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

Abstract

To provide nitrogen-containing porous carbon having high catalytic activity under a low humidification condition while suppressing poisoning of a catalyst when used for an electrode catalyst.SOLUTION: The present invention relates to a nitrogen-containing porous carbon. In the nitrogen-containing porous carbon, a part of a carbon element in a skeleton of a carbon material is substituted with a nitrogen element. The most frequent pore size in the pore size range of 2. 0nm or more and 50. 0nm or less is 2. 0nm or more and 30. 0nm or less. In the nitrogen-containing porous carbon, the ratio of the mass of nitrogen element to the mass of carbon element is 0.005 or more. The BET specific surface area measured by a nitrogen-adsorption method is 400m2 / g or more.SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[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 the 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. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2014 / 175099 Brochure [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-124769 [Patent Document 3] Japanese Patent Publication No. 2022-89016 Summary of the Invention [Problem to be solved by the invention]

[0007] 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 water 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 the catalyst is easily poisoned 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 humidification conditions while suppressing catalyst poisoning. [Means for solving the problem]

[0008] The present invention provides a nitrogen-containing porous carbon, The nitrogen-containing porous carbon is a carbon material in which a part of carbon elements in the skeleton thereof is substituted with nitrogen elements, The most common pore size in the 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 element to the mass of carbon element is 0.005 or more, The BET specific surface area measured by nitrogen adsorption method is 400m 2 The above-mentioned problems have been solved by providing a nitrogen-containing porous carbon having a porosity of 1 / g or more.

[0009] The present invention also provides a method for producing a carbonaceous material by solid-phase mixing of a nitrogen-containing compound with a porous carbonaceous material; and calcining the mixture in an inert gas atmosphere. DETAILED DESCRIPTION OF THE INVENTION

[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 the main component. "Containing carbon as the 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" refer to pores with a diameter of more than 50.0 nm. "micropores" refer 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 most frequent pore size will be explained 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, which is obtained by analyzing the nitrogen adsorption / desorption isotherm by 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, more preferably 1.30 mL / g or more, and even more preferably 1.40 mL / g or more per unit mass of the nitrogen-containing porous carbon. 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 2.00 mL / g or less per unit mass of the nitrogen-containing porous carbon. 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 explained 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, 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 per unit mass of the nitrogen-containing porous carbon. 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 nitrogen-containing porous carbon. Having a 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 explained 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., nitrogen atoms 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 form (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, a relatively high ratio of the number of N1-type nitrogen elements to the number of all nitrogen elements is preferable. The reason for this is that N1-type nitrogen elements exhibit basicity, and for example, when a sulfo group in 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. While an upper limit of the ratio is ideally 1.00, the above effect is fully 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 making the above-described effect prominent, 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 explained 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 m² or less, from the viewpoint of facilitating the placement of metals in the pores and suppressing catalyst poisoning. 2 / g or more is preferable, and 500m 2 / g or more is more preferable, and 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 is more preferable, and 2 It is more preferable that the saturation coefficient is 1 / g or less. 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 preferably has controlled degrees of hydrophilicity and hydrophobicity. 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. When the ratio is defined as BET(H2O / N2), the nitrogen-containing porous carbon preferably has a BET(H2O / N2) value of 0.010 or more, more preferably 0.050 or more, and even more preferably 0.100 or more, from the viewpoint of exhibiting proton conductivity under low humidification conditions. Furthermore, from the viewpoint of suppressing the flooding phenomenon, the BET(H2O / N2) value 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] In order to set the BET (H2O / N2) value 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 reduces the likelihood of the catalytic metal supported in the pores coming into contact with the ionomer, thereby suppressing catalyst poisoning.

[0030] The fuel cell electrode catalyst 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 0.20 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 contact between the catalytic metal supported in the pores and 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 200m 2 / g or more is more preferable, and 2 / g or more. From the viewpoint of suppressing a decrease in the durability of the electrode catalyst for fuel cells, it is more preferable that the BET specific surface area of ​​the electrode catalyst for fuel cells is 1000 m 2 / g or less, and 2 / g or less is more preferable, 2 It is more preferable that the saturation coefficient is 1 / g or less.

[0032] The fuel cell electrode catalyst preferably has a BET (H2O / N2) value within a predetermined range. Specifically, from the viewpoint of exhibiting proton conductivity under low humidification conditions, the BET (H2O / N2) value is preferably 0.050 or more, more preferably 0.100 or more, and even more preferably 0.150 or more. Furthermore, from the viewpoint of suppressing the flooding phenomenon, the BET (H2O / N2) value 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 the carbon material 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 the ionomer, 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 the nitrogen adsorption / desorption isotherm 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 object 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 in 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. The most frequent pore size of the carbon material in 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 properties such as the mode pore diameter, 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. 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 ​​in the above-mentioned ranges, such as the mode pore diameter, pore volume, and BET specific surface area. 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. For example, an organic compound having at least one amino group is preferably used as such a nitrogen-containing compound, 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 standpoints of cost and safety, the nitrogen-containing compound is preferably at least one selected from melamine, urea, and pyrazinamide, and particularly preferably melamine.

[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 for solvent removal before the calcination 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 (e.g., two-roll and three-roll), high-speed agitators (e.g., Henschel mixers and super mixers), fluid energy mills (e.g., micronizers and jet mills), attritors, particle composite devices (e.g., Nanocura®, Nobilta®, and Mechanofusion®) manufactured by Hosokawa Micron Corporation, and powder surface modification devices (e.g., Hybridization System®, 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 in the mixture is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, even more preferably 4.0% by mass or more, and particularly preferably 15.0% by mass or more. The content of the carbon material in the mixture is preferably 99.0% by mass or less, even more preferably 98.0% by mass or less, and even more preferably 96.0% by 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 in the mixture is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 4.0% by mass or more. The content of the nitrogen-containing compound in the mixture is preferably 99.0% by mass or less, more preferably 98.0% by mass or less, even more preferably 96.0% by mass or less, and particularly preferably 85.0% by mass or less. By having the content of the nitrogen-containing compound 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 nitrogen elements for carbon elements in the skeleton of the carbon material, the firing temperature 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 firing temperature 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 nitrogen elements for carbon elements in the skeleton of the carbon material, the firing time is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 3 hours or more, and from the same viewpoint, the firing time is preferably 96 hours or less, more preferably 72 hours or less, and even more preferably 48 hours or less.

[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 the 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 produced during the reaction, the carbon element can be sufficiently substituted for the nitrogen element even in the pores of the carbon material. In contrast to this, it has been common practice in the past to sinter a mixture of a carbon material without pores (i.e., a solid carbon material) and a nitrogen-containing compound. However, in conventional methods, the carbon elements inside the carbon material are not substituted, and only the carbon elements on the surface are substituted with nitrogen elements, so the substitution could not be carried out sufficiently. The present inventors were the first to discover that this problem can be solved by solid-phase mixing a carbon material with pores and a nitrogen-containing compound and sintering 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 thus formed 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 having a current collecting function. Furthermore, the gas diffusion layer has the function of supplying sufficient gas to the electrode catalyst layer. The gas diffusion layer can be made of porous materials such as carbon paper and carbon cloth, etc. Specifically, the gas diffusion layer can be made of carbon cloth woven from 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 some parts are substituted with proton conductor functional groups, 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 the grooves formed therein are perpendicular to each other, or may be arranged 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 formed by arranging tens to hundreds of such configurations 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, a method for producing the same, and an electrode catalyst for a fuel cell are further disclosed. [1] Nitrogen-containing porous carbon, The nitrogen-containing porous carbon is a carbon material in which a part of carbon elements in the skeleton thereof is substituted with nitrogen elements, The most common pore size in the 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 element to the mass of carbon element is 0.005 or more, The BET specific surface area measured by nitrogen adsorption method is 400m 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 the 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 size is 2.0 nm or more and 10.0 nm or less. [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 or more and 0.500 or less. [5] A step of solid-phase mixing a nitrogen-containing compound and a porous carbon material 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 including a platinum group element supported on the nitrogen-containing porous carbon. [Example]

[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 the mixture A carbon material having pores (mesoporous carbon) was prepared as the carbon material. The most common 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 mixed in a solid state to obtain a mixture. The solid state mixing was carried out using a mixer at 2000 rpm for 1 minute.

[0061] (2) Calcination of the mixture The mixture was fired in a tubular furnace under a nitrogen gas atmosphere. This resulted in the substitution of some of the carbon elements in the carbon material skeleton with nitrogen elements, yielding the desired nitrogen-containing porous carbon. The firing temperature and firing 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 amount of melamine was changed to 0.1 g in Example 1. Other than this, the same procedure as in Example 1 was carried out to obtain the target nitrogen-containing porous carbon.

[0065] Examples 8 and 9 In Example 1, hollow carbon black, Ketjenblack (registered trademark), was used instead of 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 Solid carbon having no pores was used instead of the mesoporous carbon in Example 1. The target nitrogen-containing porous carbon was obtained in the same manner as in Example 1 except for this.

[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 most frequent pore size and pore volume of the carbon materials used in the examples and comparative examples were measured by the following methods. The most frequent pore size and pore volume of the carbon material used in comparative example 1 were not measured because the carbon material was a solid carbon without pores. The nitrogen-containing porous carbons obtained in the Examples and Comparative Examples were measured for their modal pore size and pore volume by the following methods. The nitrogen-containing porous carbon obtained in Comparative Example 1 was not measured for its modal pore size and pore volume. Furthermore, for the nitrogen-containing porous carbons obtained in the examples and comparative examples, the BET specific surface area, the mass of carbon (C) element, the mass of nitrogen (N) element, and the ratio of the number of N1-type nitrogen elements to the number of all nitrogen elements were measured by the following methods. In comparative examples 2 to 4, no nitrogen-containing compound was used, so the mass of C element, the 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 measured by the following method. The "power generation performance" here 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 catalyst obtained in the examples and comparative examples were measured by the same methods as those for the nitrogen-containing porous carbon. The results are shown in Tables 1 and 2.

[0069] [Most frequent pore size] The most frequent pore size was measured by analyzing the nitrogen adsorption / desorption isotherm. A gas adsorption analyzer (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 most frequent pore size of the nitrogen-containing porous carbon in the range of 2.0 nm to 50.0 nm was calculated. The analysis software used was 3Flex version 5.02, which came with the gas adsorption analyzer.

[0070] [Pore volume] The pore volume was measured by analyzing the nitrogen adsorption / desorption isotherm. A gas adsorption analyzer (3Flex manufactured by Micromeritics) was used for the nitrogen adsorption / desorption measurements. 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. The analysis software used was 3Flex version 5.02, which came with the gas adsorption analyzer.

[0071] [BET specific surface area] The BET specific surface area was measured by the nitrogen adsorption method as follows: As a pretreatment, nitrogen-containing porous carbon was heated at 400°C for 3 hours under reduced pressure of 20 Pa. A gas adsorption measurement device (3Flex manufactured by Micromeritics) was used for nitrogen adsorption / desorption measurement, and the specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method from the adsorption isotherm at liquid nitrogen temperature (77 K). The BET specific surface area was measured by the water vapor adsorption method according to the following procedure. As a pretreatment, the nitrogen-containing porous carbon was heated at 100°C for 4 hours under reduced pressure of 20 Pa. Then, a gas adsorption measuring device (3Flex manufactured by Micromeritics) was used for water vapor adsorption / desorption measurement, and the specific surface area was calculated from the adsorption isotherm by the BET method.

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

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

[0074] [Power generation performance] The power generation performance of the fuel cells provided with the electrode catalysts in which the 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 (Tanaka Kikinzoku Kogyo Co., Ltd.), 15.8 g of ultrapure water, and 20.0 g of ethanol were added, and the mixture was heated under 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 (Thinky ARE310). 25% Aquivion (D72-25BS, Sigma-Aldrich) was then added as an ionomer and stirred under the same conditions as above. The I / C ratio, which is the ratio of the mass I of the ionomer to the mass C of the support contained in the electrode catalyst, was set to 0.7. The resulting dispersion 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 catalyst amount per unit area of ​​the electrode catalyst layer was 0.2 mg / cm. 2 The obtained electrode catalyst layer was used as a cathode catalyst layer. The anode catalyst layer was obtained using 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 (9.4 g of a mixed solution was obtained). 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 and stirred under the same conditions as above. The ratio of the mass of the ionomer (I) to the mass of the support (C) contained in the electrode catalyst, I / C, was set to 0.7. The resulting dispersion was applied to a polytetrafluoroethylene sheet using a bar coater, and the coating was dried at 60°C. The amount of catalyst per unit area of ​​the anode catalyst layer was 0.05 mg / cm 2 It was decided. The anode catalyst layer and cathode catalyst layer formed on the polytetrafluoroethylene sheet were cut into squares measuring 54 mm on each side, and then stacked on a Nafion (registered trademark) (NR-211, manufactured by Chemours) electrolyte membrane. 2 Transfer was performed by heat pressing for 2 minutes in the atmosphere under the conditions of In this way, a cathode catalyst layer and an anode catalyst layer were formed on each side of the solid electrolyte membrane made of Nafion, thereby obtaining 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 separators made of carbon plates with gas flow channels formed therein, to produce a polymer electrolyte fuel cell. The fuel cell thus obtained corresponds to the JARI standard cell. Hydrogen gas was supplied to the anode side of the obtained fuel cell, and air was supplied to the cathode side. The flow rates were set so that the utilization rate of the hydrogen gas was 70% and the utilization rate of the air was 40%. The gases were humidified using an external humidifier 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 so that the relative humidity was 88% RH on the anode side and 42% RH on the cathode side. The relationship between cell voltage and current density at this time was evaluated, and the current density was set to 1.0 A / cm. 2 The voltage (V) was calculated when the voltage was 100 V. The higher the obtained value, the higher the catalytic activity.

[0075] [Table 1]

[0076] [Table 2]

[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. [Industrial Applicability]

[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. A nitrogen-containing porous carbon, The nitrogen-containing porous carbon is a carbon material in which a part of carbon elements in the skeleton thereof is substituted with nitrogen elements, The most common pore size in the 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 element to the mass of carbon element is 0.005 or more, The BET specific surface area measured by nitrogen adsorption method is 400 m ２ / g or more.

2. 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 a nitrogen adsorption / desorption isotherm by the BJH method, is 0.60 or more.

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

4. 3. The nitrogen-containing porous carbon according to claim 1, 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 step of solid-phase mixing a nitrogen-containing compound with a porous carbon material to obtain a mixture; and calcining 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. 3. 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 on the carbon.

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