Porous silicon nitride composite material, fuel cell electrode, and method for producing porous silicon nitride composite material

Abstract

A porous silicon nitride composite material containing silicon nitride (Si3N4) and a carbon material, having a BET specific surface area of ​​50 m 2 / g or more 400m 2 / g or less, and the electrical conductivity is in the range of 1.0 S / cm or more and 25 S / cm or less.

Description

[Technical Field]

[0001] The present invention relates to a porous silicon nitride composite material, an electrode for a fuel cell, and a method for producing the porous silicon nitride composite material. This application claims priority based on Japanese Patent Application No. 2024-077467, filed on May 10, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] A fuel cell is a device that generates electricity and heat through a chemical reaction that converts hydrogen and oxygen into water. There are several types of fuel cells, including phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and polymer electrolyte fuel cells (PEFCs).

[0003] Among these, a polymer electrolyte fuel cell (PEFC) generally has a structure in which a catalyst layer constituting an anode (fuel electrode) is provided on one side of a solid polymer electrolyte membrane and a catalyst layer constituting a cathode (air electrode) is provided on the other side, with a gas diffusion layer attached to the outside of each catalyst layer. The catalyst layer is composed of a catalyst-supported carrier in which particulate catalyst containing, for example, a precious metal is highly dispersed and supported on the surface of nano-level support particles.

[0004] Currently, carbon-based materials with high specific surface area and high electrical conductivity are used as catalyst supports. However, deterioration of catalytic performance due to corrosion of the carbon support has become a major problem in the cathode and anode. Therefore, there is an urgent need to develop a material with high specific surface area, high electrical conductivity, and excellent durability to replace carbon.

[0005] For example, Patent Document 1 discloses an electrode catalyst comprising (A) Group 13-doped SiC, in which SiC is doped with a Group 13 (Group 3B) element, (B) conductive carbon particles, and (C) a noble metal supported on the surface of the (A) Group 13-doped SiC. The Group 13 element doped into SiC is, for example, aluminum (Al), and the doping amount of the Group 13 element in the (A) Group 13-doped SiC is 1 to 5 mol %, and the ratio of the (A) Group 13-doped SiC to the (B) conductive carbon particles [(A):(B)] is 1:9 to 5:5 by weight.

[0006] Patent Document 2 discloses a method for producing conductive silicon carbide porous bodies with different resistivities, which includes an oxidation treatment step in which a sintered body of conductive porous silicon carbide ceramics is heated in an oxidizing atmosphere at a predetermined heating temperature for a predetermined heating time to form a silicon dioxide layer on the surface of the silicon carbide particles, and the heating temperature and / or heating time in the oxidation treatment step is changed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-149008 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-051748 Summary of the Invention [Problem to be solved by the invention]

[0008] Fuel cell electrodes that achieve high efficiency and high output require supports that have both a large specific surface area and high electrical conductivity. However, although Patent Document 1 describes a silicon carbide composite material that contains silicon carbide particles with a noble metal supported on the surface and conductive carbon particles to impart electrical conductivity, it does not mention the specific surface area or specific electrical conductivity of the porous silicon carbide, leaving room for improvement and not being sufficient as a support to replace carbon.

[0009] Patent Document 2 describes a conductive silicon carbide porous body having a resistivity of 3.4 to 21.7 Ω·cm (0.046 to 0.29 S / cm), but this resistivity range is insufficient, and there is no mention of the specific surface area of ​​the porous body. Patent Document 2 also describes a porous silicon carbide composite material obtained by adding a binder, lubricant, and water to a raw material mixture of silicon carbide, silicon nitride, and graphite, kneading the mixture, extruding, and firing it in a non-oxidizing atmosphere. However, this manufacturing method does not allow for control of pore size, making it impossible to produce various porous silicon carbide composite materials with different pore sizes and specific surface areas. Therefore, there is still room for further investigation into its use as a support to replace carbon.

[0010] Furthermore, all of these documents only discuss catalyst-supporting supports for fuel cell electrodes using silicon carbide composite materials, and therefore do not disclose at all what kind of performance would be exhibited if new materials other than carbon-based materials or silicon carbide composite materials were used.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a porous silicon nitride composite material having both a high BET specific surface area and high electrical conductivity, a fuel cell electrode, and a method for producing the porous silicon nitride composite material. [Means for solving the problem]

[0012] The inventors discovered that by preparing a precursor gel by adding a carbon material or organic polymer as a carbon source during the sol-gel reaction of an organic alkoxysilane aqueous solution in the presence of a surfactant, while taking care not to interfere with the formation of the porous gel, and then firing the precursor gel under a nitrogen atmosphere, a porous silicon nitride composite material was produced in which a mesoscopic pore structure (mesopores) developed into a macroscopic pore structure (macropores) and in which the carbon material was distributed at the nanoscale. Specifically, by adjusting the porous structure during the preparation of the precursor gel and then producing the porous silicon nitride composite, it was found that the pore size and specific surface area of ​​the porous silicon nitride composite material could be controlled. Furthermore, by gradually progressing the condensation polymerization reaction of polysilsesquioxane while adjusting the pH to form a precursor gel with a porous structure, the carbon material could be densely dispersed and distributed, resulting in a porous silicon nitride composite material with higher conductivity.

[0013] That is, the present invention provides the following configurations. (1) Aspect 1 of the present invention is a porous silicon nitride composite material containing silicon nitride (Si3N4) and a carbon material, and having a BET specific surface area of ​​50 m 2 / g or more 400m 2 / g or less and a conductivity in the range of 1.0 S / cm or more and 25 S / cm or less.

[0014] (2) In the second aspect of the present invention, the total pore volume is 0.5 cm 3 / g or more 2.5cm 3 / g or less.

[0015] (3) A third aspect of the present invention is the porous silicon nitride composite material of the first or second aspect, wherein the pore size is 50 nm or more and 500 nm or less.

[0016] (4) A fourth aspect of the present invention is the porous silicon nitride composite material of any one of the first to third aspects, wherein the mass ratio ([SiN]:[C]) of the porous silicon nitride composite material to the carbon (C) contained in the porous silicon nitride composite material is in the range of 95:5 to 50:50.

[0017] (5) A fifth aspect of the present invention is the porous silicon nitride composite material of any one of the first to fourth aspects, wherein the content of the carbon material is 5% by mass or more and 50% by mass or less.

[0018] (6) A sixth aspect of the present invention is the porous silicon nitride composite material of any one of the first to fifth aspects, wherein the carbon material is one or more selected from the group consisting of carbon black, carbon nanofibers, carbon nanotubes, and low-crystalline nanocarbons.

[0019] (7) A seventh aspect of the present invention is an electrode for a fuel cell, having a layer containing the porous silicon nitride composite material according to any one of the first to sixth aspects.

[0020] (8) Aspect 8 of the present invention is a method for producing a porous silicon nitride composite material, comprising the steps of: (A) adding an organic alkoxysilane to an acidic aqueous solution containing a surfactant and a pH adjuster, and further adding a carbon material or an organic polymer, to form a gel containing the carbon material or the organic polymer through a sol-gel reaction of the organic alkoxysilane; (B) washing the gel with alcohol; (C) drying the washed gel to form a porous silicon nitride composite material precursor; and (D) firing the porous silicon nitride composite material precursor in a nitrogen-containing atmosphere to obtain a composite material containing silicon nitride (Si3N4) and the carbon material.

[0021] (9) A ninth aspect of the present invention is the method for producing a porous silicon nitride composite material according to the eighth aspect, wherein in step (D), the porous silicon nitride composite material precursor is fired at 1450°C or higher and 2000°C or lower.

[0022] (10) A tenth aspect of the present invention is the method for producing a porous silicon nitride composite material according to the eighth or ninth aspect, wherein the organic alkoxysilane is represented by the following formula (1) or formula (2): R 1 -SiR 2 x (OR 3 ) 3-X ···(1) (However, in the formula, R 1 is any group selected from a methyl group, an ethyl group, a vinyl group, and a phenyl group, and R 2 is a methyl group, R 3 represents a methyl group or an ethyl group, and the integer x is 0 or 1. R 4 -(SiR 5 y (OR 6 ) 3-y )2···(2) (However, in the formula, R 4 R contains any group selected from a methylene group, an ethylene group, a hexylene group, a vinylene group, a phenylene group, and a biphenylene group; 5 is a methyl group, R 6 represents a methyl group or an ethyl group, and the integer y is 0 or 1.

[0023] (11) Aspect 11 of the present invention is the method for producing a porous silicon nitride composite material according to any one of aspects 8 to 10, wherein the mass ratio of the carbon material or the organic polymer to the organic alkoxysilane is 2.5-50:97.5-50.

[0024] (12) A twelfth aspect of the present invention is the method for producing a porous silicon nitride composite material according to any one of the eighth to eleventh aspects, wherein the carbon material is one or more selected from carbon black, carbon nanofibers, carbon nanotubes, and low-crystalline nanocarbons.

[0025] (13) A thirteenth aspect of the present invention is the method for producing a porous silicon nitride composite material according to the eleventh aspect, wherein the organic polymer is one or more selected from the group consisting of phenolic resin, polystyrene, and polydivinylbenzene. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a porous silicon nitride composite material having both a high BET specific surface area and high electrical conductivity, an electrode for a fuel cell, and a method for producing the porous silicon nitride composite material. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a flow chart showing a step-by-step method for manufacturing a porous silicon nitride composite material according to one embodiment of the present invention. [Figure 2] 1 is a SEM photograph in Example 1. [Figure 3] 1 shows the results of X-ray diffraction analysis using XRD in Example 1. [Figure 4] 1 shows the results of X-ray diffraction analysis using XRD in Reference Example 1. [Figure 5] 1 shows the results of Si-NMR in Reference Example 1. [Figure 6] 1 shows the results of X-ray diffraction analysis using XRD in Reference Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, a porous silicon nitride composite material, a fuel cell electrode, and a method for producing the porous silicon nitride composite material according to one embodiment of the present invention will be described with reference to the drawings. Note that the following embodiment is specifically described to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Furthermore, the drawings used in the following description may show essential parts enlarged for convenience in order to make the features of the present invention easier to understand, and the dimensional proportions of the components may not necessarily be the same as those in reality.

[0029] <Structure of porous silicon nitride composite material> The porous silicon nitride composite material according to this embodiment is a porous silicon nitride composite material containing silicon nitride (Si3N4) and a carbon material, and has a BET specific surface area of ​​50 m 2 / g or more 400m 2 / g or less, and the electrical conductivity is in the range of 1.0 S / cm or more and 25 S / cm or less.

[0030] The form of the porous silicon nitride composite material is not particularly limited, but may be, for example, powder, particulate, fibrous or needle-like, with powder or particulate being preferred. When the porous silicon nitride composite material is in the form of powder or particles, the particle size of the porous silicon nitride composite material is not particularly limited, but the particle size D of 50% of the cumulative particle size in the volume-based cumulative particle size distribution is 50 For example, the thickness is preferably 0.05 μm or more and 50 μm or less, more preferably 0.1 μm or more and 10 μm or less, and even more preferably 0.1 μm or more and 2 μm or less.

[0031] Particle size D of porous silicon nitride composite 50 means a value measured in accordance with JIS Z8825-1:2013, for example, the particle diameter D measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, SALD-7000) 50 This means:

[0032] The porous silicon nitride composite material of this embodiment has a BET specific surface area of ​​50 m 2 / g or more, and 100m 2 / g or more, and 150m 2 / g or more. 2 When the BET specific surface area is 400 m / g or more, the amount of catalyst particles supported on the support surface is sufficiently secured, and when the porous silicon nitride composite material is used in a fuel cell electrode, desired characteristics such as output and efficiency can be obtained.2 / g or less, the proportion of mesopores suitable for supporting the catalyst increases, and the catalyst particle utilization rate can be further improved.

[0033] The total pore volume of the porous silicon nitride composite is 0.5 cm 3 / g or more 2.5cm 3 / g or less, and 3 / g or more 2.5cm 3 / g or less is more preferable, and 0.9 cm 3 / g or more 2.5cm 3 It is particularly preferred that the total pore volume of the porous silicon nitride composite material is 0.5 cm3 / g or less. 3 When the SiO2 content is 1 / g or more, the reaction gas and electrolyte can easily flow through the catalyst layer, and the catalytic efficiency can be improved.

[0034] The pore diameter of the porous silicon nitride composite material is preferably 50 nm or more and 500 nm or less, more preferably 80 nm or more and 400 nm or less, and particularly preferably 100 nm or more and 300 nm or less. When the pore diameter of the porous silicon nitride composite material is 50 nm or more and 500 nm or less, the flow of the reactant gas and electrolyte within the catalyst layer is facilitated, thereby improving the catalytic efficiency. In particular, when the pore diameter of the porous silicon nitride composite material is 50 nm or more, the supply of the reactant gas and electrolyte to the supported catalyst particles is stabilized, preventing a decrease in the catalyst particle utilization rate.

[0035] The BET specific surface area, total pore volume, and pore diameter of the porous silicon nitride composite material described above can be calculated as measured values ​​by a gas adsorption method. For example, they refer to values ​​calculated from the amount of adsorption and condensation of a noncorrosive gas when a noncorrosive gas such as nitrogen or argon is adsorbed while changing the relative pressure in an adsorption isotherm using a constant volume method.

[0036] [Carbon materials] The carbon material contained in the silicon nitride is not particularly limited, and may be one or more selected from, for example, carbon black, carbon nanofiber, carbon nanotube, and low-crystalline nanocarbon. Of these, carbon black is preferred as the carbon material from the viewpoint of achieving high conductivity and manufacturability.

[0037] When the carbon material is carbon black, the average diameter of the primary particles of the carbon material is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm, and even more preferably 30 nm to 50 nm. When the average diameter of the primary particles of the carbon material is 10 nm to 200 nm, good electrical conductivity can be achieved.

[0038] When the carbon material is composed of carbon nanofibers or carbon nanotubes, the average diameter of the carbon material is preferably 10 nm or more and 200 nm or less, and the length of the carbon material is preferably 1 μm or more and 20 μm or less.

[0039] The shape and size of the carbon material retained in the porous silicon nitride composite material can be measured, for example, by observation using a transmission electron microscope or a scanning electron microscope. The average diameter of the primary particles can be determined, for example, from microscope images using image analysis particle size distribution measurement software.

[0040] The content of the carbon material held in the porous silicon nitride is preferably 5% by mass to 50% by mass, more preferably 8% by mass to 45% by mass, and even more preferably 10% by mass to 40% by mass. When the content of the carbon material in the porous silicon nitride composite material is 5% by mass to 50% by mass, high electrical conductivity is achieved, and corrosion of the carbon is suppressed, thereby improving the durability of the catalyst cycle.

[0041] The mass ratio ([Si3N4]:[C]) of silicon nitride to carbon (C) contained in this porous silicon nitride composite material is in the range of 95:5 to 50:50. When this ratio is in the range of 95:5 to 50:50, high conductivity is achieved in the porous silicon nitride composite material, while corrosion of the carbon and the like are suppressed, improving the durability of the catalytic cycle. Note that the carbon (C) in the above mass ratio is the carbon in the porous silicon nitride composite material.

[0042] The carbon (C) content in the porous silicon nitride composite material refers to a value measured by, for example, determining the ratio of contained elements through elemental analysis, thermogravimetric differential thermal analysis (TG-DTA) under atmospheric conditions, and SEM-EDS (Energy Dispersive X-ray Spectroscopy) analysis.

[0043] The silicon (Si) content in the porous silicon nitride composite material refers to a value measured by, for example, determining the ratio of contained elements through elemental analysis or by SEM-EDS (Energy Dispersive X-ray Spectroscopy) analysis.

[0044] The nitrogen (N) content in the porous silicon nitride composite material refers to a value measured by, for example, determining the ratio of contained elements through elemental analysis or by SEM-EDS (Energy Dispersive X-ray Spectroscopy) analysis.

[0045] [Silicon nitride] The average diameter of the primary particles of silicon nitride in the porous silicon nitride composite material is preferably 20 nm to 5 μm, more preferably 30 nm to 2 μm, and even more preferably 40 nm to 1 μm. When the average diameter of the primary particles of silicon nitride is 20 nm to 5 μm, good voids can be obtained when the material is used as an electrode, which is preferable.

[0046] The particle size of silicon nitride in the porous silicon nitride composite material can be measured, for example, by observation using a transmission electron microscope or a scanning electron microscope. The average diameter of the primary particles can be determined, for example, from microscope images using image analysis particle size distribution measurement software.

[0047] [Silicon oxide] In the porous silicon nitride composite material of this embodiment, a domain made of silicon oxide formed by oxidation of silicon nitride may be further formed in a portion of the composite material. Examples of silicon oxide include silicon monoxide (SiO) and silicon dioxide (SiO2). The form of the domain is not particularly limited, but may be amorphous, for example. The domain made of silicon oxide is dispersed and arranged on the surface of the porous silicon nitride composite material. Furthermore, it is preferable that the domain made of silicon oxide is formed on the surface of silicon nitride (Si3N4) in the porous silicon nitride composite material. This makes it possible to suppress oxidation of the silicon nitride surface.

[0048] [Characteristics of porous silicon nitride composites] The porous silicon nitride composite material of this embodiment has an electrical conductivity of 1.0 S / cm or more and 25 S / cm or less, preferably 5.0 S / cm or more and 25 S / cm or less, more preferably 10 S / cm or more and 25 S / cm or less, and even more preferably 15 S / cm or more and 25 S / cm or less. The higher the electrical conductivity of the porous silicon nitride, the better the porous silicon nitride composite material for fuel cells that can be provided. However, if the amount of carbon material that contributes to improving electrical conductivity is too high, oxidation corrosion of the carbon component may occur in the catalytic cycle, reducing durability.

[0049] <Method of manufacturing porous silicon nitride composite material> FIG. 1 is a flow chart showing the steps of a method for producing a porous silicon nitride composite material according to one embodiment of the present invention. A method for producing a porous silicon nitride composite material according to one embodiment of the present invention includes a gel-forming step (step (A)), a washing step (step (B)), a porous silicon nitride precursor-forming step (step (C)), and a calcination step (step (D)).

[0050] Note that, on the premise that the porous silicon nitride composite material according to this embodiment is obtained, other steps may be performed before or after each step. According to the manufacturing method described below, it is possible to obtain a porous silicon nitride composite material having the desired mass ratio by a single firing.

[0051] [Process (A)] In step (A), for example, an organic alkoxysilane is added to an acidic aqueous solution containing a surfactant and a pH adjuster, and then a carbon material or an organic polymer is added to form a gel containing the carbon material or the organic polymer through a sol-gel reaction of the organic alkoxysilane. For example, a hydrolyzable organic alkoxysilane is hydrolyzed to produce a hydrolyzate, and the pH of the reaction system is then increased to carry out a polycondensation reaction of the organic alkoxysilane, thereby obtaining a polysilsesquioxane.

[0052] The pH suitable for the polycondensation reaction varies depending on the isoelectric point of the organic alkoxysilane used, but if the pH is too high, the reaction efficiency decreases and gel formation may become difficult. This sol-gel reaction is preferably carried out at a temperature of 25°C to 80°C, more preferably 30°C to 70°C, and even more preferably 40°C to 60°C. By carrying out the polycondensation reaction within this temperature range, polysilsesquioxane can be obtained as a wet gel containing water as a solvent therein.

[0053] Furthermore, by gradually increasing the pH with a pH adjuster while allowing the polysilsesquioxane condensation polymerization reaction to proceed, the carbon material or organic polymer can be dispersed more densely in the porous structure of the precursor gel, thereby further improving the dispersibility of the carbon material in the three-dimensional structure of the porous silicon nitride composite material.

[0054] The content of the surfactant in the acidic aqueous solution is preferably 0.1% by mass to 50% by mass, more preferably 0.5% by mass to 35% by mass, and even more preferably 2% by mass to 15% by mass.

[0055] The surfactant is not particularly limited, and examples thereof include nonionic surfactants and / or cationic surfactants. By appropriately selecting and using either or both of the nonionic surfactant and the cationic surfactant as the surfactant, the desired BET specific surface area and pore diameter can be obtained.

[0056] Examples of nonionic surfactants include polyethylene glycol types (ether types, ester-ether types) and polyhydric alcohol types. Examples of polyethylene glycol-type nonionic surfactants include Pluronic (registered trademark) types. Examples of cationic surfactants include amine salt types and quaternary ammonium salt types. By adjusting the surfactant content to 0.1% by mass or more and 50% by mass or less in relation to the acidic aqueous solution, a porous polysilsesquioxane gel with a large BET specific surface area and developed mesopores to macropores can be formed.

[0057] The content of the pH adjuster in the acidic aqueous solution is preferably 5% by mass to 50% by mass, more preferably 5.5% by mass to 35% by mass, and even more preferably 6% by mass to 23% by mass. By setting the content of the pH adjuster in the acidic aqueous solution to 5% by mass to 50% by mass, a porous polysilsesquioxane gel having high skeletal strength and flexibility can be formed.

[0058] The pH adjuster is not particularly limited, but examples thereof include substances containing any one selected from urea, ammonia, and sodium hydroxide.

[0059] The acidic aqueous solution is not particularly limited, but examples thereof include aqueous solutions of hydrochloric acid, nitric acid, acetic acid, and the like.

[0060] The organic alkoxysilane is preferably represented by the following formula (1) or (2): By using the organic alkoxysilane represented by the following formula (1) or (2), porous silicon nitride having a desired three-dimensional framework structure can be easily formed. R 1 -SiR 2 x (OR 3 ) 3-x ···(1) (However, in the formula, R 1 is any group selected from a methyl group, an ethyl group, a vinyl group, and a phenyl group, and R 2 is a methyl group, R 3 represents a methyl group or an ethyl group, and the integer x is 0 or 1. R 4 -(SiR 5 y (OR 6 ) 3-y )2···(2) (However, in the formula, R 4 R contains any group selected from a methylene group, an ethylene group, a hexylene group, a vinylene group, a phenylene group, and a biphenylene group; 5 is a methyl group, R 6 represents a methyl group or an ethyl group, and the integer y is 0 or 1.

[0061] Specific examples of the organic alkoxysilane represented by the above formula (1) include methyltrimethoxysilane, methyltriethoxysilane, dimethyldimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, methylethyldimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, methylvinyldimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, and methylphenyldimethoxysilane. Specific examples of the organic alkoxysilane represented by the formula (2) include bis(trimethoxysilyl)methane, bis(triethoxysilyl)methane, bis(methyldimethoxysilyl)methane, bis(methyldiethoxysilyl)methane, 1,2-bis(trimethoxysilyl)ethane, 1,2-bis(triethoxysilyl)ethane, 1,2-bis(methyldimethoxysilyl)ethane, 1,2-bis(methyldiethoxysilyl)ethane, 1,6-bis(trimethoxysilyl)hexane, 1,6-bis(triethoxysilyl)hexane, 1,6-bis(methyldimethoxysilyl)hexane, 1,6-bis(methyldiethoxysilyl)hexane, Examples of suitable organic alkoxysilanes include 1,2-bis(trimethoxysilyl)ethene, 1,2-bis(triethoxysilyl)ethene, 1,2-bis(methyldimethoxysilyl)ethene, 1,2-bis(methyldiethoxysilyl)ethene, 1,4-bis(trimethoxysilyl)benzene, 1,4-bis(triethoxysilyl)benzene, 1,4-bis(methyldimethoxysilyl)benzene, 1,4-bis(methyldiethoxysilyl)benzene, 4,4'-bis(trimethoxysilyl)biphenyl, 4,4'-bis(triethoxysilyl)biphenyl, 4,4'-bis(methyldimethoxysilyl)biphenyl, and 4,4'-bis(methyldiethoxysilyl)biphenyl. The above ethene derivatives include cis / trans geometric isomers, and either isomer can be used. The organic alkoxysilanes used may be of one type or multiple types.

[0062] In this step (A), a carbon material or an organic polymer is further added to the acidic aqueous solution to form a gel containing the carbon material or the organic polymer. By adding the carbon material or the organic polymer during the sol-gel reaction of the alkoxysilane, the precursor formed in step (C) can be calcined in step (D) to arrange the carbon material at the nano level in a porous three-dimensional structural framework, thereby imparting excellent electrical conductivity to porous silicon nitride, which is inherently an insulator or semiconductor. The organic polymer undergoes thermal decomposition by calcination in step (D), and is retained in the porous silicon nitride as low-crystalline nanocarbon, making it possible to impart electrical conductivity.

[0063] In the above step (A), the carbon material or organic polymer is preferably added to the acidic aqueous solution so that the mass ratio of the carbon material or organic polymer to the organic alkoxysilane is 2.5-50:97.5-50. The mass ratio of the carbon material or organic polymer to the organic alkoxysilane is more preferably 3-30:70-97, and even more preferably 5-20:80-95.

[0064] By setting the mass ratio of the carbon material or organic polymer to the organic alkoxysilane within the above range, it is possible to achieve both a larger BET specific surface area and higher electrical conductivity. When the amount of the carbon material or organic polymer added is equal to or less than the amount of the organic alkoxysilane added, separation from the sol-gel reaction system is suppressed, and the formation of a gel composed of polysilsesquioxane can be promoted.

[0065] The carbon material is not particularly limited, and may be composed of one or more selected from, for example, carbon black, carbon nanofiber, carbon nanotube, and low-crystalline nanocarbon. Among these, from the viewpoints of realizing high conductivity and manufacturability, the carbon material is preferably carbon black.

[0066] The organic polymer is not particularly limited, but may be composed of one or more selected from, for example, phenolic resin, polystyrene, and polydivinylbenzene.

[0067] [Process (B)] In step (B), the gel obtained in step (A) is washed with alcohol. The alcohol used for washing is not particularly limited, but examples thereof include methanol, ethanol, 1-propanol, and 2-propanol. This allows unnecessary surfactants to be removed from the acidic aqueous solution, and water in the acidic aqueous solution to be replaced with alcohol.

[0068] After washing with alcohol, the solvent may be replaced with a hydrocarbon solvent such as hexane or heptane. In step (B), water, which is a high surface tension solvent, is replaced with alcohol or a hydrocarbon solvent, which is a low surface tension solvent. This can suppress network shrinkage during the drying step at room temperature and normal pressure in step (C) described below, making it easier to form a porous gel structure.

[0069] [Process (C)] In step (C), the washed gel is dried to form a porous silicon nitride precursor that will become porous silicon nitride in a subsequent step. Methods for this step (C) include supercritical drying using carbon dioxide at 80°C and 14 MPa, drying at room temperature and atmospheric pressure, and vacuum drying at 20°C to 80°C. Among these, drying at room temperature and atmospheric pressure is preferred because it is inexpensive to produce and, when a polysilsesquioxane with high skeletal strength and flexibility is formed, it can produce a high-density porous silicon nitride precursor with developed mesopores.

[0070] [Process (D)] In step (D), the porous silicon nitride precursor containing the carbon material or organic polymer is calcined to obtain a composite material containing silicon nitride (Si3N4) and a carbon material. In this step, nitrogen gas is supplied to create a nitrogen atmosphere, which supplies nitrogen atoms and causes a carbothermal reduction reaction to form a silicon nitride skeleton.

[0071] The firing can be carried out by a known conventional method without any particular limitation, but for example, firing is carried out by raising the temperature at a rate of 2.5°C per minute in a nitrogen gas atmosphere and maintaining the reached maximum temperature for a certain period of time. The maximum firing temperature is preferably 1450°C or higher and 2000°C or lower, and more preferably 1500°C or higher and 1800°C or lower.

[0072] The time for which the maximum temperature is maintained during firing may be determined appropriately based on the time effective for obtaining a porous silicon nitride composite material. For example, 5 minutes to 16 hours is preferable, 10 minutes to 10 hours is more preferable, and 30 minutes to 3 hours is particularly preferable. The firing may be carried out in two or more stages. That is, in the first stage, firing is carried out for a certain period of time at a temperature lower than the maximum temperature, and then the temperature is raised again and firing is carried out. The firing may be carried out at atmospheric pressure. The nitrogen gas atmosphere may contain an inert gas other than nitrogen gas.

[0073] The calcination can be carried out in a fixed-bed or fluidized-bed carbonization furnace, and the heating method and type of the carbonization furnace are not particularly limited as long as the furnace has the function of raising the temperature to a predetermined temperature. Examples of the carbonization furnace include a lead hammer furnace, a tunnel furnace, and a single furnace.

[0074] In step (D), a carbon material or an organic polymer may be further mixed with the porous silicon nitride precursor and the mixture may be calcined. When an organic polymer is mixed with the porous silicon nitride precursor in step (D), similar to the case of mixing in step (A), thermal decomposition proceeds by calcination, and the organic polymer is retained in the porous silicon nitride as low-crystalline nanocarbon.

[0075] [Process (E)] In this embodiment, step (E) can be further performed as a subsequent step to step (D). In this step (E), the composite material is heat-treated in an oxygen atmosphere to form a domain of silicon oxide in a part of the composite material. The heat treatment in this step can be performed by a known, conventional method and under any conditions as long as it is performed in an oxygen atmosphere. For example, the temperature is increased at a rate of 10°C per minute in air, and the maximum temperature reached is maintained for a certain period of time.

[0076] The heat treatment temperature is preferably 500° C. to 800° C., more preferably 550° C. to 750° C., and even more preferably 600° C. to 700° C. The time for which the heat treatment temperature is maintained may be determined appropriately based on the time effective for forming a domain made of silicon oxide in a part of the porous silicon nitride composite material, and is, for example, preferably 15 minutes to 4 hours, more preferably 20 minutes to 3 hours, and even more preferably 30 minutes to 2 hours.

[0077] In this process, a portion of the silicon in the porous silicon nitride composite material is oxidized to silicon oxide, and domains consisting of the silicon oxide are formed on the surface of the silicon nitride in the porous silicon nitride composite material. Examples of silicon oxide include silicon monoxide (SiO) and silicon dioxide (SiO2). The morphology of the domains is not particularly limited, but may be amorphous, for example.

[0078] The effect of this process is to further increase the specific surface area of ​​the porous silicon nitride composite material while maintaining a certain level of electrical conductivity compared to a porous silicon nitride composite material that is not heat-treated in an oxygen atmosphere. Furthermore, the stability of the porous silicon nitride composite material can be improved by pre-oxidizing the areas of silicon nitride that are prone to deterioration.

[0079] Although the embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Example]

[0080] <Verification example 1> Verification Example 1 of the present invention will be described below. The present invention is not limited to the verification example shown below. Each "Example" below is an embodiment of the present invention, and each "Reference Example" is an embodiment of the prior art. Values ​​in the table mean "parts by weight" unless otherwise specified.

[0081] Example 1 [Synthesis of porous silicon nitride composites] 6 g of 5 mM acetic acid solution (Kanto Chemical Co., Ltd.), 0.8 g of Pluronic® F-127 (BASF Ltd.), 0.5 g of urea (Kanto Chemical Co., Ltd.), and 0.24 g of Ketjen Black (Lion Specialty Chemicals, product name "EC-600") were placed in a vial and stirred at room temperature for 10 minutes. 5 g of methyltrimethoxysilane (Kanto Chemical Co., Ltd.) was then added and stirred at room temperature for 30 minutes.

[0082] The mixture was then reacted at 60°C for four days to yield a wet gel. The resulting wet gel was washed with methanol (Kanto Chemical Co., Ltd.) and dried at room temperature and atmospheric pressure for three days, followed by further drying at 80°C and atmospheric pressure for six hours to yield 3.5 g of porous silicon nitride precursor 1. 1 g of this porous silicon nitride precursor was mixed with 0.15 g of Ketjen Black (Lion Specialty Chemicals, product name "Carbon ECP"), then placed in a tubular furnace and sintered in a nitrogen atmosphere, heating it to 1500°C at a rate of 2.5°C / min and holding for two hours to yield porous silicon nitride composite material 1. From the SEM photograph shown in FIG. 2 and the X-ray diffraction results using XRD shown in FIG. 3, it was confirmed that the obtained material was a porous silicon nitride composite material.

[0083] Example 2 Porous silicon nitride composite material 2 was obtained in the same manner as in Example 1, except that the amount of Ketjen black (carbon ECP) ​​mixed with precursor 1 was changed to 0.3 g.

[0084] Example 3 Porous silicon nitride composite material 3 was obtained in the same manner as in Example 1, except that the amount of Ketjen black (carbon ECP) ​​mixed with precursor 1 was changed to 0.4 g.

[0085] Example 4 Porous silicon nitride composite material 4 was obtained in the same manner as in Example 1, except that the amount of Ketjen black (carbon ECP) ​​mixed with precursor 1 was changed to 0.5 g.

[0086] (Reference example 1) The same procedure as in Example 3 was carried out except that the firing temperature was changed to 1400°C. The X-ray diffraction results using XRD shown in Figure 4 and the Si-NMR results shown in Figure 5 indicate that the obtained material was a composite material consisting of porous silicon nitroxycarbide (SiCNO) and carbon. (Reference example 2) The procedure was the same as in Example 1, except that the firing was carried out in an argon gas atmosphere. The X-ray diffraction results using XRD shown in Figure 6 indicate that the material obtained was a composite material made of silicon carbide (SiC) and carbon.

[0087] For each sample of Examples 1 to 4 and Reference Examples 1 and 2, the following measurement items were measured.

[0088] [Element composition ratio] Carbon tape was attached to a metal plate, and the sample was placed on top of the tape, and the sample was observed using a SEM-EDS (JMC7000, manufactured by JEOL Corporation).

[0089] [Measurement of BET specific surface area, pore volume and pore diameter] 0.04 g of electrode catalyst or catalyst powder was weighed and placed in a sample tube, and pretreated by vacuum drying at 100°C for 6 hours. After pretreatment, nitrogen was adsorbed onto the sample at -196°C while changing the relative pressure using a specific surface area / pore size distribution device (Microtrack BEL Corporation: device name "BELSORP-mini II").

[0090] [Measurement of carbon material content] The weight loss was calculated from the weight loss in the temperature range from 600 to 800°C by thermogravimetric differential thermal analysis (TG-DTA) in air.

[0091] [Conductivity measurement] The electrode catalyst or catalyst powder was introduced into a powder resistivity measurement system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: device name "MCP-PD51"), and the sample was pressurized using the attached hydraulic pump. After the pressure reached 12 kN, the resistivity was measured using a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: device name "Loresta GX"), and the conductivity was calculated from the resistivity using the following formula (1). Conductivity (S / cm) = (Resistivity (Ω cm)) -1 ···(1)

[0092] The results of each of the above measurement items are summarized in Table 1.

[0093] [Table 1]

[0094] According to the results shown in Table 1, in Example 1, the BET specific surface area of ​​the porous silicon nitride composite material was 50 m 2 / g or more 400m 2 / g or less and electrical conductivity in the range of 1.0 S / cm or more and 25 S / cm or less, and it was found that a porous silicon nitride composite material having both an appropriate range of BET specific surface area and high electrical conductivity could be obtained.

[0095] On the other hand, porous silicon nitroxycarbide was produced in Reference Example 1. In Reference Example 2, silicon carbide was produced. [Industrial Applicability]

[0096] The porous silicon nitride composite material, fuel cell electrode, and method for producing the porous silicon nitride composite material of the present invention can realize a porous silicon nitride composite material having both a larger BET specific surface area and high electrical conductivity, and when used as an electrode material for the catalyst layer of a fuel cell electrode, can realize a fuel cell with high power generation efficiency, and therefore has industrial applicability.

Claims

1. Silicon nitride (Si ３ N ４ ) and a carbon material, BET specific surface area is 50m ２ / g or more 400m ２ / g or less and a conductivity in the range of 1.0 S / cm or more and 25 S / cm or less.

2. The total pore volume is 0.5 cm ３ / g or more 2.5cm ３ 2. The porous silicon nitride composite material of claim 1, wherein the Mo content is in the range of 0.1 / g or less.

3. 3. The porous silicon nitride composite material according to claim 1, wherein the pore diameter is 50 nm or more and 500 nm or less.

4. The mass ratio of the porous silicon nitride composite material to the carbon (C) contained in the porous silicon nitride composite material ([Si ３ N ４ 2. The porous silicon nitride composite material of claim 1, wherein the ratio of [C] to [C] is in the range of 95:5 to 50:

50.

5. 2. The porous silicon nitride composite material according to claim 1, wherein the content of the carbon material is 5% by mass or more and 50% by mass or less.

6. 2. The porous silicon nitride composite material according to claim 1, wherein the carbon material is one or more selected from the group consisting of carbon black, carbon nanofibers, carbon nanotubes, and low-crystalline nanocarbons.

7. 3. An electrode for a fuel cell, comprising a layer comprising the porous silicon nitride composite material according to claim 1 or 2.

8. a step (A) of adding an organic alkoxysilane to an acidic aqueous solution containing a surfactant and a pH adjuster, and further adding a carbon material or an organic polymer to the aqueous solution, thereby forming a gel containing the carbon material or the organic polymer through a sol-gel reaction of the organic alkoxysilane; (B) washing the gel with alcohol; (C) drying the washed gel to form a porous silicon nitride composite precursor; The porous silicon nitride composite precursor is fired in a nitrogen-containing atmosphere to form silicon nitride (Si ３ N ４ (D) obtaining a composite material containing the carbon material; and In the step (D), the porous silicon nitride composite precursor is fired at a temperature of 1450°C or higher and 2000°C or lower.

9. 9. The method for producing a porous silicon nitride composite material according to claim 8, wherein the organic alkoxysilane is represented by the following formula (1) or (2): R １ -SiR ２ ｘ (OR ３ ) ３－Ｘ ・・・(1) (wherein R １ is any group selected from a methyl group, an ethyl group, a vinyl group, and a phenyl group, and R ２ is a methyl group, R ３ represents a methyl group or an ethyl group. In the formula, the integer x is 0 or 1. R ４ -(SiR ５ ｙ (OR ６ ) ３－ｙ ) ２ ・・・(2) (wherein R ４ contains any group selected from a methylene group, an ethylene group, a hexylene group, a vinylene group, a phenylene group, and a biphenylene group, and R ５ is a methyl group, R ６ represents a methyl group or an ethyl group. In the formula, the integer y is 0 or 1.

10. 9. The method for producing a porous silicon nitride composite material according to claim 8, wherein a mass ratio of the carbon material or the organic polymer to the organic alkoxysilane is 2.5-50:97.5-50.

11. 9. The method for producing a porous silicon nitride composite material according to claim 8, wherein the carbon material is one or more selected from the group consisting of carbon black, carbon nanofiber, carbon nanotube, and low-crystalline nanocarbon.

12. 11. The method for producing a porous silicon nitride composite material according to claim 10, wherein the organic polymer is one or more selected from the group consisting of phenolic resin, polystyrene, and polydivinylbenzene.

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