymerization method using a peroxide of a radical generator, a vinyl fluoride compound having the above ion exchange group precursor and tetrafluoroethylene (TFE) using a polymerization solvent such as a fluorinated hydrocarbon Polymerization by filling and dissolving fluorinated olefin gas (solution polymerization), Polymerization using vinyl fluoride compound itself as polymerization solvent without using solvents such as fluorine-containing hydrocarbons (bulk polymerization) , Using a surfactant aqueous solution as a medium, a method of polymerizing by filling and reacting a vinyl fluoride compound and a fluorinated olefin gas (emulsion polymerization), an aqueous solution of a co-emulsifier such as a surfactant and alcohol, A method of polymerization (emulsion polymerization) and suspension by filling a gas of vinyl fluoride compound and fluorinated olefin, emulsifying and reacting them. A method of polymerization by reacting filled suspending fluoride vinyl compound and the fluorinated olefin gas in an aqueous solution of Jozai (suspension) and the like are known.

  In the present embodiment, any of those prepared by any of the polymerization methods described above can be used. Moreover, the polymer of a block shape or a taper shape obtained by adjusting superposition | polymerization conditions, such as the supply amount of TFE gas, may be sufficient.

  In addition, the fluorine-based polymer electrolyte polymer precursor is prepared by removing impurities generated in the resin molecular structure during the polymerization reaction or a portion (CO group, H bond portion, etc.) that is structurally susceptible to oxidation by a known method. It may be treated under and the part may be fluorinated.

  The molecular weight of the resin precursor is 0.05 to 50 (melt flow index (MFI) value measured according to ASTM: D1238 (measurement conditions: temperature 270 ° C., load 2160 g). g / 10 minutes). The preferable range of MFI of the precursor resin is 0.1 to 30 (g / 10 minutes), and the more preferable range is 0.5 to 20 (g / 10 minutes).

Fluoropolymer electrolyte polymer resin precursor is extruded with a nozzle or die using an extruder and then subjected to hydrolysis treatment or as a product produced by polymerization, that is, dispersed liquid, or precipitated, filtered After making it into the made powdery thing, a hydrolysis process is performed. The shape of the resin precursor is not particularly limited, but from the viewpoint of increasing the treatment speed in the hydrolysis treatment and acid treatment described later, it is in the form of pellets of 0.5 cm ³ or less, or in the form of dispersed liquid or powder particles. Among them, it is preferable to use a powdery product after polymerization. From the viewpoint of cost, an extruded film-like resin precursor may be used.

  The resin precursor obtained as described above and molded as necessary is subsequently immersed in a basic reaction liquid and hydrolyzed. The basic reaction solution used for the hydrolysis treatment is not particularly limited, but is an aqueous solution of an amine compound such as dimethylamine, diethylamine, monomethylamine and monoethylamine, or hydroxide of an alkali metal or alkaline earth metal. An aqueous solution of the product is preferable, and an aqueous solution of sodium hydroxide and potassium hydroxide is particularly preferable. When the alkali metal or alkaline earth metal hydroxide is used, the content is not particularly limited, but is preferably 10 to 30% by mass with respect to the entire reaction solution. More preferably, the reaction solution further contains a swellable organic compound such as methyl alcohol, ethyl alcohol, acetone and DMSO. The content of the swellable organic compound is preferably 1 to 30% by mass with respect to the entire reaction solution.

  The resin precursor is hydrolyzed in the basic reaction liquid, then sufficiently washed with warm water and the like, and then acid-treated. The acid used for the acid treatment is not particularly limited, but preferred are mineral acids such as hydrochloric acid, sulfuric acid and nitric acid, and organic acids such as oxalic acid, acetic acid, formic acid and trifluoroacetic acid, and a mixture of these acids and water. Is more preferable. Moreover, the said acids may be used independently or may use 2 or more types together. The basic reaction solution used in the hydrolysis treatment may be removed in advance before the acid treatment, for example, by treatment with a cation exchange resin.

By the acid treatment, the resin precursor is protonated to generate ion exchange groups. For example, W of the PFSA resin precursor is protonated by acid treatment to become SO ₃ H. The fluorine-based polymer electrolyte polymer obtained by the hydrolysis and acid treatment can be dispersed or dissolved in a protic organic solvent, water, or a mixed solvent of both.

  The electrolyte membrane having a laminated structure of three or more layers in the present embodiment may be produced by molding the films forming each layer and then laminating them, or by extruding the resin forming each layer simultaneously or You may produce by casting.

  The electrolyte membrane for a redox flow secondary battery in the present embodiment has a laminated structure of three or more layers, and includes an outer layer adjacent to the positive electrode and the negative electrode, and an intermediate layer not adjacent to the positive electrode and the negative electrode. In this embodiment, the equivalent mass EW of the fluorine-based polymer electrolyte polymer constituting the outer layer (dry mass in grams of the fluorine-based polymer electrolyte polymer per equivalent of ion-exchange groups) is adjusted to 300 to 900 (g / eq). It is preferable to adjust to 500 to 900 (g / eq). The equivalent mass EW of the polymer electrolyte polymer constituting the intermediate layer is configured to be higher than the equivalent mass EW constituting the outer layer, and is preferably adjusted to 600 to 1300 (g / eq), more preferably 700 to 1200. Adjust to (g / eq).

  By adjusting the equivalent mass EW of the fluoropolymer electrolyte polymer constituting the outer layer and the intermediate layer to each of the above ranges, it is possible to impart excellent hydrophilicity to the ion exchange resin composition containing the same, and its resin composition Electrolyte membranes obtained by using materials have low electrical resistance, high hydrophilicity, and many smaller clusters (small parts where ion exchange groups coordinate and / or adsorb water molecules), and have high oxidation resistance It tends to exhibit (hydroxy radical resistance), low electrical resistance, and good ion selective permeability.

  The equivalent mass EW of the entire fluoropolymer electrolyte polymer is preferably 300 (g / eq) or more from the viewpoint of hydrophilicity and water resistance of the film, and 1300 (g / eq) from the viewpoint of hydrophilicity and electrical resistance of the film. eq) It is preferable that it is below. In addition, when the EW of the fluoropolymer electrolyte polymer is close to the lower limit, the resin can be obtained by directly or indirectly partially cross-linking the molecules of the ion exchange groups on the side chain of the membrane. It may be modified to control solubility and excessive swelling.

Examples of the partial crosslinking reaction include a reaction between an ion exchange group and a functional group or main chain of another molecule, a reaction between ion exchange groups, an oxidation-resistant low molecular compound, an oligomer, or a high molecular substance. In some cases, it may be a reaction with a salt (including an ionic bond with a SO ₃ H group) forming substance. Examples of the oxidation-resistant low molecular weight compound, oligomer or polymer substance include polyhydric alcohols and organic diamines.

  When the partial crosslinking reaction is performed, the EW of the fluorine-based polymer electrolyte polymer may be about 280. That is, the water solubility should be reduced (water resistance improved) without sacrificing the ion exchange group (in other words, EW). The same applies when the fluorine-based polymer electrolyte polymer is in a low melt flow region (polymer region) and there are many intermolecular entanglements.

Moreover, the functional group (for example, SO ₂ F group) before hydrolysis of the fluorine-based polymer electrolyte polymer is partially imidized (including alkylimidation) partially (including intermolecular). Good.

  The equivalent mass EW of the fluorine-based polymer electrolyte polymer can be measured by subjecting the fluorine-based polymer electrolyte polymer to salt substitution and back titrating the solution with an alkaline solution.

  Further, the equivalent mass EW of the fluorine-based polymer electrolyte polymer can be adjusted by the copolymerization ratio of the fluorine-based monomer, the selection of the monomer type, and the like.

Nafion is a fluorine-based resin described in Patent Document described above (Nafion: DuPont _{TM), - (CF 2 -CF 2)} - and repeating units represented by, - (CF ₂ -CF (- And a repeating unit represented by O— (CF ₂ CFXO) _n — (CF ₂ ) _m —SO ₃ H)) —, wherein X═CF ₃ , n = 1, and m = 2. , EW is known to be 893-1030. However, when Nafion is used as a material for the electrolyte membrane of a redox flow secondary battery, the hydrophilicity is insufficient, the electric resistance is high, the selective ion permeability and the current efficiency tend to deteriorate.

  As content of the fluorine-type polymer electrolyte polymer contained in the ion exchange resin composition which forms the electrolyte membrane in this embodiment, Preferably it is about 33.3-100 mass%, More preferably, it is 40-100 mass%, More preferably, it is 50-99.5 mass%.

  When the ion exchange resin composition in the present embodiment contains a basic polymer (including a low molecular weight substance such as an oligomer) in addition to the above-described fluorine-based polymer electrolyte polymer, chemical stability as the resin composition is achieved. Tend to increase the property (mainly oxidation resistance). These compounds partially form an ion complex in the form of fine particles or close to molecular dispersion in the resin composition to form an ion cross-linked structure. In particular, when the EW of the fluoropolymer electrolyte polymer is low (300 to 500 g / eq), it is preferable from the viewpoint of balance between water resistance and electric resistance.

  In addition, the fluorine-based polymer electrolyte polymer is a partial salt (total ion-exchange group equivalent of 0.01 to 0.01) with an alkali metal, alkaline earth metal, or other radical-degradable transition metal (Ce compound, Mn compound, etc.). May be used alone or in combination with a basic polymer.

  As the fluorine-based polymer electrolyte polymer in the present embodiment, fluorine-based resins other than PFSA resins (resins containing carboxylic acid, phosphoric acid, etc. and other known fluorine-based resins) can be used. When two or more of these resins are used, they may be mixed in a solvent or dispersed in a medium, or resin precursors may be extruded and mixed.

  In the electrolyte membrane in the present embodiment, the equilibrium moisture content of the outer layer adjacent to the positive electrode and the negative electrode is set higher than the equilibrium moisture content of the intermediate layer not adjacent to the positive electrode or the negative electrode. The equilibrium moisture content of the outer layer of the electrolyte membrane is preferably 15% by mass or more, more preferably 20% by mass or more. Moreover, as an upper limit, it is 150 mass% or less, More preferably, it is 100 mass% or less. On the other hand, the equilibrium moisture content of the intermediate layer of the electrolyte membrane is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 1% by mass or more. Moreover, as an upper limit, it is 100 mass% or less, More preferably, it is 50 mass% or less, More preferably, it is 20 mass% or less.

  When the equilibrium moisture content of the entire electrolyte membrane is 5% by mass or more, the electric resistance, current efficiency, oxidation resistance, and ion selective permeability of the membrane tend to be good. On the other hand, when the equilibrium moisture content is 50% by mass or less, the dimensional stability and strength of the membrane are improved, and the increase in water-soluble components tends to be suppressed. The equilibrium moisture content of the electrolyte membrane is based on a membrane obtained by forming a resin composition from a dispersion of water and an alcohol-based solvent and drying at 160 ° C. or lower, and the equilibrium is 23 ° C. and 50% relative humidity (RH). It is expressed as saturated water absorption (Wc).

  The equilibrium moisture content of the electrolyte membrane can be adjusted by the same method as that for EW described above.

  The method for producing an electrolyte membrane (film formation method) in the present embodiment is not particularly limited, and a known extrusion method or cast film formation can be used. The electrolyte membrane has a laminated structure of three or more layers, and the performance of the electrolyte membrane can be improved by laminating films having different properties (for example, resins having different functional groups).

  In addition, the electrolyte membrane formed by the above method is thoroughly washed with water (or treated with a dilute aqueous acidic solution such as hydrochloric acid, nitric acid, sulfuric acid or the like before washing with water if necessary) to remove impurities. The film is preferably heat-treated in air (preferably in an inert gas) at 130 to 200 ° C., preferably 140 to 180 ° C., more preferably 150 to 170 ° C. for 1 to 30 minutes. The heat treatment time is more preferably 2 to 20 minutes, further preferably 3 to 15 minutes, and particularly preferably about 5 to 10 minutes.

  One of the reasons for performing the above treatment is that the raw material-derived particles (between the primary particles and the secondary particles) and the molecules are not sufficiently entangled with each other in the state as they are at the time of film formation. For the purpose of entanglement of water, it is particularly useful for water resistance (especially lowering the hot water-dissolved component ratio), stabilizing the saturated water absorption rate of water, and generating stable clusters. It is also useful from the viewpoint of improving the film strength. This is particularly useful when the cast film forming method is used.

  Another reason is that by forming minute intermolecular cross-links between molecules of the fluorine-based polymer electrolyte polymer, it contributes to water resistance and stable cluster formation, and the cluster diameter is made uniform and It is estimated that there is an effect to make it smaller.

  Furthermore, the ion exchange group of the fluorine-based polyelectrolyte polymer in the resin composition reacts with an active reaction site (such as an aromatic ring) of other additives (including the resin) at least a part thereof. Therefore, it is presumed that minute cross-links are generated and stabilized (particularly due to the reaction of ion-exchange groups existing in the vicinity of other resin components that are dispersed additives). The degree of this cross-linking is preferably 0.001 to 5%, more preferably 0.1 to 3%, still more preferably 0.2 to 0.2% in terms of EW (degree of EW reduction before and after heat treatment). About 2%.

  In addition, when excessive treatment exceeding the upper limit of the above treatment conditions (time, temperature) is performed, defluorination, dehydrofluoric acid, desulfonic acid, thermal oxidation sites, etc. increase, and defects in the molecular structure occur. From that point of view, the oxidation degradation resistance tends to deteriorate during actual use as an electrolytic membrane. On the other hand, if it is less than the lower limit of the processing conditions, the effect of the above-described main processing may be insufficient.

  The electrolyte membrane in this embodiment has excellent ion selective permeability, low electrical resistance, excellent durability (mainly hydroxyl radical oxidation resistance), and was excellent as a diaphragm for redox flow secondary batteries. Demonstrate performance. In addition, each physical property in this specification can be measured according to the method described in the following Examples, unless otherwise specified.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist.

[Measuring method]
(1) Melt flow index of PFSA resin precursor Based on ASTM: D1238, measurement was carried out under conditions of measurement: temperature 270 ° C. and load 2160 g.

(2) Measurement of equivalent mass EW of PFSA resin 0.3 g of PFSA resin was immersed in 30 mL of a saturated aqueous NaCl solution at 25 ° C. and left for 30 minutes with stirring. Subsequently, the free protons in the saturated NaCl aqueous solution were subjected to neutralization titration with 0.01N aqueous sodium hydroxide solution using phenolphthalein as an indicator. The end point of the neutralization titration is set to pH 7, and the PFSA resin component in which the counter ion of the ion exchange group is in a sodium ion state is rinsed with pure water obtained after the neutralization titration, and further at 160 ° C. with an upper dish dryer. Dried and weighed. The amount of sodium hydroxide required for neutralization is M (mmol), and the mass of the PFSA resin in which the counter ion of the ion exchange group is in a sodium ion state is W (mg). g / eq).
EW = (W / M) −22
After the above operation was repeated five times, the measurement values were obtained by arithmetically averaging the three values except for the maximum and minimum values of the calculated five EW values.

(3) Measurement of equilibrium moisture content A dispersion of PFSA resin was applied on a clear glass plate, dried at 150 ° C. for about 10 minutes, peeled to form a film of about 30 μm, and this was immersed in water at 23 ° C. The equilibrium moisture content was measured after standing for about 3 hours and then leaving it in a room at 23 ° C. and a relative humidity (RH) of 50% for 24 hours. As a standard dry film, an 80 ° C. vacuum dry film was used. The equilibrium moisture content was calculated from the change in mass of the membrane.

(4) Measurement of maximum water content A dispersion of PFSA resin was applied on a clear glass plate, dried at 150 ° C. for about 10 minutes, and peeled to form a film of about 30 μm. The maximum water content was measured when the surface water was wiped off and measured for about 3 hours. As a standard dry film, an 80 ° C. vacuum dry film was used. The maximum water content was calculated from the change in mass of the membrane.

(5) Charging / discharging test Redox flow secondary batteries have liquid-permeable porous collector electrodes (for negative electrode and positive electrode) arranged on both sides of the diaphragm on both sides of the diaphragm. One was sandwiched and separated by a diaphragm, the positive electrode cell chamber, the other was the negative electrode cell chamber, and the thickness of both cell chambers was secured with a spacer. The positive electrode cell chamber is composed of a sulfuric acid electrolyte containing vanadium tetravalent (V ⁴⁺ ) and pentavalent (V ⁵⁺ ), and the negative electrode cell chamber is composed of vanadium trivalent (V ³⁺ ) and the same. A negative electrode electrolyte containing divalent (V ²⁺ ) was circulated to charge and discharge the battery. At this time, at the time of charging, V ⁴⁺ is oxidized to V ⁵⁺ because vanadium ions emit electrons in the positive electrode cell chamber, and V ³⁺ is changed to V ² by electrons returning through the outer path in the negative cell chamber. Reduced to ⁺ . In this oxidation-reduction reaction, protons (H ⁺ ) are excessive in the positive electrode cell chamber, while protons (H ⁺ ) are insufficient in the negative electrode cell chamber. The diaphragm selectively moves excess protons in the positive electrode cell chamber to the negative electrode chamber, thereby maintaining electrical neutrality. The reverse reaction proceeds during discharge. The battery efficiency (energy efficiency) (%) at this time is expressed as a ratio (%) obtained by dividing the discharge power amount by the charge power amount, and both the power amounts are the ion resistance of the battery cell and the ion selective permeability of the diaphragm. Others depend on current loss. The current efficiency (%) is expressed as a ratio (%) obtained by dividing the amount of discharged electricity by the amount of charged electricity, and both the amounts of electricity depend on the ion selective permeability of the diaphragm and other current losses. Battery efficiency is expressed as the product of current efficiency and voltage efficiency. A decrease in internal resistance, ie, cell electrical resistivity, improves voltage efficiency, and an improvement in ion selective permeability and other reductions in current loss improve current efficiency, and thus are important indicators in redox flow secondary batteries.
The charge / discharge experiment was performed using the battery obtained as described above. Use an aqueous electrolyte with a total vanadium concentration of 2 M / L and a total sulfate radical concentration of 4 M / L, and the thickness of the installed positive and negative electrode cell chambers is 5 mm, respectively, between the porous electrode and the diaphragm. Was used by sandwiching a felt made of carbon fiber having a thickness of 5 mm and a bulk density of about 0.1 g / cm ³ . The charge / discharge experiment was conducted at a current density of 80 mA / cm ² .
The cell electrical resistivity was determined by measuring the DC resistance value at an AC voltage of 10 mV and a frequency of 20 kHz at the start of discharge using the AC impedance method, and multiplying this by the electrode area.

Example 1
(1) Preparation of PFSA resin precursor A 10% aqueous solution of C ₇ F ₁₅ COONH ₄ and pure water were charged into a stainless steel stirring autoclave, and after sufficient vacuum and nitrogen substitution, tetrafluoroethylene (CF ₂ = CF ₂ ) (hereinafter also abbreviated as “TFE”) Gas was introduced and the pressure was increased to 0.7 MPa at a cage pressure. Subsequently, polymerization was initiated by injecting an aqueous ammonium persulfate solution. In order to replenish the TFE consumed by the polymerization, the TFE gas is continuously supplied to keep the pressure of the autoclave at 0.7 MPa, which corresponds to 0.70 times in mass ratio with respect to the supplied TFE. An amount of CF ₂ ═CFO (CF ₂ ) ₂ —SO ₂ F is continuously supplied to perform polymerization, and the polymerization conditions are adjusted to the optimum ranges, thereby obtaining various perfluorocarbon sulfonic acid resin precursor powders. It was. The MFI of the obtained PFSA resin precursor powder was 1.5 (g / 10 min) for both A1 and A2.

(2) Preparation of perfluorocarbon sulfonic acid resin and dispersion thereof The obtained PFSA resin precursor powder was dissolved in an aqueous solution in which potassium hydroxide (15% by mass) and methyl alcohol (50% by mass) were dissolved at 80 ° C. For 20 hours to perform hydrolysis treatment. Then, it was immersed in 60 degreeC water for 5 hours. Next, the treatment of immersing in a 2N aqueous hydrochloric acid solution at 60 ° C. for 1 hour was repeated 5 times by updating the aqueous hydrochloric acid solution every time, and then washed with ion-exchanged water and dried. Thereby, a PFSA resin having a sulfonic acid group (SO ₃ H) and a structure represented by the formula (1) was obtained. The EW of the obtained PFSA resin was 527 (g / eq) for A1 and 910 (g / eq) for A2.

The obtained PFSA resin was placed in a 5 L autoclave together with an aqueous ethanol solution (water: ethanol = 50: 50 (mass ratio)), sealed, heated to 160 ° C. with stirring with a blade, and held for 5 hours. Thereafter, the autoclave was naturally cooled to prepare a 5% by mass uniform PFSA resin dispersion. Next, 100 g of pure water was added to 100 g of these PFSA resin dispersions and stirred, and then the liquid was concentrated to a solid content concentration of 20% by mass while heating and stirring at 80 ° C.
The obtained PFSA resin dispersion was used as dispersion (ASF1) and dispersion (ASF2) in the same order as described above.

(3) Production of electrolyte membrane The obtained dispersion liquid (ASF1 and ASF2) was cast on a polyimide film as a carrier sheet by a known ordinary method, and hot air at 120 ° C. (20 minutes) was applied. The film was obtained by almost completely removing the solvent and drying.
The equilibrium water content of the obtained membrane was 23% by mass for the membrane obtained from ASF1, and 7% by mass for the membrane obtained from ASF2. The maximum water content of the electrolyte membrane in water at 25 ° C. for 3 hours was 50% by mass for the membrane obtained from ASF1, and 15% by mass for the membrane obtained from ASF2. Here, the maximum moisture content indicates the maximum value observed when measuring the equilibrium moisture content.
Next, the film obtained from ASF2 is used as the intermediate layer, the film obtained from ASF1 is used as the outer layer, and the outer layer-intermediate layer-outer layer are laminated in this order, and bonded together at 25 ° C. with a vacuum laminator to form a three-layer structure. An electrolyte membrane was obtained.
This was further heat-treated in a hot air atmosphere at 160 ° C. for 10 minutes to obtain an electrolyte membrane having a thickness of 50 μm. The change rate of EW before and after the heat treatment of the obtained electrolyte membrane was about 0.2 to 0.3%.

Next, a charge / discharge test was performed using the electrolyte membrane as a diaphragm of a vanadium redox flow secondary battery. A charge / discharge experiment was performed after sufficiently equilibrating in the electrolytic solution, and then the cell electrical resistivity and current efficiency were measured after the battery was stabilized. The current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 98.3 / 0.95, indicating an excellent tendency.

Next, an endurance test was performed by carrying out 200 cycles of charge / discharge and examining the change. As a result, the current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 98.2 / 0.95, showing a very small change and excellent oxidation resistance.

(Example 2)
Example 1 except that Nafion DE2021 (manufactured by DuPont, 20% solution, EW1050, membrane equilibrium water content 4% by mass) was used instead of the 20% PFSA resin dispersion (ASF2) as the electrolyte polymer constituting the intermediate layer. In the same manner, an electrolyte membrane was obtained.
In addition, as a result of conducting a charge / discharge test in the same manner as in Example 1, the current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 97.5 / 0.90. Further, as a durability test, as a result of performing 200 cycles of charge and discharge, the current efficiency was 97.3% and the cell electric resistance was 0.90 Ω · cm ² .

(Comparative Example 1)
As the electrolyte polymer constituting the outer layer, Nafion DE2021 (manufactured by DuPont, 20% solution, EW1050, membrane equilibrium water content 4% by mass) is used instead of the 20% PFSA resin dispersion (ASF1), and the electrolyte polymer constituting the intermediate layer In the same manner as in Example 1 except that Nafion DE2020 (manufactured by DuPont, 20% solution, EW970, membrane equilibrium water content 6 mass%) is used instead of the 20% PFSA resin dispersion (ASF2). It was.
Moreover, as a result of conducting the charge / discharge test by the same method as in Example 1, the current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 94.5 / 1.20, which was compared with that of the Example. It was inferior. Moreover, as a result of carrying out 200 cycles of charging / discharging as an endurance test, the current efficiency was 86.0%, the cell electric resistance was 1.30 Ω · cm ² , and the durability was inferior to that of the examples.

(Comparative Example 2)
As the electrolyte polymer constituting the outer layer, Nafion DE2021 (manufactured by DuPont, 20% solution, EW1050, membrane equilibrium water content 4% by mass) is used instead of the 20% PFSA resin dispersion (ASF1), and the electrolyte polymer constituting the intermediate layer In the same manner as in Example 1, except that Nafion DE2021 (manufactured by DuPont, 20% solution, EW1050, membrane equilibrium water content 4 mass%) is used instead of the 20% PFSA resin dispersion (ASF2). It was.
In addition, as a result of conducting a charge / discharge test in the same manner as in Example 1, the current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 93.2 / 1.22, which was compared with that of the Example. It was inferior. Moreover, as a durability test, as a result of carrying out 200 cycles of charging and discharging, the current efficiency was 84.3%, the cell electric resistance was 1.40 Ω · cm ² , and the durability was inferior to that of the examples.

(Comparative Example 3)
Comparative Example 2 except that 20% PFSA resin dispersion (ASF1) was used instead of Nafion DE2021 (DuPont, 20% solution, EW1050, membrane equilibrium water content 4% by mass) as the electrolyte polymer constituting the intermediate layer In the same manner, an electrolyte membrane was obtained.
Moreover, as a result of conducting the charge / discharge test by the same method as in Example 1, the current efficiency (%) / cell electrical resistivity (Ω · cm ² ) was 95.1 / 1.15, which was compared with that of the Example. It was inferior. In addition, as a durability test, as a result of performing 200 cycles of charging and discharging, the current efficiency was 89.8%, the cell electric resistance was 1.20 Ω · cm ² , and the durability was inferior to that of the examples.

  Table 1 shows the results of Examples 1-2 and Comparative Examples 1-3.

  The electrolyte membrane of the present invention has excellent ion selective permeability, low electrical resistance, and excellent durability (mainly hydroxyl radical oxidation resistance), and is industrially used as a diaphragm for redox flow secondary batteries. Has availability.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode cell chamber 3 Negative electrode 4 Negative electrode cell chamber 5 Electrolyte membrane 6 Electrolyzer 7 Positive electrode electrolyte tank 8 Negative electrode electrolyte tank 9 AC / DC converter 10 Redox flow secondary battery

Claims (8)

A positive electrode cell chamber including a positive electrode made of a carbon electrode;
A negative electrode cell chamber including a negative electrode made of a carbon electrode;
An electrolyte membrane as a diaphragm for separating and separating the positive electrode cell chamber and the negative electrode cell chamber;
Having an electrolytic cell containing
The positive electrode cell chamber contains a positive electrode electrolyte containing an active material, and the negative electrode cell chamber contains a negative electrode electrolyte containing an active material,
A redox flow secondary battery that charges and discharges based on a valence change of an active material in the electrolyte solution,
The electrolyte membrane includes an ion exchange resin composition mainly composed of a fluorine-based polymer electrolyte polymer having a structure represented by the following formula (1):
The redox flow secondary battery in which the electrolyte membrane has a laminated structure of three or more layers, and the equilibrium water content of the outer layer adjacent to the positive electrode and the negative electrode is higher than the equilibrium water content of the intermediate layer not adjacent to the positive electrode or the negative electrode .
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, Selected from the group consisting of C1-C10 perfluoroalkyl groups and fluorochloroalkyl groups A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1. D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time).   The redox flow secondary battery according to claim 1, wherein the redox flow secondary battery is a vanadium redox flow secondary battery using a sulfuric acid electrolyte containing vanadium as a positive electrode and a negative electrode electrolyte. The redox flow secondary battery according to claim 1 or 2, wherein the fluorine-based polymer electrolyte polymer is a perfluorocarbon sulfonic acid resin (PFSA) having a structure represented by the following formula (2).
_{- [CF 2 CF 2] a} - [CF 2 -CF ((- O- (CF 2) m -X 4)] g - (2)
(In the formula (2), a and g represents a number satisfying 0 ≦ a <1,0 <g ≦ 1, a + g = 1, m represents an integer of 1 to 6, the X ⁴ is SO ₃ H Show.)   Equivalent mass EW (dry mass in grams per equivalent of ion-exchange groups) of the fluorine-based polymer electrolyte polymer constituting the outer layer is 300 to 900 g / eq, equivalent mass of the fluorine-based polymer electrolyte polymer constituting the intermediate layer Is 600 to 1300 g / eq, the equilibrium moisture content of the outer layer is 15 to 150 mass%, and the equilibrium moisture content of the intermediate layer is 0.1 to 100 mass%. The redox flow secondary battery as described. An ion exchange resin composition mainly comprising a fluorine-based polymer electrolyte polymer having a structure represented by the following formula (1);
An electrolyte membrane for a redox flow secondary battery having a laminated structure of three or more layers, wherein an equilibrium water content of an outer layer adjacent to the positive electrode and the negative electrode is higher than an equilibrium water content of an intermediate layer not adjacent to the positive electrode or the negative electrode.
_{^{- [CF 2 CX 1 X 2}} ] a - [CF 2 -CF ((- O-CF 2 -CF (CF 2 X 3)) b -O c - (CFR 1) d - (CFR 2) e - ( CF ₂ ) _f −X ⁴ )] _g − (1)
(In the formula (1), X ^1, X ² and X ³ are each independently, .X ⁴ indicating one or more members selected from the group consisting of perfluoroalkyl group having halogen atoms and 1 to 3 carbon atoms Represents COOZ, SO ₃ Z, PO ₃ Z ₂ or PO ₃ HZ, where Z is a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an amine (NH ₄ , NH ₃ R ₁ , NH ₂ R ₁ R ₂ , NHR ₁ R ₂ R ₃ , NR ₁ R ₂ R ₃ R ₄ ), R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of an alkyl group and an arene group. Any one or more selected, where X ⁴ is PO ₃ Z ₂ , Z may be the same or different, R ¹ and R ² are each independently a halogen atom, From the group consisting of a C 1-10 perfluoroalkyl group and a fluorochloroalkyl group A and g are numbers satisfying 0 ≦ a <1, 0 <g ≦ 1, and a + g = 1, b is an integer of 0 to 8, and c is 0 or 1 D, e and f each independently represent an integer of 0 to 6 (provided that d, e and f are not 0 at the same time). The electrolyte membrane for a redox flow secondary battery according to claim 5, wherein the fluoropolymer electrolyte polymer is a perfluorocarbon sulfonic acid resin (PFSA) having a structure represented by the following formula (2).
_{- [CF 2 CF 2] a} - [CF 2 -CF ((- O- (CF 2) m -X 4)] g - (2)
(In the formula (2), a and g represents a number satisfying 0 ≦ a <1,0 <g ≦ 1, a + g = 1, m represents an integer of 1 to 6, the X ⁴ is SO ₃ H Show.)   Equivalent mass EW (dry mass in grams per equivalent of ion-exchange groups) of the fluorine-based polymer electrolyte polymer constituting the outer layer is 300 to 900 g / eq, equivalent mass of the fluorine-based polymer electrolyte polymer constituting the intermediate layer Is 600 to 1300 g / eq, the equilibrium moisture content of the electrolyte membrane of the outer layer is 15 to 150 mass%, and the equilibrium moisture content of the intermediate layer is 0.1 to 100 mass%. Electrolyte membrane for redox flow secondary battery.   The electrolyte membrane for a redox flow secondary battery according to any one of claims 5 to 7, wherein the electrolyte membrane is heat-treated at 130 to 200 ° C for 1 to 60 minutes.