Nitrogen-containing porous carbon, method for producing same, and electrode catalyst for fuel cell

Abstract

This invention relates to nitrogen-containing porous carbon. In the nitrogen-containing porous carbon, a part of a carbon element in the skeleton of a carbon material is substituted with a nitrogen element. A most frequent pore diameter of the nitrogen-containing porous carbon in a pore diameter range of 2.0 nm-50.0 nm inclusive is 2.0 nm-30.0 nm inclusive. In the nitrogen-containing porous carbon, the ratio of the mass of the nitrogen element to the mass of the carbon element is 0.005 or more. The nitrogen-containing porous carbon has a BET specific surface area of 400 m2 / g or more as measured by a nitrogen adsorption method.

Description

Nitrogen-containing porous carbon, its manufacturing method, and fuel cell electrode catalyst

[0001] The present invention relates to a nitrogen-containing porous carbon, a method for producing the same, and an electrode catalyst for a fuel cell.

[0002] In recent years, growing interest in environmental issues has led to growing expectations for hydrogen as an energy source. In electrochemical cells such as polymer electrolyte fuel cells that use hydrogen as fuel, and electrochemical cells such as water electrolysis devices for producing hydrogen, catalyst-supported supports in which a noble metal catalyst, such as platinum, is supported on a support are used as electrode catalysts. From the viewpoint of improving the catalytic activity of these electrode catalysts, carbonaceous materials, which have a large specific surface area, are often used as catalyst supports.

[0003] For example, Patent Document 1 proposes using a porous carbon material as a catalyst support and supporting catalyst particles in the pores of the porous carbon material. The document states that supporting catalyst particles in the pores makes it difficult for the catalyst to come into contact with the ionomer, thereby suppressing catalyst poisoning by the ionomer.

[0004] Patent Document 2 describes nitrogen-containing porous carbon in which nitrogen atoms are introduced into the carbon skeleton of porous carbon having pores. The document states that this nitrogen-containing porous carbon can achieve both high catalytic activity and long life when used as a catalyst support contained in an electrode of a polymer electrolyte fuel cell, for example.

[0005] As another example of a material having nitrogen elements introduced therein, Patent Document 3 describes a carbon material in which at least a portion of the carbon elements in the carbon skeleton is substituted with nitrogen elements, and the document states that this carbon material has excellent electrical conductivity.

[0006] International Publication No. 2014 / 175099 Pamphlet Japanese Patent Application Laid-Open No. 2016-124769 Japanese Patent Application Laid-Open No. 2022-89016

[0007] In order to improve the performance of electrochemical cells, it is important to improve the performance of the electrode catalyst. However, when the carbon materials described in Patent Documents 1 and 2 are used as electrode catalysts, there is a problem that the amount of moisture present in the pores is insufficient under low-humidity conditions (e.g., 50% RH or less), resulting in a decrease in catalytic activity. Furthermore, when the carbon material described in Patent Document 3 is used as an electrode catalyst, there is a problem that catalyst poisoning is likely to occur due to contact between the catalyst and the ionomer. Therefore, an object of the present invention is to provide a nitrogen-containing porous carbon that, when used as an electrode catalyst, exhibits high catalytic activity under low-humidity conditions while suppressing catalyst poisoning.

[0008] The present invention relates to a nitrogen-containing porous carbon, wherein a portion of carbon elements in a skeleton of a carbon material is substituted with nitrogen elements, the mode pore size within a pore size range of 2.0 nm to 50.0 nm is 2.0 nm to 30.0 nm, the ratio of the mass of nitrogen elements to the mass of carbon elements is 0.005 or more, and the BET specific surface area measured by a nitrogen adsorption method is 400 m ２ The above-mentioned problems have been solved by providing a nitrogen-containing porous carbon having a molecular weight of 1 / 2 or more.

[0009] The present invention also provides a method for producing nitrogen-containing porous carbon, comprising the steps of: solid-phase mixing a nitrogen-containing compound with a carbon material having pores to obtain a mixture; and firing the mixture in an inert gas atmosphere.

[0010] The present invention will be described below based on preferred embodiments. The present invention relates to a nitrogen-containing porous carbon. The nitrogen-containing porous carbon of the present invention is configured to contain a carbon material. The carbon material occupies the majority of the nitrogen-containing porous carbon. The nitrogen-containing porous carbon of the present invention can be used as an electrode catalyst by supporting a metal thereon.

[0011] In the nitrogen-containing porous carbon of the present invention, the carbon material preferably contains carbon as a main component. "Containing carbon as a main component" means that the proportion of carbon in the carbon material is 80 mass % or more. When carbon constitutes a large proportion of the components contained in the carbon material, some of the carbon elements in the carbon skeleton are more likely to be substituted with nitrogen elements, improving electrical conductivity. As a result, catalytic activity can be increased. From this perspective, the carbon contained in the carbon material is preferably conductive carbon.

[0012] The nitrogen-containing porous carbon of the present invention has a large number of pores. The pores may or may not have an interconnected pore structure. When the nitrogen-containing porous carbon of the present invention is used as an electrode catalyst, the pores preferably have an interconnected pore structure from the viewpoint of enhancing catalytic activity. That is, the pores preferably open at the surface of the nitrogen-containing porous carbon, extend toward the interior of the nitrogen-containing porous carbon, and have the other end open at the surface of the nitrogen-containing porous carbon. The pores may be branched inside the nitrogen-containing porous carbon. The pores may also intersect with other pores inside the nitrogen-containing porous carbon. When the pores have an interconnected pore structure, for example, when a metal is supported in the pores of the nitrogen-containing porous carbon to form an electrode catalyst, a larger amount of reactant, such as oxygen, can be supplied to the metal located in the pores through the multiple openings. As a result, the catalytic activity of the electrode catalyst can be enhanced. The interconnected pore structure can be confirmed, for example, by observing the nitrogen-containing porous carbon with a transmission electron microscope.

[0013] The nitrogen-containing porous carbon of the present invention preferably has mainly mesopores. In this specification, "mesopores" refers to pores with a diameter of 2.0 nm or more and 50.0 nm or less. When the nitrogen-containing porous carbon has mesopores, when a metal is supported on the nitrogen-containing porous carbon, the metal is more likely to be successfully arranged in the mesopores. As a result, catalyst poisoning can be suppressed. The nitrogen-containing porous carbon may have macropores and / or micropores, as long as the effects of the present invention are not impaired. In this specification, "macropores" refers to pores with a diameter of more than 50.0 nm. "Micropores" refers to pores with a diameter of less than 2.0 nm.

[0014] The nitrogen-containing porous carbon of the present invention preferably has a mode pore size within a predetermined range. Specifically, the mode pore size of the nitrogen-containing porous carbon within the pore size range of 2.0 nm to 50.0 nm is preferably 2.0 nm or more, more preferably 3.0 nm or more, and even more preferably 3.5 nm or more. Furthermore, the mode pore size of the nitrogen-containing porous carbon within the pore size range of 2.0 nm to 50.0 nm is preferably 30.0 nm or less, more preferably 25.0 nm or less, even more preferably 15.0 nm or less, and particularly preferably 10.0 nm or less. When the mode pore size is within the above range, the specific surface area of ​​the nitrogen-containing porous carbon increases, and when used as an electrode catalyst, it becomes possible to support a large amount of catalytic metal particles. Furthermore, the catalytic metal supported in the pores is less likely to come into contact with the ionomer, thereby suppressing catalyst poisoning. The method for measuring the mode pore size will be described in the Examples below.

[0015] The nitrogen-containing porous carbon of the present invention preferably has a ratio of the pore volume in the pore diameter range of 2.0 nm to 30.0 nm to the total pore volume, obtained by analyzing nitrogen adsorption / desorption isotherms using the BJH method (hereinafter also referred to as the "pore volume ratio"), within a predetermined range. Specifically, when a metal is supported on the nitrogen-containing porous carbon, the pore volume ratio is preferably 0.60 or more, more preferably 0.62 or more, and even more preferably 0.65 or more, from the viewpoint of facilitating successful placement of the metal in the pores and suppressing catalyst poisoning. The upper limit of the pore volume ratio is not particularly limited. For example, the pore volume ratio may be 1.00 or less, 0.99 or less, 0.90 or less, or 0.80 or less.

[0016] The nitrogen-containing porous carbon of the present invention preferably has a total pore volume within a predetermined range, as determined by analyzing the nitrogen adsorption / desorption isotherm using the BJH method. Specifically, the total pore volume is preferably 1.00 mL / g or more per unit mass of the nitrogen-containing porous carbon, more preferably 1.30 mL / g or more, and even more preferably 1.40 mL / g or more. Furthermore, the total pore volume is preferably 4.00 mL / g or less per unit mass of the nitrogen-containing porous carbon, more preferably 3.00 mL / g or less, and even more preferably 2.00 mL / g or less. Having a total pore volume within the above range facilitates successful placement of a metal in the pores when the nitrogen-containing porous carbon supports the metal. As a result, catalyst poisoning can be suppressed. The method for measuring the total pore volume will be described in the Examples below.

[0017] The nitrogen-containing porous carbon of the present invention preferably has a pore volume within a predetermined range for pore diameters of 2.0 nm to 30.0 nm, as determined by analyzing nitrogen adsorption / desorption isotherms using the BJH method. Specifically, the pore volume is preferably 0.40 mL / g or more per unit mass of the nitrogen-containing porous carbon, more preferably 0.50 mL / g or more, even more preferably 0.60 mL / g or more, even more preferably 0.80 mL / g or more, and particularly preferably 0.90 mL / g or more. Furthermore, the pore volume is preferably 2.00 mL / g or less per unit mass of the nitrogen-containing porous carbon, more preferably 1.80 mL / g or less, and even more preferably 1.60 mL / g or less. Having the pore volume within the above range facilitates successful placement of a metal in the pores when the nitrogen-containing porous carbon is loaded with the metal. As a result, catalyst poisoning can be suppressed. The method for measuring the pore volume will be described in the Examples below.

[0018] The nitrogen-containing porous carbon of the present invention generally preferably has a basic skeleton of a hexagonal carbon network, in which carbon atoms are covalently bonded in a polygonal shape to form a network plane. That is, the nitrogen-containing porous carbon preferably has a graphite skeleton. The nitrogen-containing porous carbon of the present invention preferably has at least a portion of the carbon atoms in the skeleton of the carbon material substituted with nitrogen atoms, and the nitrogen atoms are incorporated into the graphite skeleton. In this case, the nitrogen-containing porous carbon may be any of (i) pyridine-type nitrogen atoms, (ii) pyrrole-type nitrogen atoms, (iii) quaternary-type nitrogen atoms (i.e., bonded to three carbon atoms), or (iv) nitrogen atoms bonded to a different element such as oxygen. In this specification, any of the above embodiments (i) to (iv) is encompassed by the term "part of the carbon atoms is substituted with nitrogen atoms." When the nitrogen atoms are present in the state (i), the nitrogen-containing porous carbon of the present invention has improved proton conductivity. When the nitrogen element is present in the state (ii), (iii) or (iv), the electron conductivity of the nitrogen-containing porous carbon of the present invention is improved by the action of the π electrons in the nitrogen element.

[0019] Depending on the bonding state and substitution position of the nitrogen element, the nitrogen element in the nitrogen-containing porous carbon is classified into N1-type nitrogen element, N2-type nitrogen element, N3-type nitrogen element, and N4-type nitrogen element. The N1-type nitrogen element is the nitrogen element of the aforementioned embodiment (i). The N2-type nitrogen element is the nitrogen element of the aforementioned embodiment (ii). The N3-type nitrogen element is the nitrogen element of the aforementioned embodiment (iii). The N4-type nitrogen element is the nitrogen element of the aforementioned embodiment (iv). In the nitrogen-containing porous carbon of the present invention, the above-mentioned effects are achieved regardless of the nitrogen element type. The classification of the nitrogen element can be confirmed, for example, by differences in peak positions in X-ray photoelectron spectroscopy (XPS). The binding energy ranges in which the spectra of each component appear are known to be 398.5±0.5 eV for N1-type nitrogen elements, 400±0.5 eV for N2-type nitrogen elements, 401±0.5 eV for N3-type nitrogen elements, and 404.5±0.5 eV for N4-type nitrogen elements.

[0020] From the viewpoint of improving proton conductivity and increasing catalytic activity, it is preferable that the ratio of the number of N1-type nitrogen elements to the number of all nitrogen elements is relatively high. The reason for this is that N1-type nitrogen elements exhibit basicity, and for example, when a sulfo group possessed by an ionomer binds to the N1-type nitrogen element, a proton path is easily formed. Furthermore, the inventors speculate that N1-type nitrogen elements exist as pyridiniums with protons adsorbed under acidic conditions, and protons can migrate through the pyridiniums, thereby enabling proton conductivity to be exhibited even without the ionomer penetrating the pores. To further enhance this advantage, the ratio is preferably 0.30 or greater, and more preferably 0.32 or greater. The upper limit of the ratio is ideally 1.00, but the above effect is sufficiently achieved when the ratio is 0.60 or less.

[0021] As described above, the nitrogen-containing porous carbon of the present invention preferably has a carbon element in the skeleton of the carbon material substituted with a nitrogen element. From the viewpoint of enhancing the aforementioned effect, it is preferable that the amount of substitution by the nitrogen element be within a predetermined range, and this substitution amount can be approximated by the mass of the nitrogen element in the nitrogen-containing porous carbon. Specifically, when the ratio of the mass of the nitrogen element to the mass of the carbon element is defined as N / C, from the viewpoint of enhancing catalytic activity, the value of N / C is preferably 0.005 or more, more preferably 0.010 or more, and even more preferably 0.013 or more. Furthermore, from the viewpoint of maintaining the carbon skeleton, the value of N / C is preferably 0.20 or less, more preferably 0.18 or less, and even more preferably 0.15 or less. The method for measuring the N / C value will be described in the Examples below.

[0022] In order to set the N / C value within the above range, it is preferable to adjust the baking temperature and baking time of the mixture in the production method described below, for example.

[0023] The nitrogen-containing porous carbon of the present invention preferably has a BET specific surface area measured by a nitrogen adsorption method within a predetermined range. Specifically, when the nitrogen-containing porous carbon is used as an electrode catalyst, the BET specific surface area of ​​the nitrogen-containing porous carbon is preferably 400 m, from the viewpoint of facilitating the placement of metal in the pores and suppressing catalyst poisoning. ２ / g or more, and ２ / g or more, and more preferably 700m ２ From the viewpoint of suppressing a decrease in durability of the nitrogen-containing porous carbon, it is more preferable that the BET specific surface area of ​​the nitrogen-containing porous carbon is 2000 m / g or more. ２ / g or less, and ２ / g or less, and more preferably 1600m ２ The method for measuring the BET specific surface area by the nitrogen adsorption method will be explained in the examples below.

[0024] The nitrogen-containing porous carbon of the present invention is preferably one in which the degree of hydrophilicity and hydrophobicity is controlled. The properties of the nitrogen-containing porous carbon can be evaluated by the ratio of the BET specific surface area measured by the water vapor adsorption method to the BET specific surface area measured by the nitrogen adsorption method. The larger the value of this ratio, the higher the degree of hydrophilicity is judged to be. The smaller the value of this ratio, the higher the degree of hydrophobicity is judged to be. The value of this ratio is calculated by BET (H ２ O / N ２ ), the nitrogen-containing porous carbon has a BET (H ２ O / N ２ ) is preferably 0.010 or more, more preferably 0.050 or more, and even more preferably 0.100 or more. ２ O / N ２ The value of BET specific surface area is preferably 0.500 or less, more preferably 0.400 or less, and even more preferably 0.300 or less. The method for measuring the BET specific surface area by the water vapor adsorption method will be explained in the examples below.

[0025] BET (H２ O / N ２ In order to set the value of (a) to be within the above range, it is preferable to control, for example, the amount of nitrogen element substitution or the state of the nitrogen element.

[0026] The nitrogen-containing porous carbon of the present invention can be used as an electrode catalyst for fuel cells by supporting catalytic metal particles on the nitrogen-containing porous carbon.

[0027] The fuel cell electrode catalyst preferably has a modal pore size within a predetermined range. Specifically, the modal pore size of the fuel cell electrode catalyst within the pore size range of 2.0 nm to 50.0 nm is preferably 2.0 nm or more, more preferably 3.0 nm or more, and even more preferably 3.5 nm or more. Furthermore, the modal pore size of the fuel cell electrode catalyst within the pore size range of 2.0 nm to 50.0 nm is preferably 30.0 nm or less, more preferably 25.0 nm or less, even more preferably 15.0 nm or less, and particularly preferably 10.0 nm or less. By having the modal pore size within the above range, it is possible to support a large amount of catalytic metal particles in the pores, thereby increasing catalytic activity. Furthermore, the catalytic metal supported in the pores is less likely to come into contact with the ionomer, thereby suppressing catalyst poisoning.

[0028] The pore volume ratio of the fuel cell electrode catalyst is preferably within a predetermined range. Specifically, from the viewpoint of making it difficult for the catalytic metal supported in the pores to come into contact with the ionomer and suppressing catalyst poisoning, the pore volume ratio is preferably 0.50 or more, more preferably 0.55 or more, and even more preferably 0.60 or more. The upper limit of the pore volume ratio of the fuel cell electrode catalyst is not particularly limited. For example, the pore volume ratio may be 1.00 or less, 0.99 or less, 0.90 or less, or 0.80 or less.

[0029] The fuel cell electrode catalyst preferably has a total pore volume within a predetermined range, as determined by analyzing the nitrogen adsorption / desorption isotherm using the BJH method. Specifically, the total pore volume is preferably 0.50 mL / g or more, more preferably 0.60 mL / g or more, and even more preferably 0.70 mL / g or more per unit mass of the fuel cell electrode catalyst. Furthermore, the total pore volume is preferably 3.00 mL / g or less, more preferably 2.00 mL / g or less, and even more preferably 1.00 mL / g or less per unit mass of the fuel cell electrode catalyst. Having a total pore volume within the above range makes it difficult for the catalytic metal supported in the pores to come into contact with the ionomer, thereby suppressing catalyst poisoning.

[0030] The fuel cell electrode catalyst preferably has a pore volume within a pore diameter range of 2.0 nm to 30.0 nm, as determined by analyzing a nitrogen adsorption / desorption isotherm using the BJH method, within a predetermined range. Specifically, the pore volume is preferably 0.20 mL / g or more, more preferably 0.30 mL / g or more, and even more preferably 0.40 mL / g or more per unit mass of the fuel cell electrode catalyst. Furthermore, the pore volume is preferably 1.50 mL / g or less, more preferably 1.00 mL / g or less, and even more preferably 0.80 mL / g or less per unit mass of the fuel cell electrode catalyst. Having the pore volume within the above range reduces the likelihood of the catalytic metal supported in the pores coming into contact with the ionomer, thereby suppressing catalyst poisoning.

[0031] The electrode catalyst for a fuel cell preferably has a BET specific surface area measured by a nitrogen adsorption method within a predetermined range. Specifically, from the viewpoint of increasing catalytic activity, the BET specific surface area of ​​the electrode catalyst for a fuel cell is preferably 150 m ２ / g or more, and ２ / g or more is more preferable, and 300m ２ From the viewpoint of suppressing a decrease in durability of the electrode catalyst for a fuel cell, it is more preferable that the BET specific surface area of ​​the electrode catalyst for a fuel cell is 1000 m ２ / g or less, and ２ / g or less is more preferable, and ２ It is more preferable that the SiO2 content is 1 / g or less.

[0032] The electrode catalyst for fuel cells is BET(H ２ O / N ２ ) is preferably within a predetermined range. Specifically, from the viewpoint of exhibiting proton conductivity under low humidification conditions, it is preferable that BET(H ２ O / N ２ ) is preferably 0.050 or more, more preferably 0.100 or more, and even more preferably 0.150 or more. ２ O / N ２ ) is preferably 0.500 or less, more preferably 0.400 or less, and even more preferably 0.300 or less.

[0033] Next, a preferred method for producing the nitrogen-containing porous carbon of the present invention will be described. First, a carbon material, which is one of the raw materials for the nitrogen-containing porous carbon, is prepared. The carbon material has pores. Examples of carbon materials that can be used include a group of carbonaceous materials called carbon black, such as activated carbon, graphite, hollow carbon, mesoporous carbon, and graphene. Examples of hollow carbon that can be used include Ketjen Black (registered trademark). From the viewpoint of supporting a large amount of catalytic metal in the pores and suppressing catalyst poisoning by ionomers, it is particularly preferable to use mesoporous carbon as the carbon material. In this specification, mesoporous carbon refers to a carbon material that has mesopores and little variation in pore size. Specifically, it refers to a carbon material in which the ratio of the pore volume in the pore diameter range of 2.0 nm to 30.0 nm to the total pore volume is 0.60 or more, as obtained by analyzing nitrogen adsorption / desorption isotherms using the BJH method.

[0034] It is preferable to use a carbon material whose physical property values ​​fall within a predetermined range, because this allows for the successful production of nitrogen-containing porous carbon having the physical property values ​​of the product to be produced, such as the most frequent pore size, pore volume, and BET specific surface area, within the above-mentioned ranges. Specifically, the most frequent pore size of the carbon material within the pore size range of 2.0 nm to 50.0 nm is preferably 2.0 nm or more, more preferably 3.0 nm or more, and even more preferably 3.5 nm or more. Furthermore, the most frequent pore size of the carbon material within the pore size range of 2.0 nm to 50.0 nm is preferably 30.0 nm or less, more preferably 25.0 nm or less, even more preferably 15.0 nm or less, and particularly preferably 10.0 nm or less.

[0035] The pore volume ratio of the carbon material is preferably 0.60 or more, more preferably 0.62 or more, from the viewpoint of successfully obtaining nitrogen-containing porous carbon having physical property values, such as the most common pore size, pore volume, and BET specific surface area, within the above-mentioned ranges. The upper limit of the pore volume ratio is not particularly limited. For example, the pore volume ratio may be 1.00 or less, 0.99 or less, 0.90 or less, or 0.80 or less.

[0036] From the viewpoint of successfully obtaining nitrogen-containing porous carbon having physical property values, such as the modal pore size, pore volume, and BET specific surface area, within the above-mentioned ranges, the total pore volume of the carbon material is preferably 1.20 mL / g or more, more preferably 1.60 mL / g or more, and even more preferably 1.80 mL / g or more per unit mass of the carbon material. Furthermore, the total pore volume is preferably 4.00 mL / g or less, more preferably 3.00 mL / g or less, and even more preferably 2.20 mL / g or less per unit mass of the carbon material.

[0037] The pore volume of the carbon material having a pore diameter in the range of 2.0 nm to 30.0 nm is preferably 0.40 mL / g or more, more preferably 1.00 mL / g or more, and even more preferably 1.30 mL / g or more per unit mass of the carbon material, from the viewpoint of successfully obtaining nitrogen-containing porous carbon having physical property values ​​such as the modal pore diameter, pore volume, and BET specific surface area in the above-mentioned ranges. Furthermore, the pore volume is preferably 2.00 mL / g or less, more preferably 1.80 mL / g or less, and even more preferably 1.60 mL / g or less per unit mass of the carbon material.

[0038] A nitrogen-containing compound, which is one of the raw materials for the nitrogen-containing porous carbon, is prepared together with the carbon material. The nitrogen-containing compound may contain at least a nitrogen element, and various compounds may be used without limitation. As such a nitrogen-containing compound, for example, an organic compound having at least one amino group is preferably used, and an organic compound having multiple amino groups is more preferably used. It is also preferable that this organic compound be solid at room temperature (25°C). Examples of such organic compounds include melamine, urea, pyrazinamide, phthalocyanine, naphthalocyanine, porphyrin, and tetraazaannulene. These nitrogen-containing compounds can be used alone or in combination of two or more. From the standpoint of cost and safety, the nitrogen-containing compound is preferably at least one selected from melamine, urea, and pyrazinamide, and melamine is particularly preferred.

[0039] Once the above raw materials are prepared, the nitrogen-containing compound and the porous carbon material are mixed to obtain a mixture. Mixing without using a liquid, i.e., solid-phase mixing, is preferred. Solid-phase mixing is preferred from the perspective of simplicity, as it eliminates the need to remove the solvent before the firing step described below. Furthermore, it is preferred from the perspective of safety, as it eliminates the need to use liquids or gases (e.g., ammonia) that are highly hazardous during industrial production. Solid-phase mixing can be performed using known mixing devices, such as roll mills such as two-roll and three-roll mills, high-speed agitators such as Henschel mixers and super mixers, fluid energy mills such as micronizers and jet mills, attritors, particle composite devices such as Nanocura (registered trademark), Nobilta (registered trademark), and Mechanofusion (registered trademark) manufactured by Hosokawa Micron Corporation, and powder surface modification devices such as Hybridization System (registered trademark), Mechano Micros, and Miraro manufactured by Nara Machinery Manufacturing Co., Ltd. The operating time and rotation speed of the mixing device can be adjusted appropriately depending on the raw materials used.

[0040] The content of the carbon material contained in the mixture is preferably 1.0 mass% or more, more preferably 2.0 mass% or more, even more preferably 4.0 mass% or more, and particularly preferably 15.0 mass% or more. The content of the carbon material contained in the mixture is preferably 99.0 mass% or less, even more preferably 98.0 mass% or less, and even more preferably 96.0 mass% or less. By having the carbon material content within the above range, carbon elements in the skeleton of the carbon material can be successfully substituted with nitrogen elements in the process described below.

[0041] The content of the nitrogen-containing compound contained in the mixture is preferably 1.0 mass% or more, more preferably 2.0 mass% or more, and even more preferably 4.0 mass% or more. The content of the nitrogen-containing compound contained in the mixture is preferably 99.0 mass% or less, more preferably 98.0 mass% or less, even more preferably 96.0 mass% or less, and particularly preferably 85.0 mass% or less. When the content of the nitrogen-containing compound is within the above range, carbon elements in the skeleton of the carbon material can be successfully substituted with nitrogen elements in the process described below.

[0042] The resulting mixture is then calcined. This causes the nitrogen element in the nitrogen-containing compound to be released and replaced with a portion of the carbon element in the skeleton of the carbon material. To ensure successful carbon element substitution, the calcination is preferably carried out in an atmosphere that is less reactive with the carbon material and the nitrogen-containing compound. Examples of such an atmosphere include an inert gas atmosphere such as nitrogen gas and argon gas.

[0043] From the viewpoint of successfully substituting carbon elements in the skeleton of the carbon material with nitrogen elements, the temperature during firing is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher. From the same viewpoint, the temperature during firing is preferably 1500°C or lower, more preferably 1200°C or lower, and even more preferably 1000°C or lower. From the viewpoint of successfully substituting carbon elements in the skeleton of the carbon material with nitrogen elements, the firing time is preferably 0.5 hours or higher, more preferably 1 hour or higher, and even more preferably 3 hours or higher. From the same viewpoint, the firing time is preferably 96 hours or lower, more preferably 72 hours or lower, and even more preferably 48 hours or lower.

[0044] In this way, the nitrogen-containing compound present in the pores or on the surface of the carbon material reacts with the carbon material, substituting the carbon element for the nitrogen element. Furthermore, ammonia produced by the reaction (e.g., condensation) between nitrogen-containing compounds reacts with the carbon material, substituting the carbon element for the nitrogen element. As a result, even without preparing dangerous ammonia gas as a raw material, by effectively utilizing the ammonia gas generated during the reaction, the carbon element for the nitrogen element can be sufficiently substituted within the pores of the carbon material. In contrast, conventionally, a mixture of a carbon material without pores (i.e., a solid carbon material) and a nitrogen-containing compound has typically been fired. However, in conventional methods, the carbon element inside the carbon material is not substituted, and only the carbon element on the surface is substituted with the nitrogen element, making it difficult to achieve sufficient substitution. The present inventors were the first to discover that the problem can be solved by solid-phase mixing of a porous carbon material with a nitrogen-containing compound and firing the mixture, as in the manufacturing method of the present invention.

[0045] The obtained nitrogen-containing porous carbon can be used by supporting a metal. There are no particular limitations on the type of metal, and various metals having catalytic activity for various electrode reactions can be used. For the nitrogen-containing porous carbon of the present invention, it is preferable to use a metal having catalytic activity, particularly for electrode reactions in fuel cells. Examples of such metals include platinum, ruthenium, iridium, palladium, rhodium, osmium, nickel, cobalt, gold, silver, and alloys containing these metals. These metals can be used alone or in combination of two or more.

[0046] The metal preferably contains at least a platinum group element. When the metal supported on the nitrogen-containing porous carbon contains a platinum group element, an electrode catalyst with high catalytic activity and electrochemical stability can be obtained. In particular, when the nitrogen-containing porous carbon of the present invention is used, for example, as an electrode for a fuel cell, high oxygen reduction activity can be obtained. For this reason, the metal preferably contains at least a platinum group element or an alloy containing such a metal, and preferably consists of a platinum group element or an alloy containing such a metal. In this specification, "platinum group element" refers to platinum, ruthenium, iridium, palladium, rhodium, and osmium.

[0047] The metal preferably contains at least platinum. Platinum is known to have high catalytic activity, particularly for oxygen reduction. Therefore, when the nitrogen-containing porous carbon of the present invention is used as an electrode for a fuel cell having a relatively low operating temperature, such as a polymer electrolyte fuel cell, the metal preferably contains platinum, thereby further increasing the catalytic activity. For this reason, the metal preferably contains at least platinum or an alloy containing platinum, and more preferably consists of platinum or an alloy containing platinum.

[0048] The nitrogen-containing porous carbon of the present invention can be used to form an electrode catalyst layer in a fuel cell. The electrode catalyst layer may contain, in addition to the nitrogen-containing porous carbon, materials similar to those known in the art, such as metals, binders for binding electrode catalysts together, and ionomers, as needed.

[0049] The electrode catalyst layer is preferably formed, for example, by the following method. First, a dispersion containing nitrogen-containing porous carbon is prepared. To prepare the dispersion, the nitrogen-containing porous carbon is mixed with a dispersion medium. During mixing, an ionomer may also be added as needed.

[0050] Examples of the dispersion medium that can be used include organic solvents such as alcohols, aromatic hydrocarbons such as toluene and benzene, aliphatic hydrocarbons such as hexane, ketones, esters, and ethers, and water.

[0051] Once the dispersion is obtained, an electrode catalyst layer is formed from this dispersion. The electrode catalyst layer is formed, for example, by applying the dispersion to an object to be coated using various coating devices to form a coating film, and then drying the coating film. The object to be coated can be, for example, a film of a fluorine-based resin such as polytetrafluoroethylene. The electrode catalyst layer formed on the object to be coated is then superimposed on, for example, a solid electrolyte membrane and heat-pressed to transfer the electrode catalyst layer to the surface of the solid electrolyte membrane. This transfer results in a catalyst-coated membrane (CCM) in which the electrode catalyst layer is disposed on one surface of the solid electrolyte membrane.

[0052] The electrode catalyst layer formed in this manner is suitable for use as an electrode catalyst layer for the anode and / or cathode of a polymer electrolyte fuel cell. The anode and cathode preferably include an electrode catalyst layer and a gas diffusion layer. The gas diffusion layer functions as a supporting current collector with a current collecting function. Furthermore, the gas diffusion layer has the function of sufficiently supplying gas to the electrode catalyst layer. For example, porous materials such as carbon paper and carbon cloth can be used as the gas diffusion layer. Specifically, the gas diffusion layer can be formed from carbon cloth woven from yarns containing a predetermined ratio of carbon fibers whose surfaces are coated with polytetrafluoroethylene and carbon fibers that are not coated.

[0053] Examples of solid electrolyte membranes include perfluorosulfonic acid polymer-based proton conductor membranes, membranes in which inorganic acids such as phosphoric acid are doped into hydrocarbon polymer compounds, organic / inorganic hybrid polymers in which a portion of the polymer is substituted with a functional group of a proton conductor, and proton conductors in which a polymer matrix is ​​impregnated with a phosphoric acid solution or a sulfuric acid solution.

[0054] A membrane electrode assembly (MEA) consisting of an electrode catalyst layer, a solid electrolyte membrane, and a gas diffusion layer has separators disposed on each side thereof to form a polymer electrolyte fuel cell. The separator may have, for example, a plurality of unidirectional protrusions (ribs) formed at predetermined intervals on the surface facing the gas diffusion layer. A groove having a rectangular cross section is formed between adjacent protrusions. This groove serves as a flow path for supplying and discharging fuel gas and oxidant gas such as air. The fuel gas and oxidant gas are supplied from a fuel gas supply means and an oxidant gas supply means, respectively. The separators disposed on each side of the membrane electrode assembly may be arranged so that their grooves intersect or are parallel to each other. The above configuration constitutes the minimum unit of a fuel cell, and a fuel cell can be constructed from a cell stack consisting of tens to hundreds of such configurations arranged side by side.

[0055] Although the present invention has been described above based on the preferred embodiments, the present invention is not limited to the above embodiments.

[0056] In relation to the above-described embodiment, the following nitrogen-containing porous carbon, its manufacturing method, and fuel cell electrode catalyst are further disclosed: [1] Nitrogen-containing porous carbon, wherein the nitrogen-containing porous carbon has a structure in which a portion of carbon elements in the skeleton of a carbon material is substituted with nitrogen elements, a mode pore size in the pore size range of 2.0 nm to 50.0 nm is 2.0 nm to 30.0 nm, a ratio of the mass of nitrogen elements to the mass of carbon elements is 0.005 or more, and a BET specific surface area measured by a nitrogen adsorption method is 400 m ２ / g or more.

[0057] [2] The nitrogen-containing porous carbon according to [1], wherein the ratio of the pore volume having a pore diameter in the range of 2.0 nm to 30.0 nm to the total pore volume, obtained by analyzing a nitrogen adsorption / desorption isotherm by the BJH method, is 0.60 or more. [3] The nitrogen-containing porous carbon according to [1] or [2], wherein the mode pore diameter is 2.0 nm to 10.0 nm. [4] The nitrogen-containing porous carbon according to any one of [1] to [3], wherein the ratio of the BET specific surface area measured by a water vapor adsorption method to the BET specific surface area measured by a nitrogen adsorption method is 0.010 to 0.500. [5] A method for producing nitrogen-containing porous carbon, comprising the steps of: solid-phase mixing a nitrogen-containing compound with a carbon material having pores to obtain a mixture; and calcining the mixture in an inert gas atmosphere.

[0058] [6] The method according to [5], wherein the nitrogen-containing compound is at least one selected from the group consisting of melamine, urea, and pyrazinamide. [7] An electrode catalyst for a fuel cell, comprising the nitrogen-containing porous carbon according to any one of [1] to [4], and a metal containing a platinum group element supported on the nitrogen-containing porous carbon.

[0059] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

[0060] Example 1 (1) Preparation of Mixture A carbon material having pores (mesoporous carbon) was prepared as the carbon material. The most frequent pore size and pore volume of the mesoporous carbon are shown in Table 1. Next, melamine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was prepared as the nitrogen-containing compound. 2.0 g of mesoporous carbon and 10.0 g of melamine were solid-phase mixed to obtain a mixture. The solid-phase mixing was performed using a mixer at 2000 rpm for 1 minute.

[0061] (2) Calcination of the mixture The mixture was calcined in a tubular furnace under a nitrogen gas atmosphere. This resulted in the substitution of a portion of the carbon elements in the skeleton of the carbon material with nitrogen elements, thereby obtaining the desired nitrogen-containing porous carbon. The calcination temperature and calcination time were as shown in Table 1.

[0062] Examples 2 and 3 In Example 1, a carbon material (mesoporous carbon) having the physical properties shown in Table 1 was used. Except for this, the target nitrogen-containing porous carbon was obtained in the same manner as in Example 1.

[0063] Examples 4 to 6 In Example 1, the firing temperature or firing time was changed to the value shown in Table 1. Other than this, the same procedure as in Example 1 was carried out to obtain the target nitrogen-containing porous carbon.

[0064] Example 7 The target nitrogen-containing porous carbon was obtained in the same manner as in Example 1, except that the amount of melamine was changed to 0.1 g.

[0065] Examples 8 and 9 In Example 1, hollow carbon black, Ketjenblack (registered trademark), was used instead of the mesoporous carbon. The most frequent pore size and pore volume of Ketjenblack are shown in Table 1. Except for this, the target nitrogen-containing porous carbon was obtained in the same manner as in Example 1.

[0066] Comparative Example 1 The intended nitrogen-containing porous carbon was obtained in the same manner as in Example 1, except that solid carbon having no pores was used instead of the mesoporous carbon in Example 1.

[0067] Comparative Examples 2 to 4 The carbon materials prepared in Examples 2, 3 and 9 were used as they were, that is, no nitrogen-containing compound was used.

[0068] [Evaluation] The modal pore size and pore volume of the carbon materials used in the Examples and Comparative Examples were measured by the following methods. Because the carbon material used in Comparative Example 1 was solid carbon without pores, the modal pore size and pore volume were not measured. The modal pore size and pore volume of the nitrogen-containing porous carbon obtained in the Examples and Comparative Examples were also measured by the following methods. The modal pore size and pore volume of the nitrogen-containing porous carbon obtained in Comparative Example 1 were not measured. The BET specific surface area, mass of carbon (C) element, mass of nitrogen (N) element, and ratio of the number of N1-type nitrogen elements to the total number of nitrogen elements were also measured by the following methods for the nitrogen-containing porous carbon obtained in the Examples and Comparative Examples. Because no nitrogen-containing compound was used in Comparative Examples 2 to 4, the mass of C element, mass of N element, and the ratio were not measured. The power generation performance of the nitrogen-containing porous carbon obtained in the Examples and Comparative Examples was also measured by the following method. Here, "power generation performance" refers to the power generation performance of a fuel cell equipped with an electrode catalyst in which a catalyst is supported on nitrogen-containing porous carbon. Furthermore, the mode pore size, pore volume, and BET specific surface area of ​​the electrode catalysts obtained in the examples and comparative examples were measured in the same manner as for the nitrogen-containing porous carbon. These results are shown in Tables 1 and 2.

[0069] [Modular pore diameter] The modal pore diameter was measured by analyzing the nitrogen adsorption / desorption isotherm. A gas adsorption measurement apparatus (3Flex manufactured by Micromeritics) was used for the nitrogen adsorption / desorption measurement. As a pretreatment, the nitrogen-containing porous carbon was heated at 400°C for 3 hours under reduced pressure of 20 Pa. The obtained adsorption isotherm was analyzed using the BJH method, and the modal pore diameter of the nitrogen-containing porous carbon in the range of 2.0 nm to 50.0 nm was calculated. 3Flex version 5.02, which was included with the gas adsorption measurement apparatus, was used as the analysis software.

[0070] [Pore Volume] The pore volume was measured by analyzing the nitrogen adsorption / desorption isotherm. A gas adsorption measurement device (3Flex manufactured by Micromeritics) was used for the nitrogen adsorption / desorption measurement. As a pretreatment, the nitrogen-containing porous carbon was heated at 400°C for 3 hours under reduced pressure of 20 Pa. The total pore volume and the pore volume in the pore diameter range of 2.0 nm to 30.0 nm were calculated using the BJH method. 3Flex version 5.02, which came with the gas adsorption measurement device, was used as the analysis software.

[0071] [BET Specific Surface Area] Measurement of the BET specific surface area by the nitrogen adsorption method was carried out according to the following procedure. As a pretreatment, the nitrogen-containing porous carbon was heated at 400°C for 3 hours under conditions reduced to 20 Pa. A gas adsorption measurement apparatus (3Flex, manufactured by Micromeritics) was used for nitrogen adsorption and desorption measurement, and the specific surface area was calculated by the BET (Brunauer-Emmett-Teller) method from the adsorption isotherm at liquid nitrogen temperature (77 K). Measurement of the BET specific surface area by the water vapor adsorption method was carried out according to the following procedure. As a pretreatment, the nitrogen-containing porous carbon was heated at 100°C for 4 hours under conditions reduced to 20 Pa. Thereafter, a gas adsorption measurement apparatus (3Flex, manufactured by Micromeritics) was used for water vapor adsorption and desorption measurement, and the specific surface area was calculated by the BET method from the adsorption isotherm.

[0072] [Mass of C element and N element] The mass of C element and the mass of N element in the nitrogen-containing porous carbon were measured by organic elemental analysis. Quantitative analysis of C and N elements was performed using a Microcoder JM10 (manufactured by J Science Lab Co., Ltd.).

[0073] [Ratio of the number of N1-type nitrogen elements to the number of all nitrogen elements] Photoelectron spectra of carbon, nitrogen, and oxygen elements on the nitrogen-containing porous carbon surface were measured using an X-ray photoelectron spectrometer (PHI Quantes, manufactured by ULVAC-PHI, Inc.). AlKα radiation was used as the X-ray source, and measurements were performed according to the following procedure. XPS narrow scan analysis was performed, and the obtained N1s spectrum was subjected to waveform separation to determine the proportions of N1-type nitrogen elements, N2-type nitrogen elements, N3-type nitrogen elements, and N4-type nitrogen elements. Then, the ratio of N1-type nitrogen elements to the number of all nitrogen elements on the nitrogen-containing porous carbon surface was calculated. The spectrum of each component was identified based on the binding energy range described above.

[0074] [Power Generation Performance] The power generation performance of fuel cells equipped with electrode catalysts in which a catalyst was supported on the nitrogen-containing porous carbon obtained in the Examples and Comparative Examples was measured as follows. First, 1.00 g of the nitrogen-containing porous carbon obtained in the Examples and Comparative Examples was placed in a glass reaction tube. 4.00 g of platinum(II) nitrate solution (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), 15.8 g of ultrapure water, and 20.0 g of ethanol were added, and the mixture was heated to reflux at 85°C and 500 rpm for 16 hours. The resulting reaction product was washed with water and dried to obtain an electrode catalyst. Next, 0.80 g of the electrode catalyst and 1 mm diameter yttrium-stabilized zirconia balls were placed in a container, and ultrapure water and 2-propanol were added as solvents in a 1:3 mass ratio, in that order. The resulting mixture was stirred at 800 rpm for 20 minutes using a planetary centrifugal mixer (ARE310, manufactured by Thinky Corporation). Furthermore, 25% Aquivion (D72-25BS, manufactured by Sigma-Aldrich) was added as an ionomer to the mixed solution, and the mixture was stirred under the same conditions as above. The value of I / C, which is the ratio of the mass I of the ionomer to the mass C of the carrier contained in the electrode catalyst, was set to 0.7. The dispersion thus obtained was applied to a polytetrafluoroethylene sheet using a bar coater, and the coating was dried at 60°C to obtain an electrode catalyst layer. The amount of catalyst per unit area of ​​the electrode catalyst layer was 0.2 mg / cm. ２The resulting electrode catalyst layer was used as a cathode catalyst layer. The anode catalyst layer was obtained by the following method. 0.64 g of platinum-supported carbon black (TEC10E50E) manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. and 10 mm diameter yttrium-stabilized zirconia balls were placed in a container, and ultrapure water and 2-propanol were added as solvents in a 1:1 mass ratio, yielding 9.4 g of a mixed solution. The resulting mixed solution was stirred at 800 rpm for 20 minutes using a planetary centrifugal mixer (ARE310 manufactured by Thinky Corporation). Furthermore, 25% Aquivion (D72-25BS manufactured by Sigma-Aldrich) was added as an ionomer to the mixed solution, and the mixture was stirred under the same conditions as above. The ratio of the mass I of the ionomer to the mass C of the support contained in the electrode catalyst, I / C, was set to 0.7. The dispersion thus obtained was applied to a polytetrafluoroethylene sheet using a bar coater, and the coating was dried at 60° C. The catalyst amount per unit area of ​​the anode catalyst layer was 0.05 mg / cm ２ The anode catalyst layer and cathode catalyst layer formed on the polytetrafluoroethylene sheet were each cut into a 54 mm square. These were then stacked on top of a Nafion (registered trademark) (NR-211, manufactured by Chemours) electrolyte membrane and subjected to a pressure of 150°C and 25 kgf / cm. ２The membrane was then heat-pressed in air for 2 minutes under these conditions, and the transfer was performed. In this way, a cathode catalyst layer and an anode catalyst layer were formed on each side of the Nafion solid electrolyte membrane, yielding a CCM. The CCM was sandwiched between a pair of gas diffusion layers (manufactured by SGL Carbon Co., Ltd., model number: 22BB). This was then sandwiched between a pair of carbon separators with gas flow channels formed therein, producing a polymer electrolyte fuel cell. The resulting fuel cell corresponds to a JARI standard cell. Hydrogen gas was supplied to the anode side of the resulting fuel cell, and air was supplied to the cathode side. The flow rates were set to achieve a hydrogen gas utilization rate of 70% and an air utilization rate of 40%. The gases were humidified using external humidifiers before being supplied to the fuel cell. The temperature of the fuel cell was also adjusted to 80°C. The humidity of the supplied gas was adjusted to a relative humidity of 88% RH on the anode side and 42% RH on the cathode side. The relationship between cell voltage and current density was evaluated, and a current density of 1.0 A / cm was obtained. ２ The voltage (V) was calculated when the voltage was 100 V. The higher the obtained value, the higher the catalytic activity.

[0075]

[0076]

[0077] As is clear from the results shown in Table 1, the electrode catalysts using the nitrogen-containing porous carbon materials of Examples 1 to 9 exhibited higher power generation performance values ​​than the electrode catalysts using the nitrogen-containing porous carbon materials or carbon materials of Comparative Examples 1 to 4. This shows that the nitrogen-containing porous carbon obtained in each Example enhances catalytic activity while suppressing catalyst poisoning.

[0078] According to the present invention, it is possible to provide a nitrogen-containing porous carbon that, when used as an electrode catalyst, exhibits high catalytic activity under low-humidity conditions (e.g., 50% RH or less) while suppressing catalyst poisoning. Furthermore, according to the present invention, it is possible to easily produce a nitrogen-containing porous carbon that, when used as an electrode catalyst, exhibits high catalytic activity under low-humidity conditions while suppressing catalyst poisoning.

Claims

1. Nitrogen-containing porous carbon, wherein a portion of the carbon elements in the skeleton of a carbon material is substituted with nitrogen elements, the mode pore size within a pore size range of 2.0 nm to 50.0 nm is 2.0 nm to 30.0 nm, the ratio of the mass of nitrogen elements to the mass of carbon elements is 0.005 or more, and the BET specific surface area measured by a nitrogen adsorption method is 400 m ２ / g or more.

2. The nitrogen-containing porous carbon according to claim 1, wherein the ratio of the pore volume having a pore diameter in the range of 2.0 nm to 30.0 nm to the total pore volume, obtained by analyzing the nitrogen adsorption / desorption isotherm using the BJH method, is 0.60 or more.

3. The nitrogen-containing porous carbon according to claim 1 or 2, wherein the most frequent pore size is 2.0 nm or more and 10.0 nm or less.

4. The nitrogen-containing porous carbon according to claim 1 or 2, wherein the ratio of the BET specific surface area measured by the water vapor adsorption method to the BET specific surface area measured by the nitrogen adsorption method is 0.010 or more and 0.500 or less.

5. A method for producing nitrogen-containing porous carbon, comprising the steps of: solid-phase mixing a nitrogen-containing compound with a carbon material having micropores to obtain a mixture; and firing the mixture in an inert gas atmosphere.

6. The method according to claim 5, wherein the nitrogen-containing compound is at least one selected from the group consisting of melamine, urea, and pyrazinamide.

7. An electrode catalyst for a fuel cell, comprising the nitrogen-containing porous carbon according to claim 1 or 2, and a metal including a platinum group element supported thereon.

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