Electrode catalyst containing porous silicon nitride composite material, fuel cell electrode, fuel cell, and method for producing said electrode catalyst

Abstract

The present invention provides an electrode catalyst, a fuel cell electrode, and a fuel cell containing a porous silicon nitride composite material that has both a high BET specific surface area and high electrical conductivity, and a method for producing an electrode catalyst containing a porous silicon nitride composite material in which the pore size can be controlled using an organic alkoxysilane of the type widely distributed as an industrial raw material. The electrode catalyst of the present invention comprises a porous silicon nitride composite material containing silicon nitride (Si3N4) and a carbon material, and particles containing a noble metal supported on the porous silicon nitride composite material. The porous silicon nitride composite material has a BET specific surface area of ​​50 m 2 / g or more 400m 2 / g or less, and the electrical conductivity is 1.0 S / cm or more and 25 S / cm or less.

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst containing a porous silicon nitride composite material, an electrode for a fuel cell, a fuel cell, and a method for producing the electrode catalyst. This application claims priority from Japanese Patent Application No. 2024-077458, filed May 10, 2004, 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 (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and polymer electrolyte fuel cells (PEFC).

[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 noble metal-supported silicon carbide particles having a silicon oxide layer on the surface of silicon carbide particles having an average primary particle diameter of 0.005 μm to 5 μm, on which noble metal particles are supported. The specific surface area of ​​the silicon carbide particles is 1.0 m 2 / g~400m 2 It is said to be / g. [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. 2011-093756 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 combine a large specific surface area with high electrical conductivity. Patent Document 1 describes an electrode catalyst that contains silicon carbide particles carrying a noble metal on their surfaces and conductive carbon particles to provide electrical conductivity, but makes no mention of durability, leaving room for improvement. In particular, there is a need for electrode catalysts that can demonstrate high durability even during start-stop cycles, which are the most susceptible to deterioration under fuel cell power generation conditions.

[0009] The above-mentioned Patent Document 2 describes precious metal-supported silicon carbide particles having a silicon oxide layer on the surface of silicon carbide supporting precious metal particles, but does not mention that the silicon carbide particles are porous, nor does it mention their electrical conductivity.

[0010] The present invention has been made in view of the above circumstances, and aims to provide an electrode catalyst, a fuel cell electrode, and a fuel cell containing a porous silicon nitride composite material having both a high BET specific surface area and high electrical conductivity, and to provide a method for producing an electrode catalyst containing a porous silicon nitride composite material whose pore size can be controlled. [Means for solving the problem]

[0011] 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 the carbon material was arranged at the nanometer level within a porous three-dimensional framework. Furthermore, by gradually progressing the polysilsesquioxane condensation polymerization reaction while adjusting the pH to form a precursor gel with a porous structure, the carbon material could be densely dispersed within the porous three-dimensional framework, resulting in a porous silicon nitride composite material with higher conductivity. Furthermore, they discovered that mixing the resulting porous silicon nitride composite material with a dispersion containing a precious metal-containing colloidal dispersion produced an electrocatalyst containing precious metal particles.

[0012] That is, the present invention provides the following configurations. [1] A porous silicon nitride composite material comprising silicon nitride (Si3N4) and a carbon material; particles containing a noble metal supported on the porous silicon nitride composite material; Equipped with The porous silicon nitride composite has a BET specific surface area of ​​50 m 2 / g or more 400m 2 / g or less, and An electrode catalyst, wherein the conductivity of the porous silicon nitride composite material is 1.0 S / cm or more and 25 S / cm or less. [2] The electrode catalyst according to [1], wherein the loading rate of the particles containing the noble metal is 10% by mass or more and 60% by mass or less when the total mass of the electrode catalyst is taken as 100% by mass. [3] The electrode catalyst according to [1] or [2], wherein the content of the carbon material is 5% by mass or more and 50% by mass or less. [4] The electrode catalyst according to any one of [1] to [3], wherein the carbon material is composed of one or more selected from carbon black, carbon nanofibers, carbon nanotubes, and low-crystalline nanocarbons. [5] The electrode catalyst according to any one of [1] to [4], wherein the particles containing the noble metal are composed of one or more kinds selected from the group consisting of platinum (Pt), platinum-cobalt alloy (PtCo), and platinum-nickel alloy (PtNi). [6] The total pore volume of the porous silicon nitride composite material is 0.5 cm 3 / g or more 2.5cm 3 / g or less. [7] The electrode catalyst according to any one of [1] to [6], wherein the pore size of the porous silicon nitride composite material is 50 nm or more and 500 nm or less. [8] The electrode catalyst according to any one of [1] to [7], wherein the mass ratio ([Si3N4]:[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. [9] A fuel cell electrode having a layer containing the electrode catalyst according to any one of [1] to [8].

[10] A fuel cell comprising the fuel cell electrode according to [9].

[11] A step (A) of adding an organic alkoxysilane to an acidic aqueous solution containing a surfactant and a pH adjuster, and then adding a carbon material or an organic polymer to the aqueous solution, and 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; a step (C) of drying the washed gel to form a porous silicon nitride precursor; and a step (D) of firing the porous silicon nitride precursor in a nitrogen-containing atmosphere to obtain a porous silicon nitride composite material containing silicon nitride (Si3N4) and the carbon material. a step (E) of mixing the porous silicon nitride composite material with a dispersion containing a colloid containing a noble metal and hydrogen peroxide solution to obtain an electrode catalyst containing particles containing a noble metal; The method for producing an electrode catalyst comprising the steps of:

[12] The method for producing an electrode catalyst according to

[11] , wherein in the step (D), the porous silicon nitride precursor is fired at 1450°C or higher and 2000°C or lower.

[13] The method for producing an electrode catalyst according to either

[11] or

[12] , wherein the organic alkoxysilane is represented by the following formula (1) or formula (2): R 1 -SiR 2 x (OR 3 ) 3-x ···(1) (R in the formula 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) (R in the formula 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.

[14] The method for producing an electrode catalyst according to any one of

[11] to

[13] , wherein the carbon material is composed of one or more types selected from carbon black, carbon nanofibers, carbon nanotubes, and low-crystalline nanocarbons.

[15] The method for producing an electrode catalyst according to any one of

[11] to

[14] , wherein the colloid containing a precious metal is composed of one or more colloids selected from the group consisting of platinum (Pt) colloid, platinum-cobalt alloy (PtCo) colloid, and platinum-nickel alloy (PtNi) colloid.

[16] The method for producing an electrocatalyst according to any one of

[11] to

[15] , wherein the mass ratio of the carbon material or the organic polymer to the organic alkoxysilane is 2.5-50:97.5-50.

[17] The method for producing an electrocatalyst according to any one of

[11] to

[16] , wherein the organic polymer is composed of one or more selected from the group consisting of phenolic resin, polystyrene, and polydivinylbenzene. [Effects of the Invention]

[0013] The present invention provides an electrode catalyst containing a porous silicon nitride composite material that has both a high BET specific surface area and high electrical conductivity, a fuel cell electrode, and a fuel cell. It also provides a method for producing an electrode catalyst containing a porous silicon nitride composite material whose pore size can be controlled using an organic alkoxysilane of the type widely available as an industrial raw material. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a flowchart illustrating an example of a method for producing an electrode catalyst according to an embodiment of the present invention. [Figure 2] 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 3]FIG. 1 shows a scanning electron microscope image of a porous silicon nitride composite material (Synthesis Example 2, SC2). [Figure 4] 1 shows an X-ray diffraction pattern of a porous silicon nitride composite material (Synthesis Example 2, SC2). [Figure 5] FIG. 10 is a transmission electron microscope image of electrode catalyst B. [Figure 6] FIG. 10 is a diagram showing the results of cyclic voltammetry (CV) measurement of electrode catalyst B. [Figure 7] FIG. 10 is a graph showing the change in electrochemically active surface area (ECSA) of electrode catalyst B with respect to the CV measurement cycle. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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.

[0016] <Electrode catalyst composition> The electrode catalyst according to this embodiment comprises a porous silicon nitride composite material containing silicon nitride (Si3N4) and a carbon material, and particles containing a noble metal supported on the porous silicon nitride composite material.

[0017] The form of the electrode catalyst is not particularly limited, but may be, for example, powder, particulate, fibrous or needle-like, with powder or particulate being preferred. When the electrode catalyst is in a powder or particulate form, the particle size of the electrode catalyst is not particularly limited. 50For example, the thickness is preferably 0.1 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.

[0018] Electrode catalyst particle diameter D 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:

[0019] The electrode catalyst of this embodiment has a BET specific surface area of ​​10 m 2 / g or more, and 50m 2 / g or more, and 2 / g or more. 2 / g or less. The BET specific surface area is 10 m 2 / g or more, the catalyst particle loading rate on the support surface is sufficiently ensured, and when the electrode catalyst 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.

[0020] The total pore volume of the electrocatalyst is 0.3 cm 3 / g or more, and 3 / g or more is more preferable, and 0.6 cm 3 It is particularly preferred that the total pore volume of the electrode catalyst is 0.3 cm3 / g or more. 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.

[0021] The pore diameter of the electrode catalyst is preferably 10 nm or more and 1000 nm or less, more preferably 20 nm or more and 500 nm or less, and particularly preferably 50 nm or more and 300 nm or less. When the pore diameter of the electrode catalyst is 10 nm or more and 1000 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 10 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.

[0022] The BET specific surface area, total pore volume, and pore diameter of the electrode catalyst can be calculated as measured values ​​by gas adsorption. For example, these values ​​are calculated from the amount of adsorption and condensation of noncorrosive gas when noncorrosive gas such as nitrogen or argon is adsorbed while changing the relative pressure in the adsorption isotherm using a constant volume method.

[0023] <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. The porous silicon nitride composite material 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.

[0024] 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.

[0025] 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:

[0026] 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 120m 2 / g or more. 2 / g or more, the catalyst particle loading rate on the support surface is sufficiently ensured, 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.

[0027] 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.

[0028] 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.

[0029] The BET specific surface area, total pore volume, and pore diameter of the porous silicon nitride composite material can be calculated as measured values ​​by a gas adsorption method, for example, by using a constant volume method to adsorb a noncorrosive gas such as nitrogen or argon while changing the relative pressure in the adsorption isotherm, and calculating the values ​​from the adsorption amount and condensation of the noncorrosive gas.

[0030] [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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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, and more preferably in the range of 80:20 to 60:40. 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 can be 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.

[0036] 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.

[0037] 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.

[0038] 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. [Silicon nitride]

[0039] 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.

[0040] 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.

[0041] [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. It is also preferable that the domain made of silicon oxide is formed on the surface of silicon nitride in the porous silicon nitride composite material. This can suppress oxidation of the silicon nitride surface.

[0042] [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.

[0043] <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)).

[0044] 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.

[0045] [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.

[0046] 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.

[0047] Furthermore, by gradually increasing the pH with a pH adjuster while the polysilsesquioxane condensation polymerization reaction proceeds, the carbon material or organic polymer can be more densely dispersed 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] The pH adjuster is not particularly limited, but examples thereof include substances containing any one selected from urea, ammonia, and sodium hydroxide.

[0053] The acidic aqueous solution is not particularly limited, but examples thereof include aqueous solutions of hydrochloric acid, nitric acid, acetic acid, and the like.

[0054] 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) (R in the formula 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) (R in the formula 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.

[0055] 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.

[0056] 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 the porous three-dimensional structural framework. As a result, excellent conductivity can be imparted 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, thereby imparting conductivity.

[0057] 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 3-30:97-30, and even more preferably 5-20:95-80.

[0058] 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.

[0059] 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.

[0060] The organic polymer is not particularly limited, but may be composed of one or more selected from, for example, phenolic resin, polystyrene, and polydivinylbenzene.

[0061] [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.

[0062] 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.

[0063] [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.

[0064] [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.

[0065] 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.

[0066] 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.

[0067] The calcination can be carried out in a fixed-bed or fluidized-bed calcination furnace, and the heating method and type of the calcination furnace are not particularly limited as long as the furnace has the function of raising the temperature to a predetermined temperature. Examples of carbonization furnaces include a lead hammer furnace, a tunnel furnace, and a single furnace.

[0068] 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.

[0069] [Particles containing precious metals] When the total mass of the electrode catalyst is taken as 100 mass%, the loading rate of the particles containing the noble metal is preferably 10 mass% to 60 mass%, more preferably 15 mass% to 60 mass%, and more preferably 30 mass% to 60 mass%. When the loading rate of the particles containing the noble metal is 10 mass% to 60 mass%, the catalytic function and durability are excellent. The loading rate of particles containing a noble metal can be calculated, for example, by subjecting the electrode catalyst to alkali melting, dissolving it in aqua regia, diluting it with ultrapure water, and then subjecting it to high-frequency induction heating optical emission spectroscopy (ICP).

[0070] The particles containing a noble metal are preferably composed of one selected from the group consisting of platinum (Pt), gold (Au), ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir), platinum-cobalt alloy (PtCo), platinum-nickel alloy (PtNi), and platinum-ruthenium alloy (PtRu). Among these, from the viewpoint of high durability at high temperatures and high catalytic function, it is more preferable to be composed of one or more selected from the group consisting of platinum (Pt), platinum-cobalt alloy (PtCo), and platinum-nickel alloy (PtNi), and it is even more preferable to be composed of platinum (Pt) or platinum-cobalt alloy (PtCo).

[0071] The particles containing a noble metal are preferably nanoparticles containing a noble metal. The average particle size of the primary particles of the particles containing a noble metal is preferably 2 nm to 10 nm, more preferably 2.5 nm to 7 nm, and even more preferably 3 nm to 5 nm. When the average particle size of the primary particles of the particles containing a noble metal is 2 nm to 10 nm, good catalytic performance can be achieved even with a small amount of noble metal.

[0072] The mass ratio ([C] / [Si]) of carbon (C) to silicon (Si) contained in the electrode catalyst is preferably 6.0 / 1.0 to 20 / 1.0, more preferably 6.5 / 1.0 to 19 / 1.0, and even more preferably 6.5 / 1.0 to 18 / 1.0. When the mass ratio ([C] / [Si]) of carbon (C) to silicon (Si) is 6.0 / 1.0 to 20 / 1.0, high conductivity can be achieved while suppressing carbon corrosion and the like, thereby improving the durability of the catalytic cycle. Note that carbon (C) in the above mass ratio refers to carbon in the carbon material contained in the porous silicon nitride.

[0073] The silicon (Si) content in the electrode catalyst can be calculated, for example, by inductively coupled plasma emission spectroscopy (ICP), and the carbon (C) content can be calculated by high-frequency combustion-infrared absorption spectroscopy.

[0074] [Electrocatalytic properties] The electrode catalyst of this embodiment has a conductivity of 0.1 S / cm or more, preferably 1.0 S / cm or more, more preferably 5.0 S / cm or more, and even more preferably 10 S / cm or more. The conductivity may be 100 S / cm or less, 70 S / cm or less, or 50 S / cm or less. The higher the conductivity of the porous silicon nitride, the better the fuel cell electrode catalyst that can be provided. However, if the amount of carbon material that contributes to improving conductivity is too high, corrosion of the carbon component may occur during the catalytic cycle, resulting in reduced durability.

[0075] <Method of manufacturing electrode catalyst> The method for producing an electrode catalyst according to this embodiment includes a gel forming step (step (A)), a washing step (step (B)), a porous silicon nitride precursor forming step (step (C)), a firing step (step (D)), and a mixing step (step (E)), as shown in Fig. 1. Note that, on the premise that an electrode catalyst according to this embodiment is obtained, steps other than those described above may be performed before or after each step. In the method for producing an electrode catalyst according to this embodiment, the gel-forming step (step (A)), the washing step (step (B)), the porous silicon nitride precursor-forming step (step (C)), and the firing step (step (D)) may be, for example, steps (A), (B), (C), and (D), respectively, of the method for producing a porous silicon nitride composite material described above. That is, the method for producing an electrode catalyst according to the present embodiment may include the method for producing a porous silicon nitride composite material according to the present embodiment, and the subsequent mixing step (step (E)) using the produced porous silicon nitride composite material.

[0076] [Process (E)] In step (E), a dispersion containing a colloid containing a precious metal and aqueous hydrogen peroxide is mixed with the porous silicon nitride composite material to obtain an electrode catalyst containing particles containing precious metal. That is, a mixed solution is obtained, and domains of silicon oxide are formed in part of the porous silicon nitride composite material, thereby obtaining an electrode catalyst containing particles containing precious metal. The colloid containing precious metal is a dispersion of particles containing precious metal in a liquid. The dispersion containing precious metal is a solution containing the colloid containing precious metal and aqueous hydrogen peroxide. The colloid containing precious metal and the dispersion containing the colloid containing precious metal and aqueous hydrogen peroxide can be prepared by known, commonly used methods.

[0077] The mixing ratio of the dispersion containing the colloid containing the precious metal and the porous silicon nitride composite material is, for example, preferably such that the mass of the supported precious metal is 10 mass % or more and 60 mass % or less, more preferably 20 mass % or more and 55 mass % or less, and even more preferably 30 mass % or more and 50 mass % or less, relative to the total mass of the electrode catalyst.

[0078] The colloid containing a noble metal is preferably composed of colloids of one or more metals selected from the group consisting of platinum (Pt), gold (Au), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir). Among these, it is more preferable to be composed of one or more metals selected from the group consisting of platinum (Pt) colloid, platinum-cobalt alloy (PtCo) colloid, and platinum-nickel alloy (PtNi) colloid, and it is even more preferable to be composed of platinum (Pt) colloid or platinum-cobalt alloy (PtCo) colloid.

[0079] In step (E), a mixture of a noble metal-containing colloid, a dispersion containing aqueous hydrogen peroxide, and a porous silicon nitride composite material is stirred. This causes the noble metal-containing colloid to be supported on the porous silicon nitride composite material. As a result, an electrode catalyst containing noble metal-containing particles can be obtained. After leaving the mixture for a predetermined period of time, the solid and liquid phases are separated. To promote solid-liquid separation, the mixture may be cooled. The solid content can be washed using a known, conventional method and under conventional conditions. The washing liquid used for washing is not particularly limited, but may be, for example, water, preferably ultrapure water. This removes ions such as chloride ions from the solid content.

[0080] In the above step (E), particles containing a noble metal are dispersed and supported on the porous silicon nitride composite material. The particles containing a noble metal are preferably composed of one or more metals selected from the group consisting of platinum (Pt), gold (Au), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir). Among these, from the viewpoints of high durability at high temperatures and high catalytic function, it is more preferable to use one or more metals selected from the group consisting of platinum (Pt), platinum-cobalt alloy (PtCo), and platinum-nickel alloy (PtNi). Furthermore, it is even more preferable to use platinum (Pt) or platinum-cobalt alloy (PtCo). Furthermore, it is preferable to support nanoparticles containing a noble metal on the porous silicon nitride composite material.

[0081] [Fuel cell electrodes and fuel cells] The fuel cell electrode according to this embodiment has a catalyst layer containing the electrode catalyst. The fuel cell electrode typically has the electrode catalyst layer and a gas diffusion layer. The fuel cell electrode may be a fuel cell negative electrode (anode) or a fuel cell positive electrode (cathode). When the fuel cell electrode is a fuel cell negative electrode, the fuel cell negative electrode has an anode catalyst layer to which a fuel such as hydrogen is supplied, and a first gas diffusion layer. When the fuel cell electrode is a fuel cell positive electrode, the fuel cell positive electrode has a cathode catalyst layer to which an oxygen-containing gas such as air is supplied, and a second gas diffusion layer.

[0082] The fuel cell according to this embodiment includes the above-described fuel cell electrode. The fuel cell includes a fuel cell electrode and an electrolyte layer. Typically, the fuel cell includes a fuel cell negative electrode (anode), a fuel cell positive electrode (cathode), an electrolyte layer disposed therebetween, a first separator disposed on the fuel cell negative electrode opposite the electrolyte layer, and a second separator disposed on the fuel cell positive electrode opposite the electrolyte layer. In this case, the anode catalyst layer is disposed between the electrolyte layer and the first gas diffusion layer, and the cathode catalyst layer is disposed between the electrolyte layer and the second gas diffusion layer.

[0083] The above-mentioned fuel cell electrode and fuel cell have a catalyst layer containing the above-mentioned electrode catalyst, which allows for high conductivity while maintaining a large BET specific surface area. Additionally, the nano-level arrangement of carbon material within silicon nitride, which has a porous three-dimensional structure framework, reduces the possibility of oxidative degradation of the carbon material, for example, in high-temperature and high-humidity environments. Therefore, the fuel cell electrode and fuel cell exhibit excellent durability.

[0084] 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]

[0085] Examples of the present invention will be described below. The present invention is not limited to the examples shown below. "Example*" is an embodiment of the present invention, and "Comparative Example*" is an embodiment of the prior art. Values ​​in the table mean "parts by weight" unless otherwise specified.

[0086] (Synthesis 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.

[0087] 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 0.5 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 SC1.

[0088] (Synthesis Example 2) Porous silicon nitride composite material SC2 was obtained in the same manner as in Synthesis Example 1, except that the amount of Ketjen black used during firing was 0.4 g. The results of nitrogen adsorption measurement, electrical conductivity measurement, and EDS analysis are shown in Table 1. Scanning electron microscope images are shown in Figure 3, and X-ray diffraction results are shown in Figure 4.

[0089] (Synthesis Example 3) A porous silicon nitride composite material SC3 was obtained in the same manner as in Synthesis Example 1, except that the amount of Ketjen black used during firing was 0.15 g.

[0090] (Comparative Synthesis Example 1) A porous silicon nitride composite material cSC1 was obtained in the same manner as in Synthesis Example 1, except that the amount of Ketjen black used during firing was 0.1 g.

[0091] Example 1 [Synthesis of electrocatalysts containing platinum nanoparticles] 0.43 g of chloroplatinic acid hexahydrate was dissolved in 60 mL of ultrapure water, and 3.1 g of sodium bisulfite was added to the solution to allow the reduction reaction to proceed. The solution was then diluted with 280 mL of ultrapure water. Next, 24 mL of 35% hydrogen peroxide was added dropwise while adding 5% aqueous sodium hydroxide to adjust the pH to approximately 5, yielding a dispersion containing platinum colloid. Subsequently, an aliquot of the colloidal dispersion was taken so that the amount of platinum (Pt) after loading was 45 mass% relative to the total weight of the electrode catalyst, including the carrier. 0.4 g of the porous silicon nitride composite material SC1 obtained in Synthesis Example 1 was added as a carrier and mixed at 90°C for 3 hours. After cooling, the mixture was subjected to solid-liquid separation. The resulting powder (solid content) was thoroughly washed with ultrapure water to remove chloride ions, and then dried at 80°C for 12 hours to obtain electrode catalyst A, in which platinum was supported on the surface of the porous silicon nitride composite material SC1.

[0092] Example 2 An electrode catalyst B was obtained in the same manner as in Example 1, except that the porous silicon nitride composite material SC2 obtained in Synthesis Example 2 was used. A transmission electron microscope image of electrode catalyst B is shown in Figure 5. It was confirmed that platinum nanoparticles with a particle diameter of approximately 3 nm were supported on the porous silicon nitroxycarbide composite material.

[0093] Example 3 An electrode catalyst C was obtained in the same manner as in Example 1, except that the porous silicon nitroxycarbide composite material SC3 obtained in Synthesis Example 3 was used.

[0094] Example 4 Electrocatalyst D was obtained in the same manner as in Example 2, except that the colloidal dispersion was dispensed so that the amount of platinum (Pt) after loading was 30 mass % relative to the total amount of the electrocatalyst including the carrier.

[0095] Example 5 An electrode catalyst E was obtained in the same manner as in Example 2, except that the colloidal dispersion was dispensed so that the amount of platinum (Pt) after loading was 20 mass % relative to the total amount of the electrode catalyst including the carrier.

[0096] Example 6 Electrocatalyst F was obtained in the same manner as in Example 2, except that the colloidal dispersion was dispensed so that the amount of platinum (Pt) after loading was 60 mass % based on the total amount of the electrocatalyst including the carrier.

[0097] Example 7 Electrocatalyst H was obtained in the same manner as in Example 2, except that the colloidal dispersion was dispensed so that the amount of platinum (Pt) after loading was 5 mass % relative to the total amount of the electrocatalyst including the carrier.

[0098] (Comparative Example 1) An electrode catalyst G was obtained in the same manner as in Example 4, except that the porous silicon nitride composite material cSC1 obtained in Comparative Synthesis Example 1 was used.

[0099] (Comparative Example 2) As the cathode catalyst, Pt / CB (Tanaka Kikinzoku Co., Ltd., TEC10E50E, Pt loading rate 46 wt%) was used.

[0100] The porous silicon nitride composite materials SC1 to SC3 and cSC1, which are the samples of Synthesis Examples 1 to 3 and Comparative Synthesis Example 1, and the electrode catalysts A to H, which are the samples of Examples 1 to 7 and Comparative Example 1, and the Pt / CB of Comparative Example 3, were subjected to measurements of the following items. The results are shown in Table 1.

[0101] [Element composition ratio] [Composition analysis of porous silicon nitride composite materials] The porous silicon nitride composites SC1 to SC3 and cSC1 were fixed to carbon tape, and the elemental ratios of [C] / [Si], [O] / [Si], and [N] / [Si] were calculated from the elemental contents of Si, C, O, and N by SEM-EDS (Energy Dispersive X-ray Spectroscopy).

[0102] [Measurement of BET specific surface area, pore volume and pore diameter] 0.04 g of powder of porous silicon nitride composite materials SC1 to SC3 and cSC1 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-Bell Corporation: device name "BELSORP-miniII").

[0103] [Conductivity measurement] The porous silicon nitride composite materials SC1 to SC3 and cSC1 were introduced into a powder resistivity measurement system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: device name "MCP-PD51"), and the samples were 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 equation (1). Conductivity (S / cm) = (Resistivity (Ω cm)) -1 ···(1)

[0104] [Measurement of Pt loading rate] The electrode catalysts A to H of Examples 1 to 7 and Comparative Example 1, and the Pt / CB of Comparative Example 3 were subjected to alkali fusion using anhydrous sodium carbonate and sodium peroxide, then dissolved in aqua regia and diluted to a specified concentration with ultrapure water, and the concentration was measured using a high-frequency induction heating optical emission spectroscopy (ICP; Shimadzu Corporation, Model ICPE-9820).

[0105] [Evaluation of catalytic performance using a rotating electrode] (Electrode preparation) A glassy carbon (GC) electrode with a diameter of 5 mm was polished using alumina paste and then ultrasonically cleaned using ultrapure water. Electrocatalysts A to H of Examples 1 to 7 and Comparative Example 1, and Pt / CB of Comparative Example 3 were added to a 99% by volume aqueous ethanol solution and dispersed using an ultrasonic homogenizer. This was dropped onto a GC disk and dried at room temperature for 12 hours. After drying, a 5% Nafion (registered trademark) solution was dropped onto the electrode catalyst on the GC disk to a dry film thickness of 50 nm, and the electrode catalyst was dried at room temperature for 12 hours.

[0106] (CV measurement) Electrode evaluation was performed using an electrochemical measurement system (Hokuto Denko, HZ-5000). After purging a 0.1 M aqueous solution of perchloric acid with nitrogen gas for 30 minutes, cleaning was performed 50 times using a reversible hydrogen electrode (RHE) as the reference electrode at a potential range of 0.05 to 1.2 V and a sweep rate of 150 mV / s. Cyclic voltammetry (CV) measurements were then performed at a potential range of 0.05 to 1.0 V and a sweep rate of 100 mV / s. Electrochemically active surface area (ECSA) analysis was performed using the hydrogen adsorption wave observed below 0.4 V. The electrochemically active surface area (ECSA) is analyzed using the method described in Non-Patent Document A below. [Non-patent document A] FC-Cubic Technology Research Association: Electrocatalyst RDE Evaluation Method, 2023 / 7 / 21, ver.1.2.1.

[0107] (Oxygen reduction activity evaluation) After purging the electrolyte with oxygen gas for over an hour, linear sweep voltammetry (LSV) was performed. Data were collected under eight conditions: temperature 25°C, potential range 0.25–1.00 V, sweep rate 5 mV / s, and rotation speed 1000–2750 rpm, increasing in 250 rpm increments. The results were analyzed using the Koutecky-Levich plot to obtain the mass activity (A / g-Pt) at 0.85 V. The mass activity analysis method is based on the method described in Non-Patent Document A above.

[0108] (Start / Stop durability evaluation) After purging the electrolyte with nitrogen gas for 30 minutes, the potential range from 1.0 to 1.5 V was swept 500 times, and CV measurements were performed in the potential range from 0.05 to 1.0 V. This measurement procedure constituted one set, and 56,000 cycles of testing were performed. The results are shown in Table 1.

[0109] [Table 1]

[0110] (Consideration) As shown in Table 1, in Examples 1 to 6, the BET specific surface area of ​​the porous silicon nitride composite material used as the support was 75 m 2 / g or more and electrical conductivity of 4.5 S / cm or more, demonstrating high electrical conductivity while maintaining a large BET specific surface area. Furthermore, as a result of catalytic performance evaluation using a rotating electrode, Examples 1 to 6 showed mass activity of 403 (A / g-Pt) or more, indicating a high current per unit mass of platinum (Pt) and high oxygen reduction activity. Furthermore, the ECSA retention rate after durability testing was 74% or more, demonstrating excellent durability.

[0111] As an example, the CV measurement results for electrode catalyst B in Example 2 are shown in Figure 6. The current value was constant for each measurement cycle. The change in ECSA for electrode catalyst B in Example 2 versus the measurement cycle is shown in Figure 7. Evaluation up to 56,000 cycles revealed that the decrease in ECSA was limited to approximately 20% of the initial value, confirming that electrode catalyst B showed little deterioration and had high potential stability.

[0112] Examples 1 to 7 showed that the higher the Pt loading rate, the higher the mass activity and ECSA retention rate tended to be due to the synergistic effect with the BET specific surface area.

[0113] Comparative Example 1 has a BET specific surface area of ​​62 m 2 / g, but the conductivity is 0.9 S / cm and the ECSA is 6 m 2 / g, which was very small and showed no catalytic activity.

[0114] In Comparative Example 2, when a Pt / CB catalyst was used as the cathode catalyst, the ECSA maintenance rate was 55%, and durability was poor. [Industrial Applicability]

[0115] The electrode catalyst, fuel cell electrode, fuel cell, 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 have industrial applicability.

Claims

1. Silicon nitride (Si ３ N ４ a porous silicon nitride composite material comprising a silicon nitride film and a carbon material; particles containing a noble metal supported on the porous silicon nitride composite material; Equipped with The BET specific surface area of ​​the porous silicon nitride composite material is 50 m ２ / g or more 400m ２ / g or less, and An electrode catalyst, wherein the conductivity of the porous silicon nitride composite material is 1.0 S / cm or more and 25 S / cm or less.

2. 2. The electrode catalyst according to claim 1, wherein a loading rate of the particles containing the noble metal is 10% by mass or more and 60% by mass or less when the total mass of the electrode catalyst is taken as 100% by mass.

3. 3. The electrode catalyst according to claim 1, wherein the content of the carbon material is 5% by mass or more and 50% by mass or less.

4. 3. The electrode catalyst 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.

5. 3. The electrode catalyst according to claim 1, wherein the particles containing a noble metal are composed of one or more types selected from the group consisting of platinum (Pt), a platinum-cobalt alloy (PtCo), and a platinum-nickel alloy (PtNi).

6. The total pore volume of the porous silicon nitride composite material is 0.5 cm ３ / g or more 2.5cm ３ The electrode catalyst according to claim 1 or 2, wherein the surface area of ​​the electrode catalyst is 100 μm / g or less.

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

8. 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 3. The electrode catalyst according to claim 1, wherein the ratio of [C] to [C] is in the range of 95:5 to 50:

50.

9. 3. An electrode for a fuel cell, comprising a layer containing the electrode catalyst according to claim 1.

10. A fuel cell comprising the fuel cell electrode according to claim 9.

11. A method for producing the electrode catalyst of claim 1, comprising: 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; Step (C) of drying the washed gel to form a porous silicon nitride precursor; The porous silicon nitride precursor is fired in a nitrogen-containing atmosphere to form silicon nitride (Si ３ N ４ (D) obtaining a porous silicon nitride composite material containing the silicon nitride and the carbon material; a step (E) of mixing the porous silicon nitride composite material with a dispersion containing a colloid containing a noble metal and hydrogen peroxide solution to obtain an electrode catalyst containing particles containing a noble metal; The method for producing an electrode catalyst comprising the steps of:

12. The method for producing an electrode catalyst according to claim 11, wherein in the step (D), the porous silicon nitride precursor is fired at 1450°C or higher and 2000°C or lower.

13. The method for producing an electrode catalyst according to claim 11 or 12, wherein the organic alkoxysilane is represented by the following formula (1) or formula (2): R １ -SiR ２ ｘ (OR ３ ) ３－ｘ ・・・(1) (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. In the formula, the integer x is 0 or 1. R ４ -(SiR ５ ｙ (OR ６ ) ３－ｙ ) ２ ・・・(2) (In the formula R ４ R includes 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. In the formula, the integer y is 0 or 1.

14. The method for producing an electrode catalyst according to claim 11 or 12, wherein the carbon material is one or more selected from the group consisting of carbon black, carbon nanofiber, carbon nanotube, and low-crystalline nanocarbon.

15. 13. The method for producing an electrode catalyst according to claim 11 or 12, wherein the colloid containing a noble metal is composed of one or more colloids selected from the group consisting of platinum (Pt) colloid, platinum-cobalt alloy (PtCo) colloid, and platinum-nickel alloy (PtNi) colloid.

16. The method for producing an electrode catalyst according to claim 11 or 12, wherein a mass ratio of the carbon material or the organic polymer to the organic alkoxysilane is 2.5 to 50:97.5 to 50.

17. The method for producing an electrode catalyst according to claim 11 or 12, wherein the organic polymer is one or more selected from the group consisting of phenolic resin, polystyrene, and polydivinylbenzene.

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