Membrane electrode assembly and fuel cell

Abstract

The present invention addresses the problem of providing a membrane electrode assembly containing a hydrocarbon-based polymer electrolyte membrane and a gas diffusion electrode, the membrane electrode assembly having low resistance and thermal resistance in a fuel cell, and good power generation performance. A membrane electrode assembly having a catalyst layer and a gas diffusion electrode on both surfaces of a hydrocarbon polymer electrolyte membrane in this order from the hydrocarbon polymer electrolyte membrane side, in which the arithmetic mean roughness of the surface of the gas diffusion electrode on the catalyst layer side is 7 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to a membrane electrode assembly including an electrolyte membrane and a gas diffusion electrode for use in a fuel cell, particularly a solid polymer fuel cell, and a fuel cell. Background Art

[0002] A solid polymer fuel cell that supplies a fuel gas containing hydrogen to the anode and an oxidizing gas containing oxygen to the cathode to obtain an electromotive force through an electrochemical reaction occurring at both electrodes is usually composed of a single cell (cell) composed of a separator, a gas diffusion electrode, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode, and a separator stacked in sequence as one unit, and a power generation unit called a stack composed of multiple single cells stacked in series. Here, the five-layer part consisting of two gas diffusion electrode layers, two catalyst layers, and one electrolyte membrane layer among the elements constituting the above-mentioned single cell is called a membrane electrode assembly (MEA: Membrane Electrode Assembly).

[0003] The gas diffusion electrode needs to have high gas diffusivity for diffusing the gas supplied from the separator to the catalyst layer, high drainage for discharging water generated by the electrochemical reaction to the separator, and high conductivity for extracting the generated current. Therefore, a gas diffusion electrode using a conductive porous substrate (hereinafter sometimes referred to as a "substrate") formed of conductive fibers such as carbon fibers and having a microporous layer (MPL: Micro Porous Layer) formed on the surface thereof is widely used (Patent Documents 1, 2).

[0004] During normal operation of a fuel cell, the following reaction occurs on the anode side.

[0005] H 2 →2H + +2e - (1)

[0006] Furthermore, the following reaction proceeds on the cathode side.

[0007] O 2 +4H + +4e - →2H 2 O (2).

[0008] In the past, the fluorine-based polymer electrolyte "Nafion (registered trademark)" (manufactured by DuPont) was widely used as a material for the electrolyte membrane. "Nafion (registered trademark)" shows low humidification and high proton conductivity through the proton conduction channel derived from the cluster structure. On the other hand, since it is manufactured through a multi-stage synthesis, it is very expensive. In addition, due to the aforementioned cluster structure, there is a large problem of hydrogen permeation (crossover) through the membrane that is originally impermeable. In addition, under the operating conditions of the fuel cell, the dry-wet cycle is repeated continuously, and especially when the electrolyte membrane is a polymer electrolyte membrane, the electrolyte membrane repeatedly swells and shrinks. At this time, since the electrolyte membrane is constrained by a separator, etc., it has the problem of wrinkling and relaxation, and the membrane breaks due to local stress concentration, and the performance of the fuel cell deteriorates. In order to overcome such problems, the development of hydrocarbon-based polymer electrolyte membranes that are cheap and have excellent membrane properties that can replace "Nafion (registered trademark)" has become active in recent years (Patent Document 3).

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2014-011163

[0012] Patent Document 2: International Publication No. 2016 / 076132

[0013] Patent Document 3: Japanese Patent Application Publication No. 2019-061863 Summary of the invention

[0014] Problems to be solved by the invention

[0015] However, when a hydrocarbon-based polymer electrolyte membrane is used in a single cell constituting a fuel cell, the electrical resistance and thermal resistance of the membrane electrode assembly become high, and the power generation performance of the fuel cell using the membrane electrode assembly may become insufficient.

[0016] Means for solving problems

[0017] In view of the above problems, the present invention provides the following membrane electrode assembly.

[0018] (1) A membrane electrode assembly having a catalyst layer and a gas diffusion electrode on both sides of a hydrocarbon-based polymer electrolyte membrane in order from one side of the hydrocarbon-based polymer electrolyte membrane, wherein the arithmetic mean roughness of the surface of the gas diffusion electrode on the catalyst layer side is not more than 7 μm.

[0019] (2) The membrane electrode assembly according to (1) above, wherein the gas diffusion electrode has a microporous layer on a conductive porous substrate, and the surface of the gas diffusion electrode on the catalyst layer side is the surface of the gas diffusion electrode having the microporous layer.

[0020] (3) The membrane electrode assembly according to (1) or (2), wherein the elastic modulus of the electrolyte membrane at 23° C. and 50% RH is 1 to 3 GPa.

[0021] (4) The membrane electrode assembly according to any one of (1) to (3) above, wherein the hydrocarbon-based polymer electrolyte membrane comprises a hydrocarbon-based polymer electrolyte having a structural unit represented by the following general formula (S1):

[0022] [Chemistry 1]

[0023]

[0024] In the general formula (S1), Ar 1 ~Ar 4 represents any divalent arylene group, Ar 1 and / or Ar 2 With ionic groups, Ar 3 and Ar 4 May or may not have an ionic group; Ar 1 ~Ar 4 The arylene group may be arbitrarily substituted, and two or more arylene groups may be used independently of each other; * represents a bonding site with the general formula (S1) or other constituent units.

[0025] (5) The membrane electrode assembly according to any one of (1) to (4) above, wherein the thickness of the gas diffusion electrode is 130 to 190 μm.

[0026] (6) The membrane electrode assembly according to any one of (1) to (5) above, wherein the elasticity of the gas diffusion electrode is 3 to 7 μm.

[0027] (7) The membrane electrode assembly according to any one of (1) to (6), wherein the fluorine atom / carbon atom ratio (F / C ratio) of the catalyst layer side surface of the gas diffusion electrode is 0.05 to 0.5.

[0028] (8) The membrane electrode assembly according to any one of (1) to (7), wherein the oxygen atom / carbon atom ratio (O / C ratio) of the surface of the catalyst layer side of the gas diffusion electrode is 0.05 or less.

[0029] (9) A fuel cell comprising the membrane electrode assembly according to any one of (1) to (8) above.

[0030] Effects of the Invention

[0031] A fuel cell incorporating the membrane electrode assembly of the present invention has low electrical resistance and thermal resistance, and therefore has higher power generation performance than conventional products. DETAILED DESCRIPTION

[0032] The membrane electrode assembly of the present invention has a structure in which a catalyst layer and a gas diffusion electrode are provided on both surfaces of a hydrocarbon-based polymer electrolyte membrane in this order from the hydrocarbon-based polymer electrolyte membrane side.

[0033] The hydrocarbon polymer electrolyte membrane used in the present invention is a membrane containing a hydrocarbon polymer electrolyte as the main component. Here, the membrane containing a hydrocarbon polymer electrolyte as the main component means that the membrane contains more than 50% by mass of a hydrocarbon polymer electrolyte, preferably more than 70% by mass, and more preferably more than 90% by mass. The hydrocarbon polymer electrolyte contained in the hydrocarbon polymer electrolyte membrane refers to a polymer with a main chain and a side chain as required, and an ionic group on the main chain or side chain, with hydrocarbon as the main constituent unit, and the main chain or side chain is substantially not fluorinated. Here, the main chain with hydrocarbon as the main constituent unit means that the total number of carbon and hydrogen atoms constituting the hydrocarbon contained in the main chain exceeds 50% relative to the total number of atoms constituting the main chain. It should be noted that substantially not fluorinated means that even if a very small part of the main chain or side chain has a fluorinated part, most of it is a polymer that is not fluorinated. Specifically, a polymer with a fluorine atom content of less than 5% by mass relative to the number average molecular weight of the polymer is included in the substantially non-fluorinated polymer.

[0034] Specific examples of the main chain structure of the hydrocarbon-based polymer electrolyte contained in the hydrocarbon-based polymer electrolyte membrane used in the present invention include polyethylene, polypropylene, polystyrene, polysulfone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyparaphenylene, polyarylene ketone, polyetherketone, polyarylene phosphine oxide, polyetherphosphine oxide, polybenzoxazole, polybenzothiazole, polybenzimidazole, polyamide, polyimide, polyetherimide, and polyimide sulfone. It should be noted that the polysulfone, polyether, polyketone, etc. described here are general terms for structures having sulfone bonds, ether bonds, and ketone bonds in their molecular chains, including polyethersulfone, polyetherketone, polyetherketoneketone, polyetheretherketone, polyetherketonesulfone, etc. The main chain of the hydrocarbon-based polymer electrolyte may have multiple structures among these structures.

[0035] Among the main chain structures of the hydrocarbon-based polymer electrolytes contained in the hydrocarbon-based polymer electrolyte membrane used in the present invention, aromatic hydrocarbon-based polymers having aromatic rings in the main chain are preferred, and in particular, aromatic polyethersulfone-based polymers, aromatic polyarylether-based polymers, and aromatic polyetherketone-based polymers are more preferred from the viewpoints of gas barrier properties, durability, and cost.

[0036] The ionic groups used in the present invention are preferably atomic groups with negative charges, and more preferably those with proton exchange capacity. As such functional groups, sulfonic acid groups, sulfonimide groups, sulfate groups, phosphonic acid groups, phosphoric acid groups, and carboxylic acid groups are preferably used. Among them, from the viewpoint of high proton conductivity, it is more preferred to have at least a sulfonic acid group, a sulfonimide group, or a sulfate group, and from the viewpoint of raw material cost, it is most preferred to have at least a sulfonic acid group.

[0037] The molecular weight of the hydrocarbon-based polymer electrolyte contained in the hydrocarbon-based polymer electrolyte membrane used in the present invention is preferably 10,000 to 5,000,000, and more preferably 10,000 to 500,000, in terms of polystyrene-converted weight average molecular weight. When the polystyrene-converted weight average molecular weight is 10,000 or more, cracks can be suppressed on the molded film, and mechanical strength, physical durability, solvent resistance, etc. are improved. On the other hand, when the polystyrene-converted weight average molecular weight is 5,000,000 or less, solubility is improved, and the solution viscosity is also reduced, so that a hydrocarbon-based polymer electrolyte membrane with good processability can be obtained.

[0038] The ion exchange capacity (IEC) of the hydrocarbon-based polymer electrolyte contained in the hydrocarbon-based polymer electrolyte membrane used in the present invention is preferably 0.1 to 5.0 meq / g from the perspective of the balance between proton conductivity and water resistance. The IEC is more preferably 1.4 to 3.5 meq / g. When the IEC is 0.1 meq / g or more, a hydrocarbon-based polymer electrolyte membrane with good proton conductivity can be obtained, and when it is 5.0 meq / g or less, a hydrocarbon-based polymer electrolyte membrane with high water resistance can be obtained.

[0039] The hydrocarbon-based polymer electrolyte contained in the hydrocarbon-based polymer electrolyte membrane used in the present invention preferably contains the following structural units (S1) having an ionic group.

[0040] [Chemistry 2]

[0041]

[0042] In the general formula (S1), Ar 1 ~Ar 4 represents any divalent arylene group, Ar 1 and / or Ar 2 With ionic groups, Ar 3 and Ar 4 May or may not have an ionic group; Ar 1 ~Ar 4 The arylene group may be arbitrarily substituted, and two or more arylene groups may be used independently of each other; * represents a bonding site with the general formula (S1) or other constituent units.

[0043] Here, as Ar 1 ~Ar 4Preferred divalent arylene groups include hydrocarbon arylene groups such as phenylene, naphthylene, biphenylene, and fluorenediyl, and heteroarylene groups such as pyridinediyl, quinoxalinediyl, and thiophenediyl, but are not limited thereto. 1 ~Ar 4 A phenylene group having a phenylene group and an ionic group is preferred, and a p-phenylene group having a p-phenylene group and an ionic group is more preferred.

[0044] The hydrocarbon-based polymer electrolyte membrane used in the present invention preferably has a glass transition temperature of 120 to 200°C, and more preferably has a glass transition temperature of 140 to 180°C. When the glass transition temperature is 120°C or higher, the thermal deformation of the hydrocarbon-based polymer electrolyte membrane is suppressed when it is heated to form a catalyst layer, so the temperature can be raised to a temperature at which the polymer electrolyte contained in the catalyst layer softens, and the adhesion between the hydrocarbon-based polymer electrolyte membrane and the catalyst layer becomes good, which can reduce the interface resistance. On the other hand, when the glass transition temperature is 200°C or lower, the hydrocarbon-based polymer electrolyte membrane softens within a temperature range in which the polymer electrolyte contained in the catalyst layer does not decompose, so the adhesion between the hydrocarbon-based polymer electrolyte membrane and the catalyst layer becomes good, which can reduce the interface resistance.

[0045] The elastic modulus of the hydrocarbon-based polymer electrolyte membrane used in the present invention at 23°C and 50% RH is preferably 1 to 3 GPa. If the elastic modulus is 1 GPa or more, the deformation of the hydrocarbon-based polymer electrolyte membrane during power generation is small, so the breakage of the hydrocarbon-based polymer electrolyte membrane itself or the catalyst layer can be avoided, and high durability can be achieved. On the other hand, if the elastic modulus is 3 GPa or less, the flexibility of the hydrocarbon-based polymer electrolyte membrane is good, and the hydrocarbon-based polymer electrolyte membrane follows the concavo-convex shape of the surface of the gas diffusion electrode, so the interface resistance between the catalyst layer arranged on the surface of the hydrocarbon-based polymer electrolyte membrane and the gas diffusion electrode is reduced, so the fuel cell using it shows good power generation performance. The elastic modulus of the hydrocarbon-based polymer electrolyte membrane can be measured using a tensile tester, a viscoelasticity measuring device, a nanoindenter, etc. It should be noted that, as a hydrocarbon-based polymer electrolyte membrane, in order to improve strength, a three-layer structure is sometimes adopted in which a reinforcing layer (a mixed layer of a porous substrate and a polymer electrolyte) containing a porous substrate with high rigidity is sandwiched between two polymer electrolyte membranes. In this case, the elastic modulus of the hydrocarbon-based polymer electrolyte membrane is defined as the elastic modulus of each hydrocarbon-based polymer electrolyte membrane existing on both sides of the reinforcing layer. As a method for measuring the elastic modulus of a hydrocarbon-based polymer electrolyte membrane in such a three-layer structure, it is preferably measured by a nanoindenter in order to exclude the influence of the reinforcing layer.

[0046] In order to obtain mechanical strength and physical durability that can withstand practical use, the membrane thickness of the hydrocarbon-based polymer electrolyte membrane is preferably 3 μm or more. In order to reduce membrane resistance, that is, to improve power generation performance, or to improve the followability of the hydrocarbon-based polymer electrolyte membrane to the gas diffusion electrode surface, it is preferably 30 μm or less. The more preferred range of membrane thickness is 5 to 10 μm. The membrane thickness of the hydrocarbon-based polymer electrolyte membrane can be controlled by the concentration of the hydrocarbon-based polymer electrolyte solution or the coating thickness during membrane formation.

[0047] The catalyst layer in the membrane electrode assembly of the present invention is a layer comprising a catalyst substance and a polymer electrolyte. Generally, the catalyst layer of a fuel cell is a layer comprising catalyst-supported carbon particles carrying a catalyst substance and a fluorine-based polymer electrolyte, and is not particularly limited as long as it is a layer that stably maintains the catalyst substance and exhibits electron conduction and proton conduction in the catalyst layer. For example, the catalyst substance can be supported on inorganic oxide particles such as silicon dioxide and zirconium oxide other than carbon particles, and the polymer electrolyte in the catalyst layer can be an electrolyte other than a fluorine-based polymer electrolyte such as a hydrocarbon-based polymer electrolyte.

[0048] Here, fluorine-based polymer electrolyte refers to a polymer having a main chain with perfluorocarbon as the main constituent unit, and an ionic group is added to the main chain or side chain. The definition of the main constituent unit here is the same as that of the previous hydrocarbon. As this fluorine-based polymer electrolyte, for example, a perfluorocarbon polymer (which may contain ether oxygen atoms) with a sulfonic acid group can be cited. Among them, preferably there is a copolymer of a repeating unit based on tetrafluoroethylene and a repeating unit of perfluorocarbon with a sulfonic acid group. As a commercially available product of this copolymer, "Nafion (registered trademark)" (DuPont system), "Aquivion (registered trademark)" (Solvay system) can be cited.

[0049] Examples of the catalyst material used in the present invention include platinum, platinum group elements such as palladium, ruthenium, iridium, rhodium, osmium, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum and other metals, or alloys thereof, or oxides, composite oxides and the like.

[0050] The type of carrier carbon particles in the catalyst-supported carbon particles used in the present invention is not particularly limited as long as they are microparticles and conductive and will not corrode or deteriorate due to reaction with the catalyst. Carbon black, graphite, activated carbon, carbon fiber, carbon nanotubes and fullerene particles can be used.

[0051] If the particle size of the catalyst-supported carbon particles used in the present invention is too small, the electron conduction path is sometimes difficult to form. In addition, if it is too large, the gas diffusivity of the catalyst layer is sometimes reduced, and the utilization rate of the catalyst is reduced. Therefore, the particle size is preferably 10 to 1000 nm, and more preferably 10 to 100 nm.

[0052] The mass ratio of the content of the polymer electrolyte in the catalyst layer used in the present invention to the content of the catalyst-supported carbon particles is preferably 0.2 to 2.5, more preferably 0.5 to 2.0. If the mass ratio is 0.2 or more, the strength of the catalyst layer is improved, and the occurrence of cracks can be suppressed. On the other hand, if the mass ratio is 2.5 or less, the gas diffusion becomes good and the power generation performance is improved.

[0053] The amount of the catalyst substance per unit area of ​​the catalyst layer is preferably 0.01 mg / cm 2 More preferably, 0.05 mg / cm 2 If 0.01mg / cm 2 If the above conditions are met, the reaction activity as a catalyst becomes sufficient and excellent power generation performance can be obtained.

[0054] The thickness of the catalyst layer is preferably 0.5 to 2000 μm. In order to obtain practical power generation performance and durability, the thickness of the catalyst layer is more preferably 1 to 30 μm.

[0055] The gas diffusion electrode in the membrane electrode assembly of the present invention preferably has a structure including a microporous layer on a conductive porous substrate. The conductive porous substrate in this case will be described below.

[0056] As the conductive porous substrate used in the membrane electrode assembly of the present invention, a porous substrate containing carbon fibers such as carbon fiber fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper, and carbon cloth can be used. Here, carbon paper refers to a sheet formed by bonding a carbon fiber paper with a resin carbide. Among them, from the viewpoint of excellent corrosion resistance, carbon felt, carbon paper, and carbon cloth are preferably used, and from the viewpoint of excellent "elasticity" of absorbing the dimensional change in the thickness direction of the hydrocarbon-based polymer electrolyte membrane, carbon paper is more preferably used.

[0057] In the present invention, as the carbon fiber used in the conductive porous substrate, polyacrylonitrile (PAN)-based, asphalt-based, rayon-based, etc. can be cited. Among them, from the viewpoint of excellent mechanical strength, PAN-based carbon fiber and asphalt-based carbon fiber are preferably used. In addition, natural fibers such as rayon fibers, acrylic fibers, and cellulose fibers, and synthetic fibers can be mixed.

[0058] In the present invention, the average diameter of the single fiber of the carbon fiber used in the conductive porous substrate is preferably 3 to 20 μm. If the average diameter is 3 μm or more, the conductive porous substrate becomes a conductive porous substrate with a large pore diameter, the drainage is improved, and flooding can be suppressed. On the other hand, if the average diameter is 20 μm or less, it becomes a conductive porous substrate with low water vapor diffusion, which can suppress drying. When the average diameter of the single fiber of the carbon fiber is 5 to 10 μm, these effects are improved, so it is more preferred.

[0059] In the present invention, the average length of the single fiber of the carbon fiber used in the conductive porous substrate is preferably 3 to 20 mm. If the average length is 3 mm or more, the mechanical strength, electrical conductivity, and thermal conductivity of the conductive porous substrate become good. On the other hand, if the average length is 20 mm or less, the dispersion of the carbon fiber during papermaking becomes good, and a homogeneous conductive porous substrate can be obtained. When the average length of the single fiber of the carbon fiber is 5 to 15 mm, these effects are improved, so it is more preferred.

[0060] The mass per unit area (weight per unit area) of the conductive porous substrate used in the present invention is preferably 20 to 50 g / m 2 If the unit area weight is 20g / m 2 On the other hand, if the weight per unit area is 50 g / m 2 The gas diffusion property of the conductive porous substrate in the direction perpendicular to the surface (hereinafter, the direction perpendicular to the surface means the thickness direction) becomes good, and a fuel cell with higher power generation performance can be obtained. The weight per unit area of ​​the conductive porous substrate is 30 to 40 g / m 2 These effects are enhanced when , so it is more preferred.

[0061] The weight per unit area of ​​the conductive porous substrate used in the present invention can be adjusted by controlling the amount of carbon fibers, resin carbide, and the like, which are the constituent materials of the conductive porous substrate.

[0062] The conductive porous substrate used in the present invention preferably uses those with water-repellent resin attached to the inside by water-repellent treatment. As the water-repellent resin, fluororesins having fluoroalkyl chains are preferred. As the above-mentioned fluororesins, PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene hexafluoropropylene copolymer), PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer), ETFE (tetrafluoroethylene ethylene copolymer), PVDF (polyvinylidene fluoride), PVF (polyvinyl fluoride) etc. can be cited, preferably PTFE or FEP showing high water repellency.

[0063] The amount of the water-repellent resin when attached to the conductive porous substrate used in the present invention is not particularly limited, but is preferably 0.1 to 20% by mass based on 100% by mass of the entire conductive porous substrate. Within this range, water repellency can be fully exerted, and on the other hand, clogging of pores as gas diffusion paths by the water-repellent resin or an increase in resistance can be suppressed.

[0064] Generally, in solid polymer fuel cells, gas diffusion electrodes require high gas diffusivity for diffusing the gas supplied from the separator to the catalyst and high drainage for discharging the water generated by the electrochemical reaction to the separator. Therefore, the conductive porous substrate used in the present invention preferably has a peak of pore diameter at 10 to 100 μm. The pore diameter and its distribution can be obtained by measuring the pore diameter distribution using a mercury porosimeter. In order to obtain the pore diameter of the conductive porous substrate, only the conductive porous substrate can be measured, or the gas diffusion electrode after the microporous layer is formed can be measured. When measuring the gas diffusion electrode, the structure of each layer is confirmed by scanning electron microscopy (SEM) observation of the cross section perpendicular to the surface of the gas diffusion electrode (the cross section parallel to the direction perpendicular to the surface), and the diameter of the pore portion of the conductive porous substrate is roughly obtained by the SEM image. Next, the peaks of the multiple pore diameters obtained by the mercury porosimeter correspond to the approximate values ​​of the above-mentioned SEM image, and the pore diameter of the conductive porous substrate is determined at the same time.

[0065] In the membrane electrode assembly of the present invention, the porosity of the conductive porous substrate is preferably 80 to 95%. If the porosity is 80% or more, the gas diffusion is improved and the power generation performance is improved. On the other hand, if the porosity is 95% or less, the mechanical strength of the conductive porous substrate is improved and the conductivity is also improved. When the porosity of the conductive porous substrate is 85 to 90%, these effects are improved, so it is more preferred. The porosity of the conductive porous substrate can be measured using a specific gravity meter or the like.

[0066] The thickness of the conductive porous substrate in the membrane electrode assembly of the present invention is preferably 90 to 180 μm. Here, the thickness of the conductive porous substrate is the thickness when the two surfaces are clamped with a pressure of 0.15 MPa. If the thickness is 90 μm or more, the mechanical strength is maintained and the operation in the manufacturing process is easy. In addition, due to the increased elasticity, the gas diffusion electrode can absorb the dimensional changes caused by the expansion and contraction of the hydrocarbon-based polymer electrolyte membrane during the use of the fuel cell, and the gas diffusion in the in-plane direction becomes good, so the power generation performance is improved. On the other hand, if the thickness of the conductive porous substrate is less than 180 μm, the gas diffusion in the direction perpendicular to the surface is improved, and the conductive path in the direction perpendicular to the surface is shortened, the conductivity becomes good, and thus a fuel cell with high power generation performance can be obtained. When the thickness of the conductive porous substrate is 110 to 150 μm, these effects are improved, so it is more preferred.

[0067] Next, the microporous layer of the gas diffusion electrode in the present invention is described when the microporous layer is formed on a conductive porous substrate. In the membrane electrode assembly of the present invention, it is preferred that the gas diffusion electrode has a microporous layer on one surface of the conductive porous substrate. As the function of the microporous layer, water management such as moisturizing of the electrolyte membrane and discharge of generated water, reduction of the interface resistance between the catalyst layer and the gas diffusion electrode, and suppression of damage to the electrolyte membrane caused by carbon fibers protruding from the conductive porous substrate can be cited. The microporous layer is preferably a layer comprising carbon particles and a water-repellent resin.

[0068] In the present invention, specific examples of carbon fine particles used in the microporous layer include carbon black, carbon nanofibers, carbon nanotubes, graphene, etc. Among them, it is preferable to use inexpensive carbon black.

[0069] In the present invention, the specific surface area of ​​the carbon particles used in the microporous layer is preferably 20 to 40 m 2 / g. If the specific surface area of ​​carbon particles is 20m 2 / g or more, the dispersibility of the carbon particles in the microporous layer coating liquid described later is improved, so a uniform microporous layer can be formed. In addition, the moisture retention and drainage of the microporous layer are improved, so the power generation performance is improved. In addition, if the specific surface area of ​​the carbon particles is 40m 2 / g or less, the oxidative corrosion reaction of carbon is suppressed, so a fuel cell with higher durability can be obtained.

[0070] The content of the water-repellent resin and carbon particles per unit volume in the microporous layer of the present invention can be measured by mass spectrometry. First, the content of the water-repellent resin in the microporous layer is determined by analyzing the degassing components when the microporous layer is heated to 1000° C. in a He atmosphere to decompose and remove the resin components. Next, the content of the carbon particles can be determined from the mass of the residue after the thermal decomposition treatment. By dividing it by the volume of the microporous layer to be measured, the content per unit volume can be determined.

[0071] In the present invention, when the carbon fine particles used in the microporous layer have a fibrous form such as carbon nanofibers or carbon nanotubes, a conductive path is formed inside the microporous layer, thereby having the effect of reducing the resistance of the microporous layer, which is preferred.

[0072] In the present invention, the water-repellent resin contained in the above-mentioned microporous layer is preferably a fluororesin having a fluoroalkyl chain from the viewpoints of chemical stability, water repellency, etc., and is the same as the fluororesin preferably used when the conductive porous substrate is subjected to a water-repellent treatment, and examples thereof include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), ETFE (tetrafluoroethylene-ethylene copolymer), PVDF (polyvinylidene fluoride), PVF (polyvinyl fluoride), etc.

[0073] Fluororesin is generally insoluble in water or organic solvents, so when preparing a coating solution for forming a microporous layer, it is preferred to use a dispersion of a water-repellent resin processed into microparticles. Examples of dispersions of water-repellent resins in microparticle form include "Polyflon (registered trademark)" D-210C, ND-110 (manufactured by Daikin Industries, Ltd.), 120-JRB, 31-JR (manufactured by Mitsui Chemours Fluoroproducts Co., Ltd.), and the like.

[0074] In the present invention, the amount of water-repellent resin filled when forming the microporous layer is preferably 10 to 100 parts by mass of water-repellent resin relative to 100 parts by mass of carbon particles. If it is within this range, water repellency can be fully exerted, and on the other hand, the water-repellent resin can be inhibited from clogging the pores as the gas diffusion path or the increase in resistance. In addition, the interface resistance between the catalyst layer and the gas diffusion layer can be reduced.

[0075] In the present invention, silicone having a siloxane bond may be used as the water-repellent resin when forming the microporous layer.

[0076] In the present invention, when forming the microporous layer, a thermosetting resin may be contained in addition to the water-repellent resin. Examples of the thermosetting resin include phenolic resin, epoxy resin, acrylate resin, furan resin, and the like.

[0077] Furthermore, in the present invention, the microporous layer may contain microparticles of iridium oxide, ruthenium oxide, titanium oxide, etc., which promote electrolysis of water under the reverse potential state when hydrogen is deficient, and may contain microparticles of cerium oxide, manganese oxide, etc., which are used to deactivate hydroxyl radicals generated by the anode electrode.

[0078] In the membrane electrode assembly of the present invention, when the gas diffusion electrode has a microporous layer on a conductive porous substrate, the weight per unit area of ​​the microporous layer is preferably 10 to 35 g / m 2 If the unit area weight of the microporous layer is 10g / m 2The above-mentioned microporous layer covers the carbon fibers protruding from the surface of the conductive porous substrate, thereby preventing the carbon fibers from damaging the electrolyte membrane. In addition, the contact resistance between the gas diffusion electrode and the catalyst layer can be reduced, and the electrolyte membrane can be prevented from drying out. In addition, the unit area weight of the microporous layer is 35 g / m 2 The microporous layer has a unit area weight of 15 to 25 g / m 2 These effects are enhanced when , so it is more preferred.

[0079] In the membrane electrode assembly of the present invention, the arithmetic mean roughness of the surface on the catalyst layer side of the gas diffusion electrode is less than 7 μm. In addition, the surface on the catalyst layer side of the gas diffusion electrode is preferably a surface with a microporous layer of the gas diffusion electrode. The hydrocarbon-based polymer electrolyte membrane used in the membrane electrode assembly of the present invention has a high elastic modulus compared to the previous fluorine-based polymer electrolyte membrane, so the catalyst layer disposed on the surface of the hydrocarbon-based polymer electrolyte membrane is not easy to follow the concave and convex surface on the microporous layer side of the gas diffusion electrode, and sometimes a non-contact portion is generated, so sometimes the interface resistance between the catalyst layer and the gas diffusion electrode rises. In contrast, it can be seen that when the arithmetic mean roughness of the surface on the microporous layer side of the gas diffusion electrode is set to 7 μm or less, the interface resistance will not increase, and good power generation performance can be obtained. On the other hand, when the arithmetic mean roughness of the surface on the microporous layer side of the gas diffusion electrode is 4 μm or more, the generated water is captured in the concave portion present on the surface in the form of water droplets, and the water droplets become a water supply source, and the drying of the catalyst layer is avoided, so the power generation performance is improved, so it is preferred.

[0080] When the gas diffusion electrode has a microporous layer on the surface of the conductive porous substrate, when the surface of the conductive porous substrate itself has large irregularities, the surface of the microporous layer side is also likely to have large irregularities. Therefore, the arithmetic mean roughness of the surface of the conductive porous substrate having the microporous layer side is also preferably smaller than a specified value, preferably 30 μm or less.

[0081] The arithmetic mean roughness (Ra) of the surface of the gas diffusion electrode and the conductive porous substrate used in the membrane electrode assembly of the present invention can be measured using a laser microscope. For example, a laser microscope "VK-X3000" (manufactured by KEYENCE) can be used to measure the 5 mm long concave and convex profile at any position on the surface, and the average value thereof can be calculated as Ra.

[0082] The thickness of the gas diffusion electrode used in the membrane electrode assembly of the present invention is preferably 130 to 190 μm. Here, the thickness of the gas diffusion electrode is the thickness when both sides are clamped with a pressure of 0.15 MPa. If the thickness of the gas diffusion electrode is 130 μm or more, the mechanical strength is maintained and the operation in the manufacturing process is easy. On the other hand, if the thickness of the gas diffusion electrode is 190 μm or less, the gas diffusion is improved and the resistance is reduced, so the power generation performance of the fuel cell is improved. The thickness of the gas diffusion electrode can be adjusted by appropriately adjusting the thickness of the conductive porous substrate and the microporous layer.

[0083] The elasticity of the gas diffusion electrode used in the membrane electrode assembly of the present invention is preferably 3 to 7 μm. Elasticity refers to the change in thickness when pressure is applied to both sides of the gas diffusion electrode, and is defined here as the difference in thickness under 1 MPa and 2 MPa when the two sides of the gas diffusion electrode are clamped and pressurized. When the elasticity of the gas diffusion electrode is 3 μm or more, the pressure when tightening the fuel cell cell becomes uniform within the surface, so the deviation in power generation is reduced. In addition, the concavo-convex shape of the surface of the gas diffusion electrode is easily reduced by pressure, so the interface resistance between the catalyst layer arranged on the surface of the hydrocarbon-based polymer electrolyte membrane and the gas diffusion electrode is reduced. On the other hand, when the elasticity of the gas diffusion electrode is 7 μm or less, the pores inside the gas diffusion electrode are not easily destroyed by the pressure when tightening the fuel cell cell, so the gas diffusion and drainage properties are maintained.

[0084] The fluorine atom / carbon atom ratio (hereinafter referred to as F / C ratio) of the surface on the catalyst layer side of the gas diffusion electrode used in the membrane electrode assembly of the present invention is preferably 0.06 to 0.4. When the F / C ratio is 0.06 or more, the water drainage of the gas diffusion electrode is improved. On the other hand, when the F / C ratio is 0.4 or less, the moisture retention of the gas diffusion electrode is improved. In addition, the interface resistance between the catalyst layer and the gas diffusion layer can also be reduced.

[0085] The oxygen atom / carbon atom ratio (hereinafter referred to as O / C ratio) of the catalyst layer side of the gas diffusion electrode used in the membrane electrode assembly of the present invention is preferably 0.05 or less. When the O / C ratio is 0.05 or less, the moisture retention is improved and the interface resistance is reduced, thereby improving the power generation performance.

[0086] The F / C ratio and the O / C ratio are determined by using an X-ray photoelectron spectrometer (XPS) to perform elemental analysis on the surface of the catalyst layer side of the gas diffusion electrode, and quantifying the elemental concentrations of fluorine (F), oxygen (O) and carbon (C) to determine the F / C ratio and the O / C ratio.

[0087] Next, an example of a method for producing the membrane electrode assembly of the present invention will be described in detail.

[0088] First, a hydrocarbon-based polymer electrolyte is dissolved in a solvent to prepare a hydrocarbon-based polymer electrolyte solution. The solution is applied to a substrate such as a glass substrate or a PET substrate, and then the solvent is removed to prepare a hydrocarbon-based polymer electrolyte membrane on the substrate. The hydrocarbon-based polymer electrolyte membrane is peeled off from the substrate to obtain a single hydrocarbon-based polymer electrolyte membrane.

[0089] As a method for coating a hydrocarbon-based polymer electrolyte solution, applicable methods include rod coating, coater coating, blade coating, direct roller coating, Meyer rod coating, gravure coating, reverse coating, air knife coating, spray coating, brush coating, dip coating, die coating, vacuum die coating, curtain coating, flow coating, spin coating, screen printing, and inkjet coating.

[0090] The viscosity of the hydrocarbon-based polymer electrolyte solution is preferably 100 to 50,000 mPa·s, more preferably 300 to 10,000 mPa·s. When the viscosity is in the above preferred range, the thickness of the hydrocarbon-based polymer electrolyte membrane can be easily made uniform, and a hydrocarbon-based polymer electrolyte membrane having excellent surface smoothness can be obtained.

[0091] As the solvent used in preparing the hydrocarbon-based polymer electrolyte solution, any solvent can be used as long as it can dissolve the hydrocarbon-based polymer electrolyte material and then remove it. For example, preferably used are non-protonic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, ester solvents such as γ-butyrolactone and butyl acetate, carbonate solvents such as ethylene carbonate and propylene carbonate, alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, or alcohol solvents such as isopropyl alcohol, water and mixtures thereof. Non-protonic polar solvents have the highest solubility and are therefore preferred.

[0092] In order to obtain a strong membrane, a preferred method is to filter a hydrocarbon-based polymer electrolyte solution prepared to a desired solid content concentration under normal pressure or pressure to remove foreign matter present in the hydrocarbon-based polymer electrolyte solution. The pore size of the filter during filtration is preferably 1 μm or less.

[0093] After applying a hydrocarbon-based polymer electrolyte solution on a substrate, a hydrocarbon-based polymer electrolyte membrane can be formed by a drying process. In the drying process, the coating of the hydrocarbon-based polymer electrolyte solution is heated to evaporate the solvent. The heating method is not particularly limited as long as it can evaporate the solvent. For example, a heating device such as an oven or a heater, a device that uses infrared rays, warm air, etc. to control the temperature near the coating, etc. can be used. In addition, heat can be conducted to the coating via the substrate. The heating temperature range is preferably close to the boiling point of the solvent and below the glass transition temperature of the hydrocarbon-based polymer electrolyte membrane. In addition, the solvent can be removed by simply reducing pressure and introducing air flow without heating.

[0094] After the drying process, in order to improve the mechanical properties of the film, heat treatment can be further performed. The temperature of the heat treatment is preferably 80 to 350°C, more preferably 100 to 200°C, and more preferably 120 to 150°C. The heat treatment time is preferably 10 seconds to 12 hours, more preferably 30 seconds to 6 hours, and more preferably 1 minute to 1 hour. If the heat treatment temperature is too low, the mechanical strength and physical durability are sometimes insufficient. On the other hand, if it is too high, the chemical decomposition of the membrane material sometimes proceeds. If the heat treatment time is less than 10 seconds, the effect of the heat treatment is insufficient. On the other hand, if it exceeds 12 hours, degradation of the membrane material is likely to occur. The heat treatment is preferably carried out under an inert gas atmosphere such as nitrogen.

[0095] The hydrocarbon-based polymer electrolyte in the solution can also use those in which the ionic groups form salts with the cations of alkali metals or alkaline earth metals such as Li, Na, K, Ca, Sr, and Ba. At this time, after forming a film on the substrate and undergoing a drying process, it is also preferred to have a process in which the cations of the alkali metal or alkaline earth metal are exchanged with protons. This process is more preferably a process in which the formed film is contacted with an acidic aqueous solution. In addition, the contact is further preferably a process in which the formed film is immersed in an acidic aqueous solution. In the above-mentioned more preferred process, the protons in the acidic aqueous solution are replaced with cations that form ionic bonds with the ionic groups, and the residual water-soluble impurities, residual monomers, solvents, residual salts, etc. are removed simultaneously. The acidic aqueous solution is not particularly limited, and sulfuric acid, hydrochloric acid, nitric acid, acetic acid, trifluoromethanesulfonic acid, methanesulfonic acid, phosphoric acid, citric acid, etc. are preferably used. The concentration of the acidic aqueous solution should also be appropriately determined, and it is preferred to use an aqueous sulfuric acid solution of more than 3% by mass and less than 30% by mass.

[0096] Furthermore, additives such as a crystal nucleating agent, a plasticizer, a stabilizer, an antioxidant or a release agent commonly used for polymer compounds may be added to the hydrocarbon-based polymer electrolyte membrane obtained by the above steps within a range not hindering the purpose of the present invention.

[0097] Furthermore, the hydrocarbon-based polymer electrolyte membrane obtained through the above process may contain various polymers, elastomers, fillers, microparticles, various additives, etc. to improve mechanical strength, thermal stability, processability, etc. within a range that does not adversely affect the aforementioned characteristics.

[0098] Next, an example of a method for forming a catalyst layer on both sides of the obtained hydrocarbon-based polymer electrolyte membrane is described in detail. It should be noted that, in the following, a method is described in which a catalyst layer is formed on both sides of the hydrocarbon-based polymer electrolyte membrane and then the gas diffusion electrodes are bonded to both sides thereof, but it may also be a method in which a catalyst layer is first formed on one side of two gas diffusion electrodes, and then the side on which the catalyst layer of the gas diffusion electrode is formed is bonded to both sides of the hydrocarbon-based polymer electrolyte membrane. Here, when a microporous layer is formed on the surface of the gas diffusion electrode, the catalyst layer is preferably formed on the surface on the microporous layer side.

[0099] The method for forming the catalyst layer is not particularly limited, but from the viewpoint of simplicity of process and reduction of process cost, it is preferably a method of applying a catalyst layer coating liquid and drying it, or a method of pre-fabricating a catalyst layer decal having a catalyst layer formed on a substrate and transferring the catalyst layer decal to a polymer electrolyte membrane and then drying it. It should be noted that, as described above, when the catalyst layer is first formed on the gas diffusion electrode, it is preferred that the catalyst layer decal is formed on one side of the gas diffusion electrode.

[0100] In the case of the method of applying the catalyst layer coating liquid, the coating method is not particularly limited as long as it is a method that can apply the catalyst layer in a target shape, and the method described in the above-mentioned coating step of the hydrocarbon-based polymer electrolyte solution can be used.

[0101] The solvent contained in the catalyst layer coating liquid is not particularly limited as long as it is a solvent for dispersing polymer electrolytes and catalyst-supported carbon particles, and is preferably a solvent that is easily evaporated and removed by heating. For example, a solvent having a boiling point of 140° C. or less is preferred. Specifically, a solvent mixed with one or more of the following can be used: alcohols such as water, propanol, butanol, ketones such as methyl ethyl ketone, hexanone, cyclohexanone, diisobutyl ketone, ethers such as diethylene glycol dimethyl ether, methoxytoluene, dibutyl ether, isopropyl acetate, butyl acetate, ethyl lactate, butyl lactate, esters such as ...

[0102] In the case of the transfer method using a catalyst layer decal, first, a catalyst layer coating liquid is applied to the decal substrate and a drying process is performed as needed to prepare the catalyst layer decal. Then, the hydrocarbon-based polymer electrolyte membrane is sandwiched by the catalyst layer decal on the cathode electrode side and the catalyst layer decal on the anode electrode side, and hot pressing is performed in a manner that the surfaces of the two decals provided with the catalyst layer are in contact with the hydrocarbon-based polymer electrolyte membrane, thereby obtaining a hydrocarbon-based polymer electrolyte membrane with a catalyst layer. The temperature and pressure of the hot pressing can be appropriately selected according to the thickness, moisture content, catalyst layer, and decal substrate of the hydrocarbon-based polymer electrolyte membrane. From the perspective of industrial productivity and suppression of thermal decomposition of hydrocarbon-based polymer electrolyte membrane materials, it is preferably carried out in the range of 40 to 250°C, and more preferably at a temperature higher than the glass transition temperature of the polymer electrolyte contained in the catalyst layer and below 200°C. From the perspective of protecting the hydrocarbon-based polymer electrolyte membrane and the electrode, the pressure in the hot pressing is preferably as weak as possible, and in the case of flat plate pressing, a pressure of 10 MPa or less is preferably used.

[0103] As the decal substrate, the same resin film or substrate as that used for the hydrocarbon polymer electrolyte membrane can be used, and fluororesin films such as PTFE, polyhexafluoropropylene, ETFE, ethylene-hexafluoropropylene copolymer, FEP, PFA, and PVDF can also be used. In addition to heat resistance and solvent resistance, fluororesin films are preferably used from the viewpoint of chemical stability and demoulding properties.

[0104] The catalyst layer can be dried by the same method as described above for drying the hydrocarbon-based polymer electrolyte solution.

[0105] Next, an example of a method for producing a membrane electrode assembly by bonding a gas diffusion electrode to both surfaces of a hydrocarbon-based polymer electrolyte membrane with a catalyst layer will be described in detail. First, a method for producing a gas diffusion electrode will be described.

[0106] The carbon fiber bundle cut to a predetermined length is spread in water to produce a uniformly dispersed carbon fiber dispersion, which is picked up and dried to produce a carbon fiber paper body. Here, the carbon fiber dispersion may contain a surfactant, a thickener, a defoaming agent, and may also contain a water-soluble resin to maintain the shape of the carbon fiber paper body. As the water-soluble resin, polyvinyl alcohol, polyvinyl acetate, etc. can be used.

[0107] When making paper in a long continuous strip, the carbon fibers can be oriented in the length direction by increasing the speed of the papermaking web, and the elastic modulus of the carbon fibers in the orientation direction becomes higher. When making a fuel cell cell, if the carbon fibers of the gas diffusion electrode are arranged so as to be orthogonal to the separator flow path, the gas diffusion electrode can be prevented from being pressed into the grooves of the flow path, thereby improving drainage.

[0108] The carbon fiber paper body obtained as described above can be used as a conductive porous substrate. In order to improve the mechanical strength and reduce the electrical resistance, it is preferred to bond the intersections of the carbon fibers with a resin carbide.

[0109] For this purpose, for example, a method of impregnating a carbon fiber paper base with a resin composition solution and then performing a heat treatment to carbonize the resin component in the resin composition can be applied.

[0110] As the resin used in the resin composition solution, thermosetting resins such as phenolic resin, epoxy resin, melamine resin, furan resin, etc. can be mentioned. In addition to the resin component and the solvent, the resin composition solution can also contain carbon powder, surfactant, etc. As the carbon powder, carbon black, graphite, lead powder, carbon nanotubes, carbon nanofibers, etc. can be mentioned.

[0111] Examples of the method for impregnating the resin composition solution include dipping, spraying, doctor blade coating, die coating, transfer, etc. By adjusting the impregnation method, a gradient distribution of the amount of the resin composition in the thickness direction of the substrate can be provided.

[0112] Then, the paper sheet impregnated with the resin composition solution is dried at 80 to 150° C. in air. Next, the paper sheet is heated at 200 to 300° C. in air to cure the thermosetting resin and decompose and remove the surfactant and the like. At this time, the flatness can be improved and the thickness can be adjusted by pressing both sides of the paper sheet with a flat plate.

[0113] Furthermore, in order to improve the conductivity of the conductive porous substrate and improve long-term durability, it is preferred to carbonize the cured resin composition under an inert atmosphere such as nitrogen at a temperature of 1000 to 2400° C. Here, if the impurities are decomposed and removed by performing a pre-carbonization treatment at a temperature of 300 to 1000° C. under an inert atmosphere before the carbonization treatment to approach the crystal structure of graphite, the crystallinity during the carbonization treatment becomes higher, and the above-mentioned performance is improved, so it is preferred.

[0114] The conductive porous substrate thus obtained can have sufficient mechanical strength and conductivity as a conductive porous substrate for a gas diffusion electrode of a fuel cell.

[0115] Furthermore, in order to improve the drainage of the conductive porous substrate, a water-repellent treatment can be performed. In addition to the method of immersing the conductive porous substrate in a dispersion of a water-repellent resin, the method of applying a water-repellent resin on the conductive porous substrate by die coating, spraying, etc. can also be applied. In addition, processing based on dry processes such as sputtering of fluororesins can also be applied. After the water-repellent treatment, a drying process can be added as needed, and then a heating process for wetting and unfolding the water-repellent resin in the conductive porous substrate. In this way, a conductive porous substrate used in the membrane electrode assembly of the present invention can be obtained.

[0116] Next, a method for forming a microporous layer on a conductive porous substrate will be described.

[0117] The microporous layer can be formed by applying a microporous layer-forming coating liquid prepared by dispersing carbon fine particles and a water-repellent resin in a solvent such as water onto a conductive porous substrate and performing a heat treatment.

[0118] When making a coating liquid for forming a microporous layer, if a dispersant, a thickener, etc. are added to the solution, the dispersion stability of the carbon particles and the water-repellent resin is improved, so it is preferred. As a dispersant, since there are few metal components, it is preferably an anionic surfactant, and as an example thereof, "Triton (registered trademark)" X-100 (NacalaiTesque (Strain) system) of a polyoxyethylene octylphenyl ether system can be cited. In addition, in order to keep the coating liquid at a high viscosity, it is effective to add a thickener. As a thickener, for example, methylcellulose, polyethylene glycol, polyvinyl alcohol, etc. can be preferably used.

[0119] These dispersants and thickeners can have two functions in the same substance, and raw materials suitable for each function can also be selected. Among them, when selecting a thickener and a dispersant separately, it is preferred to select a thickener that does not impair the dispersibility of the carbon particles and the dispersibility of the water-repellent resin.

[0120] Furthermore, if necessary, fine particles of iridium oxide, ruthenium oxide, titanium oxide, or the like for promoting electrolysis of water, fine particles of cerium oxide, manganese oxide, or the like for inactivating radicals, and the like are also added.

[0121] The above mixture is kneaded using a homogenizer, a planetary mixer, an ultrasonic disperser or the like to obtain a microporous layer-forming coating liquid.

[0122] The coating liquid for forming the microporous layer can be applied to the conductive porous substrate using various commercially available coating devices, such as screen printing, rotary screen printing, gravure printing, gravure printing, spray coating, die coating, rod coating, blade coating, and roll blade coating.

[0123] After the microporous layer is coated on the conductive porous substrate, the coating liquid is dried at a temperature of 60 to 150°C, and then heated at a temperature of 250 to 380°C to promote the decomposition and removal of additives such as dispersants and thickeners and the melting of the water-repellent resin. At this time, not only the water-repellent resin in the microporous layer, but also the water-repellent resin attached to the conductive porous substrate is melted, and they wet and spread on the surface of carbon fibers, resin carbides, carbon particles, etc., which are other constituent materials. Thereby, the drainage of the conductive porous substrate and the microporous layer is improved, and thus the performance of the fuel cell using it is improved.

[0124] In this way, a gas diffusion electrode having a microporous layer formed on a conductive porous substrate is obtained.

[0125] Next, the surface of the microporous layer of the gas diffusion electrode is bonded to both surfaces of the hydrocarbon-based polymer electrolyte membrane with a catalyst layer to produce a membrane electrode assembly (MEA). As a bonding method, flat plate pressing, roll pressing, etc. can be cited, and the bonding can be performed while heating. Here, the two gas diffusion electrodes may be the same or different.

[0126] Furthermore, the obtained MEA was sandwiched from both sides by two separators, one for the anode and the other for the cathode, each having a gas flow path formed therein, to produce a fuel cell.

[0127] In the flow path of the separator, an inlet / outlet for supplying hydrogen is provided on the anode side, and an inlet / outlet for supplying air is provided on the cathode side. In addition, the separator is made of conductive materials such as stainless steel and carbon, so that electrical input and output can be achieved by connecting electrical wiring. Furthermore, by providing a passage inside the separator that can supply circulating water, the single cell can be maintained at a specified temperature.

[0128] By preparing a group of about 200 to 500 fuel cell cells thus manufactured and connecting them in series, a high voltage of 40 to 200 kV can be generated, and such a fuel cell stack can be used as a power source for fuel cell vehicles, etc.

[0129] Example

[0130] The present invention is specifically described below by way of examples, but the present invention is not limited to the modes shown here. The performance evaluation methods performed in the examples and the methods for preparing the materials used in the examples are as follows. It should be noted that in each evaluation, the number of measurements n is 1 if there is no special record of the number of measurements n.

[0131] <Method for measuring elastic modulus of electrolyte membrane>

[0132] The electrolyte membrane cut into 10 mm x 30 mm was held at both ends in the longitudinal direction and subjected to a tensile test at 23°C and 50% RH to measure the relationship between stress and strain and calculate the elastic modulus from the slope until the strain reached 5%.

[0133] <Method for measuring thickness of conductive porous substrate or gas diffusion electrode>

[0134] The thickness of the conductive porous substrate or the gas diffusion electrode was measured with a micrometer applying a load of 0.15 MPa. The measurement was performed at 9 random locations within the sample, and the average value was taken as the thickness of the sample.

[0135] <Method for measuring elasticity of gas diffusion electrode>

[0136] The gas diffusion electrode was cut into a size of 40 mm × 40 mm, and clamped by a metal rigid body with a smooth surface, and the change in thickness of the gas diffusion electrode from the thickness of the gas diffusion electrode when an average pressure of 1.0 MPa was applied to the thickness of the gas diffusion electrode when an average pressure of 2.0 MPa was applied was taken as elasticity. It should be noted that the number n for measurement was set to 3.

[0137] <Method for measuring the arithmetic mean roughness of the gas diffusion electrode surface>

[0138] The arithmetic mean roughness (Ra) of the gas diffusion electrode surface over a length of 5 mm was measured using a laser microscope "VK-X3000" (manufactured by KEYENCE Corporation). Ra was measured at four random locations on a 30 mm x 30 mm test piece, and the average value was calculated. The number of measurements n was set to 3.

[0139] <Method for measuring F / C ratio on the surface of gas diffusion electrode>

[0140] Using an X-ray photoelectron spectrometer (XPS) Model 5400 (manufactured by ULVAC-PHI Co., Ltd.), five points were randomly selected from the surface of the microporous layer side of the gas diffusion electrode for elemental analysis. The elemental concentrations [%] of fluorine (F) and carbon (C) were quantified, the F / C ratio was determined, and the average value of the five points was calculated.

[0141] <Measurement of surface O / C ratio>

[0142] Using an X-ray photoelectron spectrometer (XPS) Model 5400 (manufactured by ULVAC-PHI Co., Ltd.), elemental analysis was performed on 5 randomly selected locations on the surface of the microporous layer side of the gas diffusion electrode. The elemental concentrations [%] of oxygen (O) and carbon (C) were quantified, the O / C ratio was determined, and the average value of the 5 locations was calculated.

[0143] <Method for evaluating power generation performance of membrane electrode assembly>

[0144] Gas diffusion electrodes (cut into 5 cm × 5 cm electrodes) were placed on the anode and cathode sides of the electrolyte membrane with a catalyst layer, and hot pressing was performed at a pressure of 0.1 MPa, 130°C, and 20 minutes to produce a membrane electrode assembly (MEA). At this time, when the gas diffusion electrode has a microporous layer, the microporous layer is placed in contact with the catalyst layer. The MEA was assembled into a single cell for a fuel cell, and hydrogen was supplied to the anode side and air was supplied to the cathode side at a temperature of 70°C. The pressure was 1A / cm 2 The current density is maintained at a power generation state for 3 hours. At this time, the fuel utilization efficiency is set to 70%, the air utilization efficiency is set to 40%, the hydrogen on the anode side and the air on the cathode side are humidified to dew points of 59°C and 60°C, respectively, and introduced into the single cell. The voltage value after 3 hours is read as the power generation voltage as an indicator of power generation performance. Furthermore, the high frequency resistance (HFR: High Frequency Resistance) of the single cell is measured using the current interrupt method (Current Interrupt Method). HFR measures the total value of the resistance inside the single cell and the proton resistance.

[0145] <Carbon fine particles for microporous layer used in Examples>

[0146] Carbon particles I: "TOKABLACK (registered trademark)" #3800 (manufactured by Tokai Carbon Co., Ltd.)

[0147] Carbon particles II: "DENKA BLACK (registered trademark)" powder (manufactured by DENKA Corporation)

[0148] Carbon particles III: "Vulcan (registered trademark)" FCX80 (manufactured by Cabot Corporation).

[0149] <Method for producing polymer electrolyte membranes A and B>

[0150] (Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (K-DHBP) represented by the following formula (G1))

[0151] [Chemistry 3]

[0152]

[0153] In a 500 ml flask equipped with a stirrer, a thermometer and a distillation tube, 49.5 g of 4,4'-dihydroxybenzophenone, 134 g of ethylene glycol, 96.9 g of trimethyl orthoformate and 0.50 g of p-toluenesulfonic acid monohydrate were added and dissolved. Then, the mixture was stirred at 80°C for 2 hours. Furthermore, the internal temperature was slowly raised to 120°C and heated until the distillation of methyl formate, methanol and trimethyl orthoformate was completely stopped. After the reaction solution was cooled to room temperature, the reaction solution was diluted with ethyl acetate, the organic layer was washed with 100 ml of 5% potassium carbonate aqueous solution, and after separation, the solvent was distilled off. 80 ml of dichloromethane was added to the residue to precipitate crystals, which were filtered and dried to obtain 52.0 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane represented by the above formula (G1). The crystals were subjected to GC analysis, and the results showed 99.9% of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane and 0.1% of 4,4′-dihydroxybenzophenone.

[0154] (Synthesis of Disodium Salt-3,3'-Disulfonate-4,4'-Difluorobenzophenone Represented by the Following Formula (G2))

[0155] [Chemistry 4]

[0156]

[0157] 109.1 g of 4,4'-difluorobenzophenone (manufactured by Sigma-Aldrich) was dissolved in fuming sulfuric acid (50% SO 3 )150mL (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and reacted at 100°C for 10h. Then, a large amount of water was added little by little, neutralized with NaOH, and 200g of salt was added to precipitate the synthesized product. The obtained precipitate was filtered out and recrystallized with ethanol aqueous solution to obtain the disodium salt-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the above formula (G2). The crystals were subjected to GC analysis, and the purity was 99.3%.

[0158] (Synthesis of an oligomer having no ionic group represented by the following general formula (G3))

[0159] [Chemistry 5]

[0160]

[0161] In formula (G3), m represents a positive integer.

[0162] In a 1000 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark water separator, 16.59 g of potassium carbonate (manufactured by Sigma-Aldrich, 120 mmol), 25.8 g of K-DHBP represented by the above formula (G1) (100 mmol), and 20.3 g of 4,4'-difluorobenzophenone (manufactured by Sigma-Aldrich, 93 mmol) were added, and after nitrogen substitution, dehydration was carried out at 160°C in 300 mL of N-methyl-2-pyrrolidone and 100 mL of toluene, and the toluene was removed by heating, and polymerization was carried out at 180°C for 1 hour. Purification was carried out by reprecipitation with a large amount of methanol to obtain an oligomer having no ionic group (terminal: hydroxyl group). The number average molecular weight was 10,000.

[0163] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark water separator, 1.1 g of potassium carbonate (Sigma-Aldrich reagent, 8 mmol) and 20.0 g (2 mmol) of the above oligomer without an ionic group (terminal: hydroxyl group) were added, and after nitrogen substitution, dehydration was carried out at 100°C in 100 mL of N-methylpyrrolidone (NMP) and 30 mL of toluene, and then the toluene was removed by heating, and 4.0 g of decafluorobiphenyl (Sigma-Aldrich reagent, 12 mmol) was added, and the reaction was carried out at 105°C for 1 hour. Purification was carried out by reprecipitation with a large amount of isopropanol to obtain the oligomer without an ionic group (terminal: fluorine group) represented by the above general formula (G3). The number average molecular weight was 11,000.

[0164] (Synthesis of an oligomer having an ionic group represented by the following general formula (G4))

[0165] [Chemistry 6]

[0166]

[0167] In formula (G4), M represents Na or K.

[0168] In a 1000 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube and a Dean-Stark water separator, 27.6 g of potassium carbonate (manufactured by Sigma-Aldrich, 200 mmol), 12.9 g (50 mmol) of K-DHBP represented by the above formula (G1), 9.3 g (50 mmol) of 4,4'-biphenol (manufactured by Sigma-Aldrich, 50 mmol), 39.3 g (93 mmol) of disodium salt-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the above formula (G2), and 17.9 g of 18-crown-6 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 82 mmol) were added, and after nitrogen substitution, dehydration was carried out in 300 mL of N-methyl-2-pyrrolidone and 100 mL of toluene at 170°C, the toluene was removed by heating, and polymerization was carried out at 180°C for 1 hour. The oligomer having an ionic group represented by the above general formula (G4) was obtained by reprecipitation with a large amount of isopropanol for purification. The number average molecular weight was 16,000.

[0169] (Manufacturing of polymer electrolyte membrane)

[0170] In a 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark water separator, 0.56 g of potassium carbonate (manufactured by Sigma-Aldrich, 4 mmol) and 16 g (1 mmol) of an oligomer having an ionic group represented by the general formula (G4) were added. After nitrogen substitution, the mixture was dehydrated at 100° C. in 100 mL of N-methyl-2-pyrrolidone and 30 mL of cyclohexane. The cyclohexane was removed by heating, and 11 g (1 mmol) of an oligomer having no ionic group represented by the general formula (G3) was added. The mixture was reacted at 105° C. for 24 hours. Purification was performed by reprecipitation with a large amount of isopropanol to obtain a block copolymer of an oligomer having an ionic group represented by the general formula (G4) and an oligomer having no ionic group represented by the general formula (G3). The weight average molecular weight was 340,000.

[0171] A 5 mass% N-methyl-2-pyrrolidone solution in which the obtained block copolymer was dissolved was subjected to direct centrifugal classification of the polymerization stock solution using a variable frequency small high-speed cooling centrifuge manufactured by Kubota Corporation (model 6930 with angle rotor RA-800, 25°C, 30 minutes, centrifugal force 20,000G). The precipitated solid (cake) and the supernatant (coating liquid) were well separated, so the supernatant was recovered. Next, vacuum distillation was performed at 80°C while stirring, and pressure filtration was performed using a 1μm polypropylene filter to obtain a hydrocarbon-based polymer electrolyte solution. The viscosity of the hydrocarbon-based polymer electrolyte solution is 1,300 mPa·s. The hydrocarbon-based polymer electrolyte solution prepared as described above was cast on a polyethylene terephthalate (PET) substrate, dried at 100°C for 4 hours, and then heat treated at 150°C for 10 minutes under nitrogen. After being immersed in a 10 mass % sulfuric acid aqueous solution at 95°C for 24 hours to carry out proton substitution and deprotection reaction, the membrane was immersed in a large excess of pure water for 24 hours, and then fully washed and peeled off from the substrate to obtain a polymer electrolyte membrane A (membrane thickness: 10 μm). The elastic modulus of the obtained polymer electrolyte membrane A was 2 GPa.

[0172] Meanwhile, a polymer electrolyte membrane B (thickness: 10 μm) was prepared using a 10 mass % dispersion of a fluorine-based polymer electrolyte "Nafion (registered trademark)" (manufactured by Sigma-Aldrich) instead of the hydrocarbon-based polymer electrolyte solution. The elastic modulus of the obtained polymer electrolyte membrane B was 0.3 GPa.

[0173] <Method for producing catalyst layer decal>

[0174] Platinum catalyst-loaded carbon particles TEC10E50E (platinum loading rate 50% by mass) manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. and a 10% by mass dispersion of fluorine-based polymer electrolyte "Nafion (registered trademark)" (manufactured by Sigma-Aldrich) were used to prepare a catalyst coating solution in a mass ratio of 2:1 between the platinum catalyst-loaded carbon particles and "Nafion (registered trademark)". The catalyst coating solution was prepared at a platinum content of 0.3 mg / cm 2 The catalyst was coated on a commercially available polytetrafluoroethylene film to produce a catalyst layer decal (catalyst layer thickness: 8 μm).

[0175] <Method for producing electrolyte membranes A and B with catalyst layers>

[0176] The catalyst layer decals prepared in the above <Method for producing catalyst layer decals> were cut into 5 cm squares, and a pair of them were prepared and overlapped in a manner of sandwiching the polymer electrolyte membrane A (cut into 7 cm squares) obtained in the above <Method for producing polymer electrolyte membranes A to B>, and heated and pressed at 150°C and 5 MPa for 3 minutes to obtain electrolyte membrane A (7 cm square) with a catalyst layer. In addition, the polymer electrolyte membrane B was also treated in the same manner to obtain electrolyte membrane B (7 cm square) with a catalyst layer.

[0177] <Method for producing conductive porous substrates i to iii>

[0178] Toray Industries, Inc. polyacrylonitrile carbon fiber "TORAYCA (registered trademark)" T300 (average fiber diameter: 7 μm) was cut into 12 mm lengths, dispersed in water, and continuously made into paper. A 10% by mass aqueous solution of polyvinyl alcohol was sprayed on the paper and dried to obtain a paper having a unit area weight of 30 g / m 2 The amount of the polyvinyl alcohol attached was 20 parts by mass relative to 100 parts by mass of the papermaking body.

[0179] Next, a resin composition solution was prepared by mixing flaky graphite (average particle size: 5 μm), phenolic resin (a mixture of resol-type phenolic resin and novolac-type phenolic resin in a mass ratio of 1:1), and methanol at a mass ratio of 5:10:85. Subsequently, the resin composition solution was continuously applied to the papermaking body by spraying so that the total amount of phenolic resin and flaky graphite was 130 parts by mass relative to 100 parts by mass of the carbon fibers in the conductive porous substrate, and dried at 100°C for 5 minutes.

[0180] Next, the paper sheet with the resin composition attached was sandwiched between the upper and lower hot plates by a press molding machine and heated and compressed at 180°C for 5 minutes. Release paper was placed between the paper sheet and the hot plate to prevent the hot plate and the paper sheet substrate from adhering to each other. In addition, spacers were placed at the edges of the upper and lower hot plates to adjust the thickness of the conductive porous substrate after heating and compression. Then, it was heated at 2,000°C in a heating furnace under a nitrogen atmosphere to be carbonized.

[0181] Furthermore, a water-repellent resin dispersion prepared by mixing 5 parts by mass of a dispersion of PTFE fine particles ("Polyflon (registered trademark)" D-210C manufactured by Daikin Industries, Ltd.) and 95 parts by mass of ion-exchanged water was sprayed and dried at 100° C. for 5 minutes to obtain a film having a thickness of 160 μm and a weight per unit area of ​​50 g / m at 0.15 MPa. 2 Conductive porous substrate i.

[0182] Next, in the above method, the weight per unit area of ​​the papermaking body containing the long strip of carbon fiber is set to 20 g / m 2 , except that the same process is carried out, the unit area weight is obtained to be 30g / m 2 The conductive porous substrate ii has a thickness of 100 μm. In addition, the weight per unit area of ​​the papermaking body is set to 35 g / m 2 The forming step was omitted, and the same process was performed except that the weight per unit area was 60 g / m 2 And the conductive porous substrate iii has a thickness of 250 μm.

[0183] (Example 1)

[0184] Using carbon particles I as carbon particles, fluororesin PTFE as water-repellent resin, "Triton (registered trademark)" X-100 (manufactured by Nacalai Tesque (co., Ltd.)) as dispersant, and water as dispersion medium, the mixing ratio is set to carbon particles / PTFE resin / dispersant = 75 parts by mass / 25 parts by mass / 150 parts by mass, and the non-decomposed components (carbon particles and water-repellent resin) are prepared in a manner that the total amount is 23% by mass. It should be noted that the dispersion "Polyflon (registered trademark)" D-210C (manufactured by Daikin Industries (co., Ltd.)" in which PTFE particles are dispersed in water is used as a supply source of PTFE. In addition, when preparing the coating liquid, a planetary mixer is used to disperse the raw materials so that the coating liquid composition becomes uniform.

[0185] The above-mentioned coating solution for forming a microporous layer is applied to the conductive porous substrate i manufactured as described in the above <Method for manufacturing conductive porous substrates i to iii> using a die coater, dried at 100°C for 10 minutes, and then heated at 350°C for 10 minutes to promote the adhesion between the water-repellent resin and the carbon particles and to decompose and remove the dispersant, etc., to manufacture a gas diffusion electrode. Here, the unit area weight of the microporous layer after heating is 20 g / cm 2 Adjust the coating amount by

[0186] The thickness of the obtained gas diffusion electrode was 180 μm, and the arithmetic mean roughness of the surface on the microporous layer side was 7 μm. In addition, the F / C ratio of the surface on the microporous layer side was 0.3, and the O / C ratio was 0.006. Furthermore, the elasticity of the gas diffusion electrode was 5.9 μm.

[0187] Next, the power generation performance of the electrolyte membrane A with a catalyst layer manufactured as described in the above <Method for manufacturing electrolyte membranes A and B with catalyst layers> was evaluated as described in the above <Method for evaluating power generation performance of membrane electrode assembly>. The power generation performance was good, at 0.72 V. In addition, the HFR was 70 mΩ·cm 2 .

[0188] (Examples 2 to 4, Comparative Examples 1 to 5)

[0189] The types of the electrolyte membrane with a catalyst layer and the conductive porous substrate, the type of carbon particles when the microporous layer is formed, and the weight per unit area of ​​the microporous layer are shown in Tables 1 and 2. Other than that, the power generation performance of the membrane electrode assembly was evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0190] (Comparative Example 6)

[0191] The power generation performance of the membrane electrode assembly was evaluated in the same manner as in Example 1 except that "MB30" manufactured by AvCarb was used as the gas diffusion electrode. The results are shown in Table 2. It should be noted that for the items marked "-", no analysis or measurement was performed.

[0192] [Table 1]

[0193] [Table 1]

[0194]

[0195] [Table 2]

[0196] [Table 2]

[0197]

Claims

1. A membrane electrode assembly comprising a catalyst layer and a gas diffusion electrode on both sides of a hydrocarbon-based polymer electrolyte membrane in this order from one side of the hydrocarbon-based polymer electrolyte membrane, in, The arithmetic mean roughness of the surface of the gas diffusion electrode on the catalyst layer side is 7 μm or less.

2. The membrane electrode assembly according to claim 1, in, The gas diffusion electrode has a structure including a microporous layer on a conductive porous substrate, and the surface of the gas diffusion electrode on the catalyst layer side is the surface of the gas diffusion electrode having the microporous layer.

3. The membrane electrode assembly according to claim 1 or 2, in, The elastic modulus of the hydrocarbon-based polymer electrolyte membrane at 23° C. and 50% RH is 1 to 3 GPa.

4. The membrane electrode assembly according to claim 1 or 2, in, The hydrocarbon-based polymer electrolyte membrane comprises a hydrocarbon-based polymer electrolyte having a structural unit represented by the following general formula (S1): [Chemistry 1] In the general formula (S1), Ar 1 ~Ar 4 represents any divalent arylene group, Ar 1 and / or Ar 2 With ionic groups, Ar 3 and Ar 4 May or may not have an ionic group; Ar 1 ~Ar 4 The arylene group may be arbitrarily substituted, and two or more arylene groups may be used independently of each other; * represents a bonding site with the general formula (S1) or other constituent units.

5. The membrane electrode assembly according to claim 1 or 2, in, The thickness of the gas diffusion electrode is 130-190 μm.

6. The membrane electrode assembly according to claim 1 or 2, in, The elasticity of the gas diffusion electrode is 3-7 μm.

7. The membrane electrode assembly according to claim 1 or 2, in, The fluorine atom / carbon atom ratio (F / C ratio) of the surface on the catalyst layer side of the gas diffusion electrode is 0.06 to 0.

4.

8. The membrane electrode assembly according to claim 1 or 2, in, The oxygen atom / carbon atom ratio (O / C ratio) of the surface of the gas diffusion electrode on the catalyst layer side is 0.05 or less.

9. A fuel cell comprising the membrane electrode assembly according to claim 1 or 2.

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