JP2016015287A - Separation membrane for liquid fuel cell and membrane-electrode assembly including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel separation membrane for a liquid fuel cell, suitable for a liquid fuel cell, specifically for an alkaline liquid fuel cell.SOLUTION: A separation membrane for a liquid fuel cell has a percolation speed in a cross sectional direction of 40 mol/h g or higher and a pressure resistance between principal surfaces for air of 60 kPa or higher. The liquid fuel cell is preferably an alkaline type. The separation membrane preferably contains a hydrophilic functional group, and the hydrophilic functional group is, for example, a hydroxyl group, a carboxyl group, an amino group, etc.

Description

  The present invention relates to a diaphragm for a liquid fuel cell and a membrane-electrode assembly including the same.

  The polymer electrolyte fuel cell (PEFC) has the advantages of being able to operate at a low temperature as a fuel cell and having a high output density, and is expected to spread in the future. The PEFC includes a diaphragm between the anode and the cathode, and a polymer electrolyte membrane having ion conductivity is used as the diaphragm. As this diaphragm, a cation exchange membrane has been used. In recent years, PEFCs using an anion exchange membrane that can generate electricity without using platinum as a catalyst have been reported (for example, Patent Document 1 and Patent Document 2).

  On the other hand, a fuel cell (liquid fuel cell) using a liquid such as methanol that is easier to handle than hydrogen as a fuel has been studied.

JP 2000-331693 A Special table 2010-51685 gazette

  In a liquid fuel cell (hereinafter referred to as an alkaline liquid fuel cell) utilizing anion movement, fuel is supplied to the anode side and an oxidant such as air is supplied to the cathode side. The reaction on the cathode side requires water in addition to the oxidizing agent. In general, this water is supplied by being humidified from the outside using an auxiliary device such as a humidifier. However, it is desirable to omit auxiliary equipment as much as possible from the viewpoint of miniaturization of the fuel cell and improvement of power generation capacity per unit volume. In order to omit the auxiliary machinery, it is conceivable to use water supplied together with fuel to the anode side and permeating the diaphragm to the cathode side. In this case, it is desired to use a diaphragm that can permeate water well.

  On the other hand, when the oxidant and the fuel are directly mixed, a side reaction occurs and power generation efficiency decreases. The diaphragm is also required to have a function of separating the oxidant and the fuel. Since a gas such as air is used as the oxidizing agent, the diaphragm needs to have good pressure resistance against the gas.

  An object of the present invention is to provide a new diaphragm for a liquid fuel cell suitable for a liquid fuel cell, particularly an alkaline liquid fuel cell.

The present invention
The water transmission rate in the cross-sectional direction is 40 mol / h · g or more,
And while supplying water to the first main surface, air is supplied to the second main surface opposite to the first main surface and the pressure between the main surfaces measured by increasing the pressure of the air is 80 kPa or more. is there,
A diaphragm for a liquid fuel cell is provided.

In another aspect, the present invention provides:
There is provided a membrane-electrode assembly (MEA) comprising a diaphragm for a liquid fuel cell of the present invention.

  The diaphragm having a high cross-sectional direction water permeability and good pressure resistance between the main surfaces according to the present invention is suitable for improving the characteristics of a liquid fuel cell, particularly an alkaline liquid fuel cell. According to the present invention, it is possible to provide a membrane-electrode assembly (MEA) utilizing the characteristics of this diaphragm.

It is sectional drawing which shows typically an example of MEA of this invention. It is a disassembled perspective view which shows typically the cell for evaluation used for a water transmission rate test and a pressure resistance test. FIG. 3 is a longitudinal sectional view of a schematic evaluation cell on the III-III plane in FIG. 2. It is a front view which shows typically the cell for evaluation used for a water transmission rate test and a pressure | voltage resistance test.

  An embodiment of the present invention will be described, but the present invention is not limited to this.

  The membrane-electrode assembly (MEA) of this embodiment is suitable for use in a liquid fuel cell. This liquid fuel cell is not particularly limited to a portion other than the MEA, and a known member can be applied.

  The liquid fuel cell described below is an alkaline liquid fuel cell provided with the MEA of the present embodiment, and a liquid fuel is supplied to the anode side and an oxidant is supplied to the cathode side. The oxidizing agent is air, for example.

The liquid fuel is a fuel dissolved in water and dissolves an electrolyte. The fuel is not particularly limited as long as it can be dissolved in water. Examples thereof include lower alcohols such as methanol and ethanol; amines such as hydrazine (hydrate) and ammonia; sodium borohydride and the like. Hydrazine (hydrate) is preferable because it is high and does not generate CO ₂ on the principle of power generation.

  The amount of fuel supplied can be controlled by the concentration of fuel in the liquid fuel and the supply speed (flow rate). The amount of fuel required varies depending on the amount of current to be extracted. Therefore, it is preferable to control the amount of fuel supplied to the anode side in accordance with the amount of current to be extracted. If an excessive amount of fuel is supplied with respect to the amount of current to be extracted, the fuel will easily pass through the diaphragm. When the permeated fuel is present on the cathode, more specifically, on the cathode catalyst, the oxidant and the fuel may directly react on the cathode catalyst to cause a side reaction, thereby reducing the power generation efficiency of the fuel cell. In order to prevent this, a fuel supply system capable of controlling the supply amount of liquid fuel may be used.

The electrolyte is not particularly limited as long as it is an electrolyte that can be dissolved in water or liquid fuel and functions as an ionic conductor in the liquid fuel cell reaction. For example, in an alkaline liquid fuel cell, an anion functions as an ionic conductor, so use an electrolyte that can dissociate hydroxide ions (OH ^- ions) such as potassium hydroxide, sodium hydroxide, and calcium hydroxide. Is preferred. Since potassium carbonate, which is a reaction product with carbon dioxide, is easily dissolved in water and hardly precipitated, potassium hydroxide is particularly preferable as the electrolyte. However, other inorganic salts, ionic liquids, etc. that can be dissolved in water and can supply ions when dissolved in water can also be used as the electrolyte.

  The concentration of the electrolyte in the liquid fuel is not particularly limited, but is, for example, 0.5 to 40%, particularly 1 to 20% on a weight basis.

  In an alkaline liquid fuel cell, water is required for the reaction at the cathode catalyst. This water is usually humidified from the outside of the battery using auxiliary equipment such as a humidifier. However, the MEA of the present embodiment has a high water transmission rate in the cross-sectional direction, and can efficiently supply water to the cathode catalyst. Therefore, use of the MEA of this embodiment makes it possible to omit auxiliary equipment for humidification, which is advantageous in reducing the size of the liquid fuel cell and improving the power generation capacity per unit volume.

  From the viewpoint of suppressing the discharge of fuel that has permeated through the diaphragm from the anode side to the cathode side, and by-products generated by direct reaction of this fuel and oxidant on the cathode catalyst, You may provide the auxiliary machines which process the gas discharged | emitted from the exit of the side.

  Hereinafter, the details of the membrane-electrode assembly (MEA) will be described.

  The MEA of the present embodiment includes the diaphragm of the present invention and a catalyst layer disposed on the surface of the diaphragm.

  In the MEA of this embodiment, a catalyst layer is disposed on the surface of the diaphragm. The diaphragm and the catalyst layer are typically integrated through a processing step such as spray application of catalyst ink. Usually, the catalyst layer includes an anode catalyst layer and a cathode catalyst layer. FIG. 1 shows an example of the MEA of this embodiment. The MEA shown in FIG. 1 includes a diaphragm 2, an anode catalyst layer 3, and a cathode catalyst layer 4. The anode catalyst layer 3 is on one main surface of the diaphragm, and the cathode catalyst layer 4 is on the other main surface of the diaphragm 2, respectively. Has been placed.

  As the catalyst layer, a catalyst layer provided in a known MEA used in an alkaline liquid fuel cell can be used. Unlike an acid fuel cell, the catalyst does not necessarily need to be a noble metal such as platinum. For example, a base metal such as nickel, cobalt, or iron can be used. The structure of a catalyst layer such as a specific catalyst included may be different or the same on the anode side (anode catalyst layer) and cathode side (cathode catalyst layer) of the MEA. In particular, for the cathode catalyst layer, a catalyst that selectively promotes the redox reaction is selected from the viewpoint of preventing the deterioration of power generation efficiency by suppressing the reaction between the fuel that has permeated through the diaphragm and the oxidant, and preventing the deterioration of members due to side reactions. It is desirable to do.

  A method for forming the catalyst layer is not particularly limited, but a CCM (Catalyst Coated on Membrane) method in which the diaphragm on which the catalyst layer is formed and the gas diffusion layer are bonded, or CCS in which the catalyst layer directly applied to the gas diffusion layer is bonded to the diaphragm. It is preferable to use the (Catalyst Coated on Substrate) method, and it is more preferable to use the CCS method from the viewpoint of short circuit prevention.

  The MEA of the present embodiment can have any member other than the diaphragm and the catalyst layer as long as the effects of the present embodiment are not impaired.

  Hereinafter, details of the diaphragm provided in the MEA will be described.

  The diaphragm for a liquid fuel cell of the present embodiment has a water permeability rate of 40 mol / h · g or more in the cross-sectional direction, and supplies water to the first main surface while supplying water to the second main surface opposite to the first main surface. The pressure resistance between the main surfaces measured by supplying air and increasing the pressure of the air is 80 kPa or more.

  The water transmission rate in the cross-sectional direction of the diaphragm is 40 mol / h · g or more, preferably 40 to 120 mol / h · g, and more preferably 50 to 110 mol / h · g. If the water permeation rate in the cross-sectional direction of the diaphragm becomes too small, water necessary for the reaction on the cathode catalyst may not permeate sufficiently, and the reaction efficiency in the cathode catalyst may be reduced. By using the diaphragm of the present embodiment in which the water permeation speed in the cross-sectional direction is in a specific range, water that has permeated the diaphragm from the anode side to the cathode side can be appropriately supplied to the cathode catalyst. As a result, the reaction efficiency on the cathode catalyst can be improved, and the power generation efficiency of the battery can be further improved. The water transmission rate in the cross-sectional direction of the diaphragm can be measured by a test described later.

  Specifically, the water transmission rate in the cross-sectional direction of the diaphragm can be measured by the following method using the evaluation cell 100 shown in FIGS.

A diaphragm 2 having a first main surface and a second main surface opposite to the first main surface is prepared, and a diaphragm is formed by a pair of gaskets 11 and 21 having square openings 11a and 21a each having a side length of 2 cm. 2 is pinched. On the outside of the gaskets 11 and 21, a pair of separators 12 and 22 with serpentine-structured flow channels 12a and 22a, a pair of current collector plates 13 and 23, and a pair of end plates 14 and 24 are arranged in this order to form the diaphragm 2 Hold it. Each member is fastened using a fixing part (not shown) such as a bolt so that air and water do not leak from each contact surface of the member, and the evaluation cell 100 is formed.
The evaluation cell 100 has flow paths 18, 19, 28, and 29. The channels 18 and 19 are channels for supplying and discharging water, respectively, and the channels 28 and 29 are channels for supplying and discharging dry air, respectively. Each flow path 18, 19, 28, 29 has an opening in the end plate. The flow paths 18 and 19 penetrate the end plate 14, the current collector plate 13, and the separator 12, and are connected to the flow path 12a.

  The evaluation cell 100 is installed so that the first main surface and the second main surface of the diaphragm 2 are along the vertical direction, and supply of water and dry air to the evaluation cell 100 in this state is started. 2 ml of water per minute is supplied to the first main surface via a pipe 38 connected to the flow path 18, and 500 ml of dry air per minute is supplied to the second main face via a pipe 48 connected to the flow path 28. Supply. By continuing to supply water as described above, the inside of the first main surface side of the cell 100 is filled with the supplied water, and the first main surface of the diaphragm 2 is always in contact with water. At this time, the evaluation cell 100 is heated using the rubber heaters 15 and 25 provided on the end plates 14 and 24 so that the temperature of the evaluation cell 100 becomes 80 ° C. Water and dry air are continuously supplied as described above, and water discharged from the pipe 39 connected to the flow path 19 is collected for 30 minutes while maintaining the temperature of the evaluation cell 100 at 80 ° C. (W2).

W1 is the weight of water supplied to the evaluation cell 100 for 30 minutes, and W2 is the weight of water recovered from the pipe 39 for 30 minutes. Also, W3 is the W3 measuring diaphragm of the same type and the membrane 2 was prepared separately from the diaphragm 2, were calculated from the weight measured 24 hours Hosei after standing in an atmosphere 23 ° C. relative humidity 55%, 1 cm of the membrane ² It is per weight.
The water transmission rate in the cross-sectional direction of the diaphragm is a value calculated by the following formula using these values.
Water permeability rate [mol / h · g] = 60 [min] / 30 [min] × (W1-W2) [g] / 18 [g / mol] / 4 [cm ² ] / W3 [g / cm ² ]

  The diaphragm of the present embodiment is a pressure resistance between main surfaces measured by supplying air to the second main surface opposite to the first main surface while increasing the pressure of the air while supplying water to the first main surface. (Pressure resistance between main surfaces of the diaphragm) is 80 kPa or more, and preferably 100 kPa or more. The pressure resistance between the main surfaces of the diaphragm is the maximum value of the pressure between the main surfaces that can maintain the diaphragm, and can be specifically measured by a test described later. If the pressure resistance between the main surfaces of the diaphragm is too small, the pressure of the oxidizing agent that is a gas may not be maintained, and the oxidizing agent may leak to the anode. If the oxidant leaks to the anode and the fuel and the oxidant are mixed directly, the power generation efficiency can be reduced.

  Specifically, the withstand pressure between the main surfaces of the diaphragm can be measured by the following method using the evaluation cell 100 shown in FIGS. This evaluation cell 100 is formed in the same manner as the evaluation cell 100 used for the measurement of the water transmission rate in the cross-sectional direction of the diaphragm described above.

  The evaluation cell 100 is installed so that the first main surface and the second main surface of the diaphragm 2 are along the vertical direction, and supply of water and dry air to the evaluation cell 100 in this state is started. 2 ml of water per minute is supplied to the first main surface via a pipe 38 connected to the flow path 18, and 500 ml of dry air per minute is supplied to the second main face via a pipe 48 connected to the flow path 28. Supply. By continuing to supply water as described above, the inside of the first main surface side of the cell 100 is filled with the supplied water, and the first main surface of the diaphragm 2 is always in contact with water. At this time, the evaluation cell 100 is heated using the rubber heaters 15 and 25 provided on the end plates 14 and 24 so that the temperature of the evaluation cell 100 becomes 80 ° C.

  Next, the pressure adjusting device 43 (for example, a valve) provided in the pipe 49 connected to the flow path 29 while maintaining the temperature of the evaluation cell 100 at 80 ° C. while continuing to supply water and dry air as described above. And the pressure of the dry air to the second main surface is increased so that the pressure of the dry air to the second main surface of the diaphragm 2 is 20 kPa. The pressure of the dry air is measured with a pressure gauge 42 provided in the pipe 49. Thereafter, the pressure regulator 43 continues to supply water and dry air as described above, and maintains the temperature of the evaluation cell 100 at 80 ° C. so that the pressure of the dry air to the second main surface can be maintained at 20 kPa. Adjust the opening. The pressure of the dry air to the second main surface is measured for 10 minutes while maintaining the supply rate of water and dry air, the temperature of the evaluation cell 100, and the opening degree of the pressure adjusting device 43. When 20 kPa cannot be maintained for 10 minutes, the withstand pressure between the main surfaces of the diaphragm is evaluated as 0 kPa. When 20 kPa can be maintained for 10 minutes, the pressure of the dry air to the second main surface is increased, and the same measurement is performed in the order of 40 kPa, 60 kPa, 80 kPa, and 100 kPa. When the pressure of the dry air to the second main surface can be maintained for 10 minutes after increasing the pressure to 100 kPa, the pressure resistance between the main surfaces of the diaphragm is set to 100 kPa. Here, the case where the pressure can be maintained means that the change in the pressure of the dry air to the second main surface is 1 kPa or less in 10 minutes.

  The liquid fuel cell membrane preferably has a thickness in the range of 5 μm to 130 μm, and more preferably in the range of 10 μm to 70 μm. If the film thickness becomes too thin, the film strength may decrease, and defects such as film breakage and pinholes may occur. Moreover, the amount of fuel permeating to the cathode side and the amount of water permeation may increase. When the film thickness becomes too thick, the resistance (film resistance) as a film in the diaphragm may be increased, and the water permeability may be decreased too much. If the water permeation amount is too small, water required for the reaction in the cathode catalyst may be insufficient, and power generation efficiency may be reduced.

  In one embodiment of the present invention, a diaphragm having such characteristics comprises a polymer substrate and hydrophilic functional groups present on the polymer substrate.

  The material of the polymer base material contained in the diaphragm of the present embodiment is not particularly limited, and a known resin can be used as long as the effects of the invention are not impaired. For example, polyolefin resins such as polyethylene and polypropylene, polystyrene resins, epoxy resins such as bisphenol A type epoxy polymers, polysulfide resins such as polyphenylene sulfide, polyether resins such as polyether ketone, polyvinylidene fluoride, ethylene tetrafluoro Fluorine resins such as ethylene and polytetrafluoroethylene can be exemplified. Among these resins, at least one selected from the group consisting of polyethylene, polypropylene, polystyrene, bisphenol A type epoxy polymer, polyphenylene sulfide, polyether ketone, polyvinylidene fluoride, ethylene tetrafluoroethylene, and polytetrafluoroethylene is included. It is preferable that at least one selected from the group consisting of polyethylene, polypropylene, polystyrene, polyphenylene sulfide and polyether ketone is included, and at least one selected from the group consisting of polyethylene, polypropylene and polystyrene is included. More preferably. From the viewpoints of contamination resistance, corrosion resistance, production cost of the polymer base material, etc., polyethylene is preferable, and low density polyethylene, high density polyethylene, and ultrahigh molecular weight polyethylene are more preferable. High-density polyethylene and ultrahigh molecular weight polyethylene are particularly preferable from the viewpoints of the strength of the polymer substrate and the improvement of heat resistance. Among these, from the viewpoint of obtaining a high-strength polymer base material, ultra high molecular weight polyethylene having a weight average molecular weight of 500,000 or more, particularly 1,000,000 or more is preferable. These resins may be used alone or in admixture of two or more.

  The polymer substrate is preferably a porous film. The average pore diameter of the porous membrane is preferably in the range of 1 nm to 1000 nm, more preferably in the range of 2 nm to 500 nm, and still more preferably in the range of 5 nm to 300 nm. If the average pore diameter becomes too large, a short circuit between the electrodes may occur. In addition, the pressure resistance of the diaphragm against gas may be reduced, making it difficult to withstand the pressure of the oxidant. Also, the amount of fuel permeating to the cathode side may increase. If the average pore diameter becomes too small, the water permeability may be reduced. If the water permeation amount is too small, water required for the reaction in the cathode catalyst may be insufficient, and power generation efficiency may be reduced. Since the average pore diameter of the entire diaphragm varies depending on the hydrophilization treatment to be performed later, it is preferable to adjust the average pore diameter of the porous membrane in consideration of the variation.

  The hydrophilic functional group contained in the diaphragm of the present embodiment is not particularly limited as long as it has hydrophilicity. For example, at least one selected from the group consisting of a hydroxyl group, a carboxyl group, an amino group, a sulfonic acid group, and a phosphoric acid group. In particular, it is a carboxyl group.

  The hydrophilic functional group is preferably obtained by performing a hydrophilic treatment. As long as the effect of the present invention is not impaired, the type of hydrophilic treatment is not particularly limited, and graft polymerization treatment, corona treatment, plasma treatment, sputtering treatment, sulfonation treatment, treatment using a surfactant or a hydrophilic polymer, etc. It may be used.

  For example, in a hydrophilic treatment using a hydrophilic polymer, a solution containing the hydrophilic polymer is applied to the polymer porous membrane, and the hydrophilic polymer membrane is formed on the surface of the polymer porous membrane and the pore walls. A hydrophilic functional group can be added to the surface of the porous membrane. In this method, the amount of the hydrophilic functional group can be adjusted by the thickness of the film formed by applying the hydrophilic polymer, and the pore diameter of the diaphragm in the film can be adjusted by the thickness of the film.

  In a preferred embodiment, the hydrophilization treatment is performed using a graft polymerization method. The graft polymerization method is preferable from the viewpoint that it can be processed in a homogeneous system. In the graft polymerization method, a monomer for forming a graft chain (hereinafter referred to as “graft monomer (M)”) is introduced into a polymer substrate. That is, the liquid fuel cell membrane of this embodiment includes a polymer substrate and a graft chain introduced into the polymer substrate, and the graft chain has a hydrophilic functional group.

  The graft monomer (M) may have a hydrophilic functional group or may have a site where a hydrophilic functional group can be introduced. That is, the hydrophilic functional group may be contained in the graft monomer (M) and may be introduced into the graft chain after the graft polymerization.

  In a preferred embodiment, the graft monomer (M) has a carbon-carbon unsaturated bond and a hydrophilic functional group. Although the graft monomer (M) is not particularly limited, for example, carboxylic acid monomers such as acrylic acid and methacrylic acid, acrylamide, methacrylamide, 2-hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxy (Meth) acrylic acid derivative monomers such as propyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, vinyl acetate monomers such as vinyl acetate, allylamine, acrylamide, methacrylamide, N-vinyl Examples thereof include nitrogen-containing monomers such as pyrrolidone and N-vinylpyridine, and styrene derivative monomers such as sodium styrenesulfonate. Among these, it is preferable that at least one selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, methacrylamide, N-vinyl pyrrolidone, N-vinyl pyridine, 2-hydroxyethyl methacrylate, and a styrene derivative monomer is included. It is preferable that at least one selected from the group consisting of acid, methacrylic acid, acrylamide, methacrylamide, N-vinylpyrrolidone, N-vinylpyridine, and 2-hydroxyethyl methacrylate is included.

  The graft monomer (M) may be used for polymerization alone or may be prepared as a solution (graft monomer (M) solution) in which the graft monomer (M) is dissolved in a solvent.

  The solvent for dissolving the graft monomer (M) is not particularly limited, but if a solvent that dissolves the graft monomer (M) but not the polymer base material is used, the graft monomer (M) and the polymer base material Easy to separate. In addition, when a solvent capable of dissolving a polymer formed only from the graft monomer (M) as a by-product is used, the polymerization solution can be kept uniform. The solubility of the graft monomer (M), the polymer formed only from the graft monomer (M), and the polymer base material in the solvent is as follows: the graft monomer (M), the polymer formed only from the graft monomer (M), and Since it may vary depending on the structure or polarity of the polymer substrate, a solvent may be appropriately selected according to the solubility of these compounds. Two or more compounds may be mixed and used as a solvent.

  Specific examples of the solvent contained in the graft monomer (M) solution include aromatic compounds such as aromatic hydrocarbons such as benzene, toluene and xylene, and phenols such as phenol and cresol. By using such a solvent, a diaphragm having a high graft rate can be obtained. In addition, since the aromatic compound dissolves the polymer composed only of the graft monomer (M) as a by-product, the polymerization solution can be kept uniform.

  The concentration of the graft monomer (M) in the graft monomer (M) solution may be determined according to the polymerizability of the graft monomer (M) and the target graft ratio. It is preferable to include 20% by weight or more of the graft monomer (M) based on the weight. By using a solution having a concentration of the graft monomer (M) of 20% by weight or more, it is easy to avoid a situation in which the graft reaction does not proceed sufficiently.

  As the graft polymerization method, it is preferable to use a radiation graft polymerization treatment method from the viewpoint that it can be treated in a homogeneous system. Specifically, it is formed by irradiating a polymer substrate with radiation, bringing the polymer substrate after irradiation and the graft monomer (M) or graft monomer (M) solution into contact with each other to cause a graft polymerization reaction. It is preferable.

  Examples of radiation applied to the polymer substrate include ionizing radiation such as α rays, β rays, γ rays, electron beams, and ultraviolet rays, and γ rays or electron beams are particularly preferable. The irradiation dose is preferably in the range of 1 kGy to 400 kGy, more preferably in the range of 10 kGy to 300 kGy. The graft rate can be controlled by the radiation dose. If the irradiation dose is too low, the graft rate may be lowered. When the irradiation dose becomes too large, the mechanical strength of the diaphragm may be lowered due to deterioration of the polymer base material or excessive polymerization reaction.

  The polymer substrate after irradiation may be held at a low temperature (for example, −30 ° C. or lower).

  In order to prevent the graft polymerization reaction from being hindered by the presence of oxygen, the graft polymerization is preferably performed in an atmosphere where the oxygen concentration is as low as possible, and is performed in an inert gas atmosphere such as argon gas or nitrogen gas. Is more preferable. The oxygen in the graft monomer (M) or the graft monomer (M) solution is preferably removed using a known method such as freeze degassing or bubbling using nitrogen gas or the like.

  The temperature at which the graft polymerization is performed is, for example, 0 ° C. to 100 ° C., particularly 40 to 80 ° C. The reaction time for carrying out the graft polymerization is, for example, about 2 minutes to 12 hours. The graft ratio can be controlled by these reaction temperature and reaction time.

  The graft ratio is, for example, in the range of 5 to 200%, and preferably in the range of 15% to 100%. By having such a graft ratio, it becomes possible to obtain a porous membrane with good water permeability. Moreover, the pressure resistance of the diaphragm may not be obtained. The graft ratio indicates the ratio of the weight difference between the weight of the film after graft polymerization (graft film) and the weight of the film before graft polymerization to the weight of the film (polymer base material) before graft polymerization.

  As an example of the graft polymerization reaction, a reaction example in a solid-liquid two-phase system will be described. First, a graft monomer (M) solution containing a graft monomer (M) and a solvent is placed in a container such as glass or stainless steel. Next, in order to remove the dissolved oxygen that inhibits the graft reaction, vacuum degassing in the graft monomer (M) solution and bubbling with an inert gas (nitrogen gas or the like) are performed. Thereafter, the polymer substrate after irradiation is charged into the graft monomer (M) solution to perform graft polymerization. A graft chain is introduced into the polymer constituting the polymer substrate by graft polymerization. Next, the obtained membrane is removed from the reaction solution and filtered. Furthermore, in order to remove the polymer consisting only of the solvent, the unreacted graft monomer (M), and the graft monomer (M), the obtained film is washed 3 to 6 times with an appropriate amount of solvent and then dried. As the solvent, a solvent that can easily dissolve the graft monomer (M) and the polymer composed only of the graft monomer (M) and does not dissolve the graft film may be used. For example, water, toluene, acetone or the like can be used as the solvent.

  In a preferred embodiment, the diaphragm is substantially free of functional groups with anion exchange capacity. “Substantially free” means that the amount of the functional group having anion exchange capacity relative to the weight of the diaphragm is 0.1 mmol / g or less, preferably 0.05 mmol / g or less. Examples of functional groups having anion exchange ability include quaternary ammonium bases and quaternary phosphonium bases.

  In the liquid fuel cell having the MEA of the present embodiment, the electrolyte dissolved in the liquid fuel bears ionic conductivity. In the liquid fuel cell generally used, an anion exchange membrane is used for the diaphragm, and the anion exchange group of the anion exchange membrane bears ion conductivity. Therefore, when the anion exchange group is deteriorated, the performance of the liquid fuel cell is lowered. On the other hand, in the MEA of this embodiment having no anion exchange group, it is not necessary to consider the deterioration of the anion exchange group.

  In another embodiment, the graft monomer (M) has a carbon-carbon unsaturated bond and a site capable of introducing a hydrophilic functional group. The site capable of introducing a hydrophilic functional group is, for example, a halogenated alkyl group such as a halogenated methyl group, a halogenated ethyl group, a halogenated propyl group or a halogenated butyl group, styrene sulfonic acid, vinyl sulfonic acid or acrylic phosphonic acid. And alkyl esters. Examples of the graft monomer (M) include styrene derivatives such as styrene, chloromethylstyrene, and bromobutylstyrene. These monomers (M) may be used alone or in admixture of two or more.

  In this embodiment, it is preferable that the graft monomer (M) does not have a functional group having anion exchange ability.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these Examples.

  The physical properties in Examples and Comparative Examples were measured using the following methods.

(A) Film thickness It measured with the 1/10000 direct reading dial type film thickness measuring device.

(B) Porosity Using the thickness measured with a 1/10000 direct reading dial type film thickness measuring device, the value calculated from the weight W per unit area S of the film, the average thickness t, and the density d by the following formula was used.
Porosity (%) = (1− (10 ⁴ × W / S / t / d)) × 100

(C) Water permeation rate The measurement was carried out according to the following procedure using the evaluation cell for fuel cell shown in FIGS.
As a measurement diaphragm, a water permeability measurement diaphragm 2 having a square main surface with a side of 4 cm and a W3 measurement diaphragm having a rectangular main surface with a short side of 2 cm and a long side of 3 cm were prepared.

The diaphragm 2 for measuring the water transmission rate was sandwiched between a pair of gaskets 11 and 21 having square openings 11a and 21a each having a length of 2 cm. A pair of separators 12 and 22 with flow paths 12a and 22a having a serpentine structure, a pair of current collector plates 13 and 23, and a pair of end plates 14 and 24 are arranged in this order on the outside of the gaskets 11 and 21 so as to sandwich the diaphragm 2 did. Each member was fastened using a fixing component (not shown) such as a bolt so that air and water did not leak from each contact surface of the member, and the evaluation cell 100 was formed.
The evaluation cell 100 has flow paths 18, 19, 28, and 29. The channels 18 and 19 are channels for supplying and discharging water, respectively, and the channels 28 and 29 are channels for supplying and discharging dry air, respectively. Each flow path 18, 19, 28, 29 has an opening in the end plate. The flow paths 18 and 19 penetrate the end plate 14, the current collector plate 13, and the separator 12, and are connected to the flow path 12a. The same applies to the flow paths 28 and 29.

  The evaluation cell 100 was installed so that the main surface of the diaphragm 2 was along the vertical direction. Supply of water and dry air to the evaluation cell 100 in this state was started. 2 ml of water per minute is supplied to the first main surface via a pipe 38 connected to the flow path 18, and 500 ml of dry air per minute is supplied to the second main face via a pipe 48 connected to the flow path 28. Supplied. At this time, the evaluation cell 100 was heated using the rubber heaters 15 and 25 provided on the end plates 14 and 24 so that the temperature of the evaluation cell 100 became 80 ° C. The temperature of the evaluation cell 100 was measured using a thermocouple 41 installed in the separator 22. Water and dry air were continuously supplied to the evaluation cell 100 as described above, and the temperature of the evaluation cell 100 was maintained at 80 ° C. for 1 hour.

Thereafter, the water and dry air are continuously supplied as described above, and the water discharged from the pipe 39 connected to the anode-side flow path 19 is collected for 30 minutes while maintaining the temperature of the evaluation cell 100 at 80 ° C. did. The weight of the collected water was W2. The weight of water supplied to the evaluation cell 100 for 30 minutes was defined as W1. Further, after leaving the diaphragm for W3 measurement to stand for 24 hours in an atmosphere of 23 ° C. and 55% relative humidity, the weight per cm ^{2 of} the diaphragm calculated from the measured weight was defined as W3.
Using these values, the water transmission rate in the cross-sectional direction of the diaphragm was calculated according to the following formula.
Water permeation rate [mol / h · g] =
60 [min] / 30 [min] × (W1-W2) [g] / 18 [g / mol] / 4 [cm ² ] / W3 [g / cm ² ]

(D) Pressure resistance test A pressure resistance test was performed using the evaluation cell for fuel cell shown in FIGS. 2 to 4 as an evaluation cell, and the pressure resistance between the main surfaces of the diaphragm was evaluated.
A diaphragm 2 having a main surface of a square having a side of 4 cm was prepared. The diaphragm 2 was sandwiched between a pair of gaskets 11 and 21 having square openings 11a and 21a each having a length of 2 cm. A pair of separators 12 and 22 with serpentine-structured flow channels 12a and 22a, a pair of current collector plates 13 and 23, and a pair of end plates 14 and 24 are arranged and sandwiched in this order on the outside of the gaskets 11 and 21. Each member was fastened using a fixing component (not shown) such as a bolt so that air and water did not leak from each contact surface of the member, and the evaluation cell 100 was formed.
The evaluation cell 100 has flow paths 18, 19, 28, and 29. The channels 18 and 19 are channels for supplying and discharging water, respectively, and the channels 28 and 29 are channels for supplying and discharging dry air, respectively. Each flow path 18, 19, 28, 29 has an opening in the end plate. The flow paths 18 and 19 penetrate the end plate 14, the current collector plate 13, and the separator 12, and are connected to the flow path 12a. The same applies to the flow paths 28 and 29.

  The evaluation cell 100 was installed so that the main surface of the diaphragm 2 was along the vertical direction. Supply of water and dry air to the evaluation cell 100 in this state was started. 2 ml of water per minute is supplied to the main surface (first main surface) on the anode side through a pipe 38 connected to the flow path 18, and 500 ml per minute is dried to the main surface (second main surface) on the cathode side. Air was supplied through a pipe 48 connected to the flow path 28. At this time, the evaluation cell 100 was heated using the rubber heaters 15 and 25 provided on the end plates 14 and 24 so that the temperature of the evaluation cell 100 became 80 ° C. The temperature of the evaluation cell 100 was measured using a thermocouple 41 installed in the separator 22. Water and dry air were continuously supplied as described above, and the temperature of the evaluation cell 100 was maintained at 80 ° C. for 1 hour.

  After maintaining the above state for 1 hour, pressure adjustment provided in the pipe 49 connected to the flow path 29 while continuing to supply water and dry air as described above and maintaining the temperature of the evaluation cell 100 at 80 ° C. The opening degree of the apparatus 43 was adjusted, and the pressure of the dry air to the second main surface was increased so that the pressure of the dry air to the second main surface of the diaphragm 2 was 20 kPa. The pressure of the dry air was measured with a pressure gauge 42 provided in the pipe 49. Thereafter, the pressure regulator 43 continues to supply water and dry air as described above, and maintains the temperature of the evaluation cell 100 at 80 ° C. so that the pressure of the dry air to the second main surface can be maintained at 20 kPa. The opening degree of was adjusted. With the water and dry air supply rates, the temperature of the evaluation cell 100, and the opening degree of the pressure adjusting device 43 maintained, the pressure of the dry air to the second main surface was measured for 10 minutes. When 20 kPa could not be maintained for 10 minutes, the pressure resistance between the main surfaces of the diaphragm was evaluated as 0 kPa. When 20 kPa can be maintained for 10 minutes, the supply of water and dry air is continued as described above, the opening of the pressure regulator 43 is adjusted while maintaining the temperature of the evaluation cell 100 at 80 ° C., and the second main The pressure of the dry air to the second main surface was increased so that the pressure of the dry air to the surface was 40 kPa. Thereafter, the pressure regulator 43 continues to supply water and dry air as described above so that the pressure of the dry air to the second main surface can be maintained at 40 kPa while maintaining the temperature of the evaluation cell 100 at 80 ° C. The opening degree of was adjusted. With the water and dry air supply rates, the temperature of the evaluation cell 100, and the opening degree of the pressure adjusting device 43 maintained, the pressure of the dry air to the second main surface was measured for 10 minutes. When 40 kPa could not be maintained for 10 minutes, the pressure resistance between the main surfaces of the diaphragm was evaluated as 20 kPa. When 40 kPa could be maintained for 10 minutes, the pressure of dry air to the second main surface was increased, and the same measurement was performed in the order of 60 kPa, 80 kPa, and 100 kPa. When the pressure of dry air on the second main surface was increased to 100 kPa and 100 kPa could be maintained for 10 minutes, the pressure resistance between the main surfaces of the diaphragm was evaluated as 100 kPa.

(E) Power generation test An electrode using platinum-supported carbon was cut into a 20 mm square shape and immersed in a mixed solution of ethylenediamine and ethanol (ethyleneamine / ethanol = 3/7 by weight) for 12 hours or more in a room temperature atmosphere. . After this electrode was air-dried, a fuel cell evaluation cell was assembled together with a diaphragm and a gasket cut into a 40 mm square shape.
Using the assembled fuel cell evaluation cell, an aqueous solution containing 10% by weight of hydrazine hydrate and 1 mol / L of potassium hydroxide as a liquid fuel containing an electrolyte, and dry air as an oxidant in a 40 ° C. atmosphere A power generation test was conducted at 2 ml of liquid fuel per minute was supplied to the anode side, and 200 ml of dry air was supplied to the cathode side. In this test, whether or not current sweep is possible, the limit current density, and the cell resistance when the maximum output density was developed (measured using the current interruption method. The internal resistance of the cell was measured from the voltage change when the current was instantaneously interrupted. ) And the maximum power density were compared.

(Example 1)
In Example 1, an ultrahigh molecular weight polyethylene substrate having a film thickness of 20 μm, a porosity of 40%, and an air permeability (Gurley value) of 173 sec / 100 ml · inch ² was used as the polymer substrate. By irradiating this ultrahigh molecular weight polyethylene substrate with an electron beam of 45 kGy, free radicals were generated. The ultra-high molecular weight polyethylene base material after electron beam irradiation was cooled to −70 ° C. and stored until the next step was performed. Next, 250 g of methacrylic acid as a graft monomer (M) and 250 g of methanol are mixed to prepare a graft monomer (M) solution, and bubbling with nitrogen gas is performed for 1 hour while maintaining the temperature at 25 ° C. The oxygen remaining in the graft monomer (M) solution was removed. Then, the ultrahigh molecular weight polyethylene substrate irradiated with the electron beam was added to the graft monomer (M) solution, and the liquid temperature was raised to 55 ° C. While maintaining the liquid temperature at 55 ° C., polymerization was performed for 6 minutes, and methacrylic acid was graft-polymerized on the ultrahigh molecular weight polyethylene substrate. Thereafter, the obtained graft porous membrane was pulled up and washed with water to wash away excess monomers, and then water on the surface portion was removed to obtain a hydrophilic diaphragm having hydrophilicity. The resulting graft porous membrane had a graft rate of 40%. Each physical property of this diaphragm was measured. A power generation test was performed using this diaphragm.

(Example 2)
Except for making graft polymerization time into 4 minutes, it implemented similarly to Example 1 and obtained the hydrophilic membrane with a graft ratio of 30%. Each physical property of this diaphragm was measured. In addition, a power generation test was performed using this diaphragm.

(Comparative Example 1)
Each ultra-high molecular weight polyethylene substrate used in Example 1 was used as a diaphragm without treatment, and each physical property was measured. In addition, a power generation test was performed using this diaphragm.

(Comparative Example 2)
A non-porous film of a copolymer of tetrafluoroethylene and ethylene (ETFE, film thickness 50 μm) was used. This ETFE film was irradiated with an electron beam of 30 kGy on each side (total 60 kGy) under vacuum at room temperature to generate free radicals. The ETFE film after electron beam irradiation was cooled to −70 ° C. and stored until the next step was performed. A monomer solution was prepared by mixing 28 g of 4- (chloromethyl) styrene and 12 g of xylene. Next, this monomer solution was bubbled with nitrogen gas to remove oxygen in the monomer solution. And the ETFE film | membrane irradiated with said electron beam was thrown into this monomer solution, and liquid temperature was raised to 70 degreeC. Graft polymerization was performed by immersing for 2 hours while maintaining the liquid temperature at 70 ° C., and chloromethylstyrene was graft-polymerized on the ETFE membrane. The graft ratio of the obtained film was 43.5%. Subsequently, the graft membrane was immersed in an aqueous trimethylamine solution at room temperature for 24 hours, whereby a quaternization treatment of the chloromethyl group portion was performed. The graft membrane after the quaternization treatment was washed with ethanol for 30 minutes, then washed with 1N hydrochloric acid in ethanol for 30 minutes, and further washed with pure water. Thus, the base material was an ETFE film, and a film having a chloride ion type quaternary ammonium base was obtained. Each physical property of this diaphragm was measured. In addition, a power generation test was performed using this diaphragm.

  Table 1 shows the water content, water transmission rate, and pressure test results, and Table 2 shows the results of the power generation test. In Table 1, PE represents ultrahigh molecular weight polyethylene, CMS represents chloromethylstyrene, and TMA represents trimethylamine.

  When the pressure resistance test was performed using the diaphragm (polyethylene porous film having no hydrophilic functional group) obtained in Comparative Example 1, the pressure of dry air of 80 kPa on the second main surface could not be maintained. However, the diaphragm after the pressure resistance test showed no change in the appearance of the diaphragm. When a power generation test was performed using the diaphragm obtained in Comparative Example 1, no current could be taken out, so another power generation test could not be performed. In Comparative Example 2 (ETFE nonporous film), current flowed, but the internal resistance of the cell was high and the limiting current density was low. On the other hand, in Examples 1-2 (polyethylene porous film), an electric current could be taken out. That is, the diaphragms obtained in Examples 1 and 2 having good water permeability in the cross-sectional direction and high pressure resistance against expectation could be used as a diaphragm for a liquid fuel cell. Moreover, the limiting current density of Examples 1 and 2 was higher than the limiting current density of Comparative Example 2. The cell resistance when the maximum output density was developed in Examples 1 and 2 was lower than that in Comparative Example 2.

1 Membrane-electrode assembly (MEA)
2 Diaphragm 3 Anode catalyst layer 4 Cathode catalyst layer 11, 21 Gasket 11 a, 21 a Opening 12, 22 Separator 12 a, 22 a Channel 13, 23 Current collector 14, 24 End plate 15, 25 Rubber heater 18, 19 Channel 28 , 29 Flow path 38, 39 Piping 41 Thermocouple 42 Pressure gauge 43 Pressure adjusting device 48, 49 Piping 100 Evaluation cell

Claims (9)

The water transmission rate in the cross-sectional direction is 40 mol / h · g or more,
And while supplying water to the first main surface, air is supplied to the second main surface opposite to the first main surface and the pressure between the main surfaces measured by increasing the pressure of the air is 80 kPa or more. is there,
Liquid fuel cell membrane. The liquid fuel cell is alkaline;
The diaphragm for liquid fuel cells according to claim 1. Substantially free of functional groups having anion exchange capacity,
The diaphragm for liquid fuel cells according to claim 1 or 2. A polymer substrate;
A hydrophilic functional group present on the polymer substrate;
Comprising
The diaphragm for liquid fuel cells according to any one of claims 1 to 3. The polymer substrate is a porous membrane;
The diaphragm for liquid fuel cells according to claim 4. The hydrophilic functional group is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic acid group, an amino group, and a phosphoric acid group;
The diaphragm for liquid fuel cells according to claim 4 or 5. The polymer substrate contains at least one selected from the group consisting of polyethylene, polypropylene, polystyrene, polyphenylene sulfide and polyether ketone,
The diaphragm for liquid fuel cells according to any one of claims 4 to 6. The hydrophilic functional group is at least selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, methacrylamide, N-vinyl pyrrolidone, N-vinyl pyridine, 2-hydroxyethyl methacrylate and a styrene derivative on the polymer substrate. Obtained by polymerizing one monomer,
The diaphragm for liquid fuel cells according to any one of claims 4 to 7.   The membrane-electrode assembly provided with the diaphragm for liquid fuel cells of any one of Claims 1-8.

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