Ion exchange membrane, composite, and electrochemical cell

Abstract

To provide an ion exchange membrane having low cell resistance and excellent durability against an electrolyte solution.SOLUTION: An ion exchange membrane includes a polymer represented by the general formula (1). (R represents a divalent aromatic group, Z represents one or more of O, S, SO2, CO, C(CH3)2, C(CF3)2, and OPhO, Y1 and Y2 each represent a hydrogen atom or a group containing a quaternary ammonium salt, provided that, when the mole fraction of the group containing a quaternary ammonium salt contained in the polymer is a%, the range of a is 10≤a≤100, and, when the copolymerization ratio with a polybenzimidazole having a different structure is m, m represents an integer from 1 to 100.)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an ion exchange membrane containing polybenzimidazole, which is particularly useful for electrochemical cells such as redox flow batteries and anion exchange membrane water electrolysis devices. [Background technology]

[0002] In recent years, new electrochemical cells with excellent energy efficiency and environmental friendliness have been attracting attention, such as redox flow batteries for storing natural energy such as solar and wind power, and water electrolysis devices for converting surplus electricity into hydrogen.

[0003] Redox flow batteries generate energy through the oxidation-reduction reaction of vanadium ions in a vanadium sulfate solution, typically circulated by a pump. Redox flow batteries use an ion-exchange membrane to maintain the ionic balance between the electrodes.

[0004] In addition to ionic conductivity, which contributes to low resistance in electrochemical cells, ion exchange membranes are also required to have properties such as electrolyte permeation prevention and mechanical strength. Conventional ion exchange membranes have used perfluorocarbon sulfonic acid polymers with sulfonic acid groups introduced, such as Nafion (registered trademark) manufactured by DuPont (USA), or crosslinked polystyrene sulfonic acid, such as Neosepta manufactured by Tokuyama Corporation. Ion exchange membranes containing hydrocarbons, such as Selemion APS manufactured by Asahi Glass Co., Ltd. and polyethersulfone with sulfonic acid groups introduced, have also attracted attention.

[0005] Ion exchange membranes containing perfluorocarbon sulfonic acid polymers such as Nafion® have the advantages of excellent chemical durability, high proton conductivity, and low cell resistance. However, Nafion® suffers from the problem of poor ion permeability. Specifically, it allows vanadium ions to pass through during charge and discharge, reducing the amount of active material in the electrolyte and significantly worsening the charge and discharge cycle. Other problems include high cost and the significant environmental impact of disposal.

[0006] On the other hand, ion exchange membranes containing hydrocarbons, such as cross-linked polystyrene sulfonic acid, have the advantage of overcoming the drawbacks of Nafion. Specifically, they have advantages such as low cost, low permeability to vanadium ions, and excellent ion permeation selectivity. However, when used in redox flow batteries, they have a problem of being unable to withstand long-term operation due to their significantly insufficient oxidation resistance.

[0007] Various types of water electrolysis equipment have been proposed, including alkaline water electrolysis using a highly concentrated alkaline aqueous solution, proton exchange membrane (PEM) water electrolysis using a polymer electrolyte, and anion exchange membrane (AEM) water electrolysis. Among these, AEM water electrolysis allows for electrode designs that do not use rare metals, which not only reduces the cost of the equipment but also has the advantage of overcoming resource constraints, making the social implementation of AEM water electrolysis equipment promising.

[0008] AEM-type water electrolysis systems require the ability to conduct hydroxide ions, so anion exchange membranes are used. Anion exchange membranes are prepared by introducing quaternary ammonium groups into the main chain skeleton of polystyrene, polyethersulfone, or the like. However, AEM water electrolysis systems operate in a strongly alkaline environment at around 80°C, which creates an environment prone to degradation due to hydrolysis of the polymer main chain and quaternary ammonium groups.

[0009] The anion exchange membranes used in AEM-type water electrolysis systems are required to have alkali resistance and low cell resistance. For example, a membrane with low cell resistance can be obtained by increasing the amount of quaternary ammonium groups introduced or by reducing the membrane thickness. However, increasing the amount of quaternary ammonium groups introduced makes the membrane more susceptible to degradation and reduces alkali resistance. Furthermore, reducing the membrane thickness reduces mechanical strength, which can lead to leaks and makes it difficult to ensure reliability during cell assembly.

[0010] As described above, in the performance of ion exchange membranes in electrochemical cells such as redox flow batteries and AEM-type water electrolysis devices, there is often a trade-off between durability against the electrolyte and reliability during cell assembly, and it has been extremely difficult to achieve both.

[0011] Therefore, various technical studies have been conducted on ion exchange membranes that can achieve both durability against the electrolyte of an electrochemical cell and reliability during cell assembly. For example, Patent Document 1 proposes a method in which polyethersulfone having sulfonic acid groups is combined with a porous substrate as a reinforcing material, and the reinforcing material compensates for oxidative degradation. Furthermore, Patent Document 2 proposes a method in which diallylammonium monomer is filled into a microporous film and then polymerized to produce an anion exchange membrane. However, these methods do not suppress degradation of the polymer itself, and although the morphology of the ion exchange membrane is maintained, long-term use inevitably leads to a decrease in mechanical strength and a decrease in the performance of the electrochemical cell due to defects in the ion exchange membrane.

[0012] Patent Document 3 proposes a method of quaternizing polybenzimidazole or polyimidazole to form an anion exchange membrane, thereby improving alkali resistance through steric hindrance. However, with this method, the polymer backbone itself is responsible for anion conductivity, resulting in poor molecular mobility and making it difficult to achieve low cell resistance. Furthermore, these quaternization processes require the use of strong alkalis or metal hydrides, which poses problems such as reduced safety, a worsening work environment, and a heavy load on equipment.

[0013] Patent Document 4 also proposes a method of improving alkali resistance by using a quaternary phosphonium salt compared to ammonium salts. However, since phosphonium salts have a large molecular weight per structural unit, it is difficult to increase the amount of ion exchange groups per polymer weight, and it may be difficult to obtain low cell resistance. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] WO13 / 027758 [Patent Document 2] WO2010 / 055889 [Patent Document 3] Special Publication No. 2019-530760 [Patent Document 4] Japanese Patent Publication No. 2021-161223 Summary of the Invention [Problem to be solved by the invention]

[0015] In view of the above circumstances, an object of the present invention is to provide an ion exchange membrane that is excellent in durability against an electrolyte and has low cell resistance. [Means for solving the problem]

[0016] The present inventors have conducted extensive research to achieve the above object and have found that an ion exchange membrane prepared from a specific compound in which a quaternary ammonium salt is introduced into the side chain of polybenzimidazole has excellent durability against electrolytes and low cell resistance.

[0017] 1. An ion exchange membrane characterized by containing a polymer represented by the following general formula (1): [ka] (R represents a divalent aromatic group, Z represents one or more of O, S, SO2, CO, C(CH3)2, C(CF3)2, and OPhO; Each of Y1 and Y2 represents a hydrogen atom or a group containing a quaternary ammonium salt, When the molar fraction of the quaternary ammonium salt-containing group contained in the polymer is a%, it is in the range of 10≦a≦100, When the copolymerization ratio with polybenzimidazole of another structure is m, m represents an integer of 1 to 100. 2. The ion exchange membrane according to 1, characterized in that it contains a polymer represented by the following general formula (2): [ka] (R represents a divalent aromatic group, X represents one or more ionic groups selected from a sulfonic acid group, a phosphonic acid group, and a carboxyl group; Z represents one or more of O, S, SO2, CO, C(CH3)2, C(CF3)2, and OPhO; Each of Y1 and Y2 represents a group containing a hydrogen atom or a quaternary ammonium salt, When the molar fraction of the quaternary ammonium salt-containing group contained in the polymer is b%, b is in the range of 10≦b≦100, When the copolymerization ratio with polybenzimidazole of a different structure is n, n represents an integer from 99 to 0. 3. An ion exchange membrane according to 1 or 2, characterized in that some of the imidazole moieties are crosslinked by covalent bonds. 4. A composite comprising the ion exchange membrane according to any one of 1 to 3 and an electrode. 5. An electrochemical cell comprising the ion exchange membrane according to any one of 1 to 3 or the composite according to 4. [Effects of the Invention]

[0018] In the present invention, by using a specific compound in which a quaternary ammonium salt is introduced onto polybenzimidazole, it is possible to provide an ion exchange membrane that not only has low cell resistance but also excellent heat resistance, processability, and durability against the electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention is described in detail below. The present invention provides an ion exchange membrane that not only has low cell resistance but also excellent heat resistance, processability, and durability against electrolytes. Specifically, a compound having a polybenzimidazole main chain skeleton, which has excellent heat resistance and durability against electrolytes, and a quaternary ammonium salt-containing group introduced into the side chain, is used as the material for the ion exchange membrane. By appropriately selecting a flexible structure (Z in general formula (1)) in the polybenzimidazole, excellent processability can be easily achieved. In some cases, sulfonic acid groups can be introduced, and an extremely strong polymer structure can be achieved through acid-base interactions between the sulfonic acid groups and the imidazole or quaternary ammonium salt-containing groups. By combining the above-mentioned effects, an ion exchange membrane can be provided that has low cell resistance and excellent durability against electrolytes.

[0020] That is, the ion exchange membrane of the present invention is preferably prepared using a polymer containing a group containing a quaternary ammonium salt, as represented by general formula (1).

[0021] [ka] R represents a divalent aromatic group; Z represents one or more of O, S, SO2, CO, C(CH3)2, C(CF3)2, and OPhO; Y1 and Y2 each represent a hydrogen atom or a group containing a quaternary ammonium salt, and when the molar fraction of the group containing a quaternary ammonium salt contained in the polymer is a %, a satisfies the range of 10≦a≦100; m represents the copolymerization ratio with polybenzimidazole of a different structure and represents an integer of 1 to 100.

[0022] In the present invention, the route for synthesizing the polybenzimidazole containing the structure represented by the above general formula (1) is not particularly limited, but it can usually be synthesized by reacting one or more compounds selected from aromatic tetramines capable of forming an imidazole ring in the compound or their derivatives with one or more compounds selected from aromatic dicarboxylic acids or their derivatives. The dicarboxylic acid used is not particularly limited, but it is also possible to use dicarboxylic acids containing sulfonic acid groups or phosphonic acid groups, or salts thereof, or a combination of two or more of these.

[0023] In the present invention, the components of the polybenzimidazole are preferably bonded by random polymerization, alternating polymerization, and / or block polymerization, and the polymerization type is not limited to one type, and two or more types of polymerization may coexist in the same compound.

[0024] In the present invention, in the polybenzimidazole having the structure shown in formula (1) above, a group containing a quaternary ammonium salt is introduced into the side chain. There are no particular limitations on the method of introduction, but considering the ease of handling and the load on the manufacturing equipment, mild conditions are preferred. For example, an introduction method can be used in which an epoxy group or a halogenated alkyl group having a group containing a quaternary ammonium salt is used, and the introduction is carried out by an epoxy ring-opening reaction or an SN2 reaction with imidazole by simply heating and stirring. Alternatively, a method can be used in which a group containing a quaternary ammonium salt is introduced after the epoxy group or halogenated alkyl group is introduced.

[0025] In the present invention, in the polybenzimidazole represented by the above formula (1), Y1 and Y2 each represent a hydrogen atom or a group containing a quaternary ammonium salt, and when the molar fraction of the group containing a quaternary ammonium salt contained in the polymer is a%, the range of a is 10≦a≦100. If a is less than 10, the amount of quaternary ammonium group introduced is insufficient, and a membrane with low cell resistance cannot be obtained. In order to prevent excessive swelling due to water absorption, a is preferably 80 or less.

[0026] In the present invention, examples of the type of group containing a quaternary ammonium salt to be introduced include ammonium, imidazolium, pyridinium, pyrazolium, pyrrolidinium, pyrrolium, pyrimidium, piperidinium, indolium, triazinium, etc. In view of hydrolysis resistance and ease of introduction, preferred examples include ammonium, imidazolium, and pyridinium.

[0027] In the present invention, specific examples of aromatic tetraamines that contain the polymer represented by the above formula (1) and that give polybenzimidazole compounds containing sulfonic acid groups include, but are not limited to, 3,3',4,4'-tetraaminodiphenyl ether, 3,3',4,4'-tetraaminodiphenyl thioether, 3,3',4,4'-tetraaminodiphenyl sulfone, 2,2-bis(3,4-diaminophenyl)propane, bis(3,4-diaminophenyl)methane, 2,2-bis(3,4-diaminophenyl)hexafluoropropane, 1,4-bis(3,4-diaminophenoxy)benzene, and derivatives thereof. Among these, 3,3',4,4'-tetraaminodiphenyl ether, 3,3',4,4'-tetraaminodiphenyl sulfone, 2,2-bis(3,4-diaminophenyl)propane, 2,2-bis(3,4-diaminophenyl)hexafluoropropane, 1,4-bis(3,4-diaminophenoxy)benzene, or polybenzimidazoles obtained from derivatives thereof, which can form the bonding units represented by formula (2) and formula (3), are particularly preferred because they can be dissolved in aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, and hexamethylphosphonamide, and therefore facilitate the processability of ion exchange membranes.

[0028] Specific examples of derivatives of these aromatic tetramines include salts with acids such as hydrochloric acid, sulfuric acid, and phosphoric acid. These compounds may be used alone or in combination. Furthermore, these compounds may contain known antioxidants such as tin(II) chloride and phosphorous compounds, as needed.

[0029] In the present invention, the effect of introducing a flexible structure (Z in general formula (1)) into the polybenzimidazole structure is not limited to excellent processability and ease of production. From the viewpoint of durability against the electrolyte, it is preferable to select Z appropriately. Z may be one or more of O, SO2, CO, C(CH3)2, C(CF3)2, and OPhO, and when oxidation resistance is taken into consideration, SO2 and CO are preferred.

[0030] In the present invention, in the structure represented by the above formula (1), R is not particularly limited as long as it is a divalent aromatic group. Examples include those derived from common dicarboxylic acids reported as polyester raw materials, such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, biphenyl dicarboxylic acid, terphenyl dicarboxylic acid, and 2,2-bis(4-carboxyphenyl)hexafluoropropane.

[0031] The compound containing the polymer represented by the general formula (1) may be a compound into which a polybenzimidazole having a structure other than that represented by formula (1) is introduced by any known method such as random copolymerization, block copolymerization, or graft polymerization, or may be a compound blended with a polymer having a structure other than that represented by formula (1).

[0032] The ion exchange membrane of the present invention preferably contains a polymer represented by the following general formula (2). [ka] R represents a divalent aromatic group; X represents one or more ionic groups selected from a sulfonic acid group, a phosphonic acid group, and a carboxyl group; Z represents one or more of O, SO2, CO, C(CH3)2, C(CF3)2, and OPhO; Y1 and Y2 each represent a hydrogen atom or a group containing a quaternary ammonium salt, and when the molar fraction of the group containing a quaternary ammonium salt contained in the polymer is b%, b satisfies the range of 10≦b≦100; n indicates the copolymerization ratio with polybenzimidazole of a different structure, and represents an integer of 99 to 0.

[0033] The ion exchange membrane of the present invention may contain an acidic group such as a sulfonic acid group, as shown in the above general formula (2). This is preferable for use in redox flow batteries, as it allows for an extremely strong polymer structure to be formed due to acid-base interaction between the sulfonic acid group and the group containing imidazole or a quaternary ammonium salt.

[0034] In the present invention, in the polybenzimidazole represented by the above formula (2), Y1 and Y2 each represent a hydrogen atom or a group containing a quaternary ammonium salt, and when the molar fraction of the group containing a quaternary ammonium salt contained in the polymer is b%, the range of b is 10≦b≦100. If b is less than 10, the amount of quaternary ammonium group introduced is insufficient, making it impossible to obtain a membrane with low cell resistance. In order to prevent excessive swelling due to water absorption, b is preferably 80 or less.

[0035] The dicarboxylic acid containing an acidic group that provides the structure of formula (2) above can be selected from aromatic dicarboxylic acids containing one to four ionic groups. Specific examples include sulfonic acid-containing dicarboxylic acids such as 2,5-dicarboxybenzenesulfonic acid, 2,5-dicarboxybenzenephosphonic acid, 3,5-dicarboxybenzenesulfonic acid, 3,5-dicarboxybenzenephosphonic acid, 2,5-dicarboxy-1,4-benzenedisulfonic acid, 4,6-dicarboxy-1,3-benzenedisulfonic acid, 2,2'-disulfo-4,4'-biphenyldicarboxylic acid, 3,3'-disulfo-4,4'-biphenyldicarboxylic acid, (3,3'-disulfo-4,4'-dicarboxylic acid)diphenyl ether, and (2,6-disulfo)-1,5-naphthalenedicarboxylic acid, as well as derivatives thereof. Examples of derivatives include alkali metal salts such as sodium and potassium, ammonium salts, and alkylammonium salts. The structure of the dicarboxylic acid containing a sulfonic acid group is not particularly limited to these.

[0036] Examples of the ionic group in the dicarboxylic acid containing the ionic group that gives the structure of the above formula (1) include a sulfonic acid group, a phosphonic acid group, a hydroxyl group, and a carboxyl group. Among these, sulfonic acid groups and phosphonic acid groups, which have a high degree of proton dissociation, are preferred. By using a sulfonic acid group or a phosphonic acid group, a stronger ionic bond can be obtained, and both low cell resistance and durability against the electrolyte can be achieved.

[0037] These compounds may be used alone or in combination. Furthermore, these compounds may contain a known antioxidant such as tin(II) chloride or a phosphorous compound, as needed.

[0038] In the present invention, the effect of introducing a flexible structure (Z in general formula (2)) into the polybenzimidazole structure is not limited to excellent processability and ease of production. From the viewpoint of durability against the electrolyte, it is preferable to select Z appropriately. Z may be one or more of O, SO2, CO, C(CH3)2, C(CF3)2, and OPhO, and when oxidation resistance is taken into consideration, SO2 and CO are preferred.

[0039] The method for synthesizing the polybenzimidazole of the present invention is not particularly limited, but it can be synthesized, for example, by dehydration cyclopolymerization using polyphosphoric acid as a solvent, as described in J.W. Wolfe, Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 11, p. 601 (1988). Polymerization using a similar mechanism using a methanesulfonic acid / phosphorus pentoxide mixed solvent system instead of polyphosphoric acid can also be applied. To synthesize polybenzimidazole compounds with high thermal stability, polymerization using polyphosphoric acid, which is commonly used, is preferred.

[0040] Furthermore, polybenzimidazole can be obtained by, for example, synthesizing a precursor polymer having a polyamide structure or the like by a reaction in an appropriate organic solvent or in the form of a mixed raw material monomer melt, and then converting the precursor polymer into the desired polybenzimidazole structure by a cyclization reaction using an appropriate heat treatment or the like.

[0041] In the present invention, the molecular weight of the polybenzimidazole is not particularly limited, but is preferably 1,000 or more, more preferably 3,000 or more. Furthermore, the molecular weight is preferably 1,000,000 or less, more preferably 200,000 or less. If the molecular weight is less than 1,000, the viscosity decreases, making it difficult to obtain molded articles with good properties from the polybenzimidazole compound. If the molecular weight exceeds 1,000,000, the viscosity increases, making it difficult to mold the polybenzimidazole. The molecular weight of the polybenzimidazole compound of the present invention can be essentially evaluated by the logarithmic viscosity measured in concentrated sulfuric acid. The logarithmic viscosity is preferably 0.25 or more, more preferably 0.40 or more. The logarithmic viscosity is preferably 10 or less, more preferably 8 or less. If the logarithmic viscosity is less than 0.25, it is difficult to obtain molded articles with good properties. If the molecular weight exceeds 10, the viscosity increases, making it difficult to mold the polybenzimidazole compound.

[0042] In the present invention, various additives such as antioxidants, heat stabilizers, lubricants, tackifiers, plasticizers, crosslinking agents, viscosity modifiers, antistatic agents, antibacterial agents, antifoaming agents, dispersants, or polymerization inhibitors may be contained in the film and the electrolyte solution as needed.

[0043] The polybenzimidazole constituting the ion exchange membrane of the present invention can be formed into a molded article by any method, such as extrusion, rolling, or casting. Among these, molding from a solution dissolved in a suitable solvent is preferred. The solvent can be appropriately selected from aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or hexamethylphosphonamide, strong acidic solvents such as phosphoric acid or sulfuric acid, and alcohols such as methanol or ethanol, but is not limited thereto. These solvents may also be used in combination to the extent possible.

[0044] Among these solvents, aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or hexamethylphosphonamide are preferred from the viewpoints of ease of processing and the absence of the need for acid resistance in processing equipment.

[0045] The compound concentration in the solution is preferably in the range of 0.1 to 50% by mass. If the compound concentration in the solution is less than 0.1% by mass, it tends to be difficult to obtain a good molded product, while if it exceeds 50% by mass, processability tends to deteriorate. A molded product can be obtained from the solution using known methods. For example, the solvent can be removed by heating, drying under reduced pressure, or immersion in a compound non-solvent that is miscible with the solvent that dissolves the compound, to obtain a molded product. When the solvent is an organic solvent, the solvent is preferably removed by heating or drying under reduced pressure. In this case, the compound can be molded into various shapes, such as fibers, films, pellets, plates, rods, pipes, balls, or blocks, in a composite with other compounds, if necessary. Combining the compound with a compound having a similar solubility behavior is preferred because it allows for good molding. The sulfonic acid groups in the molded product obtained in this manner may be in the form of a salt with a cationic species, but they can also be converted to free sulfonic acid groups by acid treatment, if necessary.

[0046] In the present invention, the most preferred method for forming an ion exchange membrane from polybenzimidazole is solution casting. An ion exchange membrane for a vanadium-based redox battery can be obtained by removing the solvent from the cast solution as described above. The solution may be an organic polar solvent such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, or dimethyl sulfoxide, or, in some cases, a strongly acidic solution or an alcohol-based solvent. For solvent removal, drying is preferred from the viewpoint of achieving uniform thickness of the ion exchange membrane. Furthermore, drying can be performed under reduced pressure at the lowest possible temperature to avoid decomposition or deterioration of the compound or solvent. Furthermore, when the solution has a high viscosity, heating the substrate or solution and casting at a high temperature reduces the viscosity of the solution, facilitating casting. The thickness of the solution in the casting process (hereinafter referred to as the cast thickness) is not particularly limited, but is preferably 10 to 1,000 μm. It is more preferably 50 to 500 μm. If the cast thickness is less than 10 μm, the ion exchange membrane for a vanadium-based redox battery tends to fail to maintain its shape, while if it is thicker than 1000 μm, the resulting ion exchange membrane tends to be uneven in thickness. Known methods can be used to control the cast thickness of the solution. For example, the thickness can be controlled by using an applicator or doctor blade to achieve a uniform thickness, or by using a glass petri dish to maintain a constant casting area and adjusting the amount and concentration of the solution. A more uniform film can be obtained from the cast solution by adjusting the solvent removal rate. For example, when heating, the evaporation rate can be slowed by performing a low-temperature treatment in the initial stage. Furthermore, when immersing in a non-solvent such as water, the solidification rate of the compound can be adjusted by leaving the solution in air or an inert gas for an appropriate period of time.

[0047] The ion exchange membrane of the present invention can be adjusted to any thickness depending on the purpose, but from the viewpoint of suppressing cell resistance, it is preferable that the membrane thickness is as thin as possible. Specifically, it is preferably 5 to 200 μm, more preferably 5 to 50 μm, and most preferably 5 to 20 μm. If the thickness is thinner than 5 μm, not only is it difficult to handle, but reliability when incorporated into a cell tends to decrease. If the thickness exceeds 200 μm, it becomes difficult to suppress cell resistance.

[0048] In the present invention, the polybenzimidazole may be used as an asymmetric membrane. The asymmetric membrane is a structure having a dense layer on one side and a porous layer on the other side. In this case, even if the membrane thickness is less than 5 μm, the dense layer facilitates handling and is preferable because it can sufficiently suppress the cell resistance. In this case, the thickness of the dense layer is preferably 10 to 200 μm, more preferably 20 to 100 μm.

[0049] In the case of an asymmetric membrane, the production method is not particularly limited, but examples thereof include the above-mentioned method of solution casting followed by solidification in a poor solvent, or the membrane can be produced by adding a predetermined amount of a high-boiling poor solvent and inducing phase separation during drying.

[0050] In the present invention, polybenzimidazole may be composited with a porous material, for example, by a method of filling the pores with the polymer by contacting the porous material after the above-mentioned solution casting, or by a method of forming an ion exchange membrane layer only on the surface.

[0051] The porous material is preferably in the form of a synthetic fiber fabric, a chemical fiber fabric, a natural fiber fabric, a synthetic fiber nonwoven fabric, a chemical fiber nonwoven fabric, paper, a porous film, a porous metal plate, or a porous ceramic plate, and more preferably in the form of a synthetic fiber fabric, a synthetic fiber nonwoven fabric, a chemical fiber nonwoven fabric, or a porous film. By using a porous material in these forms, impregnation with the polymer can be carried out efficiently and handling during production is improved. [Example]

[0052] The present invention will be specifically described below using examples, but the present invention is not limited to these examples. Various measurements were carried out as follows.

[0053] Battery characteristics: 10cm vertically (fluid flow direction), 1cm width, electrode area 10cm 2 A small cell was fabricated, and repeated charging and discharging was performed at a constant current density. The current efficiency, cell resistance, energy efficiency, and voltage efficiency were calculated as follows. The positive electrode electrolyte was a 2.5 mol / L aqueous solution of 1.5 mol / L vanadium oxysulfate, and the negative electrode electrolyte was a 2.5 mol / L aqueous solution of 1.5 mol / L vanadium sulfate. The amount of electrolyte was in large excess relative to the cell and piping. The liquid flow rate was 6.2 ml per minute, and measurements were performed at 30°C.

[0054] (a) Current efficiency: η I In a test consisting of one cycle starting with charging and ending with discharging, the current density was 80 mA / cm per electrode geometric area. 2 (1260mA), the amount of electricity required to charge up to 1.6V is Q1 coulomb, the amount of electricity extracted by constant current discharge down to 1.0V is Q2 coulomb, and the current efficiency η I Ask for.

[0055]

number

[0056] (b) Cell resistance: R V in the negative electrode solution 3+ V 2+ The theoretical amount of electricity Q required to completely reduce th The ratio of the amount of electricity extracted by discharging to the amount of charge is taken as the charge rate, and the charge rate is calculated using Equation 2.

[0057]

number

[0058] The charging voltage V corresponding to the amount of electricity when the charging rate is 50% C50 , discharge voltage V D50 are calculated from the electric quantity-voltage curve, and the cell resistance R (Ω cm) relative to the electrode geometric area is calculated using Equation 3. 2 ) is found.

number

[0059] (c) Voltage efficiency: η V Using the cell resistance R obtained by the above method, the voltage efficiency η is calculated by the simple method of Equation 4. V Ask for.

[0060]

number

[0061] (d) Energy efficiency: η E The aforementioned current efficiency η I and voltage efficiency η V Using Equation 5, the energy efficiency η E Ask for.

number

[0062] Oxidation resistance test: The membrane was immersed in an electrolyte containing 2.5 mol / L sulfuric acid containing 0.9 mol / L pentavalent vanadium ions and incubated at 70°C for 5 hours. After the test, the membrane was checked for the presence or absence of membrane shape, and then subjected to the above-mentioned method. I was calculated, and the current efficiency loss was calculated according to Equation 5.

[0063] (e) Current efficiency loss: Δη I

number

[0064] Example 1 1.500 g of 3,3',4,4'-tetraaminodiphenyl sulfone, 0.894 g of isophthalic acid, 14.74 g of polyphosphoric acid (75% phosphorus pentoxide content), and 2.88 g of phosphorus pentoxide were weighed into a polymerization vessel. Nitrogen was introduced, and the mixture was heated to 100°C in an oil bath with slow stirring. After holding at 100°C for 1 hour, the mixture was heated to 150°C for 1 hour and then to 200°C for 4 hours of polymerization. After polymerization was complete, the mixture was allowed to cool, water was added, and the polymer was removed. It was then repeatedly washed with water using a household mixer until the pH was neutral on a pH test paper. The resulting polymer was dried overnight under reduced pressure at 120°C.

[0065] 1 g of the polymer obtained in Example 1 was dissolved in 10 ml of NMP, and 0.5 g of glycidyltrimethylammonium chloride was added, followed by stirring at 80°C for 8 hours. After cooling to room temperature, the solution was cast onto a glass plate on a hot plate to a film thickness of approximately 300 μm, and dried at 100°C for 1 hour. The film was peeled off from the glass plate and immersed in water overnight or longer. The thickness of the resulting film was 20 μm. The polymer structure obtained in Example 1 is (3).

[0066] [ka]

[0067] Example 2 A polymer was obtained in the same manner as in Example 1, except that 1.500 g of TAS, 0.447 g of isophthalic acid, 0.728 g of sulfoisophthalic acid, 16.59 g of polyphosphoric acid (phosphorus pentoxide content 75%), and 2.97 g of phosphorus pentoxide were weighed into a polymerization vessel.

[0068] 1 g of the polymer obtained in Example 1 was dissolved in 10 ml of NMP, and 0.5 g of glycidyl trimethylammonium chloride and 0.03 g of ethylene glycol diglycidyl ether were added. The solution was cast onto a glass plate on a hot plate to a film thickness of approximately 300 μm and dried at 100°C for 1 hour. The film was peeled off from the glass plate and immersed in water overnight or longer. The thickness of the resulting film was 20 μm. The polymer structure obtained in Example 1 is (4).

[0069] [ka]

[0070] (Comparative Example 1) A solution of polybenzimidazole (Sato Light Industrial Co., Ltd.) in N,N-dimethylacetamide was cast onto a glass plate on a hot plate, adjusting the thickness. The solvent was evaporated until a film formed, and the plate was then immersed in water overnight to prepare a film with an average thickness of 20 μm. The polymer structure was (5).

[0071] [ka]

[0072] The ion exchange membranes prepared in Examples 1 and 2 and Comparative Example 1 were sandwiched between carbon electrode materials (XF30A manufactured by Toyobo Co., Ltd.) and screwed together at eight points to assemble the cells. The electrode materials were 10 cm in the vertical direction (flow direction), 1 cm in the width direction, and had an electrode area of ​​10 cm. 2 A small cell with a constant current density was fabricated and repeatedly charged and discharged to test the performance of the ion exchange membrane. The current value during charging and discharging was set to 1280 mA, and the current density was 80 mA / cm. 2 The upper limit voltage during charging was 1.6 V, and the lower limit voltage during discharging was 1.0 V. A 2.5 mol / L aqueous solution of 1.5 mol / L vanadium oxysulfate was used as the positive electrode electrolyte, and a 2.5 mol / L aqueous solution of 1.5 mol / L vanadium sulfate was used as the negative electrode electrolyte. The amount of electrolyte was in large excess relative to the cell and piping. The liquid flow rate was 6.2 ml per minute, and measurements were performed at 30°C.

[0073] The ion exchange membranes prepared in Examples 1 and 2 and Comparative Example 1 were subjected to battery performance and oxidation resistance tests, and the results are shown in Table 1.

[0074] [Table 1]

[0075] As is clear from the results in Table 1, the ion exchange membranes containing the ionic group-containing polybenzimidazoles of Examples 1 and 2 exhibited low cell resistance and very high current efficiency. Furthermore, even after the oxidation resistance test, the membrane shape was maintained and almost no loss in current efficiency was observed. These results demonstrate that the ion exchange membranes containing the polybenzimidazole of the present invention were able to achieve both low cell resistance and durability against the electrolyte.

[0076] In contrast, the ion exchange membrane containing polybenzimidazole not containing ionic groups (Comparative Example 1) was fragmented and could not maintain its membrane shape. [Industrial Applicability]

[0077] The ion exchange membrane prepared using the polybenzimidazole containing a specific ionic group of the present invention can exhibit a long life and low cell resistance, and can therefore be suitably used in electrochemical cells and the like.

Claims

1. An ion exchange membrane characterized by containing a polymer represented by the following general formula (1): 【Chemistry 1】 (R represents a divalent aromatic group, Z is O, S, SO ２ , CO, C(CH ３ ) ２ , C(CF ３ ) ２ , OPhO, Each of Y1 and Y2 represents a hydrogen atom or a group containing a quaternary ammonium salt, When the molar fraction of the quaternary ammonium salt-containing group contained in the polymer is a%, the molar fraction a is in the range of 10≦a≦100, When the copolymerization ratio with polybenzimidazole having a different structure is m, m represents an integer of 1 to 100.

2. 2. The ion exchange membrane according to claim 1, comprising a polymer represented by the following general formula (2): 【Chemistry 2】 (R represents a divalent aromatic group, X represents one or more ionic groups selected from a sulfonic acid group, a phosphonic acid group, and a carboxyl group; Z is O, S, SO ２ , CO, C(CH ３ ) ２ , C(CF ３ ) ２ , OPhO, Each of Y1 and Y2 represents a hydrogen atom or a group containing a quaternary ammonium salt, When the molar fraction of the quaternary ammonium salt-containing group contained in the polymer is defined as b%, b is in the range of 10≦b≦100, When the copolymerization ratio with polybenzimidazole of another structure is n, n represents an integer from 99 to 0.

3. 3. The ion exchange membrane according to claim 1, wherein a portion of the imidazole moieties is crosslinked by a covalent bond.

4. A composite comprising the ion exchange membrane according to any one of claims 1 to 3 and an electrode.

5. An electrochemical cell comprising the ion exchange membrane according to any one of claims 1 to 3 or the composite according to claim 4.

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