Electrolyte membrane and redox flow battery using the same

Abstract

An electrolyte membrane is constituted of at least an A layer and a B layer, the A layer contains an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer, the B layer contains an ion-conductive hydrocarbon polymer, and the ion-conductive hydrocarbon polymer is dispersed in the A layer. An electrolyte membrane excellent in oxidation resistance is provided. Further, by using the electrolyte membrane as a separator, an electrolyte membrane for a redox flow battery capable of achieving high power efficiency and stable charge and discharge even in long-time use is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrolyte membrane, and more particularly to an electrolyte membrane suitable for a redox flow battery. BACKGROUND

[0002] In recent years, the use of renewable energy sources such as sunlight and wind power is increasing. On the other hand, the power generation amount of sunlight and wind speed greatly fluctuates, and a large-scale battery system is required to achieve output stabilization of renewable energy sources, power load equalization, and stabilization of the power system. As a battery, a lead storage battery, a sodium-sulfur (NAS) battery, a lithium ion battery, and a redox flow battery are known. Among them, the redox flow battery, which circulates an electrolyte solution containing an active material by a pump to perform charge and discharge, can store the electrolyte solution in an external tank, and thus is easily large-sized, and is more easily regenerated than other batteries, and for the above reasons, the use thereof is expanding.

[0003] The redox flow battery is a battery that repeatedly performs charge and discharge by a pump in the battery using an oxidation-reduction reaction on an electrode. As an active material contained in an electrolyte solution, iron-chromium, chromium-bromine, titanium-manganese, bromine-zinc, and the like can be given, and in particular, in the case where a vanadium system having different valence numbers is used as an active material of a positive electrode and a negative electrode, a side reaction in an electrode reaction such as generation of a precipitate is less likely to occur, and it is also considered to be suitable for long-term use in view of less degradation of the electrolyte solution.

[0004] As a separator that separates a positive electrode containing an electrode and a positive electrode active material from a negative electrode containing an electrode and a negative electrode active material, a polymer electrolyte membrane is used. As a characteristic required for the electrolyte membrane, low permeability of the active material can be given for the purpose of maintaining the output power and the energy efficiency of the battery. In addition, it is required to be able to stably charge and discharge for a long time when assembled as a battery.

[0005] Since the electrolyte membrane is exposed to a high potential environment of 1.5 V or more at the time of charge and discharge, particularly at the anode (positive electrode side), the electrolyte membrane is required to have electrical oxidation resistance. In the case where the oxidation resistance is insufficient, there is a concern that the electrolyte membrane is degraded during charge and discharge, and the charge and discharge characteristics are reduced over time.

[0006] In Patent Literature 1, a adhesion promoting layer containing an ion conductive polymer and a non-ion conductive polymer is provided in order to improve the adhesion between a polymer electrolyte membrane (PEM) and a catalyst layer in a membrane electrode assembly (MEA) of a fuel cell.

[0007] In Patent Literature 2, an electrolyte membrane is proposed in which, in an ion conductive membrane containing a first layer and a second layer, the first layer contains a perfluorosulfonic acid polymer, and the second layer contains a sulfonated hydrocarbon polymer, thereby reducing hydrogen crossover in a fuel cell.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2008-512844

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-502629 SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] In the configuration described in Patent Document 1, there is a problem that, in the case of use as an electrolyte membrane for a redox flow battery, durability cannot be sufficiently obtained.

[0014] The electrolyte membrane described in Patent Document 2 has a problem that, in the case of applying the electrolyte membrane to a redox flow battery, the thickness of the hydrocarbon electrolyte membrane as the second layer is 2 μm or less in the total thickness of 5 μm to 50 μm, and the current efficiency (power efficiency) decreases along with an increase in vanadium ion permeability, and mechanical durability decreases.

[0015] Therefore, an object of the present application is to provide, for example, an electrolyte membrane suitable for a redox flow battery. That is, an object of the present application is to provide an electrolyte membrane which can achieve high power efficiency and stable charge and discharge even in the case of long-term use as a redox flow battery.

[0016] MEANS FOR SOLVING THE PROBLEMS

[0017] In order to solve the above problems, the electrolyte membrane of the present application has any one of the following [1] or [2]. That is,

[0018] [1] An electrolyte membrane composed of at least an A layer and a B layer, the A layer containing an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer, the B layer containing an ion-conductive hydrocarbon polymer, the ion-conductive hydrocarbon polymer being dispersed in the A layer;

[0019] or,

[0020] [2] An electrolyte membrane composed of at least an A layer and a B layer, the A layer containing an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer, the B layer containing an ion-conductive hydrocarbon polymer, the electrolyte membrane having a lowest crystalline melting peak (Tm) of less than 175°C in the third run in differential scanning calorimetry (DSC) measurement.

[0021] In addition, the redox flow battery of the present application has the following configuration. That is,

[0022] A redox flow battery using the above electrolyte membrane as a separator that separates a positive electrode from a negative electrode.

[0023] For the electrolyte membrane of the above [1] of the present application, it is preferable that the ion-conducting hydrocarbon polymer is dispersed in the A layer with a dispersed diameter of 0.55 μm or less.

[0024] For the electrolyte membrane of the above [1] of the present application, it is preferable that, in a case where a thickness of the A layer is denoted as (t1) and a shortest distance from a surface layer of the A layer to the hydrocarbon polymer in the A layer is denoted as (t2), the following formula (1) is satisfied.

[0025] (t1) - (t2) > 0 (1)

[0026] For the electrolyte membrane of the above [1] of the present application, it is preferable that a lowest crystalline melting peak (Tm) in a third run in differential scanning calorimetry (DSC) measurement is less than 175°C.

[0027] For the electrolyte membrane of the present application, it is preferable that the non-ion-conducting fluorinated polymer is at least one selected from the group consisting of polyvinylidene fluoride and a copolymer of vinylidene fluoride and other fluorine-containing monomer.

[0028] For the electrolyte membrane of the present application, it is preferable that the non-ion-conducting fluorinated polymer is a copolymer of vinylidene fluoride and other fluorine-containing monomer.

[0029] For the electrolyte membrane of the present application, it is preferable that the non-ion-conducting fluorinated polymer is a copolymer of vinylidene fluoride and hexafluoropropylene.

[0030] For the electrolyte membrane of the present application, it is preferable that a mass ratio of the ion-conducting fluorinated polymer to the non-ion-conducting fluorinated polymer in the A layer is 25:75 to 55:45.

[0031] For the electrolyte membrane of the present application, it is preferable that the ion-conducting hydrocarbon polymer contains structural units represented by the following formulae (1) and (2).

[0032] -0-Ar1-O-Ar2-O- (1)

[0033] -0-Ar1-O-Ar3-O- (2)

[0034] In the formulae, Ar1, Ar2 are represented by the following formula (3), and Ar3 is represented by the following formula (4).

[0035]

[0036]

[0037] In the formula, X1and X2independently represent a ketone group (— (C=0)— ), an ether group (— O— ), a sulfonic acid group (— SO2— ), a fluorine-containing carbon group (— C(CF3)2— ), and X1and X2may contain a plurality of the above structures. In addition, X1may be a protecting group. Y represents an ionic group.

[0038] For the electrolyte membrane of the present application, it is preferable that the ion-conducting hydrocarbon polymer is a random copolymer.

[0039] For the electrolyte membrane of the present application, it is preferable that the weight average molecular weight of the ion-conducting hydrocarbon polymer is 300,000 or more.

[0040] The electrolyte membrane of the present application has a unit area active material permeation amount of 4 valence vanadium concentration 1.5 mol · L －1 , sulfuric acid concentration 3.0 mol · L －1 aqueous solution of 1,800 x 10 －10 cm 2 / minute or less.

[0041] The electrolyte membrane of the present application has a tensile elastic modulus of 0.5 GPa or more at 23°C x 50% RH.

[0042] For the electrolyte membrane of the present application, it is preferable that the A layer is laminated only on one side of the B layer.

[0043] The electrolyte membrane of the present application is preferably an electrolyte membrane for a redox flow battery.

[0044] It is preferable that the redox flow battery of the present application is configured with the A layer side of the electrolyte membrane as the positive electrode side.

[0045] Effects of the Invention

[0046] According to the present application, an electrolyte membrane having excellent oxidation resistance can be provided. Furthermore, by using the electrolyte membrane as a separator for a redox flow battery, high power efficiency and stable charge and discharge even in long-term use can be achieved. That is, the redox flow battery using the electrolyte membrane of the present application has high power efficiency, and the durability in long-term driving is improved.

[0047] In addition, the electrolyte membrane of the present application is not limited to a redox flow battery, and can be widely used in fuel cells, water electrolysis devices, electrochemical hydrogen compression devices, and the like. BRIEF DESCRIPTION OF DRAWINGS

[0048] [ Figure 1is a cross-sectional enlarged photograph of an electrolyte membrane as one embodiment of the present application. DETAILED DESCRIPTION

[0049] Hereinafter, the present application will be described in detail.

[0050] In a redox flow battery, an electrode of high potential directly contacts with an electrolyte membrane, and thus, in the case of long-time driving, electrons are directly taken from not only an active material but also the electrolyte membrane, and there is a problem that the electrolyte membrane is electrochemically oxidized and deteriorated. In order to solve the above problem, the basic configuration of the electrolyte membrane of the present application is that at least one layer of an A layer containing an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer is provided on a B layer containing an ion-conductive hydrocarbon polymer, and the A layer is not easily oxidized even in a high potential environment. By adopting the above basic configuration of the electrolyte membrane, the above problem can be solved, and stable charge and discharge can be performed even in long-time use.

[0051] In order to suppress the deterioration (expansion, peeling, whitening, etc.) of the electrolyte membrane in long-time driving of a redox flow battery and further improve durability, the electrolyte membrane of the present application has the following characteristic configuration in addition to the above basic configuration. That is,

[0052] [Embodiment 1] characterized in that the ion-conductive hydrocarbon polymer is dispersed in the A layer.

[0053] [Embodiment 2] characterized in that the lowest crystalline melting peak (Tm) in the third run in differential scanning calorimetry (DSC) measurement is less than 175°C.

[0054] Hereinafter, the A layer containing an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer is sometimes simply referred to as "A layer", and the B layer containing a hydrocarbon polymer is sometimes simply referred to as "B layer".

[0055] In the present application, the ion-conducting fluorinated polymer refers to a polymer having a main chain with perfluorocarbon as a main structural unit, and an ionic group imparted to the main chain or side chain. As such a fluorine-based polymer electrolyte, for example, a perfluorocarbon polymer having a sulfonic acid group (may also contain an etheric oxygen atom) can be given. Among them, a copolymer containing a repeating unit based on tetrafluoroethylene, and a repeating unit of perfluorocarbon having a sulfonic acid group is preferable. As a commercial product of such a copolymer, a perfluorocarbon sulfonic acid-based polymer, a polytrifluorostyrene sulfonic acid-based polymer, a perfluorocarbon phosphonic acid-based polymer, a trifluorostyrene sulfonic acid-based polymer, an ethylene tetrafluoroethylene-g-styrene sulfonic acid-based polymer, an ethylene-tetrafluoroethylene copolymer, a polyvinylidene fluoride-perfluorocarbon sulfonic acid-based polymer, an ethylene-tetrafluoroethylene copolymer, a resin based on a polymer of trifluorostyrene or the like, etc. of "Nafion" (registered trademark: manufactured by DuPont), "Acquion" (registered trademark: manufactured by Solvay), etc. are given. From the viewpoint of power generation performance such as heat resistance, chemical stability, etc., a fluorine-based polymer electrolyte composed of a perfluorocarbon sulfonic acid-based polymer is particularly preferable.

[0056] In addition, as a non-ion-conducting fluorinated polymer, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and trifluoroethylene, a copolymer of vinylidene fluoride and tetrafluoroethylene, etc. can be given. Among them, from the viewpoint of making the dispersion diameter of the ion-conducting hydrocarbon polymer in the A layer smaller, or the viewpoint of making the crystalline melting peak (Tm) of the electrolyte membrane lower, a copolymer of vinylidene fluoride and other fluorine-containing monomer (fluoroalkylene, etc. as described above) is preferable, and a copolymer of vinylidene fluoride and hexafluoropropylene is particularly preferable.

[0057] The electrolyte membrane of Embodiment 1 of the present application is characterized in addition to the above-described basic constitution in that the ion-conducting hydrocarbon polymer is dispersed in the A layer. That is, it is characterized in that a part of the ion-conducting hydrocarbon polymer of the B layer is dispersed in the A layer.

[0058] The fact that the ion-conducting hydrocarbon polymer is dispersed in the A layer can be confirmed, for example, by observing the cross section of the electrolyte membrane with an electron microscope (transmission electron microscope (TEM), scanning electron microscope (SEM)).

[0059] Figure 1 is a partial enlarged photograph of the cross section of the electrolyte membrane as one mode of the present application (a photograph showing a part of the A layer and the B layer). It can be confirmed that the ion-conducting hydrocarbon polymer (P) is dispersed in the A layer (A-layer).

[0060] From the viewpoint of suppressing the deterioration of the electrolyte membrane in the long-term driving of the redox flow battery, improving the durability, and the like, it is preferable, for example, that the ion-conductive hydrocarbon polymer be dispersed at a high density in the interface-side region of the A layer and the B layer. Here, the so-called interface-side region is, for example, appropriately set to a region between the interface of the A layer and the B layer and 50% of the thickness of the A layer. For example, in the case where the thickness of the A layer is 5 μm, the interface-side region is between the interface of the A layer and the B layer and 2.5 μm. The above interface-side region is further preferably set to a region of up to 40% of the thickness of the A layer, and more preferably set to a region of up to 30% of the thickness of the A layer.

[0061] It is preferable that more than 70% of the total number of the dispersed ion-conductive hydrocarbon polymers be present in the above interface-side region, more preferably more than 80%, further preferably more than 90%, and particularly preferably more than 95%. On the other hand, from the viewpoint of the durability of the long-term operation, it is preferable that no ion-conductive hydrocarbon polymer be present in the surface layer of the A layer (the side opposite to the B layer).

[0062] In the present application, the so-called dispersion of the ion-conductive hydrocarbon polymer is, for example, appropriately set to a dispersed diameter of 0.10 μm or more. If the dispersed diameter is 0.10 μm or more, the ion-conductive hydrocarbon polymer is dispersed in the electrolyte membrane in a state where the ion-conductive hydrocarbon polymer is not aggregated, and thus the ion-conductive hydrocarbon polymer is dispersed in the electrolyte membrane in a state where the ion-conductive hydrocarbon polymer is not aggregated. Figure 1 The cross-sectional observation of the electrolyte membrane as shown in the drawing (including the entire region in the thickness direction of the A layer and at least a part of the B layer) can be easily confirmed, and it is also presumed that this is the minimum size that affects the characteristics (performance) of the electrolyte membrane, the effects of the present application, and the like.

[0063] The electrolyte membrane of the present application as described above can be obtained, for example, by applying (wet coating) the A layer on the B layer. At the time of applying the A layer, the B layer can be in a dry state (wet on dry (WOD) method), or can be in a wet state (wet on wet (WOW) method).

[0064] As described above, from the viewpoint of dispersing the ion-conductive hydrocarbon polymer at a high density in the interface-side region, the WOD method is preferable. Furthermore, as the solvent of the A layer coating liquid, it is preferable to use the same kind of solvent as that used at the time of forming the B layer. As the above solvent, a non-protic polar solvent is preferable, and further, N-methyl-2-pyrrolidone (hereinafter, NMP), dimethyl sulfoxide (hereinafter, DMSO), N,N-dimethylformamide (hereinafter, DMF), N,N-dimethylacetamide (hereinafter, DMAc), and the like are suitable, but the present application is not limited thereto.

[0065] The coating of the A layer can be performed directly on the B layer or through other layers, but is preferably performed directly. In the case of being sandwiched by other layers, the other layers are preferably extremely thin films, and specifically, a thickness of 1 μm or less is appropriate, and a thickness of 0.5 μm or less is preferred. That is, the electrolyte membrane of the present application preferably has the A layer directly laminated on the B layer.

[0066] In the electrolyte membrane of the present application, from the viewpoint of suppressing the deterioration of the electrolyte membrane in the long-time driving of the redox flow battery, improving the durability, and the like, the dispersed diameter of the ion-conducting hydrocarbon polymer dispersed in the A layer is preferably small. Specifically, the dispersed diameter of the ion-conducting hydrocarbon polymer dispersed in the A layer is preferably 0.55 μm or less, more preferably 0.50 μm or less, further preferably 0.40 μm or less, particularly preferably 0.30 μm or less, and most preferably 0.25 μm or less. The lower limit is not particularly limited, but as described above, about 0.10 μm is appropriate. Here, the dispersed diameter of the ion-conducting hydrocarbon polymer refers to the number average dispersed diameter.

[0067] Here, the dispersed diameter is confirmed by transmission electron microscopy (TEM) observation, scanning electron microscopy (SEM) observation, or the like for the cross section of the film, and if it is a circular shape, the diameter of the circle can be taken as the dispersed diameter, and if it is an elliptical, spindle-like, or the like deformed dispersed shape, the diameter of the circumscribed circle can be taken as the dispersed diameter.

[0068] From the viewpoint of making the dispersed diameter of the ion-conducting hydrocarbon polymer in the A layer small, the content ratio of the non-ion-conducting fluorinated polymer in the A layer with respect to the entire components (total amount of solid components) of the A layer is preferably 25% by mass or more and 75% by mass or less, preferably 30% by mass or more and 70% by mass or less, and particularly preferably 35% by mass or more and 65% by mass or less. In addition, from the above viewpoint, the mass ratio of the ion-conducting fluorinated polymer to the non-ion-conducting fluorinated polymer in the A layer is preferably in the range of 25:75 to 55:45, more preferably in the range of 30:70 to 50:50, and particularly preferably in the range of 35:65 to 45:55.

[0069] In addition, from the viewpoint of making the dispersed diameter of the ion-conducting hydrocarbon polymer in the A layer small, the lowest crystalline melting peak (Tm) in the third run in the differential scanning calorimetry (DSC) measurement of the electrolyte membrane is preferably less than 175°C. The above crystalline melting peak (Tm) is more preferably less than 160°C, further preferably less than 150°C, particularly preferably less than 140°C, and most preferably less than 135°C. Details are described later.

[0070] The electrolyte membrane of the present application preferably has substantially no ion-conducting hydrocarbon polymer in the surface layer of the A layer. Specifically, where the thickness of the A layer is denoted as (tl) and the shortest distance from the surface layer to the hydrocarbon polymer in the A layer is denoted as (t2), it is preferable to satisfy the following equation (1).

[0071] (tl) - (t2) > 0 • • • (1)

[0072] Here, the expression "substantially no ion-conducting hydrocarbon polymer in the surface layer of the A layer" means that there is no ion-conducting hydrocarbon polymer having a dispersed diameter of 0.1 μm or more. The above (t2) is the shortest distance from the surface layer to the hydrocarbon polymer having a dispersed diameter of 0.1 μm or more.

[0073] By satisfying the above equation (1), there is no ion-conducting hydrocarbon polymer in the surface layer of the A layer, and thus, even if the electrolyte membrane is in direct contact with an electrode of high potential, electrochemical oxidative degradation is less likely to occur, and stable charge and discharge can be performed even over a long period of time, and thus, is preferable. Further, the relationship between (tl) and (t2) is more preferably satisfied by the following equation (2). It is further preferable to satisfy the following equation (3), and most preferable to satisfy the following equation (4).

[0074] 0.3 x (tl) < (t2) • • • (2)

[0075] 0.5 x (tl) < (t2) • • • (3)

[0076] 0.8 x (tl) < (t2) • • • (4)

[0077] That is, the dispersion of the ion-conducting hydrocarbon polymer in the A layer is preferably such that it is present at a high density in the region on the interface side between the A layer and the B layer, and is substantially absent in the surface layer of the A layer.

[0078] The above equations (1) to (4) can be obtained, for example, by applying a coating liquid of the A layer onto the B layer.

[0079] Further, as indicated by equations (2) to (4), it is preferable that the ratio of t2 (the shortest distance from the surface layer to the hydrocarbon polymer) to tl (the thickness of the A layer) be large, and the above ratio can be controlled by adjusting, for example, the solvent of the coating liquid of the A layer, the solid content concentration of the coating liquid, the drying speed after application, and the like. For example, the above ratio can be increased by making the solvent of the coating liquid of the A layer the same as the solvent used when forming the B layer, or by making the solid content concentration of the coating liquid high, or by making the drying speed fast.

[0080] The electrolyte membrane of Embodiment 2 of the present application is characterized in that, in addition to the above-described basic constitution, the lowest crystalline melting peak (Tm) in the third run in differential scanning calorimetry (DSC) measurement is less than 175°C. By using such an electrolyte membrane of the present application, the degradation of the electrolyte membrane in long-term operation can be suppressed, and the durability is improved.

[0081] The electrolyte membrane of the present application preferably has a lowest crystalline melting peak (Tm) in the third run in differential scanning calorimetry (DSC) measurement of less than 160°C, more preferably less than 150°C, further preferably less than 140°C, and most preferably less than 135°C. The lower limit of the above-described crystalline melting peak (Tm) is about 80°C. In the above-described preferable range, the degradation of the electrolyte membrane can be suppressed, and the durability is improved.

[0082] The adjustment of the above-described crystalline melting peak (Tm) of the electrolyte membrane can be performed, for example, by using a crystalline fluorinated polymer as the non-ionically conductive fluorinated polymer contained in the A layer, and controlling the melting point of the crystalline fluorinated polymer. Specifically, a crystalline fluorinated polymer having a melting point of less than 175°C is preferable, a crystalline fluorinated polymer having a melting point of less than 150°C is more preferable, and a crystalline fluorinated polymer having a melting point of less than 135°C is particularly preferable. When the melting point of the crystalline fluorinated polymer is low, there is a tendency that the adhesion between the A layer and the B layer is improved. As the crystalline fluorinated polymer having a melting point of less than 175°C, a copolymer of vinylidene fluoride and hexafluoropropylene can be preferably used, but the present application is not limited thereto.

[0083] When the above-described crystalline melting peak (Tm) of the electrolyte membrane is less than 175°C, the ionically conductive hydrocarbon polymer is easily dispersed in the A layer, and when the above-described crystalline melting peak (Tm) is further lowered, there is a tendency that the dispersion diameter of the ionically conductive hydrocarbon polymer in the A layer is reduced.

[0084] In the present application, the A layer preferably contains an ionically conductive fluorinated polymer and a non-ionically conductive fluorinated polymer as main components. That is, the total amount of the ionically conductive fluorinated polymer and the non-ionically conductive fluorinated polymer is preferably 60% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and particularly preferably 90% by mass or more, of the total amount of all components (total amount of solid components) of the A layer. The upper limit is 100% by mass.

[0085] Further, the proportion of the non-ionically conductive fluorinated polymer in the A layer is more preferably 25% by mass or more, further preferably 30% by mass or more, and particularly preferably 35% by mass or more, of the total amount of the solid components in the A layer. Further, the proportion is preferably 75% by mass or less, more preferably 70% by mass or less, and further preferably 65% by mass or less. When the non-ionically conductive polymer in the A layer is within the above preferable range, the ion conductivity in the A layer is sufficient, and when incorporated as a redox flow battery, the resistance is prevented from increasing, and sufficient charge / discharge characteristics can be obtained. Further, the adhesion of the A layer to the layer containing the ionically conductive hydrocarbon polymer is sufficient, and peeling between the layers and the like during charge / discharge can be suppressed.

[0086] Further, as described above, the mass ratio of the ionically conductive fluorinated polymer to the non-ionically conductive fluorinated polymer in the A layer is preferably in the range of 25:75 to 55:45, more preferably in the range of 30:70 to 50:50, and particularly preferably in the range of 35:65 to 45:55. When the mass ratio of the ionically conductive polymer to the non-ionically conductive polymer in the A layer is within the above preferable range, the ion conductivity in the A layer is sufficient, and when incorporated as a redox flow battery, the resistance is prevented from increasing, and sufficient charge / discharge characteristics can be obtained. Further, the adhesion of the A layer to the layer containing the ionically conductive hydrocarbon polymer is sufficient, and swelling, peeling, and the like between the layers during charge / discharge can be suppressed.

[0087] In the present application, the content of the ionically conductive hydrocarbon polymer in the B layer is preferably 70% by mass or more, more preferably 80% by mass or more, and particularly preferably 90% by mass or more, of the total amount of the electrolyte components (ionically conductive polymers) contained in the B layer. The upper limit is 100% by mass. Further, the B layer can contain a non-ionically conductive polymer, such as a fluorinated polymer, a hydrocarbon polymer, and the like. The content of the non-ionically conductive polymer in this case is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 10 parts by mass or less, per 100 parts by mass of the ionically conductive hydrocarbon polymer.

[0088] The thickness of the A layer is preferably 1 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, and particularly preferably 3 μm or more and 10 μm or less, from the viewpoint of high power efficiency and durability in long-time driving.

[0089] The thickness of the B layer is preferably 10 μm or more and 300 μm or less, more preferably 20 μm or more and 200 μm or less, and particularly preferably 30 μm or more and 100 μm or less, from the viewpoint of high power efficiency and durability in long-time driving.

[0090] Regarding the thickness of the electrolyte membrane of the present invention, from the viewpoint of durability, it is preferably 15 μm or more, more preferably 20 μm or more, further preferably 25 μm or more, and particularly preferably 30 μm or more. On the other hand, from the viewpoint of voltage efficiency, it is preferably 500 μm or less, more preferably 400 μm or less, further preferably 300 μm or less, and particularly preferably 200 μm or less. The thickness of the electrolyte membrane can be controlled by the solution concentration or the coating thickness on the supporting substrate.

[0091] In the electrolyte membrane of the present invention, layer A can be stacked on one or both sides of layer B, but preferably layer A is stacked on only one side of layer B. By stacking layer A only on one side of layer B, the resistance of the electrolyte membrane can be suppressed to a low level, and durability can be improved.

[0092] The electrolyte membrane of this invention is suitable for a vanadium tetravalent ion concentration of 1.5 mol·L⁻¹. －1 Sulfuric acid concentration 3.0 mol·L －1 The preferred permeability of the active substance per unit area in the aqueous solution is 1,800 × 10⁻⁶. －10 cm 2 / min or less. More preferably 800×10 －10 cm 2 Below / min, further preferably 300×10 －10 cm 2 Below / min, 50×10 is particularly preferred. －10 cm 2 For speeds below / min, the optimal value is 30 × 10. －10 cm 2 / min or less. If the permeation rate is within the preferred range described above, there is no situation where the active material located at the positive electrode permeates (penetrates) to the negative electrode side and causes self-discharge. Therefore, the current efficiency will not decrease, and the energy efficiency will not decrease even after repeated charge-discharge cycles. Furthermore, while there is no particular limitation on the lower limit, from the viewpoint of ensuring proton conductivity, 1×10⁻⁶ is preferred. －11 cm 2 / min or more. It should be noted that, in order to keep the permeation of the active material within the above range, it is preferable to use an ion-conducting hydrocarbon polymer in the B layer, and more preferably to contain structural units represented by chemical formulas (1) and (2) described later.

[0093] The tensile elastic modulus of the electrolyte membrane of the present application at 23°C x 50% RH is preferably 0.5 GPa or more. More preferably, it is 1.0 GPa or more, and further preferably, it is 1.5 GPa or more. When the tensile elastic modulus is in the above-mentioned preferable range, the workability at the time of assembly into a redox flow battery becomes good. In addition, the upper limit of the tensile elastic modulus is not particularly set, and when the tensile elastic modulus of a general polymer membrane is considered, the upper limit is considered to be 10 GPa or less. Here, in order to make the tensile elastic modulus 0.5 GPa or more, it is preferable to use an ion-conducting hydrocarbon polymer in the B layer, and more preferably, it contains the structural units represented by Chemical Formulas (1) and (2) described later.

[0094] In the electrolyte membrane of the present application, as the ion-conducting hydrocarbon polymer constituting the B layer, a hydrocarbon polymer having an aromatic ring in the main chain is preferable. As the hydrocarbon electrolyte membrane having an aromatic ring in the polymer main chain, polymers such as polysulfone, polyethersulfone, polyphenylene ether, polyarylene ether-based polymer, polyphenylene sulfide, polyphenylene sulfide sulfone, poly-p-phenylene, polyarylene-based polymer, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, polyether phosphine oxide, polybenzimidazole, polybenzoxazole, polybenzothiazole, aromatic polyamide, polyimide, polyetherimide, polyimide sulfone, and the like can be given. Note that, the polysulfone, polyethersulfone, polyether ketone, and the like referred to here are general terms for polymers having a sulfone bond, ether bond, and ketone bond in the molecular chain, and include polyether ketone ketone, polyether ether ketone, polyether ether ketone ketone, polyether ketone ether ketone ketone, polyether ketone sulfone, and the like, and are not limited to a specific polymer structure.

[0095] Among these polymers, from the aspects of mechanical strength, physical durability, processability, and hydrolysis resistance, poly sulfone, polyethersulfone, polyphenylene ether, polyarylene ether-based polymer, polyphenylene sulfide, polyphenylene sulfide sulfone, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, polyether phosphine oxide, and the like are more preferable. Among these, from the aspects of mechanical strength, physical durability, and manufacturing cost, an aromatic polyether-based polymer is further preferable. From the aspects of crystallinity due to good packing of the main chain skeletal structure and extremely strong intermolecular cohesive force, and the properties of being completely insoluble in general solvents, and from the aspects of tensile strength, elongation, tear strength, and fatigue resistance, an aromatic polyether ketone (PEK)-based polymer is particularly preferable.

[0096] ​An aromatic polyether ketone (PEK) polymer shows crystallinity due to its good stacking property and strong intermolecular cohesive force, and has a property of being completely insoluble in a general solvent, and is not suitable for solution film formation. However, by containing a protecting group in the polymer, the crystallinity of the PEK polymer can be reduced, and solubility in an organic solvent can be imparted, and thus solution film formation can be made possible. Note that after being formed into a film or the like, in order to increase the intermolecular cohesive force of the molecular chain of the polymer, a part of the protecting group is deprotected, and thus an electrolyte membrane in which the mechanical properties such as heat water resistance, tensile strength, elongation, tear strength, fatigue resistance, and the barrier property against active substances such as vanadium, titanium, and manganese are greatly improved can be obtained. In the case where the manufacturing process is performed, the polymer electrolyte membrane of the present application has the following characteristics in particular: in addition to high proton conductivity, film formation property (processability), manufacturing cost, and the barrier property against active substances, heat water resistance, and mechanical properties can be simultaneously achieved.

[0097] The ion-conducting hydrocarbon polymer of the present application more preferably contains structural units represented by the following formulas (1) and (2).

[0098] -O-Ar1-O-Ar2-O- (1)

[0099] -O-Ar1-O-Ar3-O- (2)

[0100] wherein Ar1, Ar2 are represented by the following formula (3), Ar3 is represented by the following formula (4), X1 and X2 each independently represent a ketone group (- (C=O) -), an ether group (-O-), a sulfonic acid group (-SO2-), a fluorine-containing carbon group (-C(CF3)2-), and X1 and X2 can contain a plurality of the above structures. In addition, X1 can be a protecting group. Y represents an ionic group. From the viewpoint of solubility in an organic solvent and physical properties when formed into a film, X1 and X2 are more preferably a ketone group and a fluorine-containing carbon group, and particularly preferably a ketone group.

[0101]

[0102]

[0103] Note that the above structure can be obtained by polycondensation of a 2-membered aromatic dihalide and a 2-membered bisphenol compound, and the content of the structural unit can be changed by the input ratio thereof. Here, Y (ionic group) includes a case where a salt is formed. As the cation forming the above salt, any metal cation, NR4 +(R is an arbitrary organic group) and the like. In the case of metal cations, no particular limitation can be imposed on the valence number thereof and the like. These ionic groups can be contained in the polymer in two or more kinds, and the combination can be appropriately determined depending on the structure of the polymer and the like. Among them, from the aspect of high proton conductivity, it is more preferable to have at least a sulfonic acid group, a sulfonimide group, a sulfuric acid group, and from the aspect of raw material cost, it is most preferable to have at least a sulfonic acid group. As a method of introducing the ionic group into the electrolyte polymer, there are a method of polymerizing using a monomer having an ionic group, and a method of introducing an ionic group by a polymer reaction, and the present application employs the method of polymerizing using a monomer having an ionic group.

[0104] Here, X1may be a protecting group as described above, and as the protecting group, there are protecting groups generally used in organic synthesis, and the so-called protecting group is a substituent temporarily introduced with the premise of removal in a later stage, is a group that protects a functional group having high reactivity, is a non-reactive group for the subsequent reaction, and is a group that can be deprotected after the reaction to recover the original functional group. That is, the protecting group is a group paired with the functional group to be protected, and for example, a tert-butyl group is sometimes used as a protecting group for a hydroxyl group, but in the same manner, in the case where a tert-butyl group is introduced in an alkylene chain, it is not referred to as a protecting group. The reaction of introducing the protecting group is referred to as protection (reaction), and the reaction of removing the protecting group is referred to as deprotection (reaction).

[0105] As such a protection reaction, for example, it is described in detail in Theodora W. Greene, "Protective Groups in Organic Synthesis", U.S.A., John Wiley & Sons, Inc., 1981, and these protection reactions can be preferably used. The reactivity of the protection reaction and the deprotection reaction, the stability of the state containing the protecting group, the manufacturing cost, and the like can be considered to be appropriately selected. In addition, as the stage of introducing the protecting group in the polymerization reaction, it can be from the monomer stage, or from the oligomer stage, or it can be the polymer stage, and can be appropriately selected.

[0106] If a specific example of a protective reaction is given, a method of protecting / deprotecting a ketone moiety with a ketal moiety, a method of protecting / deprotecting a ketone moiety with a heteroatom analog of a ketal moiety, such as a ketothioacetal, can be given. With respect to these methods, they are described in Chapter 4 of the above-mentioned "Protective Groups in Organic Synthesis". In addition, a method of protecting / deprotecting between a sulfonic acid and a soluble ester derivative, a method of introducing a tert-butyl group as a soluble group in an aromatic ring and de-tert-butylation with an acid to thereby protect / deprotect, and the like can be given. In order to improve solubility in a general solvent, reduce crystallinity, from the viewpoint of steric hindrance, an aliphatic group, particularly an aliphatic group including a cyclic moiety, can be preferably used as a protecting group.

[0107] As the position of the functional group to which the protecting group is introduced, the main chain of the polymer is more preferable. With respect to the material of the high-molecular electrolyte membrane of the present application, by introducing the protecting group in the main chain portion of the polymer having good stackability, the processability is improved. Here, the functional group present on the main chain of the polymer is defined as a functional group in which the polymer chain is broken if the functional group is removed. For example, if the ketone group of an aromatic polyether ketone is removed, the benzene ring is broken from the benzene ring. More specifically, the structures represented by the following (P1) to (P7) can be introduced into the polymer chain by using a 2-membered bisphenol compound as a monomer as shown in the structures. Note that in the case where the bisphenol compound represented by the following formula is used, it can be considered that the structure of Ar1 represented by the above formula (1), (2) is used. In addition, after being formed into a film, the ketal moiety can be changed to a ketone group by deprotection.

[0108]

[0109] Here, if Ar2 is a structure from a 2-membered aromatic dihalide and Ar3 is a structure from an aromatic dihalide having an ionic group, a compound as shown below can be used as each monomer.

[0110] As specific examples of the aromatic dihalide, 3,3'-disulfonate-4,4'-dichlorodiphenyl sulfone, 3,3'-disulfonate-4,4'-difluorodiphenyl sulfone, 3,3'-disulfonate-4,4'-dichlorodiphenyl ketone, 3,3'-disulfonate-4,4'-difluorodiphenyl ketone, 3,3'-disulfonate-4,4'-dichlorodiphenyl phenyl phosphine oxide, 3,3'-disulfonate-4,4'-difluorodiphenyl phenyl phosphine oxide, and the like can be given. Among them, from the viewpoint of the manufacturing cost, the effect of suppressing the permeation of the active material, 3,3'-disulfonate-4,4'-dichlorodiphenyl ketone and 3,3'-disulfonate-4,4'-difluorodiphenyl ketone are more preferable, and from the viewpoint of the polymerization activity, 3,3'-disulfonate-4,4'-difluorodiphenyl ketone is most preferable.

[0111] In addition, as specific examples of the aromatic dihalide having an ionic group, 3,3'-disulfonate-4,4'-dichlorodiphenyl sulfone, 3,3'-disulfonate-4,4'-difluorodiphenyl sulfone, 3,3'-disulfonate-4,4'-dichlorodiphenyl ketone, 3,3'-disulfonate-4,4'-difluorodiphenyl ketone, 3,3'-disulfonate-4,4'-dichlorodiphenyl phenyl phosphine oxide, 3,3'-disulfonate-4,4'-difluorodiphenyl phenyl phosphine oxide, and the like can be given. Among them, from the viewpoint of the manufacturing cost, the effect of suppressing the permeation of the active material, 3,3'-disulfonate-4,4'-dichlorodiphenyl ketone and 3,3'-disulfonate-4,4'-difluorodiphenyl ketone are more preferable, and from the viewpoint of the polymerization activity, 3,3'-disulfonate-4,4'-difluorodiphenyl ketone is most preferable.

[0112] As for the polymerization method of the aromatic polyether-based polymer preferably used in the present application, any method can be used as long as it is a method that can substantially achieve sufficient high molecular weight, and is not particularly limited, and for example, an aromatic nucleophilic substitution reaction of an aromatic dihalide with a 2-valent phenol compound can be used for the synthesis. In addition, an aromatic nucleophilic substitution reaction of a halogenated aromatic phenol compound can be used for the introduction of the 5th component.

[0113] The polymerization of the aromatic polyether-based polymer by the aromatic nucleophilic substitution reaction for obtaining the preferred material of the polymer electrolyte membrane of the present application can be carried out by reacting the above-mentioned monomer mixture in the presence of an alkaline compound. The polymerization can be carried out at a temperature ranging from 0 to 350°C, but is preferably at a temperature ranging from 50 to 250°C. In the case where the polymerization temperature is in the above-mentioned preferred range, the reaction sufficiently proceeds, on the other hand, the polymer is not decomposed. The reaction can be carried out without a solvent, but is preferably carried out in a solvent. As the solvent which can be used, non-protic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoric triamide, and the like can be mentioned, but are not limited to these solvents, as long as a solvent which can be used as a stable solvent in the aromatic nucleophilic substitution reaction. These organic solvents can be used alone or as a mixture of two or more.

[0114] As the alkaline compound, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, and the like can be mentioned, as long as a phenoxide structure which makes the aromatic diol active is formed, and can be used without being limited to these alkaline compounds.

[0115] In the aromatic nucleophilic substitution reaction, water is sometimes generated as a by-product. At this time, toluene or the like can be coexisting in the reaction system regardless of the polymerization solvent, and thus the water is removed to the outside of the system in the form of an azeotrope. As the method of removing the water to the outside of the system, a water absorbent such as a molecular sieve can be used. The azeotrope which can be used for removing the reaction water or the water introduced in the reaction is generally any inactive compound which does not substantially hinder the polymerization, co-distills with water, and boils between about 25°C and about 250°C. Typical azeotropes include benzene, toluene, xylene, chlorobenzene, dichloromethane, dichlorobenzene, trichlorobenzene, and the like. Of course, it is beneficial to select such an azeotrope whose boiling point is lower than that of the bipolar solvent used. The azeotrope is often unnecessary in general, but when a high reaction temperature, for example, a temperature of 200°C or higher is employed, particularly when an inactive gas is continuously dispersed in the reaction mixture, the azeotrope is often necessary. In general, the reaction is preferably carried out in an inactive atmosphere, in the absence of oxygen.

[0116] In the case where the aromatic nucleophilic substitution reaction is carried out in a solvent, it is preferred that the monomers are charged in such a manner that the resulting polymer concentration becomes 5 to 50% by weight. In the case where the polymer concentration is in the above-mentioned preferred range, the polymerization degree sufficiently increases, on the other hand, the viscosity of the reaction system is moderate, and the post-treatment of the reactants is easily carried out.

[0117] After the polymerization reaction, the solvent is removed from the reaction solution by evaporation, and the residue is washed as necessary, whereby the desired polymer is obtained. Alternatively, the reaction solution can be added to a solvent in which the polymer has low solubility and in which the inorganic salt produced by the side reaction has high solubility, whereby the inorganic salt is removed and the polymer is precipitated as a solid, and the polymer is obtained by filtration of the precipitate. The recovered polymer can be washed with water, alcohol or another solvent as appropriate, and dried. If the desired molecular weight is obtained, a phenoxide or halide capping agent that forms a stable end group can be introduced as appropriate for the halide or phenoxide end group.

[0118] In the present application, from the viewpoint of processability, it is necessary to introduce the protecting group in such a manner that it is not deprotected until the film formation stage, and thus, polymerization and purification need to be performed under conditions in which the protecting group can stably exist. For example, in the case where a ketal is used as the protecting group, a deprotection reaction occurs under acidic conditions, and thus, it is necessary to maintain the system as neutral or basic.

[0119] In the present application, the method of deprotecting at least a part of the ketone site protected by the ketal to form a ketone site is not particularly limited. The above deprotection reaction can be performed under heterogeneous or homogeneous conditions in the presence of water and an acid, but from the viewpoint of mechanical strength and solvent resistance, a method of performing acid treatment after molding into a film or the like is more preferable. Specifically, the deprotection can be performed by dipping the molded film in an aqueous hydrochloric acid solution, and the concentration of the acid and the temperature of the aqueous solution can be appropriately selected.

[0120] Although the weight ratio of the aqueous acid necessary for the polymer is preferably 1 to 100 times, a larger amount of water can also be used. The acid catalyst is preferably used at a concentration of 0.1 to 50% by weight of the water present. As a suitable acid catalyst, strong inorganic acids such as hydrochloric acid, nitric acid, fluorosulfonic acid, sulfuric acid, and strong organic acids such as p-toluenesulfonic acid and trifluoromethanesulfonic acid can be mentioned. The amount of the acid catalyst and excess water, the reaction pressure, and the like can be appropriately selected depending on the film thickness of the polymer and the like.

[0121] For example, by dipping in an aqueous acid solution such as 6N hydrochloric acid and heating at 95°C for 1 to 48 hours, almost the entire amount can be easily deprotected. In addition, even by dipping in an aqueous 1N hydrochloric acid solution at 25°C for 24 hours, most of the protecting groups can be deprotected. However, as the conditions for deprotection, these conditions are not limiting, and deprotection can also be performed using an acid gas, an organic acid, or the like, or by heat treatment.

[0122] Generally, if an electrode of high potential is in direct contact with a hydrocarbon-based polymer, there is a concern that the polymer is deteriorated by being electrochemically oxidized. In particular, the electrode on the positive electrode side becomes high potential, and thus, when the electrolyte membrane of the present application is assembled into a redox flow battery, it is preferable to arrange the A layer containing the ion-conducting fluorinated polymer and the non-ion-conducting fluorinated polymer toward the positive electrode side, whereby deterioration can be effectively suppressed. As a method for confirming the A layer, the spectra of the surface and the back of the electrolyte membrane can be collected by FT-IR ATR method, and confirmation can be made based on whether the peak intensity from the ion-conducting fluorinated polymer or the non-ion-conducting fluorinated polymer is different between the surface and the back. Alternatively, a section can be prepared in the thickness direction of the electrolyte membrane, and the cross-sectional image observed by transmission electron microscope (TEM) or scanning electron microscope (SEM) can be used to perform elemental analysis of the fluorine content on the surface of the membrane and in the interior of the membrane by EDX, and confirmation can be made based on whether the concentration of fluorine atoms is different between the surface and the center of the membrane.

[0123] In addition, in the electrolyte membrane of the present application, a crystallization nucleating agent, a plasticizer, a stabilizer or a mold release agent, an antioxidant, or the like, which are generally used for high molecular compounds, can be added within a range not deviating from the object of the present application.

[0124] In addition, in the electrolyte membrane of the present application, various polymers, elastomers, fillers, microparticles, various additives, or the like, can be contained within a range not adversely affecting the above-described various characteristics, for the purpose of improving mechanical strength, thermal stability, processability, or the like. In addition, reinforcement can be performed using a microporous membrane, a nonwoven fabric, a mesh, or the like.

[0125] The amount of sulfonic acid groups in the electrolyte membrane can be shown as the value of sulfonic acid group density (mmol / g). For the sulfonic acid group density of the electrolyte membrane in the present application, from the aspects of proton conductivity, active material barrier property, and mechanical strength, it is preferable to be 1.0 to 3.0 mmol / g, more preferable to be 1.1 to 2.5 mmol / g from the aspect of active material barrier property, and further preferable to be 1.2 to 2.0 mmol / g. If the sulfonic acid group density is in the above preferable range, the proton conductivity is high, sufficient voltage efficiency can be obtained, and on the other hand, when used as an electrolyte membrane for a redox flow battery, the mechanical strength when containing water is sufficient.

[0126] Here, the sulfonic acid group density is the number of moles of the sulfonic acid group introduced per 1 g of the dried polymer electrolyte material, and the larger the value, the more the amount of the sulfonic acid group. The sulfonic acid group density can be calculated by elemental analysis and neutralization titration. Among them, the elemental analysis method is preferred in terms of ease of measurement, but in the case of containing a source of sulfur other than the sulfonic acid group, etc., the ion exchange capacity can also be calculated by the neutralization titration method. The electrolyte membrane of the present application includes a complex composed of a polymer having an ionic group and a component other than the same, as described later, in which case the sulfonic acid group density is also calculated based on the total amount of the complex.

[0127] In the electrolyte membrane of the present application, other components such as non-active polymers, organic or inorganic compounds, which do not have conductivity or ion conductivity, can be contained within a range that does not interfere with the purpose of the present application.

[0128] In the electrolyte membrane of the present application, the content of the group having a protecting group is not particularly limited, but from the aspects of mechanical properties, active material barrier properties, and chemical stability, it is preferred to be a small amount, and most preferably all are deprotected. The content of the protecting group can be measured by nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA), gas evolution analysis using temperature programmed desorption-mass spectrometry (TPD-MS), thermal cracking gas chromatography, thermal cracking GC-MS, infrared absorption spectroscopy (IR), etc. In the case where the amount of the protecting group contained in the electrolyte membrane is large, nuclear magnetic resonance spectroscopy (NMR) is suitable for the quantification of the protecting group because it has solvent solubility. However, in the case where the amount of the protecting group is a very small amount and is solvent-insoluble, it is sometimes difficult to accurately quantify by NMR. In such a case, gas evolution analysis using temperature programmed desorption-mass spectrometry (TPD-MS), or thermal cracking gas chromatography, thermal cracking GC-MS becomes a suitable quantification method.

[0129] The electrolyte membrane of the present application refers to a molded body containing the electrolyte material of the present application. In the present application, as the specific shape of the molded body, in addition to the film type (including films, sheets, and film-like objects), various forms such as plate-like, fiber-like, hollow filament-like, particle-like, block-like, microporous-like, coating-like, foamed body-like, etc. can be taken depending on the use. Since the design freedom of the polymer can be improved and various properties such as mechanical properties, solvent resistance, etc. can be improved, it can be adapted to a wide range of uses. It is particularly suitable when the polymer electrolyte molded body is of the film type.

[0130] The weight-average molecular weight (WM) of the hydrocarbon polymer constituting the electrolyte membrane of the present invention is preferably 300,000 or more. If the WM of the polymer constituting the electrolyte membrane of the present invention is within the above-mentioned preferred range, cracks are less likely to occur in the molded membrane, resulting in sufficient mechanical strength and good chemical stability. More preferably, the WM of the polymer constituting the electrolyte membrane of the present invention is 350,000 or more, further preferably 400,000 or more, and particularly preferably 500,000 or more. On the other hand, there is no particular upper limit to the WM; when it is 5,000,000 or less, the solubility is sufficient, the solution viscosity is moderate, and the processability is good. The WM used herein represents the relative WM as determined by gel permeation chromatography using NMP solvent (NMP solvent containing 10 mmol / L lithium bromide) as the mobile phase, relative to the molecular weight of standard polystyrene.

[0131] It should be noted that the chemical structure of the polymer constituting the electrolyte membrane of the present invention can be determined using infrared absorption spectroscopy, based on the values ​​of 1,030 to 1,045 cm⁻¹. －1 S=O absorption of 1,160–1,190 cm⁻¹ and 1,130–1,250 cm⁻¹ －1 C-O-C absorption, 1,640–1,660 cm⁻¹ －1 Their composition ratio can be confirmed by C=O absorption, etc., and can be determined by neutralization titration of sulfonic acid groups and elemental analysis. Alternatively, nuclear magnetic resonance spectroscopy (NMR) can be used. 1 The structure was confirmed by ¹H-NMR, for example, based on the peak of aromatic protons in the range of 6.8–8.0 ppm. Alternatively, solution analysis can be used. 13 C-NMR, solid 13 C-NMR was used to confirm the position and arrangement of the sulfonic acid groups.

[0132] The electrolyte membrane of the present invention is sometimes insoluble in common organic solvents, such as deuterated dimethyl sulfoxide and deuterated chloroform, but can be measured using deuterated sulfuric acid. Thus, it is possible to identify polymers that have been sulfonated at the monomer stage with controlled sulfonation positions, or post-sulfonated polymers with uncontrolled sulfonation positions. However, when electron-withdrawing groups such as ketones and sulfonates are not adjacent, sulfonation reactions occur during sample preparation and measurement, making it difficult to determine the precise sulfonation position of the sample.

[0133] The ion-conducting hydrocarbon polymer that can be used as an electrolyte material in layer B can be a block copolymer or a random copolymer, but a random copolymer is preferred. By using a random copolymer as the ion-conducting hydrocarbon polymer constituting layer B, the permeation of vanadium ions can be suppressed. In addition, compared with block copolymers, random copolymers tend to have a smaller dispersion diameter in layer A, and are therefore preferred.

[0134] As the ion-conducting hydrocarbon-based polymer usable in the B layer, for example, (i) a random copolymer preferably containing the structural units represented by the above-mentioned formulae (1) and (2), (ii) more preferably the above-mentioned formula (3) and formula (4) wherein X1and X2are a ketone group, (iii) further preferably the above-mentioned random copolymer of formulae (1) and (2), and (iv) particularly preferably a random copolymer having a weight average molecular weight of 300,000 or more.

[0135] The electrolyte membrane of the present application is characterized in that it is obtained by deprotecting at least a part of the protecting group contained in the molded body after molding. The method of converting into a membrane is not particularly limited, and can be a method of film formation from a solution state at the stage of having a ketal or the like protecting group or a method of film formation from a molten state or the like. In the case of the former, for example, the following method can be exemplified: the high-molecular electrolyte material is dissolved in a solvent such as NMP, the solution is cast-coated on a support substrate such as a PET film, a glass plate or the like, and the solvent is removed, whereby film formation is performed.

[0136] As the solvent for film formation, any solvent can be used as long as it can dissolve the high-molecular electrolyte material and can be removed thereafter, and for example, non-protic polar solvents such as DMAc, DMF, NMP, DMSO, sulfolane, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoric triamide, ester-based solvents such as γ-butyrolactone, butyl acetate, carbonate-based solvents such as ethylene carbonate, propylene carbonate, alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, alcohol-based solvents such as isopropyl alcohol, water and mixtures thereof can be suitably used, and non-protic polar solvents are the most preferable in terms of solubility.

[0137] In order to obtain a strong and tough membrane, a preferable method is to remove foreign matter present in the high-molecular electrolyte solution by performing filtration under normal pressure or pressurized filtration or the like on a polymer solution prepared at a necessary solid content concentration. The filtration material used here is not particularly limited, and a glass filter, a metallic filter is suitable. In the filtration, the pore diameter of the smallest filter through which the polymer solution passes is preferably 1 μm or less. By performing the filtration, the mixing of foreign matter is not allowed, the occurrence of a film breakage is prevented, and the durability becomes sufficient.

[0138] Next, it is preferable to perform heat treatment after at least a part of the ionic groups of the obtained polymer electrolyte membrane is made into a state of a metal salt. If the polymer electrolyte material used is a material that is polymerized in a state of a metal salt at the time of polymerization, it is preferable to directly perform film formation and heat treatment. The metal of the metal salt is a metal that can form a salt with sulfonic acid, but from the aspects of price and environmental load, it is preferable to be Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W, etc., among which, it is more preferable to be Li, Na, K, Ca, Sr, Ba, and further preferable to be Li, Na, K. The temperature of the heat treatment is preferably 150 to 550°C, further preferably 160 to 400°C, and particularly preferably 180 to 350°C.

[0139] The heat treatment time is preferably 10 seconds to 12 hours, further preferably 30 seconds to 6 hours, and particularly preferably 1 minute to 1 hour. If the heat treatment temperature is in the above preferable range, the effects of suppressing the permeation of the active material, the elastic modulus, and the breaking strength become sufficient. On the other hand, deterioration of the film material is less likely to occur. If the heat treatment time is in the above preferable range, the effects of the heat treatment become sufficient. On the other hand, deterioration of the film material is less likely to occur. The polymer electrolyte membrane obtained by the heat treatment can be impregnated in an acidic aqueous solution as needed, whereby proton substitution is performed. By molding using this method, the polymer electrolyte membrane of the present application can simultaneously achieve proton conductivity and active material barrier properties, and mechanical properties and long-term durability in a more favorable balance.

[0140] The electrolyte membrane of the present application is obtained by forming a ketone site by deprotecting at least a part of a ketol acetal-protected ketone site after forming a polymer solution composed of an aromatic polyether ketone-based polymer into a film shape using the above method. According to this method, solution film formation of a low sulfonic acid group amount polymer having insufficient solubility can be performed, and simultaneous achievement of proton conductivity and active material barrier property effects, excellent mechanical properties, and excellent dimensional stability can be achieved.

[0141] In the present application, the method of deprotecting part or all of the ketone site protected by the ketal is not particularly limited, and for example, a method of performing acid treatment after molding can be given. Specifically, the deprotection can be performed by immersing the molded film in hydrochloric acid, an aqueous sulfuric acid solution, and the concentration of the acid, the temperature of the aqueous solution can be appropriately selected in consideration of the ease of penetration into the molded body and the like. For example, the deprotection can be easily performed by immersing in an acidic aqueous solution such as a 6N aqueous hydrochloric acid solution and heating at 95°C for 8 hours. However, the conditions for deprotection are not limited to these conditions, and the deprotection can be performed by heat treatment, or by using an acidic gas, an organic acid, or the like.

[0142] The electrolyte membrane of the present application is a laminated membrane composed of at least an A layer containing an ion-conductive fluorinated polymer and a non-ion-conductive fluorinated polymer and a B layer containing an ion-conductive hydrocarbon polymer. As a method for producing the same, a method of forming the A layer on the B layer containing the ion-conductive hydrocarbon polymer formed by the method described above can be given. As a solvent used when the A layer is applied, the same solvent as that used when the B layer containing the ion-conductive hydrocarbon polymer is formed as described above is preferably used. As a method for casting and applying the A layer and the B layer, a method such as doctor blade coating, direct roll coating, Meyer bar coating, gravure coating, reverse coating, air knife coating, spray coating, brush coating, dip coating, die coating, vacuum die coating, curtain coating, flow coating, spin coating, screen printing, inkjet coating and the like can be employed. In addition, the concentration of the polymer electrolyte solution of the salt type used is preferably 3 to 40 mass%, more preferably 5 to 30 mass%. In the case where the solution viscosity is in the above-mentioned preferable range, the solution has good retention and is less likely to produce a liquid flow, on the other hand, the surface smoothness of the electrolyte membrane is good.

[0143] The electrolyte membrane of the present application can be applied to various uses. For example, it can be applied to medical uses such as extracorporeal circulation columns, artificial skins and the like, uses for filtration, ion exchange resin uses, various structural material uses, electrochemical uses. In addition, it is also suitable as an artificial muscle. Among them, it is more preferably utilized for various electrochemical uses. As electrochemical uses, for example, fuel cells, redox flow batteries, water electrolysis devices, electrochemical hydrogen compression devices, chlor-alkali electrolysis devices and the like can be given, and among them, the redox flow battery is most preferable.

[0144] The redox flow battery of this embodiment includes an electrolyzer comprising: a positive electrode cell containing a positive electrode made of a carbon electrode; a negative electrode cell containing a negative electrode made of a carbon electrode; and an electrolyte membrane serving as a separator to isolate and separate the positive electrode cell from the negative electrode cell. The positive electrode cell contains a positive electrolyte containing active material, and the negative electrode cell contains a negative electrolyte containing active material. The positive and negative electrolytes containing active material are stored, for example, in positive and negative electrolyte tanks and supplied to each cell using a pump or the like.

[0145] The redox flow battery of this embodiment can have the following structure: liquid-permeable and porous carbon electrodes (for negative and positive electrodes) are respectively arranged on both sides of a separator, and clamped by compression. One side separated by the separator is used as the positive electrode cell, and the other side is used as the negative electrode cell. The thickness of the two cell cells is ensured by a gasket. When clamped by compression, the porous carbon electrodes are compressed, and the electrolyte membrane is deformed at the same time. As a result, the contact area with the electrodes increases, and the movement of active materials and electrons and protons flowing through the electrodes is smooth, thereby improving voltage efficiency. As a porous carbon material, known electrodes can be used. Examples include electrodes based on carbon fibers, such as non-woven fabric (carbon felt) and paper. If a carbon felt electrode is used, the following effects are achieved: (1) even when an aqueous solution is used in the electrolyte, oxygen is not easily generated even when the potential for oxygen generation changes during charging; (2) large surface area; (3) excellent electrolyte flowability. The improved voltage efficiency achieved by combining nonwoven fabric (carbon felt) with the electrolyte membrane of the present invention is preferred in this respect. Furthermore, the compressibility of the carbon material is preferably 20% or more and less than 70%.

[0146] In the case of a vanadium-type redox flow battery, by circulating a mixture of tetravalent vanadium (V2) into the positive electrode cell, 4+ ) and pentavalent vanadium (V 5+ The positive electrode electrolyte is composed of sulfuric acid electrolyte, and the negative electrode unit chamber is circulated with trivalent vanadium (V). 3+ ) and divalent vanadium (V 2+ The negative electrode electrolyte is used for charging and discharging of the battery. During charging, vanadium ions release electrons in the positive electrode cell, thus increasing Vo. 4+ Oxidized to V 5+ In the negative electrode cell, electrons returning through the external path are used to convert V 3+ Restore to V 2+ In this redox reaction, the protons (H+) in the positive electrode unit chamber... +) becomes excessive, the diaphragm selectively moves the excessive protons of the positive electrode cell chamber to the negative electrode chamber, thereby maintaining electrical neutrality. During discharge, the reverse reaction thereof is performed. The charge storage performance in charge and discharge of the redox flow battery can be judged based on the energy efficiency. In the redox flow battery using a membrane that achieves both low internal resistance and prevention of permeation of the active material, the amount of charge accumulated in the active material is extracted without waste, and the internal resistance loss per unit amount of charge can also be suppressed, and thus the final charge storage performance (energy efficiency) of the battery is improved. If the active material permeates through the diaphragm during operation of the redox flow battery, the ions whose valence has changed due to charging cannot contribute to discharge, and thus the current efficiency (power efficiency) decreases. By using the diaphragm for a redox flow battery according to the present embodiment, permeation of the active material can be suppressed, and thus operation can be performed with high current efficiency (power efficiency).

[0147] On the other hand, the internal resistance of the redox flow battery depends on the proton permeability of the membrane, and by using a membrane with high proton permeability, the internal resistance of the battery is reduced. If the internal resistance is reduced, the voltage required during charging decreases, and during discharge, operation can be performed at a higher voltage. As described above, the diaphragm for a redox flow battery according to the present embodiment achieves high current efficiency (high power efficiency) by suppressing permeation of the active material while maintaining low internal resistance, and the resulting energy efficiency of the redox flow battery is high.

[0148] Example

[0149] Hereinafter, the present application will be described in more detail using examples, but the present application is not limited thereto. Note that the measurement conditions of each property are described below. In addition, chemical structural formulas are inserted in the present example, but the chemical structural formulas are inserted for the purpose of helping the reader to understand, and do not necessarily accurately represent the chemical structure of the polymerization component, the accurate composition, the arrangement, the position, the number, the molecular weight of the sulfonic acid group, and the like, and are not limited thereto.

[0150] (1) Nuclear magnetic resonance spectrum (NMR)

[0151] Measurement was performed under the following measurement conditions 1 Measurement of H-NMR was performed to confirm the structure.

[0152] Apparatus: EX-270 manufactured by JEOL Ltd.

[0153] Resonance frequency: 270 MHz 1 H-NMR)

[0154] Measurement temperature: room temperature

[0155] Dissolution solvent: DMSO-d6

[0156] Internal standard substance: TMS (0 ppm)

[0157] Cumulative number: 16 times

[0158] (2) Weight average molecular weight

[0159] The weight average molecular weight of the polymer was measured by GPC. As an integrated device of an ultraviolet detector and a differential refractometer, HLC-8022 GPC manufactured by Tosoh Corporation was used, and as a GPC column, TSK gel Super HM-H (inner diameter 6.0 mm, length 15 cm) manufactured by Tosoh Corporation was used. The weight average molecular weight was calculated by conversion to standard polystyrene under the following conditions: NMP solvent (NMP solvent containing 10 mmol / L of lithium bromide) was used, the sample concentration was 0.1 wt%, the flow rate was 0.2 mL / min, and the temperature was 40°C.

[0160] (3) Measurement method of purity

[0161] Quantitative analysis was performed by gas chromatography (GC) under the following conditions.

[0162] Column: DB-5 (manufactured by Agilent Technologies), L = 30 m, Φ = 0.53 mm, D = 1.50 μm

[0163] Carrier: helium (linear velocity = 35.0 cm / sec)

[0164] The analysis conditions were as follows.

[0165] Inj. temp.: 300°C

[0166] Detct. temp.: 320°C

[0167] Oven: 50°C x 1 minute

[0168] Rate: 10°C / min

[0169] Final: 300°C x 15 minutes

[0170] SP ratio: 50:1

[0171] (4) Dispersed diameter, thickness of A layer and B layer

[0172] (5) Measurement method of molecular weight distributionThe electrolyte membrane was cut along the vertical direction, and a cross-sectional sample was prepared using a freeze-ultra-thin sectioning method. In order to make the contrast of the image clear, staining with osmic acid, ruthenic acid, phosphotungstic acid, or the like can also be performed. Using a transmission electron microscope (TEM, Hitachi Ltd. HT7700), the cross section was observed under the condition of an acceleration voltage of 200 kV. The obtained photograph was inputted into an image analyzer (Leica MICROSYSTEMS, Leica Application Suite LAS ver4.6) in the form of an image, and for dispersion having a dispersion diameter of 0.1 μm or more, the circumscribed circle of each dispersion was taken, and the number average of the diameters thereof was taken as the dispersion diameter. Note that in a case where it was not possible to determine whether the dispersion was a hydrocarbon polymer, mapping of the fluorine element was performed using EDX, and the dispersion in which no fluorine atom was detected was regarded as a hydrocarbon polymer. Further, using the same method as above, the thickness of the A layer and the B layer was measured at 5 places each and averaged.

[0173] (5) Distance of hydrocarbon polymer in the A layer from the surface layer of the A layer (t2)

[0174] From the results of TEM-EDX observation above, the distance from the surface layer of the A layer to the hydrocarbon polymer (dispersion diameter of 0.1 μm or more) in the A layer was confirmed at 5 points, and the minimum value was taken. Next, it was confirmed whether the thickness of the A layer (t1) and (t2) satisfied the following equation. In a case where the following equation was satisfied, it was indicated as O in Table 1.

[0175] (t1) - (t2) > 0...(1)

[0176] (6) Crystalline melting peak temperature (Tm)

[0177] For the electrolyte membrane, measurement was performed in accordance with JIS K 7121 (1987) and JIS K 7122 (1987), using a differential scanning calorimeter "Temperature Modulation DSC7000X" manufactured by Hitachi High-Tech Corporation as the measuring device, and "SSC / 5200" manufactured by Disk Session as the data analyzer, in accordance with the following essentials. The value of the lowest crystalline melting peak temperature in the results of the third run was taken.

[0178] Sample amount: 10 mg

[0179] Rising and falling speed: 10°C / min

[0180] Measurement mode: lamp mode

[0181] Measurement procedure:

[0182] First run: 25°C to 120°C, held for 60 minutes

[0183] Second run: 120℃~25℃, maintain for 10 minutes

[0184] Third run: 25℃~210℃, maintain for 0 minutes

[0185] Fourth run: 210℃~25℃, maintain for 10 minutes.

[0186] (7) Transmission rate of tetravalent vanadium ions

[0187] An electrolyte membrane (6.6 cm) is sandwiched between H-type batteries. 2 ), and 1.5 mol·L⁻¹ was added to one side. －1 Magnesium sulfate / 3.5 mol·L －1 A sulfuric acid aqueous solution, with 1.5 mol·L⁻¹ added on the other side. －1 Vanadium(IV) sulfate / 3.5 mol·L －1 70 mL of sulfuric acid aqueous solution was prepared. The mixture was stirred using a magnetic stirrer at 25 °C and 300 rpm. The concentration of tetravalent vanadium ions dissolved in the magnesium sulfate solution after 4 days was determined by the absorbance at 765 nm using a UV spectrophotometer (Hitachi, Ltd., U-3010).

[0188] Pre-prepared 3.5 mol·L⁻¹ vanadium(IV) sulfate solutions of different concentrations －1 The absorbance of the sulfuric acid aqueous solution was measured using the UV spectrophotometer described above, and a standard curve was prepared based on the relationship between concentration and absorbance. The concentration of tetravalent vanadium ions that were transmitted was then quantified using this standard curve.

[0189] Next, the permeation rate of tetravalent vanadium ions is calculated using the following formula. Here, the specific gravity (density) of the sulfuric acid aqueous solution used is assumed to be the same as that of water.

[0190] Vanadium tetravalent ion permeation (×10) －10 cm 2 / minute) = Vanadium ion concentration that has passed through (mol) / (membrane area (cm²) 2 () × Transmission time (minutes) / 1.5 (×10) －3 mol / cm 3 ) × film thickness (cm)

[0191] (8) Tensile test (Young's modulus, elongation at break, tensile strength)

[0192] Tensile tests were conducted under the following conditions, and the average value of the tests with n=5 was used as Young's modulus, elongation at break, and fracture stress.

[0193] Measuring device: AutoGrav AG-IS (manufactured by Shimadzu Corporation)

[0194] Load: 100 N

[0195] Stretching speed: 10 mm / min

[0196] Test piece: width 10 mm x length 50 mm

[0197] Distance between samples: 30 mm

[0198] Test environment: 23°C x 50% RH

[0199] Number of tests: n = 5

[0200] (9) Dimensional change rate

[0201] A composite electrolyte membrane was cut into a square of about 5 cm x about 5 cm, and after standing for 24 hours in a temperature-controlled and humidity-controlled atmosphere at a temperature of 23°C ± 5°C and a humidity of 50% ± 5%, the length in the direction of membrane formation (MD) and the length in the direction orthogonal to the direction of membrane formation (TD) (MD1 and TD1) were measured with a vernier caliper. After the electrolyte membrane was immersed in hot water at 80°C for 8 hours, the length in the MD direction and the length in the TD direction (MD2 and TD2) were again measured with a vernier caliper, and the dimensional change rates in the MD direction and the TD direction (λMD and λTD) and the dimensional change rate in the plane direction (λxy) (%) were calculated from the following formula.

[0202] λMD = (MD2 - MD1) / MD1 x 100

[0203] λTD = (TD2 - TD1) / TD1 x 100

[0204] λxy = (λMD + λTD) / 2

[0205] (10) Ion exchange capacity (IEC)

[0206] After the electrolyte membrane, which had been subjected to proton replacement and sufficiently washed with pure water, was wiped of moisture on the membrane surface, it was vacuum-dried at 100°C for 12 hours or more, and the dried weight was obtained. Next, 50 mL of a 5% by weight sodium sulfate aqueous solution was added to the electrolyte, and ion exchange was performed by standing for 12 hours. Next, the produced sulfuric acid was titrated using a 0.01 mol / L sodium hydroxide aqueous solution. As an indicator, a commercially available phenothalin solution for titration 0.1 w / v% was added, and the point at which the color became light purple red was taken as the end point. Finally, the ion exchange capacity was calculated from the following formula.

[0207] Ion exchange capacity (meq / g) = (concentration of sodium hydroxide aqueous solution (mmol / mL) x amount of titration (mL)) / dried weight of test piece (g)

[0208] (11) Redox flow battery charge-discharge test

[0209] For the electrolyte membranes produced in the examples and comparative examples, charge-discharge tests were performed as described below, respectively.

[0210] The electrolyte membranes were sandwiched with two carbon electrode materials composed of carbon felt (thickness 3.0 mm) to assemble evaluation batteries. The evaluation batteries were small batteries having an effective electrode area of 50 cm 2 , and had piping for supplying electrolyte solutions to each electrode. The electrolyte solution of each electrode was supplied to the carbon electrode material constituting each electrode through the piping provided in the battery and passed through the voids in the carbon electrode material.

[0211] The performance of the electrolyte membranes was tested by repeatedly performing charge-discharge at a constant current density. The current density at the time of charge-discharge was set to 80 mA / cm 2 . The upper limit voltage at the time of charge was set to 1.55 V, and the lower limit voltage at the time of discharge was set to 1.0 V. The positive electrode electrolyte solution was a 4.3 mol / L aqueous sulfuric acid solution of 1.7 mol / L vanadyl sulfate, and the negative electrode electrolyte solution was a 4.3 mol / L aqueous sulfuric acid solution of 1.7 mol / L vanadium sulfate. The amount of electrolyte solution was set to 50 mL in both the positive and negative electrodes. The liquid flow rate was set to 40 mL per minute, and the test was performed in a constant-temperature bath at 35°C to 1,000 cycles. Among them, the discharge capacity at the 1st cycle was taken as a reference, and at the time point at which the capacity decreased by 30%, the electrolyte solution was withdrawn from the tank, replaced with a new one, and the test was performed.

[0212] Here, in the charge-discharge test, the current density per unit electrode area was set to 80 mA / cm 2 (4,000 mA), the amount of electricity required for charging up to 1.55 V was recorded as Q1 coulombs, and the amount of electricity extracted in constant-current discharge up to 1.0 V was recorded as Q2 coulombs. In addition, the amount of electricity required for charging up to 1.55 V was recorded as W1, and the amount of electricity extracted in constant-current discharge up to 1.0 V was recorded as W2.

[0213] Then, using these Q1, Q2, and W1, W2, the current efficiency, power efficiency, and voltage efficiency at the 5th cycle were calculated based on the following formulas as initial performance. In addition, the power efficiency after 1,000 cycles (VE1000) was similarly calculated, and the reduction rate compared to the initial power efficiency (VE) was calculated from the following formula.

[0214] Current efficiency (CE) = Q2 x 100 / Q1

[0215] Power efficiency (VE) = W2 x 100 / W1

[0216] Voltage efficiency (EE) = Power efficiency / Current efficiency

[0217] Rate of decrease in power efficiency (%) = {(VE - VE1000) / VE} x 100

[0218] (12) Durability

[0219] After the above redox flow battery charge-discharge test was performed for 800 cycles and 1,000 cycles, the battery was disassembled, the appearance of the electrolyte membrane was observed, and evaluation was performed in 5 ranks based on the following criteria. A is the best, B and C are good, D is the allowable level, and E is the unallowable level. Details are described below.

[0220] A: No change even at 1,000 cycles

[0221] B: No change at 800 cycles, but slight swelling and peeling were confirmed at 1,000 cycles.

[0222] C: No change at 800 cycles, but slight swelling and peeling were confirmed at 1,000 cycles.

[0223] D: Slight swelling and peeling were confirmed at 800 cycles, but whitening did not occur at 1,000 cycles.

[0224] E: Slight whitening was present at 800 cycles, and severe whitening occurred at 1,000 cycles. (If the deterioration of the ion-conducting hydrocarbon polymer of the electrolyte membrane progresses, whitening occurs, and the power efficiency rapidly decreases.)

[0225] (13) Non-ion-conducting fluorinated polymer

[0226] As the non-ion-conducting fluorinated polymer, the following polyvinylidene fluoride (homopolymer or copolymer) was used.

[0227] • PVDF 2751: "Kynar" (registered trademark) 2751: copolymer of vinylidene fluoride and hexafluoropropylene, manufactured by Arkema Co.

[0228] • PVDF 21510: "Solef" (registered trademark) 21510: copolymer of vinylidene fluoride and hexafluoropropylene, manufactured by Solvay Co.

[0229] • PVDF 7300: "Kureha KF Polymer" (registered trademark) #7300: homopolymer of vinylidene fluoride, manufactured by Kureha Corporation

[0230] (14) Ion-conducting hydrocarbon polymer

[0231] • "Nafion" (registered trademark) D2020 (manufactured by Chemours Co.)

[0232] • "Aquivion" (registered trademark) D79-25BS (Solvay Specialty Polymers USA, LLC)

[0233] (15) Hydrocarbon polymer having ion conductivity

[0234] <SYNTHESIS EXAMPLE 1>

[0235] Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (hereinafter, K-DHBP) represented by the following general formula

[0236]

[0237] In a 500 mL flask equipped with a stirrer, a thermometer, and a distillation tube, 4,4'-dihydroxybenzophenone 49.5 g, ethylene glycol 134 g, trimethyl orthoformate 96.9 g, and p-toluenesulfonic acid monohydrate 0.50 g were charged and dissolved. Thereafter, the temperature was kept at 78 to 82°C and stirred for 2 hours. Further, the internal temperature was slowly increased to 120°C, and heating was performed until the distillation of methyl formate, methanol, and trimethyl orthoformate was completely stopped. After the reaction solution was cooled to room temperature, the reaction solution was diluted with ethyl acetate, and the organic layer was washed with 5% potassium carbonate aqueous solution 100 mL and separated. Then, the solvent was distilled off. In the residue, dichloromethane 80 mL was added, and crystallization was performed. The obtained crystal was filtered and dried to obtain 52.0 g of a dry crystal. GC analysis of the crystal showed that the purity was 99.8% of K-DHBP and 0.2% of 4,4'-dihydroxybenzophenone.

[0238] <SYNTHESIS EXAMPLE 2>

[0239] Synthesis of 3,3'-disodium sulfonate-4,4'-difluorobenzophenone represented by the following general formula

[0240]

[0241] 4,4'-difluorobenzophenone 109.1 g (Aldrich Reagent) was reacted in fuming sulfuric acid (50% SO3) 150 mL (Wako Pure Chemical Industries, Ltd. Reagent) at 100°C for 10 hours. Thereafter, it was added to a large amount of water little by little, neutralized with NaOH, and then 200 g of salt was added to precipitate the product. The obtained precipitate was filtered and recrystallized in an ethanol aqueous solution to obtain 3,3'-disodium sulfonate-4,4'-difluorobenzophenone. The purity was 99.3%. The structure was confirmed by 1 H-NMR. As for the impurities, quantitative analysis was performed by capillary electrophoresis (organic matter) and ion chromatography (inorganic matter).

[0242] <SYNTHESIS EXAMPLE 3> Synthesis of random copolymer r1

[0243] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, were placed potassium carbonate 13.82 g (Aldrich reagent, 100 mmol), K-DHBP obtained in Synthesis Example 1 above 20.66 g (80 mmol, 50 mol%), 3,3'-disulfonate sodium-4,4'-difluorobenzophone obtained in Synthesis Example 2 above 13.51 g (32 mmol, 20 mol%), and 4,4'-difluorobenzophone 10.47 g (Aldrich reagent, 48 mmol, 30 mol%). After nitrogen substitution, dehydration was performed at 180°C in NMP 90 mL and toluene 45 mL, and then the toluene was removed by heating. Polymerization was performed at 200°C for 3 hours. Purification was performed by reprecipitation with a large amount of water to obtain random copolymer r1. The weight average molecular weight of the random copolymer was 410,000.

[0244] <SYNTHESIS EXAMPLE 4> Synthesis of random copolymer r2

[0245] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, were placed potassium carbonate 13.82 g (Aldrich reagent, 100 mmol), K-DHBP obtained in Synthesis Example 1 above 16.53 g (64 mmol, 40 mol%), 2,2-bis(4-hydroxyphenyl)hexafluoropropane 5.39 g (Tokyo Chemical Industry Co., Ltd. reagent, 16 mmol, 10 mol%), 3,3'-disulfonate sodium-4,4'-difluorobenzophone obtained in Synthesis Example 2 above 13.51 g (32 mmol, 20 mol%), and 4,4'-difluorobenzophone 10.47 g (Aldrich reagent, 48 mmol, 30 mol%). After nitrogen substitution, dehydration was performed at 180°C in NMP 90 mL and toluene 45 mL, and then the toluene was removed by heating. Polymerization was performed at 200°C for 3 hours. Purification was performed by reprecipitation with a large amount of water to obtain random copolymer r2. The weight average molecular weight of the random copolymer was 390,000.

[0246] <SYNTHESIS EXAMPLE 5> Synthesis of random copolymer r3

[0247] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, potassium carbonate 13.82 g (Aldrich reagent, 100 mmol), K-DHBP 20.66 g (80 mmol, 50 mol%) obtained in Synthesis Example 1 above, 3,3'-disulfonate sodium-4,4'-difluorobenzophenone 11.82 g (28 mmol, 17.5 mol%) obtained in Synthesis Example 2 above, and 4,4'-difluorobenzophenone 11.35 g (Aldrich reagent, 52 mmol, 32.5 mol%) were charged, and after nitrogen substitution, dehydration was performed at 180°C in NMP 90 mL and toluene 45 mL, and then the toluene was removed by warming, and polymerization was performed at 200°C for 3 hours. Purification was performed by reprecipitation with a large amount of water, and a random copolymer r3 was obtained. The weight average molecular weight of the random copolymer was 400,000.

[0248] <SYNTHESIS EXAMPLE 6> Synthesis of block copolymer b1

[0249] (Synthesis of ion-group-free oligomer a1' represented by the following general formula)

[0250] In a 1,000 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, potassium carbonate 16.59 g (Aldrich reagent, 120 mmol), K-DHBP 25.8 g (100 mmol), and 4,4'-difluorobenzophenone 20.3 g (Aldrich reagent, 93 mmol) were charged, and after nitrogen substitution, dehydration was performed at 160°C in NMP 300 mL and toluene 100 mL, and then the toluene was removed by warming, and polymerization was performed at 180°C for 1 hour. Purification was performed by reprecipitation with a large amount of methanol, and an ion-group-free oligomer a1 (terminal hydroxyl group) was obtained. The number average molecular weight was 10,000.

[0251] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, potassium carbonate 1.1 g (Aldrich reagent, 8 mmol), ion-group-free oligomer a1 (terminal hydroxyl group) 20.0 g (2 mmol) above were charged, and after nitrogen substitution, dehydration was performed at 100°C in NMP 100 mL and cyclohexane 30 mL, and then the cyclohexane was removed by warming, and decafluorobiphenyl 4.0 g (Aldrich reagent, 12 mmol) was charged, and reaction was performed at 105°C for 1 hour. Purification was performed by reprecipitation with a large amount of isopropyl alcohol, and an ion-group-free oligomer a1' (terminal fluoro group) represented by the following formula was obtained. The number average molecular weight was 11,000, and the number average molecular weight of the ion-group-free oligomer a1' was calculated by subtracting the value of the linker site (molecular weight 630) from 10,400.

[0252]

[0253] (Synthesis of ion-containing oligomer a2 represented by the following formula)

[0254] In a 1,000 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, potassium carbonate 27.6 g (Aldrich Reagent, 200 mmol), K-DHBP 12.9 g (50 mmol), and 4,4'-dihydroxybiphenyl 9.3 g (Aldrich Reagent, 50 mmol), 3,3'-disulfonato-4,4'-difluorobenzophenone 39.3 g (93 mmol), and 18-crown ether-6 7.9 g (Wako Pure Chemical Industries, 82 mmol) were charged, and after nitrogen replacement, dehydration was performed at 170°C in NMP 300 mL and toluene 100 mL, and then the toluene was removed by warming, and polymerization was performed at 180°C for 1 hour. Purification was performed by reprecipitation with a large amount of isopropyl alcohol, and thus ion-containing oligomer a2 (terminal hydroxyl group) represented by the following formula (G4) was obtained. The number average molecular weight was 16,000.

[0255]

[0256]

[0257] (In the above formula, M represents Na or K).

[0258] (Synthesis of block polymer b1 containing ion-containing oligomer a2 as segment (Al), oligomer al as segment (A2) not containing an ion-containing group, and octafluoro- diphenylene as a linker site)

[0259] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap, potassium carbonate 0.56 g (Aldrich Reagent, 4 mmol), ion-containing oligomer a2 (terminal hydroxyl group) 16 g (1 mmol) were charged, and after nitrogen replacement, dehydration was performed at 100°C in NMP 100 mL and cyclohexane 30 mL, and then the cyclohexane was removed by warming, and oligomer al' (terminal fluoro group) not containing an ion-containing group 11 g (1 mmol) was charged, and a reaction was performed at 105°C for 24 hours. Purification was performed by reprecipitation with a large amount of isopropyl alcohol, and thus block copolymer bl was obtained. The weight average molecular weight was 340,000.

[0260] (Example 1)

[0261] Ion-conducting hydrocarbon polymer 14 obtained in Synthesis Example 3 was dissolved in NMP to prepare a polymer solution, and pressure filtration was performed using a glass fiber filter.

[0262] The polymer solution obtained above was cast-coated on a polyethylene terephthalate substrate using a die coater, dried at 115°C for 10 minutes and at 150°C for 10 minutes, to obtain a polyketol ketone film. Subsequently, after a proton replacement, deprotection reaction was performed by immersing in 10 mass% sulfuric acid aqueous solution at 25°C for 24 hours, and the B layer was obtained by sufficiently washing by immersing in a large excess of pure water for 24 hours.

[0263] Using a die coater, a solution A obtained by dissolving "Nafion" (registered trademark) (after NMP replacement of a commercially available solution of D2020 manufactured by Chemours) and PVDF 2751 in NMP (solid content ratio: "Nafion" / PVDF = 40 mass% / 60 mass%, solid content concentration 10 mass%) was coated on one side of the B layer obtained above. Subsequently, drying was performed at 80°C for 10 min, at 100°C for 5 min, and at 120°C for 5 min, and then, the A layer was produced by sufficiently washing by immersing in a large excess of pure water for 24 hours and drying at room temperature, thereby obtaining an electrolyte membrane (film thickness 55 μm). The properties of the membrane are shown in Table 1.

[0264] [Table 1]

[0265]

[0266] (Examples 2 to 15)

[0267] The kind of the ion conductive hydrocarbon polymer used for the B layer, the thickness of the B layer, the thickness of the A layer, the kind of the PVDF used for the A layer, and the ratio of ("Nafion" or "Aquivion") / (PVDF) were changed as shown in Tables 1 and 2, and otherwise, the electrolyte membrane was obtained by the same method as in Example 1. The properties of the membrane are shown in Tables 1 and 2.

[0268] [Table 2]

[0269]

[0270] (Example 16)

[0271] A polymer solution was prepared by dissolving the ion conductive hydrocarbon polymer obtained in Synthesis Example 4 and PVDF 7300 in NMP so as to be 90 mass% and 10 mass%, respectively, and pressure filtration was performed using a glass fiber filter. The B layer was obtained by the same method as in Example 1 using the polymer solution obtained above.

[0272] Using a die coater, the one side of the B layer obtained above was coated with a solution A (solid content ratio: Nafion / PVDF = 40 mass% / 60 mass%, solid content concentration 10 mass%) obtained by dissolving "Nafion" (registered trademark) (after NMP replacement of a commercially available solution of D2020 manufactured by Chemours) and PVDF 21510 in NMP. Subsequently, drying was performed at 80°C for 10 min, at 100°C for 5 min, at 120°C for 5 min, and then, the membrane was sufficiently washed by immersion in a large excess of pure water for 24 hours, and drying was performed at room temperature, whereby the A layer was produced, and an electrolyte membrane (film thickness 55 μm) was obtained. The properties of the membrane are shown together in Table 2.

[0273] (Comparative Example 1)

[0274] Using the polymer obtained in Synthesis Example 3, the same operation as described in Example 1 was performed to produce the B layer, and an electrolyte membrane composed only of the B layer was obtained. The properties of the membrane are shown together in Table 2.

[0275] Explanation of Reference Signs

[0276] P: ion-conductive hydrocarbon polymer

[0277] Industrial Applicability

[0278] The electrolyte membrane of the present application can be applied to various electrochemical devices (for example, redox flow batteries, fuel cells, water electrolysis devices, chlor-alkali electrolysis devices, etc.). Among these devices, it is suitable as a separator for a redox flow battery, and in particular, it can be suitably used for a redox flow battery using vanadium as an active material.

[0279] A redox flow battery produced using the electrolyte membrane of the present application can be preferably used as a secondary battery for storing electric power generated by solar power generation, wind power generation, etc.

Claims

1. An electrolyte membrane comprising at least a layer A and a layer B, wherein layer A comprises an ion-conducting fluorinated polymer and a non-ion-conducting fluorinated polymer, layer B comprises an ion-conducting hydrocarbon polymer, the thickness of layer A being 2 μm or more and 15 μm or less, and the ion-conducting hydrocarbon polymer being dispersed in layer A. The mass ratio of ion-conductive fluorinated polymers to non-ion-conductive fluorinated polymers in layer A is 25:75 to 55:

45.

2. The electrolyte membrane according to claim 1, wherein the ion-conducting hydrocarbon polymer is dispersed in layer A with a dispersion diameter of less than 0.55 μm.

3. The electrolyte membrane according to claim 1 or 2, wherein the thickness of layer A is denoted as t1 and the shortest distance from the surface of layer A to the hydrocarbon polymer in layer A is denoted as t2, satisfies the following formula (1). (t1)-(t2)>0···(1).

4. The electrolyte membrane according to claim 1 or 2, wherein the lowest crystallization melting peak Tm in the third run of differential scanning calorimetry (DSC) measurement is less than 175°C.

5. The electrolyte membrane according to claim 1 or 2, wherein the nonionic conductive fluorinated polymer is at least one selected from polyvinylidene fluoride and copolymers of polyvinylidene fluoride with other fluorinated monomers.

6. The electrolyte membrane according to claim 1 or 2, wherein the non-ionic conductive fluorinated polymer is a copolymer of vinylidene fluoride and other fluorinated monomers.

7. The electrolyte membrane according to claim 1 or 2, wherein the non-ionic conductive fluorinated polymer is a copolymer of vinylidene fluoride and hexafluoropropylene.

8. The electrolyte membrane according to claim 1 or 2, wherein the ion-conducting hydrocarbon polymer contains structural units represented by the following formulas (1) and (2), -O-Ar1-O-Ar2-O- (1) -O-Ar1-O-Ar3-O- (2) In formulas (1) and (2), Ar1 and Ar2 are represented by the following chemical formula (3), and Ar3 is represented by the following chemical formula (4). In equations (3) and (4), X1 and X2 independently represent -(C=O)-, -O-, -SO2-, and fluorocarbon groups, respectively. X1 and X2 can contain multiple of the above structures, and Y represents an ionic group.

9. The electrolyte membrane according to claim 8, wherein the fluorocarbon group is -C(CF3)2-.

10. The electrolyte membrane according to claim 1 or 2, wherein the ion-conducting hydrocarbon polymer contains structural units represented by the following formulas (1) and (2). -O-Ar1-O-Ar2-O- (1) -O-Ar1-O-Ar3-O- ((2) In formulas (1) and (2), Ar1 and Ar2 are represented by the following chemical formula (3), and Ar3 is represented by the following chemical formula (4). In formulas (3) and (4), X1 is a group that is deprotected after the reaction and reverts to -(C=O)-, X2 is -(C=O)-, -O-, -SO2- or a fluorocarbon group, and X1 and X2 can each independently contain multiple of the above structures, and Y represents an ionic group.

11. The electrolyte membrane according to claim 10, wherein the fluorocarbon group is -C(CF3)2-.

12. The electrolyte membrane according to claim 1 or 2, wherein the ion-conducting hydrocarbon polymer is a random copolymer.

13. The electrolyte membrane according to claim 1 or 2, wherein the weight average molecular weight of the ion-conducting hydrocarbon polymer is 300,000 or more.

14. The electrolyte membrane according to claim 1 or 2, wherein the electrolyte has a vanadium ion concentration of 1.5 mol·L⁻¹. －1 Sulfuric acid concentration 3.0 mol·L －1 The permeability of active material per unit area in the aqueous solution is 1800 × 10⁻⁶. －10 cm 2 / min or less.

15. The electrolyte membrane according to claim 1 or 2 has a tensile modulus of elasticity of 0.5 GPa or higher at 23°C and 50% RH.

16. The electrolyte membrane according to claim 1 or 2, wherein layer A is stacked only on one side of layer B.

17. The electrolyte membrane according to claim 1 or 2, wherein it is an electrolyte membrane for a redox flow battery.

18. A redox flow battery that uses the electrolyte membrane according to any one of claims 1 to 17 as a separator to isolate and separate the positive electrode from the negative electrode.

19. The redox flow battery according to claim 18, wherein the electrolyte membrane A layer side is configured as the positive electrode side.

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