Electrode material for fuel cells, membrane electrode assembly for fuel cells, and fuel cell

Abstract

To provide a novel fuel cell electrode material.SOLUTION: An electrode material for a fuel cell is made of carbon fiber, and the carbon fiber is coated with fibrous nanocarbon supporting a catalyst metal.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to an electrode material used in a fuel cell, and to a membrane electrode assembly and a fuel cell using the electrode material. [Background technology]

[0002] A fuel cell is a device that electrochemically converts the chemical energy of fuels such as hydrogen and methanol directly into electrical energy without converting it into heat. Fuel cells use hydrogen and oxygen as raw materials, and because they generate only electricity, water, and heat during power generation, they have attracted attention as an environmentally friendly energy conversion device.

[0003] Fuel cells are classified according to the type of electrolyte and fuel they use into polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), alkaline electrolyte fuel cells (AFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), direct methanol fuel cells (DMFCs), etc. PEFCs in particular have high power generation efficiency even when operated at low temperatures, and are therefore expected to be put to practical use and become widespread in applications such as automobiles, homes, and mobile devices.

[0004] However, fuel cells, which can operate at low temperatures, require the use of precious metals such as platinum as catalysts to promote reactions at the electrodes. To achieve high catalytic activity with a small amount of precious metal, development is underway to reduce the particle size of precious metals or to support them in a highly dispersed state on electrode materials.

[0005] Carbon materials are used as catalyst supports in fuel cells, and the amount and utilization rate of the supported catalytic metal can be controlled by selecting the carbon material. Therefore, there is a need to develop carbon materials with properties that can improve the performance of electrode catalysts.

[0006] In Japanese Patent Publication No. 5854314 (Patent Document 1), the present inventors proposed a technology using the carbon material Marimo Carbon as a catalyst support for fuel cells. Marimo Carbon is a carbon material in which carbon nanofilaments (CNFs) grow radially and isotropically from diamond microparticles as nuclei, resulting in spherical, algae-like particles. The CNFs that make up Marimo Carbon are highly crystalline and have a fibrous structure consisting of stacked cup-shaped (or cone-shaped) graphene units. Patent Document 1 shows that platinum particles were effectively supported using this Marimo Carbon. For this reason, it is expected to serve as a catalyst support that can replace amorphous carbon materials such as activated carbon and carbon black. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5854314 Summary of the Invention [Problem to be solved by the invention]

[0008] On the other hand, while the inventors were investigating the practical application of Marimo Carbon as a catalyst support, they discovered that because Marimo Carbon appears to be a powder, it complicates the process of producing fuel cell components.

[0009] The essential basic structure of a polymer electrolyte fuel cell is composed of a pair of electrodes and an electrolyte disposed between the electrodes. A membrane electrode assembly (MEA), which integrates the electrodes and an electrolyte membrane, is typically used to fabricate a fuel cell. The MEA of a PEFC is fabricated by attaching an anode (negative electrode) and a cathode (positive electrode) to each side of a polymer electrolyte membrane. The electrodes are composed of an electrode catalyst layer that contacts the electrolyte membrane to cause an electrode reaction, and a gas diffusion layer is disposed on the outside of the electrode catalyst layer to deliver hydrogen and oxygen to the electrode catalyst layer. While the term "electrode" sometimes includes the gas diffusion layer, in this specification, the term "electrode" refers to the electrode catalyst layer, and the gas diffusion layer is a component distinct from the electrode.

[0010] When fabricating an MEA using a powdered carbon material such as Marimo Carbon as a catalyst support, a catalytic metal such as platinum is first supported on a powdered support (such as Marimo Carbon) to create an electrode catalyst. The resulting electrode catalyst is then mixed with a proton-conducting material (such as an ionomer) to form a slurry, which is then sprayed onto a Teflon sheet or similar to form a sheet. This sheet is then pressure-bonded to the electrolyte membrane to create an MEA in which the electrode catalyst layer (electrode) and electrolyte membrane are integrated. Carbon fiber paper (CFP) or similar is then pressure-bonded onto the electrode catalyst layer of the resulting MEA as a gas diffusion layer. The powdered catalyst support thus complicates the manufacturing process, necessitating a simplification of the process.

[0011] Furthermore, because Marimo Carbon is a spherical microparticle with a size of 10 μm or more, consisting of densely packed CNFs, it is difficult for the raw material gases (hydrogen and oxygen) to diffuse (supply) into the interior of Marimo Carbon, and it is also difficult to diffuse (remove) the reaction product (water) produced inside Marimo Carbon. Furthermore, it is difficult to infiltrate proton-conducting materials into Marimo Carbon, and ensuring proton conductivity inside Marimo Carbon has been a challenge. Therefore, there is room for further improvement before Marimo Carbon can be used as a carbon material for effective electrode reactions.

[0012] Therefore, an object of the present invention is to provide a novel electrode material for a fuel cell using a technique different from conventional techniques. Another object of the present invention is to provide a membrane electrode assembly and a fuel cell using such an electrode material. [Means for solving the problem]

[0013] To achieve the above objective, the present inventors investigated the use of a carbon composite material, in which fibrous nanocarbon is deposited on a carbon fiber substrate, as a catalyst support instead of Marimo Carbon. The substrate used in this carbon composite material has spaces between the carbon fibers, allowing fibrous nanocarbon to be deposited even within the substrate, and facilitating the diffusion (supply) of raw material gases into the substrate and the diffusion (removal) of reaction products from the substrate. Furthermore, such a carbon composite material can easily impart proton conductivity to the substrate by dripping or immersing it in a proton-conducting material. Furthermore, because the carbon composite material can be directly layered on the electrolyte membrane instead of in powder form, the MEA manufacturing process can be simplified. Thus, the present inventors discovered that a carbon composite material, in which fibrous nanocarbon is deposited on a carbon fiber substrate, is suitable as a catalyst support for fuel cells, leading to the completion of the present invention.

[0014] Therefore, a first aspect of the present invention is an electrode material for a fuel cell made of carbon fiber, characterized in that the carbon fiber is coated with fibrous nanocarbon carrying a catalytic metal.

[0015] In a preferred embodiment of the fuel cell electrode material of the present invention, the carbon fiber coated with the fibrous nanocarbon carrying the catalytic metal is further coated with a proton-conductive material.

[0016] A second aspect of the present invention is a membrane electrode assembly for a fuel cell, comprising a pair of electrode catalyst layers and an electrolyte membrane disposed between the electrode catalyst layers, wherein at least one of the pair of electrode catalyst layers contains the electrode material according to the first aspect of the present invention.

[0017] In a preferred embodiment of the membrane electrode assembly for a fuel cell of the present invention, the electrolyte membrane is a proton-conducting polymer membrane.

[0018] A third aspect of the present invention is a fuel cell comprising a pair of electrode catalyst layers and an electrolyte disposed between the electrode catalyst layers, wherein at least one of the pair of electrode catalyst layers contains the electrode material according to the first aspect of the present invention.

[0019] A fourth aspect of the present invention is a fuel cell comprising the membrane electrode assembly according to the second aspect of the present invention.

[0020] In a preferred embodiment of the fuel cell of the present invention, the electrode catalyst layer containing the electrode material has a gas diffusion function. [Effects of the Invention]

[0021] According to the first aspect of the present invention, a novel electrode material for a fuel cell can be provided. According to the second to fourth aspects of the present invention, a membrane electrode assembly and a fuel cell using such an electrode material can be provided. [Brief explanation of the drawings]

[0022] [Figure 1]SEM images of CFP (top) and CNFs / CFP (bottom) are shown. [Figure 2] TEM image of CNF is shown. [Figure 3] 1 shows the relationship between the amount of fibrous nanocarbon precipitated (amount of carbon precipitated) and the synthesis temperature. [Figure 4] SEM images of CNFs / CFP are shown. (a) shows the surface of CNFs / CFP, and (b) shows the cross section of CNFs / CFP. [Figure 5] The fiber diameter distribution of CNFs synthesized at 450°C and 550°C is shown. (a) is for 450°C, and (b) is for 550°C. [Figure 6] The volume resistivity of CFP and CNFs / CFP is shown. [Figure 7] SEM images of Pd / CNFs / CFP (a) and Pd / CFP (b) are shown. [Figure 8] An SEM image of Pd / CFP is shown. [Figure 9] SEM images of Pd / CNFs / CFP are shown. [Figure 10] The reactivity of Pd / CNFs / CFP and Pd / CFP to hydrogen is compared. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described in detail below.

[0024] One aspect of the present invention is a fuel cell electrode material made of carbon fibers, characterized in that the carbon fibers are coated with fibrous nanocarbon carrying a catalytic metal. In this specification, this fuel cell electrode material is also referred to as the "electrode material of the present invention."

[0025] The electrode material of the present invention includes a material made of carbon fiber. The material made of carbon fiber is an aggregate of a plurality of carbon fibers, and in the present invention, functions as a support for fibrous nanocarbon carrying a catalytic metal. For this reason, the material made of carbon fiber can also be called a substrate. This substrate also has spaces formed between the carbon fibers, and is also called a porous substrate. By using a substrate in which spaces are formed between the carbon fibers, the fibrous nanocarbon carrying a catalytic metal can be supported even inside the substrate. The thickness of the substrate is preferably 0.15 to 0.4 mm.

[0026] The carbon fiber material is preferably a planar material because it is used as an electrode material for a fuel cell. Specific examples of the carbon fiber material include carbon fiber woven fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper (CFP), and carbon cloth. Among these, CFP is preferred.

[0027] The carbon fibers constituting the substrate serve as a support for the fibrous nanocarbon, and therefore have a larger diameter than the fibrous nanocarbon, typically on the order of micrometers. For example, the diameter of a single carbon fiber is 5 to 10 μm. The diameter of a single carbon fiber can be determined in accordance with JIS R7607:2000.

[0028] The gas permeability of carbon fiber materials is preferably in the range of 100 to 10,000 ml·mm / (cm 2 ·hr·mmAq), and more preferably 500 to 5000 ml·mm / (cm 2 ·hr·mmAq), and most preferably 1000 to 3000 ml·mm / (cm 2 In this specification, gas permeability can be measured by the isobaric method in accordance with JIS K 7126-2. Oxygen gas is used as the test gas.

[0029] In the electrode material of the present invention, the carbon fibers are coated with fibrous nanocarbon supporting a catalytic metal. Here, "carbon fibers coated with fibrous nanocarbon" means that the carbon fibers are entirely or partially coated with fibrous nanocarbon. Specific examples include an embodiment in which some of the carbon fibers among all the carbon fibers are entirely or partially covered with fibrous nanocarbon, and an embodiment in which all the carbon fibers are entirely or partially covered with fibrous nanocarbon. In the electrode material of the present invention, it is preferable that the entire carbon fibers constituting the substrate are uniformly covered with fibrous nanocarbon. In the electrode material of the present invention, the substrate is an aggregate of carbon fibers, and spaces are formed between the carbon fibers, so that the carbon fibers present inside the substrate can also be coated with fibrous nanocarbon.

[0030] The electrode material of the present invention preferably has high electrical conductivity, and the volume resistivity before supporting the catalyst metal (i.e., the volume resistivity of the carbon composite material) is preferably 10 ―8 ~10 8 mΩ·cm, and more preferably 10 ―8 ~10 4 mΩ·cm, and more preferably 10 ―8 ~100mΩ·cm, most preferably 10 ―8 In this specification, the volume resistivity can be measured using either a contact or non-contact type electrical resistance measuring device.

[0031] In this specification, fibrous nanocarbon refers to a fibrous carbon material with a diameter on the order of nanometers, and is synonymous with carbon nanofilaments (CNFs), and may also be referred to as CNFs. Fibrous nanocarbon is a carbon material with a fibrous structure formed by stacking graphene as a structural unit. Because the structural unit is a graphene-like structure, fibrous nanocarbon is highly crystalline. Unlike amorphous carbon materials such as activated carbon and carbon black, fibrous nanocarbon does not undergo structural changes even with repeated use, contributing to a longer lifespan of power generation performance. Furthermore, because fibrous nanocarbon has a fibrous structure formed by stacking graphene, countless graphene edges exist on its surface, which serve as support sites for catalytic metals. This promotes the reduction in particle size and high dispersion of catalytic metal particles, thereby increasing the number of electrode reaction sites. Graphene stacking structures include a structure in which cup-shaped graphene is stacked and a structure in which coin-shaped graphene is stacked, and these can be produced by adjusting the synthesis conditions of the fibrous nanocarbon. Therefore, the CNFs of the present invention significantly differ from so-called carbon nanotubes in that graphene edges are regularly exposed over the entire surface of the fibrous structure. In this research field, there are fibrous nanocarbons with internal structures resembling bamboo nodes, referred to as "bamboo-like structures," and some reports have claimed that these have cup-stacked structures. However, these actually possess graphene edges that would be classified as multi-walled carbon nanotubes, but the exposure is irregular and partial, significantly different from the CNFs structure provided by the present invention.

[0032] The average fiber diameter of the fibrous nanocarbon is preferably 5 to 300 nm, more preferably 10 to 100 nm, and even more preferably 10 to 40 nm. The fiber diameter of the fibrous nanocarbon can be determined from SEM images obtained by a scanning electron microscope (SEM). For example, approximately 10 to 20 SEM images observed at approximately 100,000 magnifications are taken, and approximately five fibrous nanocarbons are selected from each SEM image with clear images and no overlaps. The fiber diameters of the selected fibrous nanocarbons are measured, and a histogram is created in which the fiber diameters are classified into 5-nm classes to determine the average fiber diameter. In an example of the present invention, the fiber diameter distribution obtained from the fiber diameter measurement results of approximately 100 CNFs synthesized by catalytic reaction of a nickel catalyst with methane was predominantly in the 15-30 nm range when the synthesis temperature was 450°C, and predominantly in the 20-40 nm range when the synthesis temperature was 550°C.

[0033] In the synthesis of fibrous nanocarbon, the fibrous nanocarbon can be densely grown on the carbon fibers constituting the substrate, thereby forming a thin layer of fibrous nanocarbon on the surface of the carbon fiber. Therefore, unlike spherical fine particles with a size of 10 μm or more such as Marimo carbon, the fibrous nanocarbon layer formed on the surface of the carbon fiber allows for easy diffusion (supply) of raw material gas into its interior and diffusion (removal) of reaction products from its interior. In the electrode material of the present invention, the lower limit of the thickness of the fibrous nanocarbon layer formed on the surface of the carbon fiber is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more. Furthermore, the upper limit of the thickness of the fibrous nanocarbon layer formed on the surface of the carbon fiber is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less.

[0034] In the electrode material of the present invention, the amount of fibrous nanocarbon present per gram of the substrate made of carbon fiber is preferably 2 to 10,000 mg / g, more preferably 5 to 5,000 mg / g, and even more preferably 10 to 500 mg / g. If the amount of fibrous nanocarbon present per gram of the substrate made of carbon fiber is within the above-specified range, the fibrous nanocarbon can be formed in a thin layer on the surface of the carbon fiber.

[0035] In the electrode material of the present invention, the catalytic metal is supported on fibrous nanocarbon. Supporting the catalytic metal on fibrous nanocarbon covering carbon fiber allows for a significantly greater catalytic metal loading than supporting the catalytic metal directly on carbon fiber. Marimo Carbon is also known as a fuel cell catalyst support made of CNFs. Marimo Carbon is composed almost entirely of CNFs, and has a significantly higher CNF content than the carbon composite material used in the present invention, so it was expected that the catalytic metal loading would be higher. However, in reality, the carbon composite material used in the present invention can support a greater catalytic metal than Marimo Carbon. This is thought to be because Marimo Carbon's large diameter (over 10 μm) makes it difficult to impregnate the entire Marimo Carbon with the catalytic metal solution, preventing the catalytic metal loading from being increased. On the other hand, in the carbon composite material used in the present invention, a thin layer of fibrous nanocarbon can be formed on the surface of the carbon fiber, so the entire layer of fibrous nanocarbon can be sufficiently impregnated with the catalytic metal stock solution, and as a result, it is thought that a larger amount of catalytic metal can be supported than in carbon materials made of CNFs such as Marimo Carbon. Therefore, the electrode material of the present invention exhibits a high catalytic effect and excellent electrode performance.

[0036] In the electrode material of the present invention, the amount of the catalytic metal supported is preferably 0.3 to 10 mass % and more preferably 0.5 to 5 mass % based on the total mass of the material made of carbon fibers coated with fibrous nanocarbon (carbon composite material). When the carbon fibers are not entirely coated with fibrous nanocarbon, the catalytic metal can also be supported on the carbon fibers.

[0037] In the electrode material of the present invention, the average particle size of the catalytic metal is preferably 0.5 to 50 nm, more preferably 1 to 30 nm, and even more preferably 2 to 20 nm. The particle size of the catalytic metal can be measured using a transmission electron microscope (TEM). If the image of the catalytic metal is not a circle, the maximum value of the distance between two points is taken as the particle size of the catalytic metal. In this specification, the average value is calculated from the particle sizes of 50 or more catalytic metal particles.

[0038] The catalyst metal is appropriately selected depending on the type of fuel cell, and examples include platinum group metals such as platinum, palladium, rhodium, ruthenium, and osmium, with platinum and palladium being most commonly used.

[0039] In the electrode material of the present invention, the carbon fiber coated with fibrous nanocarbon carrying a catalytic metal is preferably further coated with a proton-conducting material. The proton-conducting material is a material capable of transferring protons, and materials that can be used in the electrolyte of a fuel cell are preferably used. For example, ionomers such as fluorine-based ionomers and hydrocarbon-based ionomers are preferred. The fluorine-based ionomer is an ionomer containing fluorine atoms in the polymer backbone, such as perfluoroalkyl sulfonic acid polymers, and DuPont's Nafion® is a preferred example. The hydrocarbon-based ionomer is a non-fluorine-based ionomer that does not contain fluorine atoms in the polymer backbone, such as ionomers in which sulfonic acid groups are introduced into aromatic polymers such as polystyrene and aromatic polyether ketone. Coating the carbon fiber coated with fibrous nanocarbon carrying a catalytic metal (catalyst-supported carbon fiber) with a proton-conducting material enables electrode reactions to occur in areas other than the contact interface with the electrolyte of the fuel cell, thereby improving the utilization rate of the supported catalytic metal. Furthermore, by coating the catalyst-supporting carbon fiber with a proton-conductive material, when the carbon fiber is directly pressure-bonded to an electrolyte membrane to form a membrane-electrode assembly, the material is expected to function as a binder that maintains the adhesion between the carbon fibers and prevents breakage of the carbon fibers, as well as a reinforcing material and a buffer material. Here, "carbon fiber coated with fibrous nanocarbon carrying a catalyst metal is covered with a proton-conductive material" means that the catalyst-supporting carbon fiber is entirely or partially coated with a proton-conductive material.

[0040] In order to leave spaces between the carbon fibers even after a material (catalyst-supported carbon composite material) made of carbon fibers coated with fibrous nanocarbon carrying a catalytic metal is coated with a proton-conductive material, it is preferable to thinly coat the catalyst-supported carbon fibers with an appropriate amount of proton-conductive material. In the electrode material of the present invention, the amount of the proton-conductive material is preferably 10 to 25 mass %, more preferably 15 to 20 mass %, of the total mass of the material made of carbon fibers coated with fibrous nanocarbon carrying a catalytic metal.

[0041] The electrode material of the present invention uses a material made of carbon fiber as a substrate, which allows for easy diffusion (supply) of raw material gases and easy diffusion (removal) of reaction products. This allows for dense growth of fibrous nanocarbon on the surface of the carbon fiber, and allows for highly dispersed catalytic metal to be supported on the fibrous nanocarbon. This structure of the electrode material of the present invention improves the utilization rate of the catalytic metal and prevents the supply of raw material gases from becoming the rate-limiting factor for the redox reaction. In addition, because of its high drainage efficiency, drainage is not rate-limiting on the output side where the reaction occurs vigorously, and no voltage drop occurs. The electrode material of the present invention contributes to increasing the output power of fuel cells.

[0042] Furthermore, the electrode material of the present invention can have a gas diffusion function (it can be used as a component in which the electrode catalyst layer and gas diffusion layer in a fuel cell are integrated), eliminating the need for a separate gas diffusion layer, which contributes to the miniaturization of fuel cells. In addition to eliminating the need for a gas diffusion layer, the electrode material of the present invention can be layered directly on the electrolyte membrane instead of in powder form, which also simplifies the MEA manufacturing process.

[0043] Furthermore, the electrode material of the present invention is highly crystalline because the structural units of the fibrous nanocarbon have a graphene-like structure. Unlike amorphous carbon materials such as activated carbon and carbon black, the structure does not change even with repeated use, contributing to a longer life of fuel cells. Furthermore, the surface of the fibrous nanocarbon contains countless graphene edges, which serve as support sites for the catalytic metal. This promotes the reduction in particle size and high dispersion of catalytic metal particles, thereby increasing the number of electrode reaction sites.

[0044] Next, a method for producing the electrode material of the present invention will be described.

[0045] One embodiment of the method for producing an electrode material of the present invention includes a step of coating carbon fibers that constitute a material made of carbon fibers with fibrous nanocarbon, and a step of supporting a catalytic metal on a material (carbon composite material) made of carbon fibers coated with fibrous nanocarbon, and preferably further includes a step of coating a material (catalyst-supported carbon composite material) made of carbon fibers coated with fibrous nanocarbon that supports a catalytic metal with a proton-conductive material.

[0046] The process of coating carbon fibers that constitute a carbon fiber material with fibrous nanocarbon is a process of forming a material (carbon composite material) made of carbon fibers coated with fibrous nanocarbon, and preferably involves vapor-phase synthesis of fibrous nanocarbon on the carbon fibers, more preferably chemical vapor-phase synthesis of fibrous nanocarbon using a catalytic reaction between a hydrocarbon gas and a transition metal catalyst. In a carbon fiber material, spaces are formed between the carbon fibers, and the gaps between the carbon fibers serve as passageways for solutions and gases. Therefore, by using a vapor-phase synthesis method, fibrous nanocarbon can be grown densely so as to completely cover each and every carbon fiber, and fibrous nanocarbon can be synthesized in the plane and thickness directions of the carbon fiber material.

[0047] In the chemical vapor synthesis of fibrous nanocarbons, a solution containing transition metal ions is preferably used to impregnate carbon fibers constituting the carbon fiber material with a transition metal catalyst in the form of fine particles. Repeated impregnation is preferred to ensure uniform support of the transition metal catalyst on the carbon fiber material. Examples of transition metals used include nickel (Ni), copper (Cu), palladium (Pd), zinc (Zn), cobalt (Co), and iron (Fe), with nickel being particularly preferred. In addition to using nickel alone as a catalyst, a catalyst containing nickel as the main component and, for example, approximately 20% copper by molar ratio can be used to obtain coin-stacked CNFs, in which graphene sheets are stacked to form a fibrous structure. Adding zinc to nickel results in smaller CNF diameters than those containing nickel alone, enabling the synthesis of cup-stacked CNFs. Adding cobalt to nickel can also be adjusted to reduce the graphene edge exposure per unit length on the CNF surface. In this way, by synthesizing CNFs using a multi-component catalyst containing nickel as the primary component and a second or third element, it is possible to control and arrange graphene edges on the surface of the fibrous structure. This structure is completely different from that of so-called carbon nanotubes. Carbon nanotubes, as their name suggests, have a hollow structure formed by rolling up graphene sheets, and no graphene edges are present on the surface. Bamboo-like structures also have a different microstructure from the CNFs of the present invention. Bamboo-like refers to fibrous carbon with a bamboo-like structure inside the fibrous structure. Compared to carbon nanotubes, exposed graphene edges are observed on the surface, but the number is significantly fewer than that of the CNFs of the present invention. Therefore, the CNFs of the present invention can be said to have a structure closer to that of a multi-walled carbon nanotube. Thus, the CNFs of the present invention have uniformly controlled microstructures, such as cup-layered and coin-layered structures, by using a nickel-based catalyst metal and by controlling conditions such as the type of reaction gas and synthesis temperature.Therefore, the CNFs of the present invention have graphene edges exposed on the surface that are uniformly and regularly arranged in the longitudinal direction of the fibrous structure, which is significantly different from tubular structures with almost no exposed graphene edges or bamboo structures with few exposed graphene edges and uneven graphene edge exposure. Examples of solvents for the solution containing transition metal ions include pure water, ethanol, and acetone. It is preferable to dry the carbon fiber after supporting the transition metal catalyst. Drying can be performed in air at a drying temperature of, for example, 300 to 400°C and for, for example, 30 to 90 minutes. Next, fibrous nanocarbon can be grown by contact reaction between the transition metal catalyst supported on the carbon fiber and a hydrocarbon gas, thereby forming a material (carbon composite material) made of carbon fiber coated with fibrous nanocarbon. Examples of hydrocarbon gases that can be used here include methane, ethane, and a mixture of methane and ethane. If necessary, the hydrocarbon gas can be appropriately mixed with a reaction aid gas or diluent gas, such as argon or hydrogen. The temperature during the catalytic reaction is, for example, 400 to 600°C, preferably 450 to 550°C, and the reaction time is, for example, 30 to 180 minutes. The catalytic reaction may be performed in a fixed bed or a fluidized bed. Furthermore, before the catalytic reaction, an annealing treatment may be performed on the material made of carbon fiber carrying a transition metal catalyst. The annealing treatment is preferably performed in an inert gas such as argon (Ar), and the treatment temperature is, for example, 350 to 450°C, and the treatment time is, for example, 30 to 90 minutes.

[0048] The process of supporting a catalytic metal on a carbon composite material involves forming a material (catalyst-supported carbon composite material) consisting of carbon fibers coated with fibrous nanocarbon supporting the catalytic metal. This allows the numerous graphene edges present on the surface of the fibrous nanocarbon that constitutes the carbon composite material to support the catalytic metal particles. The catalytic metal is preferably supported by treating the carbon composite material in a solution containing catalytic metal ions using an impregnation method or a nanocolloid method. To ensure uniform support of the catalytic metal on the carbon composite material, repeated impregnation is preferred. In the nanocolloid method, it is preferable to optimize the amount and concentration of the reducing agent, the addition method, and the stirring method during the reaction by examining the optimum conditions. Examples of solvents for the solution containing catalytic metal ions include solutions prepared by appropriately mixing pure water (ion-exchanged water) with ethanol, acetone, or the like. The resulting catalyst-supported carbon composite material may also be reduced in a hydrogen stream. It is preferable to carry out the treatment in an inert gas such as hydrogen or argon (Ar), the treatment temperature is, for example, 200 to 600° C. depending on the type of metal, and the treatment time is, for example, 30 to 60 minutes.

[0049] The process of coating a catalyst-supported carbon composite material with a proton-conductive material is a process of further coating carbon fibers coated with fibrous nanocarbon carrying a catalytic metal with a proton-conductive material. This allows electrode reactions to occur in areas other than the contact interface with the electrolyte that constitutes the fuel cell, improving the utilization rate of the supported catalytic metal. The carbon fibers that constitute the catalyst-supported carbon composite material can be coated with the proton-conductive material by dropping or immersing the proton-conductive material in the proton-conductive material. A solvent may be used for dropping or immersing the proton-conductive material. The amount of proton-conductive material is optimized depending on the microstructure of the fibrous nanocarbon material.

[0050] Another aspect of the present invention is a membrane electrode assembly for a fuel cell, which includes a pair of electrode catalyst layers and an electrolyte membrane disposed between the electrode catalyst layers. In this specification, this membrane electrode assembly for a fuel cell is also referred to as the "membrane electrode assembly of the present invention."

[0051] In the membrane electrode assembly of the present invention, one of the pair of electrode catalyst layers is an electrode catalyst layer that constitutes the anode of the fuel cell, and the other is an electrode catalyst layer that constitutes the cathode of the fuel cell. In the membrane electrode assembly of the present invention, at least one of the pair of electrode catalyst layers contains the above-mentioned electrode material of the present invention, and preferably both of the pair of electrode catalyst layers contain the above-mentioned electrode material of the present invention. Since the electrode material of the present invention can have a gas diffusion function, the membrane electrode assembly of the present invention does not necessarily require the provision of a gas diffusion layer on the electrode catalyst layer. In a preferred embodiment of the membrane electrode assembly of the present invention, the electrode catalyst layer containing the electrode material of the present invention is an electrode catalyst layer that has a gas diffusion function.

[0052] In the membrane electrode assembly of the present invention, the electrolyte membrane is preferably a proton-conducting polymer membrane. A proton-conducting material such as that described in the electrode material of the present invention can be used as the proton-conducting polymer membrane. For example, it is preferable to use an electrolyte membrane made of an ionomer such as a fluorine-based ionomer or a hydrocarbon-based ionomer. Specific examples of fluorine-based ionomers include perfluoroalkylsulfonic acid-based polymers, and Nafion (registered trademark) from DuPont is preferably used. Specific examples of hydrocarbon-based ionomers include ionomers in which sulfonic acid groups are introduced into aromatic polymers such as polystyrene and aromatic polyether ketone.

[0053] In the membrane electrode assembly of the present invention, the electrode material of the present invention can be directly attached to the electrolyte membrane, thereby allowing the electrode catalyst layer to be disposed on the electrolyte membrane, eliminating the need for slurrying the electrode catalyst and preparing and transferring a thin film, thereby simplifying the manufacturing process of the membrane electrode assembly (MEA). Furthermore, the electrode material of the present invention can have a gas diffusion function, making it unnecessary to provide a gas diffusion layer on the electrode catalyst layer, which also simplifies the manufacturing process of the MEA.

[0054] Another aspect of the present invention is a fuel cell comprising a pair of electrode catalyst layers and an electrolyte disposed between the electrode catalyst layers. In this specification, this fuel cell is also referred to as the "fuel cell of the present invention." The fuel cell of the present invention can be used as a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), an alkaline electrolyte fuel cell (AFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), a direct methanol fuel cell (DMFC), or other fuel cell, but is preferably a PEFC or a DMFC, and most preferably a PEFC.

[0055] In one embodiment of the fuel cell of the present invention, at least one of a pair of electrode catalyst layers contains the electrode material of the present invention. In another embodiment of the fuel cell of the present invention, the fuel cell comprises the membrane electrode assembly of the present invention.

[0056] In the fuel cell of the present invention, one of the pair of electrode catalyst layers constitutes the anode, and the other constitutes the cathode. At the anode, oxidation of hydrogen usually occurs. At the cathode, reduction of oxygen usually occurs to produce water. In the case of a DMFC, a similar reaction occurs at the cathode, but at the anode, methanol is oxidized by supplying methanol and water, producing carbon dioxide.

[0057] In an embodiment in which at least one of a pair of electrode catalyst layers in a fuel cell of the present invention comprises the electrode material of the present invention, both of the pair of electrode catalyst layers preferably comprise the electrode material of the present invention. The electrode catalyst layer comprising the electrode material of the present invention is preferably an electrode catalyst layer having a gas diffusion function. Furthermore, in this embodiment, the electrolyte is preferably composed of an electrolyte membrane, and the electrolyte membrane is preferably a proton-conducting polymer membrane. A proton-conducting material such as that described in the electrode material of the present invention can be used as the proton-conducting polymer membrane. For example, an electrolyte membrane made of an ionomer such as a fluorine-based ionomer or a hydrocarbon-based ionomer is preferably used. Specific examples of fluorine-based ionomers include perfluoroalkylsulfonic acid-based polymers, and DuPont's Nafion® is preferably used. Specific examples of hydrocarbon-based ionomers include ionomers in which sulfonic acid groups are introduced into aromatic polymers such as polystyrene and aromatic polyether ketone.

[0058] When the fuel cell of the present invention comprises the membrane electrode assembly of the present invention, the pair of electrode catalyst layers in the fuel cell are the pair of electrode catalyst layers constituting the membrane electrode assembly of the present invention. In this embodiment, the membrane electrode assembly of the present invention is one in which at least one of the pair of electrode catalyst layers contains the electrode material of the present invention, and preferably both of the pair of electrode catalyst layers contain the electrode material of the present invention, and here, the electrode catalyst layer containing the electrode material of the present invention is preferably an electrode catalyst layer having a gas diffusion function. Furthermore, when the fuel cell of the present invention comprises the membrane electrode assembly of the present invention, the electrolyte of the fuel cell is the electrolyte membrane constituting the membrane electrode assembly of the present invention.

[0059] The fuel cell of the present invention preferably includes a gasket when the electrolyte has a portion not covered by the electrode catalyst layer. The gasket can be disposed on the surface of the electrolyte not covered by the electrode catalyst layer. One embodiment of the fuel cell of the present invention further includes a pair of gaskets, wherein the pair of gaskets are disposed so as to cover the surface of the electrolyte not covered by the pair of electrode catalyst layers. The gaskets are preferably disposed along the outer periphery of the electrode catalyst layer. Various polymer films such as polyethylene terephthalate and polyamide can be used for the gaskets.

[0060] The fuel cell of the present invention may include a pair of separators disposed on the outer sides of the pair of electrode catalyst layers. When the fuel cell of the present invention includes a pair of gaskets, the pair of separators may be disposed on the outer sides of the pair of electrode catalyst layers and the pair of gaskets. An electrolyte, an anode, a cathode, and other components required for power generation are disposed between the pair of separators, and the separators can be used to separate the fuel cell units. The separators are preferably composed of conductive flat plates, and may be made of carbon-based materials or metal materials such as steel, stainless steel, titanium, and aluminum.

[0061] The fuel cell of the present invention may include a pair of current collecting members on the outer sides of the pair of electrode catalyst layers. The pair of current collecting members is preferably disposed on the outer sides of the pair of separators. The current collecting members are members for extracting electricity generated by the electrode reaction to the outside, and are preferably formed of conductive flat plates, and metal materials such as steel, stainless steel, titanium, and aluminum can be used.

[0062] The fuel cell of the present invention may include a pair of clamping members on the outer sides of the pair of electrode catalyst layers. The pair of clamping members are preferably disposed on the outer sides of the pair of current collecting members, and an insulating member is preferably disposed between the current collecting members and the clamping members. The clamping members are members for clamping components such as the electrolyte and electrodes between the clamping members, and are preferably formed of flat plates, and metal materials such as steel, stainless steel, titanium, and aluminum can be used.

[0063] The fuel cell of the present invention has cell components including an electrolyte, an electrode catalyst layer, and if necessary, gaskets, separators, current collectors, fastening members, etc., and multiple cell components can be integrated in parallel or series to obtain the desired voltage and current. A single cell component is sometimes called a fuel cell, a fuel cell composed of multiple cell components is sometimes called a fuel cell stack, and a fuel cell composed of multiple stacks is sometimes called a fuel cell module. [Example]

[0064] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0065] <Reference Example: Synthesis and Evaluation of Carbon Composite Materials> (experiment) CFP (Toray, TGP-H-060, 0.19 mm thick, single fiber diameter: approximately 6 μm) was pretreated by heating at 350 °C for 30 min in air. This CFP was then cut into 10 mm × 30 mm pieces to serve as substrates. The process for supporting the Ni catalyst on the substrate was as follows: First, the substrate was immersed in an impregnation solution of Ni(NO3)2·6H2O dissolved in ethanol for 30 min, removed from the solution, and dried in air at 350 °C for 60 min. This procedure was repeated twice, except that the substrate was placed face down in the second impregnation. This resulted in a substrate (Ni / CFP) supported with the Ni catalyst. The Ni / CFP was then introduced into a fixed-bed flow reactor and annealed at 400 °C for 60 min in an Ar gas atmosphere to decompose and remove nitrate ions contained in the impregnation solution and to form Ni catalyst particles. After annealing, the temperature was raised to the reaction temperature (500°C) in Ar, and as soon as it reached 500°C, Ar gas was switched to CH4 gas, and a catalytic reaction was carried out for 60 minutes, synthesizing a carbon composite material (CNFs / CFP) in which the carbon fibers constituting the CFP were coated with CNFs.

[0066] (Results and Discussion) Figure 1 shows an SEM image of CFP (top) and an SEM image of CNFs / CFP (bottom). The SEM image of CNFs / CFP shows that CNFs are uniformly deposited on the surface of the carbon fibers. In this experiment, the transition metal catalyst was evenly supported in the in-plane and thickness directions of the CFP by two impregnations, which is thought to have achieved an even coating of the carbon fibers that make up the CFP with CNFs. This experiment was repeated several dozen times, and carbon composite materials in which the carbon fibers were evenly coated with fibrous nanocarbon were synthesized with good reproducibility. Figure 2 shows a TEM image of the CNF, which shows that the CNF microstructure is composed of stacked cup-shaped graphene.

[0067] Example 1: Synthesis and evaluation of catalyst-supported carbon composite material (experiment) CFP (Toray, TGP-H-060, 0.19 mm thick, single fiber diameter: approximately 6 μm) was heat-treated in air at 350 °C for 30 min and then cut into 1 cm × 3 cm pieces for use as substrates. Ni catalyst loading was performed by impregnation using nickel nitrate hexahydrate as the catalyst precursor and ethanol as the solvent. The impregnated substrate was dried in air at 350 °C for 60 min to obtain Ni / CFP. CNFs were synthesized using a fixed-bed flow reactor. First, the Ni / CFP was introduced into the reactor and annealed at 400 °C for 60 min in Ar. The temperature was then raised to the synthesis temperature and maintained at this temperature for CNFs synthesis. CH4 was used as the reaction gas, and the synthesis time was 60 min. The synthesis temperature was set between 450 °C and 600 °C. Pd particles were loaded onto the resulting CNFs / CFP by impregnation using palladium acetate as the catalyst precursor and acetone as the solvent. The impregnated CNFs / CFP was air-dried and then heat-treated in Ar at 250°C for 30 min to obtain a Pd / CNFs / CFP catalyst-supported carbon composite. The morphology of the sample was examined using a scanning electron microscope (SEM), and the electrical resistance was measured using a four-probe method. Although not shown, it was confirmed by a transmission electron microscope (TEM) that the CNFs had a structure in which cup-shaped graphene was stacked, similar to the reference example.

[0068] (Results and Discussion) Figure 3 shows the relationship between the amount of fibrous nanocarbon precipitated (amount of carbon precipitated) and synthesis temperature. As can be seen from Figure 3, fibrous nanocarbon precipitated stably at temperatures between 450 and 550°C. Approximately 5 mg of fibrous nanocarbon precipitated on approximately 25 mg of CFP with a size of 1 cm x 3 cm, and the mass of CNFs / CFP was approximately 20% greater than the mass of CFP. When this material was used as an electrode, the amount of CNFs precipitated was approximately 100% for an electrode area of ​​1 cm. 2 The average daily intake was 1.7 mg per 1000 mg. Figure 4 shows an SEM image of CNFs / CFP. Figure 4(a) shows the surface of the CNFs / CFP, and Figure 4(b) shows a cross-section of the CNFs / CFP. As can be seen from Figure 4(a), the CNFs grew densely and were generated to uniformly cover the carbon fibers that make up the CFP. Furthermore, as can be seen from Figure 4(b), CNFs were also generated in the thickness direction of the CFP, allowing the entire CFP to be coated with CNFs. In this case, the thickness of the CNFs layer covering the carbon fibers was approximately 2 μm. Figure 5 shows the fiber diameter distributions of CNFs synthesized at 450°C and 550°C. Figure 5(a) shows the results for 450°C, and Figure 5(b) shows the results for 550°C. The fiber diameters of CNFs synthesized at 450°C were mostly distributed in the 15-30 nm range. On the other hand, the fiber diameters of CNFs synthesized at 550°C were mostly distributed in the 20-40 nm range. At 550°C, the fiber diameter distribution was more widespread toward larger diameters than at 450°C, and fiber diameters of 40 nm or greater, which were rarely observed at 450°C, were also present. It is known that the fiber diameter of CNFs is dependent on the size of the transition metal catalyst (NMRodriguez; J. Mater. Res., 8, 3233 (1993)). At higher temperatures, sintering of the transition metal catalyst is thought to occur, resulting in larger CNF diameters. Figure 6 shows the volume resistivity of CFP and CNFs / CFP measured in the plane direction. The volume resistivity of CFP was approximately 6.0 mΩ·cm to 7.0 mΩ·cm. On the other hand, the volume resistivity of CNFs / CFP was approximately 5.3 mΩ·cm to 6.0 mΩ·cm, indicating that the volume resistivity decreased by adding CNFs. The nominal volume resistivity of CFP was 5.8 mΩ·cm, suggesting that the value of CNFs was equal to or lower than that of CFP. Figure 7 shows SEM images of the catalyst-supported carbon composite Pd / CNFs / CFP (a) and Pd / CFP (b), in which palladium is supported on CFP. In the Pd / CNFs / CFP, approximately 0.9 mass% Pd was supported relative to the CNFs / CFP. Palladium particles with sizes ranging from 5 to 20 nm were observed on the surface of the CNFs. The Pd loading on CFP for Pd / CFP was performed in the same manner as for Pd / CNFs / CFP. Approximately 0.3 mass% Pd was supported relative to the CFP, with Pd particle sizes ranging from 100 to 200 nm, approximately 40 times larger than in the Pd / CNFs / CFP case. The CNFs / CFP supported more Pd than the CFP, and the Pd particle size was also reduced.

[0069] (summary) CNFs / CFP could be synthesized stably at temperatures between 450 and 550°C when Ni was used as the transition metal catalyst and CH4 as the reaction gas. This suggests that the fiber diameter of CNFs can be controlled by the synthesis temperature. Furthermore, CNFs / CFP had lower electrical resistance than CFP, and the supported Pd particles could be made smaller.

[0070] Example 2: Palladium loading on CFP and CNFs / CFP (experiment) A palladium solution was prepared by dissolving 100 mg of palladium acetate in 12 mL of acetone. 5 mL of the prepared palladium solution was added to a 3 cm inner diameter, 1.5 cm high Petri dish, and 1 cm square pieces of CFP and CNFs / CFP were immersed in the solution. The CFP was a Toray TGP-H-060 (0.19 mm thick), and the CNFs / CFP was the CNFs / CFP synthesized in Experimental Example 1. After 60 minutes, the CFP and CNFs / CFP were removed and placed on a quartz boat for 2 hours of air drying. After air drying, the samples were introduced into a fixed-bed flow reactor and annealed at 250 °C for 60 minutes in Ar. The weight of the palladium was determined from the weight change before and after impregnation and annealing. The morphology of the prepared samples was evaluated using a scanning electron microscope (SEM). Although not shown, it was confirmed by a transmission electron microscope (TEM) that the CNFs had a structure in which cup-shaped graphene was stacked, similar to the reference example.

[0071] (result) Tables 1 and 2 show the CFP and CNFs / CFP before and after impregnation, as well as the amount of palladium supported. In the case of CFP, the mass increased by 0.0232 mg after impregnation, and 0.281 mass% of palladium was supported relative to the substrate (CFP). In the case of CNFs / CFP, the mass increased by 0.0918 mg after impregnation, and 0.879 mass% of palladium was supported relative to the composite carbon material (CNFs / CFP). The mass of CNFs contained in the composite carbon material (CNFs / CFP) was 1.3928 mg, and the mass of palladium relative to CNFs was 6.183 mass%. CNFs / CFP supported 0.0686 mg more palladium than CFP.

[0072] [Table 1]

[0073] In Table 1, "Before impregnation" represents the mass of the CFP before it was impregnated with the palladium solution, "After impregnation" represents the mass of Pd / CFP after it was impregnated with the palladium solution and annealed, "Mass of palladium" represents the mass of palladium supported on the CFP, and "Supported amount" represents the amount of palladium supported (mass %) relative to the mass of the CFP.

[0074] [Table 2]

[0075] In Table 2, "before impregnation" represents the mass of CNFs / CFP before impregnation with the palladium solution, "after impregnation" represents the mass of Pd / CNFs / CFP after impregnation with the palladium solution and annealing treatment, "mass of palladium" represents the mass of palladium supported on CNFs / CFP, "amount supported relative to CNFs / CFP" represents the amount of palladium supported (mass%) relative to the mass of CNFs / CFP, "mass of CNFs" represents the mass of CNFs contained in CNFs / CFP, and "amount supported relative to CNFs" represents the amount of palladium supported (mass%) relative to the mass of CNFs.

[0076] Figure 8 shows an SEM image of Pd / CFP, and Figure 9 shows an SEM image of Pd / CNFs / CFP. As can be seen from Figure 8, Pd particles with a diameter of approximately 90 nm were supported on the CFP, covering the surface of the carbon fiber. Also, as can be seen from Figure 9, in the CNFs / CFP, Pd particles with a diameter of approximately 5 nm to 20 nm were present on the surface of the CNFs, and the Pd particles were smaller than those in the CFP.

[0077] Example 3: Comparison of reactivity of Pd / CNFs / CFP and Pd / CFP to hydrogen (experiment) The reactivity to hydrogen was investigated using Pd / CFP and Pd / CNFs / CFP. Unlike in Example 2, Pd loading was performed by sputtering. The same material as used in Example 2 was used for CFP. CNFs were synthesized on CFP under the same conditions as in Example 2 to obtain CNFs / CFP. Hydrogen gas was fed at 100 sccm (100 cc per minute) under atmospheric pressure into a measurement vessel filled with nitrogen gas, and temperature changes were measured by bringing K-type thermocouples close to each of the Pd / CFP and Pd / CNFs / CFP. The results are shown in Figure 10. (result) As can be seen from Figure 10, Pd / CNFs / CFP is more susceptible to reaction with hydrogen than Pd / CFP. In other words, the reaction starts earlier and at a faster rate with Pd / CNFs / CFP than with Pd / CFP. The reason for this is thought to be that the growth of CNFs on the CFP results in an increase in the number of active sites due to the miniaturization of Pd. Pd is supported by sputtering, so unlike impregnation methods, there is no Pd in ​​the thickness direction of the CNFs / CFP, and this is not a result of utilizing the entire thickness of the CNFs / CFP, but there is a significant change even just near the surface. The amount of Pd sputtered is about 40 nm in terms of film thickness (approximately 0.5 g / m2 by mass). 2 )

Claims

1. A fuel cell electrode material made of carbon fiber, the carbon fiber being coated with fibrous nanocarbon carrying a catalytic metal, The average fiber diameter of the fibrous nanocarbon is 10 nm to 40 nm, the fibrous nanocarbon has a fibrous structure formed by stacking graphene, and graphene edges are exposed on the entire surface of the fibrous structure; The electrode material for a fuel cell is characterized in that it has a gas diffusion function.

2. 2. The fuel cell electrode material according to claim 1, wherein the carbon fiber coated with the fibrous nanocarbon carrying the catalytic metal is further coated with a proton-conductive material.

3. 3. A membrane electrode assembly for a fuel cell, comprising a pair of electrode catalyst layers and an electrolyte membrane disposed between the electrode catalyst layers, wherein at least one of the pair of electrode catalyst layers comprises the electrode material according to claim 1 or 2.

4. 4. The membrane electrode assembly according to claim 3, wherein the electrolyte membrane is a proton-conducting polymer membrane.

5. A fuel cell comprising a pair of electrode catalyst layers and an electrolyte disposed between the electrode catalyst layers, wherein at least one of the pair of electrode catalyst layers contains the electrode material according to claim 1 or 2.

6. A fuel cell comprising the membrane electrode assembly according to claim 3 or 4.

7. 7. The fuel cell according to claim 5, wherein the electrode catalyst layer containing the electrode material has a gas diffusion function.

8. 3. The fuel cell electrode material according to claim 1, wherein the thickness of the fibrous nanocarbon layer formed on the surface of the carbon fiber is 0.1 μm or more.

9. 9. The fuel cell electrode material according to claim 1, wherein the thickness of the fibrous nanocarbon layer formed on the surface of the carbon fiber is 10 μm or less.

10. 10. The fuel cell electrode material according to any one of claims 1, 2, and 8 to 9, wherein the electrode material comprises a substrate containing carbon fiber, and the amount of the fibrous nanocarbon present per gram of the substrate containing the carbon fiber is 2 to 10,000 mg / g.

11. A fuel cell electrode material made of carbon fiber, the carbon fiber being coated with fibrous nanocarbon carrying a catalytic metal, The average fiber diameter of the fibrous nanocarbon is 10 nm to 40 nm, the fibrous nanocarbon has a fibrous structure formed by stacking graphene, and graphene edges are exposed on the entire surface of the fibrous structure; The electrode material is contained in at least one of a pair of electrode catalyst layers disposed between an electrolyte membrane, and is used as a member in which the electrode catalyst layer and a gas diffusion layer are integrated in a fuel cell. The fuel cell electrode material.

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