JP7528445B2 - Membrane Electrode Assembly - Google Patents

Description

The present invention relates to a membrane electrode assembly for a fuel cell having anode and cathode electrode substrates and an electrolyte membrane sandwiched between them, and relates to a membrane electrode assembly having a gas diffusion layer with high water retention and thus excellent power generation performance even under low humidification conditions.

Fuel cells are a type of power generation device that extracts electrical energy by electrochemically oxidizing fuels such as hydrogen and methanol, and have been attracting attention in recent years as a clean energy source. Solid polymer electrolyte fuel cells in particular have a low standard operating temperature of around 100°C and a high energy density, so they are expected to be used in a wide range of applications as power generation devices for relatively small-scale distributed power generation facilities and mobile objects such as automobiles and ships. They are also attracting attention as a power source for small mobile and portable devices, and are expected to be installed in mobile phones and personal computers, replacing secondary batteries such as nickel-metal hydride batteries and lithium-ion batteries.

A fuel cell is usually constructed by a gas diffusion layer that supplies fuel gas and oxidizing gas to the catalyst layer, anode and cathode catalyst layers where the reaction responsible for generating electricity occurs, and a polymer electrolyte membrane that serves as a proton conductor between the anode catalyst layer and the cathode catalyst layer, which constitute a membrane electrode assembly (hereinafter sometimes abbreviated as MEA). The MEA is sandwiched between separators to form a cell as a unit. The required characteristics of the polymer electrolyte membrane are, first of all, high proton conductivity, and in particular, high proton conductivity even under high temperature and low humidity conditions. Conventionally, Nafion (registered trademark) (manufactured by DuPont), a perfluorosulfonic acid-based polymer, has been widely used for polymer electrolyte membranes. While Nafion (registered trademark) exhibits high proton conductivity through proton conducting channels due to its cluster structure, there was a problem with its proton conductivity under low humidity conditions. Meanwhile, the development of hydrocarbon-based polymer electrolyte membranes that can replace Nafion (registered trademark) has been intensified in recent years. In particular, in order to improve proton conductivity, several attempts have been made to form a microphase-separated structure using a block copolymer consisting of hydrophobic and hydrophilic segments, but proton conductivity under low humidity conditions remains an issue.

In light of these circumstances, it is important to manage the water in the membrane electrode composite (especially the water content of the electrolyte membrane) in fuel cells. The gas diffusion layer is responsible for managing the water in the membrane electrode composite, and is the component that controls the water retention in the membrane electrode composite and the discharge to the separator of the water generated during the electrochemical reaction and the water contained in the supply gas. For this reason, efforts have been made to control the water in fuel cells by controlling the behavior of the water in the gas diffusion layer.

Patent Document 1 discloses a carbon sheet for gas diffusion layers that can prevent bending into the separator flow path while maintaining high drainage. The carbon sheet is divided into three equal parts in the direction perpendicular to the sheet surface (hereinafter, the direction perpendicular to the surface), and the layers obtained are layer X, which is closest to one surface and has the highest layer packing rate, layer Y, which is closest to the other surface and has a lower layer packing rate than layer X, and layer Z, which is located between layer X and layer Y. The layer packing rates of the layers decrease in the order of layer X, layer Y, and layer Z. However, since Patent Document 1 aims to improve drainage from the gas diffusion layer, it does not mention the water retention of the membrane electrode assembly, and does not have the effect of improving power generation performance under low humidification conditions.

Patent Document 2 also describes a gas diffusion layer that contains carbon black in order to increase the water retention of the fuel cell, and that has an organic substance interposed in the gaps formed between the carbon black particles as a result of the aggregation of the carbon black particles. However, Patent Document 2 describes the introduction of a paste containing carbon black into the carbon sheet by impregnation or coating, and does not mention the filling rate. In addition, if carbon black is introduced into all the pores in the carbon sheet by the above method, there is a concern that the gas diffusibility of the entire carbon sheet will decrease.

Furthermore, Patent Document 3 describes the use of an anode gas diffusion layer made of a sheet-like, rubber-like porous material mainly composed of conductive particles and polymer resin, with a porosity of 60% or less, and a cathode gas diffusion layer with a porosity greater than that of the anode gas diffusion layer, for the purpose of improving power generation performance under high temperature and low humidification operating conditions. However, Patent Document 3 does not mention the packing rate distribution in the gas diffusion layer, and it is difficult to control the packing rate distribution in the anode gas diffusion layer, which has a sheet-like, rubber-like shape.

Patent No. 5954506 JP 2010-40399 A JP 2011-146407 A

In view of the background of the conventional technology, the present invention provides a fuel cell that can improve power generation performance by sufficiently humidifying the polymer electrolyte membrane and maintaining high proton conductivity even under high temperature and low humidity conditions.

In order to solve the above problems, the present invention employs the following means.
That is, the membrane electrode composite of the present invention is a membrane electrode composite having an electrolyte membrane, and a catalyst layer and a gas diffusion layer provided on both sides of the electrolyte membrane, the gas diffusion layer being composed of at least a porous carbon sheet and a microporous layer, and the porous carbon sheet is divided into three equal parts in the direction perpendicular to the sheet surface to obtain layers X, Y, and Z, where the layer closest to the electrolyte membrane side is layer X, the layer closest to the other surface is layer Y, and the layer located between layers X and Y is layer Z, the layer packing rate of layer X and layer Y is equal, and layer Z is smaller than layers X and Y.

The present invention provides a membrane electrode assembly that has high power generation performance under high temperature and low humidity conditions.

FIG. 2 is a schematic cross-sectional view of a carbon sheet used in the membrane electrode assembly of the present invention. FIG. 4 is a schematic cross-sectional view for illustrating the configuration of a membrane electrode assembly according to a second embodiment of the present invention. FIG. 4 is a schematic cross-sectional view for illustrating the configuration of a membrane electrode assembly according to a second embodiment of the present invention.

The present invention will be described in detail below.

[Membrane Electrode Assembly]
The membrane electrode assembly of the present invention includes a polymer electrolyte membrane, catalyst layers provided on both sides of the polymer electrolyte membrane, and a gas diffusion layer provided in contact with the catalyst layer on the side opposite to the polymer electrolyte membrane. In this case, the porous carbon sheet included in the gas diffusion layer is divided into three equal parts in the vertical direction of the sheet surface to obtain layers X, Y, and Z. When the layer closest to the electrolyte membrane side is layer X, the layer closest to the other surface is layer Y, and the layer located between layers X and Y is layer Z, the layer packing rate of layer X and layer Y is equal and layer Z is smaller than layers X and Y, thereby increasing the water retention of the membrane electrode assembly and realizing high power generation performance even under high temperature and low humidification conditions.

Here, the method for producing the MEA is as follows:
These methods can be broadly classified into two types: (I) a method in which a gas diffusion electrode (GDE) having a catalyst layer formed on one side of a gas diffusion layer is prepared, and then laminated with an electrolyte membrane; and (II) a method in which a catalyst-coated electrolyte membrane (CCM) is prepared, and then laminated with a gas diffusion layer.

In the case of method (I), a gas diffusion electrode (GDE) is used in which a catalyst layer is formed on one surface of a gas diffusion layer having the characteristics described in the present application. In this case, the effect of the present invention can be obtained by forming a catalyst layer on the surface closest to the layer with the second highest packing rate among the layers obtained by dividing the porous carbon sheet into three equal parts in the direction perpendicular to the surface in the section from the surface having a 50% packing rate closest to one surface of the porous carbon sheet contained in the gas diffusion layer to the surface having a 50% packing rate closest to the other surface.

In the case of method (II), the anode and cathode electrode substrates (gas diffusion layers) are arranged and joined so that the catalyst layer-forming surface of the CCM is in direct contact with the gas diffusion layer. The catalyst layer is generally formed from carbon black particles carrying platinum, but is not limited to this.

The method for joining the polymer electrolyte membrane and the gas diffusion layer is not particularly limited, and known methods (e.g., the chemical plating method described in Electrochemistry, 1985, 53, p. 269, and the hot press joining method for gas diffusion electrodes described in Electrochemical Science and Technology, 1988, 135, 9, p. 2209, edited by the Electrochemical Society (J. Electrochem. Soc.)) can be used.

When the polymer electrolyte membrane and the gas diffusion layer are integrated by pressing, the temperature and pressure may be appropriately selected depending on the thickness of the electrolyte membrane, the moisture content, the catalyst layer, and the electrode substrate. Specific pressing methods include roll pressing with a specified pressure and clearance, and flat pressing with a specified pressure, which are preferably performed in the range of 0°C to 250°C from the viewpoints of industrial productivity and suppression of thermal decomposition of polymer materials having ionic groups. It is preferable to apply as little pressure as possible from the viewpoint of protecting the electrolyte membrane and electrodes, and in the case of flat pressing, a pressure of 10 MPa or less is preferable. One of the preferable options from the viewpoint of preventing short circuits between the anode and cathode electrodes is to stack the electrodes and the electrolyte membrane to form a fuel cell without performing the composite process by pressing. In this method, when power generation is repeated as a fuel cell, deterioration of the electrolyte membrane, which is assumed to be caused by the short circuit, tends to be suppressed, and the durability of the fuel cell is good.

Specifically, it is preferable to manufacture an MEA by stacking the electrolyte membrane, gas diffusion layer, and catalyst layer as shown in Figures 2 and 3, and pressing them at a constant temperature and pressure. Such stacking and pressing may be performed on both sides simultaneously, or one side at a time.

A method for continuously producing a membrane electrode assembly includes a method in which a roll-shaped electrolyte membrane is produced, then laminated with a gas diffusion layer, and pressed at a constant temperature and pressure. When laminating film-like members such as a substrate, an electrolyte membrane, or an electrolyte membrane with a substrate, it is preferable to carry out the lamination while applying tension to each film-like member, and the tension can be changed by a method such as providing a tension cut between each process. For example, a tension cut may be provided by installing a motor, clutch, brake, etc. on a roll, and it is preferable to provide a detection means for detecting the tension applied to the film. Examples of rollers used for tension cut include a nip roller, a suction roller, or a combination of multiple rollers. A nip roller sandwiches a film between the rollers, and controls the feed speed of the film by the frictional force generated by the sandwiching pressure, so that the pressure applied to the film can be changed before and after the roller. Suction rollers are rollers with many holes on the surface, or rollers wrapped with wire to create a net or lattice-like shape, which attract the film-like material by creating negative pressure inside the roller, and the friction generated by the suction force controls the feed speed of the film-like material, making it possible to change the pressure applied to the film-like material before and after the roller.

[Polymer electrolyte membrane]
The polymer electrolyte membrane (hereinafter sometimes simply referred to as "electrolyte membrane") contained in the membrane electrode assembly of the present invention is not particularly limited, but is preferably an electrolyte membrane made of an aromatic hydrocarbon-based polymer.

Traditionally, perfluorosulfonic acid-based polymers have been widely used in polymer electrolyte membranes. While this polymer exhibits high proton conductivity through proton-conducting channels resulting from its cluster structure, it is very expensive because it is manufactured through a multi-step synthesis, and in addition, the aforementioned cluster structure poses the problem of large fuel crossover. Other issues that have been pointed out include the loss of mechanical strength and physical durability of the membrane due to swelling and drying, a low softening point that means it cannot be used at high temperatures, and problems with disposal after use and difficulties in recycling the material.

To overcome these drawbacks, there has been active development in recent years of hydrocarbon-based polymer electrolyte membranes that can replace perfluorosulfonic acid-based polymers, are inexpensive, suppress fuel crossover, have excellent mechanical strength, have a high softening point, and can withstand use at high temperatures. In particular, several attempts have been made to form a microphase-separated structure using block copolymers consisting of hydrophobic and hydrophilic segments in order to improve low-humidity proton conductivity.

By using a polymer with this structure, the mechanical strength is improved due to hydrophobic interactions and aggregation between the hydrophobic segments, and the ionic groups in the hydrophilic segments are clustered due to electrostatic interactions, forming ion-conducting channels, thereby improving proton conductivity.

Proposed mechanisms for the movement of protons in these electrolyte membranes include the vehicle mechanism, in which the hydronium ions themselves, which are hydrated protons, move, and the Grothus mechanism, in which protons bound to a substrate hop to another substrate. Under low humidity conditions with few water molecules, the movement by hopping of sulfonic acid groups based on the Grothus mechanism becomes dominant.

Under these circumstances, in the case of fluorine-based electrolyte membranes, etc., the acid dissociation constant of the sulfonic acid groups contained in the molecular structure is small, and protons are easily dissociated, so proton conduction by hopping is likely to proceed. On the other hand, in hydrocarbon-based electrolyte membranes, the acid dissociation constant of the sulfonic acid groups in the molecules is larger than that of fluorine-based electrolyte membranes, and proton dissociation is less likely to occur, so the decrease in proton conductivity under low humidification conditions is greater than that of fluorine-based electrolyte membranes.

The acid dissociation constant referred to here is an index for expressing the acid strength of a substance, and is expressed by the negative common logarithm pKa of the equilibrium constant in the dissociation reaction in which a proton is released from an acid. The smaller the pKa value, the easier it is to release a proton, indicating a stronger acid. The acid dissociation constant in this application is a value in water at 25°C.

As described above, the decrease in proton conductivity under low humidification conditions is greater in hydrocarbon-based electrolyte membranes with large acid dissociation constants than in fluorine-based electrolyte membranes. Therefore, the effects of the present invention are more pronounced when a membrane electrode assembly is formed from a hydrocarbon-based electrolyte membrane and a gas diffusion layer with high water retention.

Specific examples of aromatic hydrocarbon polymers include polymers having an aromatic ring in the main chain, such as polysulfone, polyethersulfone, polyphenylene oxide, polyarylene ether polymers, polyphenylene sulfide, polyphenylene sulfide sulfone, polyparaphenylene, polyarylene polymers, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, polyether phosphine oxide, polybenzoxazole, polybenzothiazole, polybenzimidazole, aromatic polyamide, polyimide, polyetherimide, and polyimide sulfone.

Polyethersulfone is a general term for polymers that have sulfone bonds in their molecular chains. Polyetherketone is a general term for polymers that have ether bonds and ketone bonds in their molecular chains, and includes polyetherketoneketone, polyetheretherketone, polyetheretherketoneketone, polyetherketoneetherketoneketone, polyetherketonesulfone, etc., and is not limited to a specific polymer structure.

Among these polymers, polysulfone, polyethersulfone, polyphenylene oxide, polyarylene ether polymers, polyphenylene sulfide, polyphenylene sulfide sulfone, polyarylene ketone, polyether ketone, polyarylene phosphine phosphate, polyether phosphine phosphate, and other polymers are often used due to their mechanical strength, physical durability, processability, and hydrolysis resistance.

The synthesis method of aromatic hydrocarbon-based electrolyte polymers is not particularly limited as long as it satisfies the above-mentioned characteristics and requirements. Such a method is described, for example, in Journal of Membrane Science, 197, 2002, p.231-242.

As an example, the preferred polymerization conditions for synthesizing an aromatic hydrocarbon polymer by polycondensation reaction are shown below. The polymerization can be carried out at a temperature range of 0 to 350°C, but a temperature of 50 to 250°C is preferable. If the temperature is lower than 0°C, the reaction tends not to proceed sufficiently, and if the temperature is higher than 350°C, the polymer tends to start to decompose. The reaction is preferably carried out in a solvent. Examples of solvents that can be used include aprotic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphontriamide, but are not limited to these, and any solvent that can be used as a stable solvent in an aromatic nucleophilic substitution reaction may be used. These organic solvents may be used alone or as a mixture of two or more types.

When the condensation reaction is carried out in a solvent, it is preferable to mix the monomers so that the resulting polymer concentration is 5 to 50% by weight. If the amount is less than 5% by weight, the degree of polymerization tends to be difficult to increase. On the other hand, if the amount is more than 50% by weight, the viscosity of the reaction system tends to become too high, making post-treatment of the reactants difficult.

Methods for introducing ionic groups into aromatic hydrocarbon polymers include a method of polymerization using a monomer having an ionic group, and a method of introducing ionic groups by polymer reaction. The method for polymerization using a monomer having an ionic group can be carried out by using a monomer having an ionic group in the repeating unit, and if necessary, an appropriate protecting group can be introduced and deprotected after polymerization.

To explain the method of introducing ionic groups, for example, the method of sulfonating aromatic rings, i.e., the method of introducing sulfonic acid groups, is described, for example, in JP-A-2-16126 or JP-A-2-208322.

Specifically, for example, aromatic rings can be sulfonated by reacting them with a sulfonating agent such as chlorosulfonic acid in a solvent such as chloroform, or by reacting them in concentrated sulfuric acid or fuming sulfuric acid. There are no particular limitations on the sulfonating agent as long as it sulfonates aromatic rings, and sulfur trioxide and the like can also be used. When sulfonating aromatic rings using this method, the degree of sulfonation can be easily controlled by the amount of sulfonating agent used, the reaction temperature, and the reaction time. Sulfonimide groups can be introduced into aromatic polymers, for example, by reacting sulfonic acid groups with sulfonamide groups.

The ionic group is preferably a functional group having a negative charge, and particularly preferably a functional group having proton exchange ability. As such functional groups, sulfonic acid groups, sulfonimide groups, sulfate groups, phosphonic acid groups, phosphoric acid groups, and carboxylic acid groups are preferably used. Here, the sulfonic acid group means a group represented by the following general formula (f1), the sulfonimide group means a group represented by the following general formula (f2) [in general formula (f2), R represents any organic group. ], the sulfate group means a group represented by the following general formula (f3), the phosphonic acid group means a group represented by the following general formula (f4), the phosphoric acid group means a group represented by the following general formula (f5) or (f6), and the carboxylic acid group means a group represented by the following general formula (f7).

Such an ionic group includes the case where the functional groups (f1) to (f7) are in the form of a salt. Examples of the cation that forms the salt include any metal cation and NR ⁴⁺ (R is any organic group). In the case of a metal cation, the valence is not particularly limited. Specific examples of preferred metal ions include Li, Na, K, Rh, Mg, Ca, Sr, Ti, Al, Fe, Pt, Rh, Ru, Ir, and Pd. Among them, Na, K, and Li, which are inexpensive and can be easily proton-substituted, are preferably used for the block copolymer used in the present invention.

Two or more types of these ionic groups can be contained in the polymer, and the combination can be appropriately determined depending on the structure of the polymer, etc. Among them, it is more preferable to use a sulfonic acid group, a sulfonimide group, or a sulfate group from the viewpoint of high proton conductivity, and it is most preferable to have a sulfonic acid group from the viewpoint of raw material costs.

The thickness of the electrolyte membrane is not particularly limited, but if it is thicker than 20 μm, the power generation performance decreases, and if it is less than 5 μm, durability and handling are significantly reduced, so a thickness of 5 μm to 20 μm is preferable. When the thickness of the electrolyte membrane is 5 μm to 20 μm, the amount of water retained in the membrane is small, and the membrane dries out quickly under low humidity conditions, resulting in a significant decrease in power generation performance, so application of the present invention is preferable.

[Catalyst layer]
The catalyst layer of the present invention is composed of an ion conductor and catalyst-supported particles in which the catalyst is supported on a carrier. As the catalyst, noble metal species such as platinum, gold, ruthenium, and iridium, which show high activity in oxidation and reduction reactions, are preferably used, but are not limited to these. As the carrier, carbon particles having electrical conductivity, high chemical stability, and a large surface area are preferable, and in particular, acetylene black, ketjen black, vulcan carbon, etc. are mentioned.

[Gas diffusion layer]
The gas diffusion layer of the present invention includes a carbon sheet and a microporous layer. That is, it can be produced by forming a microporous layer on a carbon sheet. In the present invention, the gas diffusion layer of the present invention can be obtained by forming a microporous layer on the surface of the carbon sheet closest to the layer X.

The microporous layer is composed of a water-repellent resin such as PTFE and a conductive filler. Carbon powder is preferred as the conductive filler. Examples of carbon powder include carbon black such as furnace black, acetylene black, lamp black, and thermal black, graphite such as flake graphite, scaly graphite, earthy graphite, artificial graphite, expanded graphite, and flake graphite, carbon nanotubes, linear carbon, and milled carbon fiber. Of these, carbon black is more preferably used as the carbon powder filler, and acetylene black is more preferably used because it contains fewer impurities.

In the present invention, from the viewpoint of increasing water retention, it is preferable to reduce the amount of water-repellent resin used in the microporous layer. In addition, the water retention of the membrane electrode assembly can be further increased by using a hydrophilic resin having binding properties instead of the water-repellent resin.

[Carbon sheet]
It is important that the carbon sheet of the present invention is porous in order to have high gas diffusivity for diffusing the gas supplied from the separator to the catalyst layer and high drainage for discharging the water generated by the electrochemical reaction to the separator. Furthermore, the carbon sheet of the present invention preferably has high electrical conductivity in order to extract the generated current. For this reason, it is preferable to use a conductive porous body in order to obtain the carbon sheet. More specifically, the porous body used to obtain the carbon sheet is preferably a porous body containing carbon fibers such as carbon fiber woven fabric, carbon paper, and carbon fiber nonwoven fabric, and a carbonaceous foamed porous body containing carbon fibers.

Among these, it is preferable to use a porous body containing carbon fibers to obtain a carbon sheet because of its excellent corrosion resistance, and furthermore, it is preferable to use carbon paper, which is made by binding a carbon fiber sheet with a carbide (binder), because it has excellent "springiness," i.e., the ability to absorb dimensional changes in the direction perpendicular to the surface of the electrolyte membrane (thickness direction).

The first embodiment of the carbon sheet of the present invention is characterized in that, when the layer closest to the electrolyte membrane side is called layer X, the layer closest to the other surface is called layer Y, and the layer located between layers X and Y is called layer Z, the layer packing ratios of layers X and Y are equal, and layer Z is smaller than layers X and Y. Here, the layer packing ratio refers to the average value obtained using the packing ratios of the surfaces that form the layers.

Because the filling rate of layer Z is lower than the other two layers, the diameter of the pores in layer Z becomes larger. This allows for effective gas diffusion and water discharge. Also, by making the filling rate of layer X and the filling rate of layer Y equal, the water generated in layer Z due to power generation is evenly supplied to layers X and Y. This allows for a balanced supply of water to the electrolyte membrane and drainage to the separator. On the other hand, because layer Y has a higher filling rate than layer Z, layer Y does not flex when the separator is pressed against it because it has a rigid flat surface. In this way, by arranging the layers so that the filling rates of layers X and Y are equal and layer Z is smaller than layers X and Y, it is possible to prevent water retention and flexing of the separator surface.

In the first embodiment of the present invention, the packing ratio of layers to achieve water retention and prevention of sagging is preferably such that, when the packing ratio of layer X is 1, the packing ratio of layer Y is 1, and the packing ratio of layer Z is 0.97 or less. Hereinafter, the packing ratio of each layer when the packing ratio of layer X is 1 is referred to as the packing ratio ratio of each layer.

The packing rate of layer Z is more preferably 0.8 or less, and even more preferably 0.7 or less, when the packing rate of layer X is 1. By reducing the packing rate of layer Z, the gas diffusion inside the carbon sheet and the drainage of water generated inside are significantly improved, resulting in good flooding resistance. There is no particular lower limit to the packing rate ratio of layer Z, but considering the strength required for post-processing and use in fuel cell stacks, it is preferable that it be 0.14 or more.

In the present invention, the filling rate of the layers X, Y, and Z is preferably 5 to 35%, since it is a carbon sheet with a well-balanced mechanical strength required for use as a gas diffusion layer for a fuel cell. When the filling rate of the layers X, Y, and Z is 5% or more, the carbon sheet resists destruction due to the force from the separator when assembled into a fuel cell stack, and the handling properties during the manufacture and advanced processing of the carbon sheet are good. When the filling rate of the layers X, Y, and Z is 35% or less, the material transfer within the carbon sheet becomes easy, the diffusion of gas and the drainage of generated water are efficiently performed, and the power generation performance of the fuel cell is significantly improved. The filling rates of the layers X and Y are more preferably 8 to 25%, and even more preferably 10 to 20%. The filling rate of the layer Z is 5 to 20%, and even more preferably 7 to 15%. By setting the filling rates of the layers X, Y, and Z to their respective preferred ranges, it is possible to optimally achieve both mechanical strength and power generation performance.
[Method of measuring the filling ratio of layer X, layer Y, and layer Z]
The method for measuring the packing ratio of the carbon sheet obtained by the above method will be specifically described below.

The filling ratios of layers X, Y, and Z are obtained by three-dimensional measurement X-ray CT. The entire area perpendicular to the surface of the carbon sheet is scanned at regular intervals from one surface of the carbon sheet to the other surface with three-dimensional X-ray CT to obtain three-dimensional data of the carbon sheet. By analyzing such three-dimensional data, the filling ratio of the measured surface can be obtained, and the filling ratio of a specific layer can be determined. Note that the above-mentioned fixed length (hereinafter referred to as slice pitch) can be set arbitrarily, but should be no more than one-third of the average diameter of the single carbon fibers that make up the carbon sheet.

The surface filling rate at a given position in the direction perpendicular to the surface of the carbon sheet is calculated by dividing the slice image at that position in the 3D data into 256 levels of maximum and minimum brightness using the image processing program "J-trim," and binarizing the area 175 levels from the minimum as the threshold. The proportion of the binarized bright side area to the total area is the surface filling rate at the given position.

The field of view measured once to calculate the surface filling rate depends on the slice pitch, but multiple measurements are performed so that the total field of view measured is ⁵ mm2 or more, and the average value is calculated to determine the layer filling rate.

The X-ray CT used for the measurement is SMX-160CTS manufactured by Shimadzu Corporation or an equivalent device. In the examples described later, since the average diameter of a single carbon fiber is 7 μm, the slice pitch is 2.1 μm, the measurement field of view is 1070 μm, and the measurement field of view is ⁵ mm2 or more to determine the packing rate of the surface, so the number of measurements when determining the packing rate of one surface was 7.

[Method of manufacturing carbon sheet]
Next, a preferred method for producing the carbon sheet of the present invention will be specifically described below, taking as a representative example carbon paper using a carbon fiber paper sheet as the porous body.

<Porous body>
Examples of carbon fibers include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among them, PAN-based carbon fibers and pitch-based carbon fibers are preferably used in the present invention because of their excellent mechanical strength.

The carbon fibers in the carbon sheet of the present invention and in the porous body such as the paper sheet used to obtain it preferably have an average single fiber diameter in the range of 3 to 20 μm, more preferably in the range of 5 to 10 μm. If the average single fiber diameter is 3 μm or more, the pore size becomes large, improving drainage and making it possible to suppress flooding. On the other hand, if the average single fiber diameter is 20 μm or less, it is preferable because it is easy to control the thickness of the carbon sheet to the preferred range described below.

The carbon fibers used in the present invention preferably have an average single fiber length in the range of 3 to 20 mm, more preferably in the range of 5 to 15 mm. If the average single fiber length is 3 mm or more, the carbon sheet will have excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, if the average single fiber length is 20 mm or less, the carbon fibers will be excellently dispersed during papermaking, resulting in a homogeneous carbon sheet. Carbon fibers having such an average single fiber length can be obtained by a method such as cutting continuous carbon fibers to the desired length.

The average diameter and average length of single fibers in carbon fibers are usually measured by directly observing the carbon fibers as the raw material, but they can also be measured by observing a carbon sheet.
The carbon fiber paper sheet formed by papermaking, which is one form of a porous body used to obtain a carbon sheet, is preferably in the form of a sheet in which the carbon fibers are randomly dispersed in a two-dimensional plane in order to maintain isotropic in-plane electrical conductivity and thermal conductivity. The carbon fiber paper sheet to obtain the carbon fiber paper sheet can be made only once or can be made by laminating the carbon fibers multiple times.

In the present invention, in order to prevent internal peeling of the carbon sheet, it is preferable to form the carbon sheet to the desired thickness continuously in one process. If a thick sheet is formed by performing a papermaking process or other process multiple times, discontinuous surfaces will be formed in the direction perpendicular to the surface, and stress will be concentrated when the carbon sheet is bent, which may cause internal peeling.

Specifically, from the viewpoint of preventing internal peeling, it is preferable that the ratio between the average diameter of the carbon fiber single fibers measured from one surface of the carbon sheet and the average diameter of the carbon fiber single fibers measured from the other surface is 0.5 or more and 1 or less. Here, when the average diameters are the same, the ratio is 1, and when the average diameters are different, the ratio is the smaller average diameter/the larger average diameter.

It is also preferable that the difference between the average length of the single carbon fiber measured from one surface and the average length of the single carbon fiber measured from the other surface is 0 mm or more and 10 mm or less. If the ratio and the difference are within these ranges, the carbon fibers can be uniformly dispersed in the porous body containing the carbon fibers, and the variation in density and thickness of the porous body can be reduced. Carbon sheets and gas diffusion layers made from such porous bodies have excellent surface smoothness, so that when they are made into a membrane electrode assembly, a uniform adhesion state with the catalyst layer and electrolyte membrane can be achieved, and good power generation performance can be obtained.

<Impregnation of Resin Composition>
When obtaining the carbon sheet of the present invention, a porous body containing carbon fibers, such as a carbon fiber paper sheet, is impregnated with a resin composition that serves as a binder.

In the present invention, methods for impregnating a porous body containing carbon fibers with a resin composition that serves as a binder include immersing the porous body in a solution containing the resin composition, applying a solution containing the resin composition to the porous body, and laminating and bonding a film made of the resin composition. Among these, the method of immersing the porous body in a solution containing the resin composition is particularly preferred because of its excellent productivity. Note that a porous body containing carbon fibers that has been impregnated with a resin composition that serves as a binder is sometimes referred to as a "pre-impregnated body."

The carbon sheet of the present invention is characterized in that the filling rates of layers X and Y are equal, and layer Z is smaller than layers X and Y. In the present invention, if the difference in the numerical values of the filling rates is 0.2% or less, they are considered to be equal. Conversely, if the difference in the numerical values of the filling rates is greater than 0.2%, one filling rate is considered to be smaller than the other filling rate. Such a carbon sheet of the present invention can be obtained by impregnating a porous body with a resin composition so that the amount of resin composition to be impregnated is equal in layers X and Y, and layer Z is smaller than layers X and Y. For this reason, the paper sheet containing carbon fibers is uniformly impregnated with the resin composition that serves as a binder by immersion or the like, and then the excess resin composition adhering to the carbon sheet is removed before drying, thereby controlling and distributing the amount of resin composition in the direction perpendicular to the surface of the carbon sheet, thereby controlling the filling rate of each layer.

As an example, the carbon fiber sheet can be immersed in a solution containing the resin composition, and then the solution containing the resin composition can be absorbed from both surfaces before drying, or a squeeze roll can be passed over both sides of the carbon fiber sheet. This allows the amount of adhesion near surface X and surface Y to be made uniform by the squeeze roll. On the other hand, since the solvent of the resin composition evaporates from the surface during the drying process, the resin composition can be distributed in large amounts on the surface, and as a result, the amount of binder in the carbon sheet can be changed so that it is equal in layers X and Y, and less in layer Z than layers X and Y.

The resin composition used in producing the pre-impregnated body is preferably a resin composition that is carbonized during firing to become a binder that is a conductive carbide. The resin composition used in producing the pre-impregnated body refers to a resin component to which a solvent or the like is added as necessary. Here, the resin component includes resins such as thermosetting resins and thermoplastic resins, and further includes additives such as carbon powder and surfactants as necessary.

More specifically, it is preferable that the carbonization yield of the resin component contained in the resin composition used in producing the pre-impregnated body is 40% by mass or more. If the carbonization yield is 40% by mass or more, the carbon sheet will have excellent mechanical properties, electrical conductivity, and thermal conductivity. There is no particular upper limit to the carbonization yield of the resin component in the resin composition, but it is usually about 60% by mass.

The resins constituting the resin component in the resin composition used to manufacture the pre-impregnated body include thermosetting resins such as phenolic resin, epoxy resin, melamine resin, and furan resin. Among these, phenolic resin is preferably used because of its high carbonization yield.

Additionally, as an additive to be added to the resin component as required, carbon powder can be used as a resin component in the resin composition used to manufacture the pre-impregnated body, for the purpose of improving the mechanical properties, electrical conductivity, and thermal conductivity of the carbon sheet. Here, examples of carbon powder that can be used include carbon black such as furnace black, acetylene black, lamp black, and thermal black; graphite such as flake graphite, phosphorus graphite, earthy graphite, artificial graphite, and expanded graphite; carbon nanotubes, linear carbon, and milled carbon fiber.

The resin composition used in producing the pre-impregnated body can be the resin component obtained by the above-mentioned configuration as it is, or can contain various solvents as necessary to improve the impregnation of porous bodies such as carbon fiber paper bodies. Here, examples of the solvent that can be used include methanol, ethanol, and isopropyl alcohol.

The resin composition used in producing the pre-impregnated body is preferably liquid at a temperature of 25°C and a pressure of 0.1 MPa. When the resin composition is liquid, it has excellent impregnation properties into the paper body, and the resulting carbon sheet has excellent mechanical properties, electrical conductivity, and thermal conductivity.

When impregnating, it is preferable to impregnate the carbon fiber in the pre-impregnated body with 30 to 400 parts by mass of the resin component, and more preferably with 50 to 300 parts by mass. When the resin component is 30 parts by mass or more per 100 parts by mass of carbon fiber in the pre-impregnated body, the carbon sheet has excellent mechanical properties, electrical conductivity, and thermal conductivity. On the other hand, when the resin component is 400 parts by mass or less per 100 parts by mass of carbon fiber in the pre-impregnated body, the carbon sheet has excellent gas diffusion in the in-plane direction and gas diffusion perpendicular to the plane.

<Bonding and heat treatment>
In the present invention, after forming a pre-impregnated body by impregnating a porous body such as a carbon fiber paper body with a resin composition, the pre-impregnated body can be laminated or heat-treated prior to carbonization.

In the present invention, multiple pre-impregnated bodies can be laminated together in order to obtain a carbon sheet with a predetermined thickness. In this case, multiple pre-impregnated bodies having the same properties can be laminated together, or multiple pre-impregnated bodies having different properties can be laminated together. Specifically, multiple pre-impregnated bodies having different average diameters and average lengths of single carbon fibers, carbon fiber weight per unit area of a porous body such as a carbon fiber paper used to obtain the pre-impregnated body, and impregnation amounts of resin components can be laminated together.

On the other hand, lamination can result in discontinuous surfaces in the direction perpendicular to the surface, which can lead to internal isolation. Therefore, in the present invention, it is preferable to use only one sheet of a porous body such as a carbon fiber paper sheet without laminating it, and to perform heat treatment on this sheet.

The pre-impregnated body can also be heat-treated to thicken and partially crosslink the resin composition in the pre-impregnated body. Methods for heat treatment that can be used include blowing hot air onto the pre-impregnated body, heating the pre-impregnated body by sandwiching it between hot plates of a press or the like, and heating the pre-impregnated body by sandwiching it between continuous belts.

<Carbonization>
In the present invention, after impregnating a porous body such as a carbon fiber paper body with a resin composition to obtain a pre-impregnated body, the resin composition is calcined in an inert atmosphere to carbonize the resin composition. This calcination can be performed using a batch-type heating furnace or a continuous-type heating furnace. The inert atmosphere can be obtained by flowing an inert gas such as nitrogen gas or argon gas into the furnace.

The maximum firing temperature is preferably in the range of 1300 to 3000°C, more preferably in the range of 1700 to 3000°C, and even more preferably in the range of 1900 to 3000°C. If the maximum temperature is 1300°C or higher, the resin components in the pre-impregnated body are carbonized, and the carbon sheet has excellent electrical and thermal conductivity. On the other hand, if the maximum temperature is 3000°C or lower, the operating costs of the heating furnace are reduced.

In the present invention, a porous body such as a carbon fiber paper sheet that is impregnated with a resin composition and then carbonized may be referred to as a "carbon fiber sintered body." In other words, a carbon sheet means a carbon fiber sintered body, and both a carbon fiber sintered body before water repellent treatment and a carbon fiber sintered body after water repellent treatment are included in the carbon sheet.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

[Synthesis of electrolyte membrane]
[Synthesis Example 1]
Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (K-DHBP) represented by the following general formula (G1):

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 charged and dissolved in a 500 ml flask equipped with a stirrer, thermometer, and distillation tube. The mixture was then stirred and kept at 78-82°C for 2 hours. The internal temperature was then gradually raised to 120°C, and the mixture was heated until the distillation of methyl formate, methanol, and trimethyl orthoformate completely stopped. After cooling the reaction solution to room temperature, the reaction solution was diluted with ethyl acetate, the organic layer was washed with 100 ml of a 5% aqueous potassium carbonate solution, and the solvent was distilled off. 80 ml of dichloromethane was added to the residue to precipitate crystals, which were then filtered and dried to obtain 52.0 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane. GC analysis of the crystals revealed that they were 99.8% 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane and 0.2% 4,4'-dihydroxybenzophenone.

[Synthesis Example 2]
Synthesis of disodium 3,3'-disulfonate-4,4'-difluorobenzophenone represented by the following general formula (G2):

109.1 g of 4,4'-difluorobenzophenone (Aldrich reagent) was reacted in 150 mL of fuming sulfuric acid (50% SO ₃ ) (Wako Pure Chemical Industries, Ltd.) at 100°C for 10 hours. Then, the mixture was gradually poured into a large amount of water, neutralized with NaOH, and 200 g of salt was added to precipitate the synthesized product. The resulting precipitate was filtered and recrystallized with an aqueous ethanol solution to obtain disodium 3,3'-disulfonate-4,4'-difluorobenzophenone represented by the above general formula (G2). The purity was 99.3%. The structure was confirmed by ¹ H-NMR. Impurities were quantitatively analyzed by capillary electrophoresis (organic substances) and ion chromatography (inorganic substances).

[Synthesis Example 3]
Synthesis of a polyetherketone-based polymer electrolyte membrane made of a polymer represented by the following general formula (G5):

Polymerization was carried out in N-methylpyrrolidone (NMP) at 210°C using 6.91 g of potassium carbonate, 7.30 g of disodium 3,3'-disulfonate-4,4'-difluorobenzophenone having an ionic group obtained in Synthesis Example 1, 10.3 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane having a hydrolyzable group obtained in Synthesis Example 2, and 5.24 g of 4,4'-difluorobenzophenone.

A 25 wt % N-methylpyrrolidone (NMP) solution in which the obtained block polymer was dissolved was pressure-filtered using a glass fiber filter, cast onto a glass substrate, dried at 100° C. for 4 hours, and then heat-treated at 150° C. for 10 minutes under nitrogen to obtain a polyketal ketone membrane. The solubility of the polymer was extremely good. The membrane was immersed in a 10 wt % aqueous sulfuric acid solution at 95° C. for 24 hours to carry out proton substitution and deprotection reactions, and then immersed in a large excess of pure water for 24 hours to thoroughly wash, to obtain a polymer electrolyte membrane.
[Measurement of proton conductivity]
The electrolyte membrane was immersed in pure water at 25°C for 24 hours, and then held in a thermohygrostat at 80°C and 25-90% relative humidity for 30 minutes at each step, and the proton conductivity was measured by a constant potential AC impedance method. A polyetherketone-based polymer electrolyte membrane using an aromatic hydrocarbon polymer and a commercially available perfluorocarbon-based electrolyte membrane (Nafion (registered trademark) 211) were used as the electrolyte membrane.

The measurement equipment used was a Solartron electrochemical measurement system (Solartron 1287 Electrochemical Interface and Solartron 1255B Frequency Response Analyzer), and proton conductivity was determined by performing constant potential impedance measurement using the two-terminal method. The AC amplitude was 50 mV. A membrane with a width of 10 mm and a length of 50 mm was used as the sample. The measurement tool was made of phenolic resin, and the measurement area was left open. Platinum plates (thickness 100 μm, two sheets) were used as electrodes. The electrodes were arranged on the front and back sides of the sample membrane, with an interelectrode distance of 10 mm, parallel to each other and perpendicular to the longitudinal direction of the sample membrane. Proton conductivity was measured at a measurement temperature of 80°C and relative humidity of 25% and 90%.

[Preparation of carbon sheet]
- Preparation of a carbon sheet with a thickness of 150 μm Toray Industries, Inc.'s polyacrylonitrile carbon fiber "TORAYCA" (registered trademark) T300 (average diameter of single fiber: 7 μm) was cut into short fibers with an average length of 12 mm, dispersed in water, and continuously made into paper by a wet papermaking method. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol as a binder was applied to the paper and dried to prepare a carbon fiber paper body with a basis weight of 10.0 g/m2. The amount of polyvinyl alcohol applied was 22 parts by mass per 100 parts by mass of the carbon fiber paper body.

Next, a resin composition in which a resol-type phenolic resin and a novolac-type phenolic resin were mixed in a mass ratio of 1:1 as the thermosetting resin, flake graphite (average particle size 5 μm) as the carbon powder, and methanol as the solvent were mixed in a ratio of thermosetting resin/carbon powder/solvent = 10 parts by mass/5 parts by mass/85 parts by mass, and the mixture was stirred for 1 minute using an ultrasonic dispersion device to obtain a uniformly dispersed resin composition.

Next, the carbon fiber paper cut to 15 cm x 12.5 cm was immersed horizontally in the impregnation liquid of the resin composition filled in an aluminum bat, and was squeezed and impregnated by sandwiching it between rolls. At this time, two rolls were placed horizontally with a certain clearance between them, and the carbon fiber paper was pulled up vertically to adjust the overall amount of resin composition attached. In addition, the amount of resin composition attached was made uniform by sandwiching it between rolls of the same shape from both sides and squeezing the liquid. After that, the pre-impregnated body was heated at a temperature of 100 ° C for 5 minutes and dried to prepare a pre-impregnated body. Next, while pressing with a flat plate press, heat treatment was performed at a temperature of 180 ° C for 5 minutes. During pressing, a spacer was placed on the flat plate press to adjust the distance between the upper and lower press plates so that the thickness of the pre-impregnated body after heat treatment was 50 μm. In this way, a high-density pre-impregnated body for carbon sheet with a thickness of 50 μm was prepared. In addition, by adjusting the amount of resin composition removed in the adhesion of the resin to this pre-impregnated body, a medium-density pre-impregnated body for carbon sheet with a thickness of 50 μm was prepared. Two sheets of pre-impregnated high-density carbon sheets and one sheet of pre-impregnated medium-density carbon sheets were stacked in the order of high density, medium density, and high density, and then heated and pressed.

The substrate, which had been subjected to a heating and pressurizing treatment using this pre-impregnated body, was then introduced into a heating furnace maintained in a nitrogen gas atmosphere with a maximum temperature of 2400°C, yielding a carbon sheet made of sintered carbon fiber.

The carbon fiber sintered body was impregnated with the water-repellent material by immersing the carbon sheet prepared above in an aqueous dispersion of PTFE resin ("Polyflon" (registered trademark) PTFE dispersion D-1E (manufactured by Daikin Industries, Ltd.)) or an aqueous dispersion of FEP resin ("Neoflon" (registered trademark) FEP dispersion ND-110 (manufactured by Daikin Industries, Ltd.)). The carbon sheet was then heated and dried for 5 minutes in a drying oven at 100°C to produce a water-repellent carbon sheet. During drying, the carbon sheet was placed vertically and the up-down direction was changed every minute. The aqueous dispersion of the water-repellent material was diluted to an appropriate concentration so that 5 parts by mass of the water-repellent material was applied to 95 parts by mass of the carbon sheet after drying. A carbon sheet with a basis weight of 45 g/m2 and a thickness of 150 μm was produced.

[Preparation of gas diffusion layer]
[material]
Carbon powder A: Acetylene black: "Denka Black" (registered trademark) (manufactured by Denki Kagaku Kogyo Co., Ltd.)
Carbon powder B: Linear carbon: "VGCF" (registered trademark) (manufactured by Showa Denko K.K.) Aspect ratio 70
Material C: Water-repellent material: PTFE resin (Polyflon (registered trademark) PTFE dispersion D-1E (manufactured by Daikin Industries, Ltd.), which is an aqueous dispersion containing 60 parts by mass of PTFE resin, is used)
Material D: Surfactant "TRITON" (registered trademark) X-100 (manufactured by Nacalai Tesque, Inc.)
The above materials were mixed using a disperser to form a filler-containing coating liquid. This filler-containing coating liquid was applied to one surface of a water-repellent carbon sheet using a slit die coater, and then heated at a temperature of 120 ° C for 10 minutes, followed by heating at a temperature of 380 ° C for 10 minutes. In this way, a microporous layer was formed on the water-repellent carbon sheet to produce a gas diffusion layer. The filler-containing coating liquid used here was prepared by adjusting the composition of the carbon coating liquid shown in the table, using carbon powder, a water-repellent material, a surfactant, and purified water, with the blending amount shown in the table being described in parts by mass. The blending amount of material C (PTFE resin) shown in the table does not represent the blending amount of the aqueous dispersion of PTFE resin, but the blending amount of the PTFE resin itself.

[High temperature and low humidity power generation evaluation (power generation performance)]
The membrane electrode assembly prepared by the method described in the Examples was set in a JARI standard cell "Ex-1" (electrode area 25 ^cm2 ) manufactured by Eiwa Co., Ltd. to prepare a module for power generation evaluation, and power generation evaluation was performed under the following conditions, sweeping the current from 0 A/ ^cm2 to 1.2 A/ ^cm2 until the voltage reached 0.2 V or less. In the present invention, the voltage at a current density of 1 A/ ^cm2 was compared. Note that a pressure of 0.7 GPa was applied when the membrane electrode assembly was set in the above cell.
Electronic load device: Kikusui Electronics Co., Ltd. Electronic load device "PLZ664WA"
Cell temperature: always 80°C
Gas humidification conditions: 30% RH or 90% RH
Gas utilization: anode is 70% of stoichiometric, cathode is 40% of stoichiometric.

[Example 1]
A polyetherketone-based polymer electrolyte membrane (thickness: 10 μm) was prepared using the synthesis method described in [Synthesis of electrolyte membrane] above. The proton conductivity of the prepared electrolyte membrane was 0.12 S/cm2 at 25% relative humidity and 8 S/cm2 at 90% relative humidity. A transfer sheet with an anode catalyst (size: 50 x 50 mm) and a transfer sheet with a cathode catalyst (size: 50 x 50 mm) were placed on both sides of this electrolyte membrane (size: 70 mm x 70 mm), and the catalyst-coated electrolyte membrane (CCM) was prepared by hot pressing at 160 °C, 4.5 MPa, and 5 min.

The anode gas diffusion layer and the cathode gas diffusion layer were produced using the production method described in [Production of gas diffusion layer] above. The carbon sheets contained in the anode gas diffusion layer and the cathode gas diffusion layer were carbon sheets having the characteristics described in [Production of carbon sheet]. The packing ratio of this carbon sheet was measured using the method described in [Method of measuring the packing ratio of layer X, layer Y, and layer Z], and the packing ratio of layer X and layer Y was 17.5%, and the packing ratio of layer Z was 16.7%.
The CCM, an anode gas diffusion layer (size: 50 mm x 50 mm), and a cathode gas diffusion layer (size: 50 mm x 50 mm) were arranged as shown in Fig. 2 (cross-sectional view). A membrane electrode assembly was fabricated by hot pressing under conditions of 160°C, 5 minutes, and 4.5 Ma.
The power generation performance was 0.50 V at 30% RH and 0.65 V at 90% RH.

[Example 2]
A polyetherketone-based polymer electrolyte membrane (thickness: 10 μm, size: 70 mm×70 mm) prepared under the same conditions as in Example 1, and an anode electrode (size: 50 mm×50 mm) and a cathode electrode (size: 50 mm×50 mm) which are gas diffusion electrodes (GDEs) were arranged as shown in FIG. 3 (cross-sectional view). For the gas diffusion layers contained in the anode and cathode gas diffusion electrodes, gas diffusion layers containing carbon sheets prepared under the same conditions as in Example 1 were used. A membrane electrode assembly was prepared by hot pressing under conditions of 160° C., 5 minutes, and 4.5 Ma.

The power generation performance was 0.50 V at 30% RH and 0.65 V at 90% RH.

[Comparative Example 1]
A carbon sheet having a uniform packing rate throughout the carbon sheet was produced according to the method described in Example 1, except that in the above [Preparation of carbon sheet], three sheets of a 50 μm-thick medium-density pre-impregnated body for carbon sheet were stacked and heated and pressed. The packing rate of this carbon sheet was measured by the method described in [Method for measuring packing rates of layers X, Y, and Z], and the packing rates of layers X, Y, and Z were found to be 16.7%. A membrane electrode assembly was produced under the same conditions as in Example 1, except that this carbon sheet was used as the carbon sheet contained in the gas diffusion layer.

The power generation performance was 0.30 V at 30% RH and 0.65 V at 90% RH.

[Comparative Example 2]
When the proton conductivity was measured using a commercially available perfluorocarbon-based polymer electrolyte membrane (film thickness: 25 μm), it was 1 S/cm2 at a relative humidity of 25% and 6 S/cm2 at a relative humidity of 90%. A membrane electrode assembly was produced under the same conditions as in Example 1, except that this perfluorocarbon-based polymer electrolyte membrane (size: 70 mm × 70 mm) was used as the electrolyte membrane.

The power generation performance was 0.50 V at 30% RH and 0.60 V at 90% RH.

[Comparative Example 3]
A membrane electrode assembly was produced under the same conditions as in Comparative Example 1, except that the same perfluorocarbon polymer electrolyte membrane as in Comparative Example 2 was used as the electrolyte membrane.

The power generation performance was 0.40 V at 30% RH and 0.60 V at 90% RH.

1: Carbon sheet 2: Surface X
3: Face Y
4: Layer X
5: Layer Z
6: Layer Y
7: Gas diffusion layer 8: Catalyst layer 9: Electrolyte membrane

Claims (3)

1. A membrane electrode composite having an electrolyte membrane, and a catalyst layer and a gas diffusion layer provided on both sides of the electrolyte membrane, the gas diffusion layer being composed of at least a porous carbon sheet and a microporous layer, the porous carbon sheet being divided into three equal parts in a direction perpendicular to a sheet surface to obtain layers X, Y, and Z, the layer closest to the electrolyte membrane side being layer X, the layer closest to the other surface being layer Y, and the layer located between layers X and Y being layer Z, the layer packing ratios obtained by three-dimensional measurement X-ray CT are equal for layer X and layer Y, and layer Z is smaller than layers X and Y, the microporous layer is formed on the surface closest to layer X, and the electrolyte membrane is made of an aromatic hydrocarbon-based polymer. The membrane electrode assembly according to claim 1, wherein the packing rate of layer Z of the gas diffusion layer is 0.97 or less when the packing rates of layers X and Y of the gas diffusion layer are 1. The membrane electrode assembly according to claim 1 or 2, characterized in that the electrolyte membrane is a block copolymer consisting of a hydrophilic segment having a sulfonic acid group and a hydrophobic segment not having a sulfonic acid group, and the acid dissociation constant of the sulfonic acid group is -10 or more.

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