Porous carbon for fuel cell catalyst support, fuel cell catalyst, and fuel cell

Abstract

The present invention pertains to a porous carbon for a fuel cell catalyst support wherein, when pressure changes of nitrogen until reaching adsorption equilibrium at prescribed relative pressures in a nitrogen adsorption isotherm measurement are converted into mass transfer coefficients using LDF approximation, and line approximation is applied to the relationship between the relative pressures and the mass transfer coefficients in the range of relative pressures from 1.0×10-4 to 1.0×10-3, the slope of an approximate line obtained thereby is at least 2.5.

Description

Porous carbon for fuel cell catalyst support, fuel cell catalyst, and fuel cell

[0001] The present invention relates to porous carbon for use as a fuel cell catalyst support, a fuel cell catalyst containing the porous carbon and a catalytic metal, and a fuel cell comprising the catalyst.

[0002] In fuel cells, metals and alloys, primarily platinum or platinum alloys, which have high reactivity, are used as catalysts for the oxidation reaction that occurs at the anode and the reduction reaction that occurs at the cathode. Carbon materials, specifically porous carbon, are generally used as supports for supporting the catalytic metals.

[0003] For example, Patent Document 1 describes a catalyst carrier for a polymer electrolyte fuel cell, which has mesopores (pores with a diameter of 2 to 50 nm) and a specific surface area of ​​600 to 1600 m 2 / g, and a specific intensity ratio between the G-band and the G'-band and a specific G'-band peak position in a Raman spectroscopy spectrum are described.

[0004] International Application Publication No. 2015 / 141810

[0005] In a fuel cell, water molecules that migrate from the anode to the cathode accompanying protons and water vapor generated by a reduction reaction at the cathode remain in the catalyst layer, resulting in insufficient diffusion and supply of reactants (i.e., oxygen gas) to the catalyst layer and reduced fuel cell output. The voltage drop loss caused by this phenomenon is called concentration overvoltage, and concentration overvoltage causes a decrease in fuel cell output. Therefore, to improve the output performance of a fuel cell, it is important to improve the gas diffusivity of the catalyst support, suppress the above phenomenon, and supply reactant gas to the electrode, thereby achieving a low concentration overvoltage. According to the inventors' studies, the porous carbon material for a polymer electrolyte fuel cell described in Patent Document 1 has a dendritic structure for improved gas diffusivity and a relatively large specific surface area of ​​mesopores for efficient support of catalytic metal particles. However, further improvements in fuel cell output performance are constantly being sought. The present invention was made in consideration of the above circumstances and aims to provide a porous carbon for a fuel cell catalyst support that suppresses the above phenomenon and achieves a low concentration overvoltage when used as a catalyst support in a fuel cell.

[0006] The present inventors have conducted extensive research into porous carbon to solve the above-mentioned problems, and as a result have completed the present invention. That is, the present invention includes the following preferred embodiments: [1] In measuring a nitrogen adsorption isotherm, the pressure change of nitrogen until adsorption equilibrium is reached at each predetermined relative pressure is converted into a mass transfer coefficient using the LDF approximation, and when the relative pressure is 1.0 × 10 －４ Above 1.0 x 10 －３ [2] A porous carbon for a fuel cell catalyst support, wherein the gradient of an approximated line obtained by linearly approximating the relationship between the relative pressure and the mass transfer coefficient in the following conditions is 2.5 or more. [3] The volume of pores with a diameter of 2 nm or more and 200 nm or less calculated from a nitrogen adsorption isotherm by the BJH method is 0.8 cm ３ [3] In the approximate line, the relative pressure is 1.0 × 10 －３ The mass transfer coefficient at this time is 3.0 × 10 －３ seconds －１[4] The porous carbon according to any one of [1] to [3], wherein the average primary particle diameter is 500 nm to 5 μm. [5] The porous carbon according to any one of [1] to [3], wherein the bulk density is 0.10 g / cm ３ [6] A fuel cell catalyst comprising the porous carbon according to any one of [1] to [5] and a catalytic metal, wherein the porous carbon supports the catalytic metal. [7] A fuel cell comprising the fuel cell catalyst according to [6].

[0007] According to the present invention, it is possible to provide a porous carbon for use as a fuel cell catalyst support, which can bring about a low concentration overvoltage when used as a catalyst support in a fuel cell.

[0008] 1 is a graph showing the relationship between relative pressure and mass transfer coefficient for the porous carbon of Example 1. FIG. 2 is a diagram showing the relationship between IV characteristics and each overvoltage in a fuel cell. FIG. 3 is a graph showing an IV curve in Example 1. FIG. 4 is a graph showing a Tafel plot and its regression line in Example 1.

[0009] Hereinafter, embodiments of the present invention will be described in detail. Note that the following description is merely illustrative of embodiments of the present invention, and is not intended to limit the present invention to the following embodiments.

[0010] [Porous Carbon] The porous carbon for a fuel cell catalyst support of the present invention is a porous carbon having a nitrogen adsorption isotherm at a predetermined relative pressure (P / P ０ The change in nitrogen pressure until adsorption equilibrium was reached was converted into a mass transfer coefficient using the LDF (Linear Driving Force) approximation. －４ Above 1.0 x 10 －３ The relationship between the relative pressure and the mass transfer coefficient in the following is approximated linearly, and the gradient of the approximated line obtained is 2.5 or more. Here, P is the adsorption equilibrium pressure, and P ０ is the saturated vapor pressure.

[0011] <Relationship between Mass Transfer Coefficient and Relative Pressure, and Mass Transfer Coefficient> The mass transfer coefficient of porous carbon is an index representing the speed at which reactant gases (e.g., oxygen) and product gases (e.g., water vapor) move through the porous carbon for use as a fuel cell catalyst. The greater the change in mass transfer coefficient with respect to a change in relative pressure, the smoother the reactant gas can move from mesopores to micropores, and the smoother the product gas can move from micropores to mesopores. Furthermore, the greater the mass transfer coefficient, the faster the reactant and product gases move within the pores of the porous carbon. Therefore, the change in mass transfer coefficient with respect to a change in relative pressure and the mass transfer coefficient serve as indicators of gas diffusivity in the porous carbon. According to IUPAC, micropores refer to pores with a diameter of less than 2 nm, and mesopores refer to pores with a diameter of 2 to 50 nm. The relationship between relative pressure and mass transfer coefficient, and the mass transfer coefficient, can be determined by creating an adsorption isotherm by adsorbing nitrogen gas onto porous carbon at -196°C and analyzing the adsorption isotherm. Specifically, as described in the Examples below, a nitrogen adsorption isotherm for porous carbon is created, and the change in nitrogen pressure until adsorption equilibrium is reached at each predetermined relative pressure is converted into a mass transfer coefficient using the LDF approximation. Next, a graph is created with the relative pressure on the x-axis and the mass transfer coefficient on the y-axis, thereby showing the relationship between the relative pressure and the mass transfer coefficient.

[0012] In this graph, the inventors have determined that the relative pressure is 1.0×10 －４ Above 1.0 x 10 －３ It has been found that porous carbon can exhibit excellent gas diffusivity when the gradient of an approximate line drawn within the following range (in this specification, this gradient may be referred to as (a)) is 2.5 or more. Here, the range of relative pressure is 1.0 × 10 －４ Above 1.0 x 10 －３ The reason for this setting is explained below. In this specification, pores with a diameter of about 2 to 200 nm are assumed as the diffusion paths for reactant gases and product gases, such as oxygen or water vapor, that diffuse into the porous carbon. If the relative pressure is too low, i.e., if the relative pressure is less than 1.0 × 10 －４On the other hand, if the relative pressure is too high, i.e., if the relative pressure is less than 1.0×10, the mass transfer coefficient is likely to reflect the diffusion behavior in pores smaller than those that effectively function as transfer paths for the reactant gases and the product gases. －３ When the relative pressure is larger than 1.0×10, the diffusion behavior in the pores is considered to be reflected in the mass transfer coefficient. －４ Above 1.0 x 10 －３ By setting the following, it is possible to evaluate the region in which the diffusion behavior of the reactant gas and product gas assumed in this specification is reflected in the mass transfer coefficient, and it is believed that the slope of the approximation line in this range can be used as one index of the gas diffusivity.

[0013] A more specific method for determining (a) will be described using the graph shown in Figure 1, which shows the relationship between relative pressure and mass transfer coefficient for the porous carbon of Example 1 described later. First, the graph showing the relationship between relative pressure and mass transfer coefficient was created as described in the Example described later, and the relative pressure of 1.0 x 10 －４ Above 1.0 x 10 －３ An approximate line is drawn in the following range (in FIG. 1, this range is indicated by a dashed line and an arrow): Next, the slope (a) of this approximate line is found.

[0014] (a) is 2.5 or more, and is usually 10.0 or less, although not limited thereto. (a) is preferably 2.5 to 10.0, 3.0 to 7.0, or 3.5 to 5.0. When (a) is equal to or greater than the lower limit, the porous carbon tends to have a pore structure suitable for the diffusion of reactant gases and the discharge of product gases when used as a catalyst support for a fuel cell, thereby exhibiting excellent gas diffusibility. Furthermore, when (a) is equal to or greater than the lower limit, the porous carbon can have many mesopores. When (a) is less than 2.5, it is difficult for the porous carbon to exhibit excellent gas diffusibility.

[0015] The relative pressure of the approximate straight line in the above range is 1.0 x 10 －３ The mass transfer coefficient (in this specification, this value may be referred to as (b)) is preferably 3.0 x 10 －３ seconds－１ or more, and is usually, but not limited to, 1.0 × 10 －２ seconds －１ (b) is more preferably 3.0 × 10 －３ ~1.0 x 10 －２ seconds －１ , 3.5 × 10 －３ ~8.0 x 10 －３ seconds －１ , 4.0 × 10 －３ ~6.0 x 10 －３ seconds －１ When (b) is equal to or greater than the lower limit, the porous carbon tends to have a pore structure suitable for diffusing reactant gases and discharging generated gases, thereby exhibiting excellent gas diffusivity. Explained using the graph shown in FIG. 1, (b) is the relative pressure of 1.0×10 －３ (the right end of the dashed line or arrow on the right) is the mass transfer coefficient.

[0016] The above-mentioned "pore structure suitable for diffusing reactant gases and discharging product gases" is considered to be a pore structure having many pores that function effectively as migration paths for reactant gases and product gases, the openings of such pores having dimensions suitable for the inflow and outflow of reactant gases and product gases, a high proportion of pores that are connected from one opening to the other (pores that have an opening at one end and the other end is not blocked), few structures (e.g., bottlenecks) that hinder the migration of reactant gases and product gases at the communication parts, and a relatively short gas migration path (a migration path for discharging product gases that are not necessary for the chemical reaction in the fuel cell and for supplying reactant gases necessary for the chemical reaction onto the catalyst metal supported on the porous carbon). Therefore, the pore structure of the porous carbon of the present invention is distinguishable from a "three-dimensional network structure" (e.g., JP 2011-1224 A) formed by mixing a carbon source and template particles (pore source), carbonizing the mixture, and removing the aggregated and connected template particles, resulting in voids (connected pores created by removing the existing template particles) and the surrounding carbon skeletons, in which the paths through which reactant gases and product gases move within the porous carbon are relatively long and there are many structures (e.g., bottlenecks) that obstruct the movement of reactant gases and product gases corresponding to the regions between the existing template particles or the gaps between the existing continuous template particles. Therefore, the "three-dimensional network structure" does not satisfy the numerical ranges of (a) and / or (b) of the porous carbon of the present invention. Furthermore, the pore structure of the porous carbon of the present invention is distinguished from pore structures such as "three-dimensional dendritic structures" (e.g., JP 2018-174078 A) formed by dendrites having a three-dimensional structure in which rod-shaped or ring-shaped bodies containing carbon are three-dimensionally branched, in which the paths through which reactant gases and product gases move within the porous carbon are relatively long and there are many structures (e.g., bottlenecks) that obstruct the movement of reactant gases and product gases corresponding to the regions between existing template particles or the gaps between existing continuous template particles. Therefore, in the case of a "three-dimensional dendritic structure," the numerical ranges of (a) and / or (b) of the porous carbon of the present invention are not satisfied. In a preferred embodiment of the present invention, it is considered that all pores are connected from the opening at one end to the opening at the other end.Furthermore, the above (a) is thought to correspond to the length of the path traveled for the reactant gas to be supplied to the porous carbon and for the product gas to be discharged from the porous carbon, and the larger the (a), the shorter the path length.The above (b) is thought to correspond to the amount of structure that hinders the movement of the reactant gas and the product gas in the communicating part, and the larger the (b), the fewer the structure that hinders the movement of the reactant gas and the product gas.

[0017] Porous carbon having (a) equal to or greater than the lower limit can be produced, for example, by heat-treating a mixture in which a carbon source and a pore source are mixed in a state where they are highly compatible, followed by carbonization to fix the phase-separated structure, and then removing the pore source from the resulting carbide. For example, in the method for producing porous carbon described below, (a) can be adjusted to equal to or greater than the lower limit or within the range by appropriately adjusting the temperature and time during which a mixture of a carbon source and a calcium compound serving as a pore source is heat-treated, or by changing the type of carbon source and appropriately adjusting the temperature and time during which the mixture is heat-treated. Furthermore, (b) can also be adjusted to equal to or greater than the lower limit or within the range by appropriately adjusting similar conditions.

[0018] <Pore volume of pores with a diameter of 2 nm or more and 200 nm or less> In the porous carbon, the lower limit of the volume of pores with a diameter of 2 nm or more and 200 nm or less (hereinafter sometimes referred to as "pore volume of 2 nm or more and 200 nm or less") calculated from a nitrogen adsorption isotherm by the BJH method is preferably 0.8 cm ３ / g or more, and the upper limit is not particularly limited, but is usually 4.00 cm ３ The pore volume of 2 nm or more and 200 nm or less is preferably 0.80 to 4.00 cm ３ / g, more preferably 0.90 to 3.90 cm ３ / g, more preferably 1.00 to 3.80 cm ３ / g, and even more preferably 1.30 to 3.70 cm ３ / g, particularly preferably 1.60 to 3.60 cm ３ / g, more particularly preferably 1.90 to 3.50 cm ３ / g, very preferably 2.00 to 3.40 cm３ / g, more preferably 2.20 to 3.30 cm ３ / g. Pores of 2 nm to 200 nm are thought to be involved in the diffusion of reactant gases and product gases and the highly dispersed support of catalytic metals on the porous carbon. A highly dispersed state refers to a state in which catalytic metals are dispersed and supported on the porous carbon at an appropriate distance. If the spatial distribution of catalytic metals within the porous carbon is sparse to a certain extent, the amount of water vapor generated per unit volume of porous carbon can be reduced, and the oxygen consumption rate per unit volume of porous carbon can be reduced, thereby enabling efficient diffusion and supply of oxygen gas and efficient discharge of water vapor. Generally, oxidation and reduction reactions in fuel cells occur on catalytic metals. Therefore, to improve catalyst utilization efficiency, catalytic metals generally have a size of several nanometers (e.g., an average particle diameter of 2 to 3 nm) to increase the specific surface area per mass. Therefore, when the pore volume of 2 nm to 200 nm is equal to or greater than the lower limit, catalytic metals can be supported on the porous carbon in a highly dispersed state, achieving good discharge of product gases and smooth supply of reactant gases.

[0019] The pore volume of 2 nm or more and 200 nm or less can be adjusted to be equal to or more than the lower limit and equal to or less than the upper limit, or within the above range, by, for example, appropriately adjusting the types and / or proportions of the carbon source and calcium compound, the temperature and / or time of the heat treatment step (phase separation step and / or carbonization step) in the method for producing porous carbon described later. The pore volume of 2 nm or more and 200 nm or less can be calculated by analyzing the nitrogen adsorption isotherm of the porous carbon by the BJH method, and can be determined, for example, by the method described in the Examples described later.

[0020] <Mode diameter> In porous carbon, the mode diameter of pores calculated from the nitrogen adsorption isotherm by the BJH method (hereinafter sometimes simply referred to as "mode diameter") may be, for example, 2 to 200 nm, 5 to 190 nm, or 10 to 180 nm, depending on the size of the supported catalytic metal. Here, "mode diameter" refers to the pore diameter with the largest occurrence ratio in the logarithmic differential pore volume distribution [dV / d(log D)] obtained by differentiating the cumulative pore volume (V) with the common logarithm of the pore diameter (D). When the mode diameter is equal to or less than the upper limit, good utilization efficiency of the supported catalyst can be achieved. The smaller the mode diameter, the more highly dispersed the catalytic metal can be supported, and the larger the mode diameter, the more excellent the gas diffusivity tends to be.

[0021] The mode diameter can be adjusted to within the above range, for example, by appropriately adjusting the types and / or proportions of the carbon source and calcium compound, the temperature and / or time of the heat treatment step (phase separation step and / or carbonization step), etc. in the method for producing porous carbon described below. The mode diameter can be calculated by analyzing the nitrogen adsorption isotherm of the porous carbon by the BJH method, and can be determined, for example, by the method described in the Examples described below.

[0022] <Bulk density> The bulk density of porous carbon indicates the degree of development of the pore structure. When the pore volumes of pores with diameters of 2 nm to 200 nm are the same, the lower the bulk density, the larger the volume of pores with diameters greater than 200 nm. Pores with diameters of 2 nm to 200 nm contribute to the movement of reactant gases and product gases and the highly dispersed support of catalytic metals on the porous carbon, while pores with diameters greater than 200 nm are thought to serve as entrances or migration paths for the smooth movement of reactant gases, product gases, and catalytic metals into pores with diameters of 2 nm to 200 nm. Therefore, in addition to the volume of pores with diameters of 2 nm to 200 nm, it is preferable that the volume of pores with diameters of 200 nm or more is large, i.e., the bulk density is small.

[0023] In one preferred embodiment, the bulk density of the porous carbon is preferably 0.10 g / cm ３The lower limit is not particularly limited, but is usually 0.001 g / cm ３ The bulk density of the porous carbon is preferably 0.001 to 0.10 g / cm ３ less than 0.004 to 0.08 g / cm ３ , more preferably 0.008 to 0.06 g / cm ３ , particularly preferably 0.01 to 0.05 g / cm ３ When the bulk density is less than or equal to the upper limit, reactant gases and product gases can move smoothly inside pores with a pore diameter of 2 nm to 200 nm, and as a result, excellent gas diffusivity is likely to be exhibited. When the bulk density is equal to or greater than the lower limit, scattering can be suppressed and handleability can be improved when the porous carbon is used as a catalyst support. The bulk density can be adjusted to be less than the upper limit or within the range by, for example, appropriately adjusting the type and / or ratio of the carbon source and calcium compound; the temperature and / or time of the heat treatment step (phase separation step and / or carbonization step) in the method for producing porous carbon described below. The bulk density can be measured by the method described in the Examples described below.

[0024] <Volume of pores with a diameter of less than 2 nm> In a preferred embodiment, the volume of pores with a diameter of less than 2 nm calculated by the DFT method from the nitrogen adsorption isotherm of the porous carbon is preferably 0.35 cm ３ / g or less, and the lower limit is not particularly limited, but is preferably 0.01 cm ３ The pore volume of pores with a diameter of less than 2 nm is preferably 0.01 to 0.35 cm ３ / g, more preferably 0.02 to 0.32 cm ３ / g, more preferably 0.04 to 0.30 cm ３ / g, particularly preferably 0.06 to 0.28 cm ３ / g, and even more preferably 0.08 to 0.25 cm ３ / g, more particularly preferably 0.10 to 0.23 cm ３ / g, even more particularly preferably 0.12 to 0.21 cm ３ / g. Pores with a pore diameter of less than 2 nm are pores that are difficult to discharge once they have retained the generated gas (water vapor). When the pore volume of pores with a diameter of less than 2 nm is equal to or less than the upper limit, it becomes difficult to retain the generated gas within the porous carbon, thereby enabling good discharge of the generated gas. Furthermore, when the pore volume of pores with a diameter of less than 2 nm is equal to or less than the upper limit, the volume of pores with a diameter of 2 to 200 nm tends to be high, and therefore excellent gas diffusibility is likely to be exhibited. The pore volume of pores with a diameter of less than 2 nm can be adjusted to be equal to or greater than the lower limit and equal to or less than the upper limit, or within the range, by appropriately adjusting, for example, the type and / or ratio of the carbon source and calcium compound; the temperature and / or time of the heat treatment step (phase separation step and / or carbonization step) in the method for producing porous carbon described below. The pore volume of pores with a diameter of less than 2 nm can be determined by analyzing the adsorption isotherm obtained by nitrogen adsorption measurement of the porous carbon using the QS-DFT method and calculating the volume of pores with a pore diameter of less than 2 nm.

[0025] <Specific surface area calculated by BET method> In a preferred embodiment, the specific surface area of ​​the porous carbon calculated by the BET method from the nitrogen adsorption isotherm is preferably 300 to 1500 m ２ / g, more preferably 400 to 1400 m ２ / g, more preferably 450 to 1300 m ２ / g, particularly preferably 500 to 1200 m ２ / g. When the specific surface area is within this range, the porous carbon tends to have many pores with a pore diameter of 2 to 200 nm, which are suitable for diffusing reactant gases and product gases and supporting the catalytic metal in a highly dispersed state, and therefore, excellent gas diffusivity is likely to be exhibited. The specific surface area can be adjusted to within this range, for example, in the method for producing porous carbon described below, by appropriately adjusting the type and / or proportion of the carbon source and calcium compound; the temperature and / or time of the heat treatment step (phase separation step and / or carbonization step); etc. The specific surface area can be calculated by measuring the adsorption isotherm of the porous carbon, analyzing this adsorption isotherm using the multipoint method using the BET equation, and approximating the resulting curve in the relative pressure range of 0.05 to 0.1.

[0026] <Calcium Content> In a preferred embodiment, the calcium content of the porous carbon is preferably 20 to 2000 ppm, more preferably 50 to 1500 ppm, and even more preferably 100 to 1000 ppm. When the calcium content is within this range, an excessive increase in the mass of the porous carbon is easily suppressed, and productivity also tends to be excellent. The calcium content can be adjusted to within this range, for example, by appropriately adjusting the conditions for the step of removing calcium compounds (e.g., the type and / or concentration of the acid used in acid washing, the time and / or temperature of acid washing, etc.) in the method for producing porous carbon described below.

[0027] <Sulfur Content and Silicon Content> In a preferred embodiment, the sulfur and silicon contents in the porous carbon are each preferably 1000 ppm or less, and although the lower limit thereof is not particularly limited, is 0 ppm. The sulfur content in the porous carbon is preferably 0 to 1000 ppm, more preferably 0 to 900 ppm, and even more preferably 0 to 800 ppm. Furthermore, in a preferred embodiment, the silicon content in the porous carbon is preferably 0 to 1000 ppm, more preferably 0 to 900 ppm, and even more preferably 0 to 800 ppm. When the sulfur and silicon contents are not more than the above upper limit values ​​or within the above ranges, excessive increases in the mass of the porous carbon tend to be suppressed, and productivity also tends to be excellent. The sulfur and silicon contents can be adjusted to be equal to or less than the upper limit or within the above ranges, for example, by appropriately selecting the type of carbon source in the method for producing porous carbon described below and / or appropriately adjusting the conditions of the step of removing calcium compounds (e.g., the type and / or concentration of the acid used in acid washing, the time and / or temperature of acid washing, etc.). The calcium, sulfur, and silicon contents can be measured by fluorescent X-ray analysis.

[0028] <Average Primary Particle Diameter> The shape of the porous carbon may be, for example, particulate, scaly, lamellar, or flaky, but is preferably particulate or flaky. In a preferred embodiment, the porous carbon is preferably particulate. In this embodiment, the average primary particle diameter of the porous carbon is preferably 500 nm to 5.0 μm, more preferably 600 nm to 4.0 μm, and particularly preferably 700 nm to 3.0 μm. When the average primary particle diameter is within the above range, the contact resistance between particles tends to be reduced, and the electrode tends to have excellent conductivity. The average primary particle diameter can be adjusted to within the above range, for example, by appropriately selecting the type of carbon source and / or appropriately adjusting the conditions of the pulverization step in the method for producing porous carbon described below. In this specification, the average primary particle diameter refers to the particle diameter at which the cumulative volume is 50% as measured by laser diffraction / scattering. When measurement by laser diffraction / scattering is not possible, the average primary particle diameter may refer to the average particle diameter obtained by measuring the particle diameters of primary particles captured in an electron microscope image and calculating the average value.

[0029] [Method for producing porous carbon] The porous carbon of the present invention can be produced by a method comprising, for example, the steps of: (1) obtaining a mixture containing a carbon source and a calcium compound; (2) heat-treating the mixture in an inert gas atmosphere to cause phase separation into the carbon source and the calcium compound; (3) heat-treating the phase-separated mixture in an inert gas atmosphere to obtain a carbide; (4) removing the calcium compound from the carbide to obtain porous carbon; and (5) optionally, pulverizing the porous carbon.

[0030] <Step (1)> The carbon source is not particularly limited. From the viewpoint of increasing compatibility with the calcium compound that serves as the pore source, the carbon source is preferably a sugar. Examples of sugars include monosaccharides such as glucose, galactose, mannose, fructose, ribose, and glucosamine; disaccharides such as sucrose, trehalose, maltose, cellobiose, maltitol, lactobionic acid, and lactosamine; and polysaccharides such as starch, cellulose, glycogen, pectin, curdlan, and guar gum. These sugars can be used alone or in combination of two or more. Among these sugars, glucose and starch are preferred from the viewpoints of ease of producing porous carbon that exhibits excellent gas diffusivity and ease of mass procurement.

[0031] The starch is not particularly limited, and starches derived from, for example, corn, cassava, potato, sweet potato, tapioca, beans, wheat, rice, etc. can be used. The amylose content of the starch is preferably 50% by mass or less, more preferably 30% by mass or less, based on the mass of the starch. The lower the amylose content of the starch, the lower the gelatinization temperature tends to be. Therefore, starch having an amylose content below the upper limit is preferred because it is more likely to gelatinize at low temperatures and to have increased compatibility with calcium compounds that serve as pore-forming materials. The amylose content can be determined, for example, by iodine colorimetry. The starch may also be modified starch. Examples of modified starches include etherified starch, esterified starch, cationized starch, and cross-linked starch. One type of starch may be used alone, or two or more types may be used in combination. When two or more starches are used in combination, the amylose content is the average amylose content of the combined starches.

[0032] The pore source is not particularly limited. From the viewpoint of easily increasing compatibility with the carbon source, the pore source is preferably a calcium compound. The calcium compound is not particularly limited, and examples thereof include calcium chloride, calcium hydroxide, calcium oxide, calcium carbonate, calcium acetate, calcium fluoride, calcium bromide, calcium iodide, calcium carbide, calcium bicarbonate, calcium nitrate, calcium sulfate, calcium silicate, calcium phosphate, calcium pyrophosphate, calcium gluconate, and calcium lactate. Among these calcium compounds, from the viewpoint of easily producing porous carbon exhibiting excellent gas diffusibility, calcium compounds having a melting point of 300°C or less (when the mixture contains water and / or a polyhydric alcohol and / or a carboxylic acid, the melting point of a eutectic compound of the calcium compound, the carbon source, and water and / or a polyhydric alcohol and / or a carboxylic acid is 300°C or less) are preferred, and at least one selected from the group consisting of calcium chloride hydrate, calcium hydroxide, calcium oxide, calcium carbonate, and calcium acetate is more preferred. Calcium chloride hydrate exists as a dihydrate, a tetrahydrate, and a hexahydrate, and any of the hydrates may be used, but the dihydrate is preferred because of its good reactivity with sugars.

[0033] The mixture containing a carbon source and a calcium compound may further contain at least one selected from the group consisting of water, a polyhydric alcohol, and a carboxylic acid. It is believed that the presence of water and / or a polyhydric alcohol and / or a carboxylic acid in the mixture allows the carbon source and the calcium compound to dissolve in each other via the water and / or the polyhydric alcohol and / or the carboxylic acid, forming a eutectic compound. This allows a calcium compound that has a melting point of 300°C or higher alone to have a melting point of 300°C or lower as the melting point of the eutectic compound. Therefore, in this specification, "the melting point of the calcium compound is 300°C or lower" also encompasses "the melting point of the eutectic compound of water and / or a polyhydric alcohol and / or a carboxylic acid with the carbon source and the calcium compound is 300°C or lower." Water is preferred due to its availability. Usable polyhydric alcohols include, for example, glycerin, ethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol. Among these polyhydric alcohols, glycerin and ethylene glycol are preferred due to their ease of dissolving calcium compounds and their availability in large quantities. Examples of carboxylic acids that can be used include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, lactic acid, malic acid, citric acid, benzoic acid, phthalic acid, salicylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, and maleic acid. Among these carboxylic acids, formic acid and acetic acid are preferred because they easily dissolve calcium compounds and are readily available in large quantities. When a mixture of two or more of water, one or more polyhydric alcohols, and one or more carboxylic acids is used, the ratio of the water, polyhydric alcohols, and two or more carboxylic acids can be appropriately changed depending on the desired properties of the porous carbon.

[0034] The method for mixing the carbon source and calcium compound, and optionally water, polyhydric alcohol and / or carboxylic acid, is not particularly limited, and they can be mixed by any mixing method.

[0035] The amount of the calcium compound mixed with the carbon source is preferably 80 to 500 parts by mass, more preferably 130 to 400 parts by mass, and even more preferably 180 to 300 parts by mass, relative to 100 parts by mass of the carbon source. When the amount of the calcium compound is within the above range, the relationship between the mass transfer coefficient and the relative pressure, and the pore volume of the resulting porous carbon can be favorable.

[0036] When the mixture containing a carbon source and a calcium compound further contains water and / or a polyhydric alcohol and / or a carboxylic acid, the amount of water and / or a polyhydric alcohol and / or a carboxylic acid (when two or more of water, one or more polyhydric alcohols, and one or more carboxylic acids are used, the total amount) is preferably 50 to 500 parts by mass, more preferably 100 to 400 parts by mass, and even more preferably 150 to 300 parts by mass per 100 parts by mass of the carbon source. However, when the mixture containing a carbon source and a calcium compound contains water but not a polyhydric alcohol or a carboxylic acid, the amount of water is preferably 10 to 500 parts by mass, more preferably 20 to 400 parts by mass, and even more preferably 50 to 300 parts by mass per 100 parts by mass of the carbon source. When the amount of water and / or a polyhydric alcohol and / or a carboxylic acid is within the above range, a eutectic compound is likely to form, thereby enabling the resulting porous carbon to have a favorable relationship between the mass transfer coefficient and the relative pressure, and a favorable pore volume.

[0037] <Step (2)> In step (2), the mixture obtained in step (1) is heat-treated in an inert gas atmosphere to separate the carbon source and the calcium compound into phases. Examples of inert gases include nitrogen, argon, and mixtures thereof. The lower the concentration of the oxidizing gas in the gas used, the better. The concentration of the oxidizing gas, particularly oxygen, is typically 1% by volume or less, more preferably 0.1% by volume or less, based on the volume of the gas used. When the concentration of the oxidizing gas is below the upper limit, oxidation of the mixture is suppressed, making it easier to obtain a structure with the desired characteristics. Furthermore, oxidative decomposition of the resulting structure can be suppressed. The supply (flow) rate of the inert gas is typically 200 to 7,000 mL / min, preferably 500 to 6,000 mL / min, and more preferably 1,000 to 5,000 mL / min per gram of the mixture.

[0038] The heat treatment temperature is preferably 150 to 500°C, more preferably 200 to less than 500°C, even more preferably 240 to 470°C, particularly preferably 250 to 450°C, even more particularly preferably greater than 300°C to 420°C, and even more particularly preferably greater than 300°C to 400°C. The present inventors have found that by heat treating the mixture obtained in step (1) in an inert gas atmosphere, preferably at a temperature within the above range, a characteristic pore structure of porous carbon can be obtained. The reason for this is not clear, but the following non-limiting mechanism of action is thought to be involved.

[0039] The heat treatment causes phase separation of the carbon source and the calcium compound, which are compatible with the carbon source through hydrogen bonding, in the mixture, triggered by the elimination of hydration water from the mixture or the dehydration reaction of the carbon source. Furthermore, as the dehydration reaction of the carbon source progresses, the carbon source hardens, and the structure in which the carbon source and the calcium compound are phase-separated is believed to be fixed. At this time, at least a portion of the phase-separated calcium compounds (preferably 50% by mass or more, more preferably almost all or all of the phase-separated calcium compounds) are continuously connected in a three-dimensional manner. In the subsequent carbonization step, the carbon source is aromatized (carbonized) to make it insoluble in hot water or hot acid. In the subsequent calcium compound removal step, the calcium compounds are removed from the carbonized product (preferably by washing with acid). Because the areas where the calcium compounds are removed become pores, the phase separation of the calcium compounds by the heat treatment is believed to result in the characteristic pore structure of porous carbon.

[0040] The temperature rise rate during the heat treatment step is preferably 2°C / min or more, and although there is no particular upper limit, it is preferably 200°C / min or less from the viewpoint of easily achieving uniform heat treatment. The temperature rise rate during the heat treatment step is preferably 2 to 200°C / min, more preferably 5 to 100°C / min, and even more preferably 10 to 50°C / min. When the temperature rise rate is equal to or greater than the lower limit, it is easy to obtain the desired pore volume and / or the desired mass transfer coefficient.

[0041] The heat treatment time is appropriately selected depending on the heat treatment temperature, the amount of inert gas supplied, etc. For example, it is 0.1 to 24 hours, preferably 0.2 to 12 hours, more preferably 0.3 to 8 hours, and even more preferably 0.4 to 2 hours. When the heat treatment time is equal to or greater than the lower limit, phase separation tends to proceed sufficiently. When the heat treatment time is equal to or less than the upper limit, it is an appropriate time from the viewpoint of economy, and is therefore preferred.

[0042] The heat treatment may be performed using various furnaces such as rotary kilns, fluidized bed furnaces, fixed bed furnaces, moving bed furnaces, and moving bed furnaces, as well as various dryers such as spray dryers, moving dryers, agitator dryers, roll dryers, and thin film evaporators. A continuous furnace or dryer may be used, which continuously introduces the material to be heat-treated and removes the material after heat treatment, or a batch furnace or dryer may be used, which does so discontinuously. The heating means may be any means capable of heating to a predetermined temperature, such as electric heating, gas combustion heating, microwave heating, high-frequency induction heating, or electrical heating. These heating means may be used alone or in combination.

[0043] <Step (3)> In step (3), the mixture phase-separated in step (2) is heat-treated in an inert gas atmosphere to carbonize it. The heat treatment is preferably carried out in two stages: a medium-temperature heat treatment (e.g., 500 to 900°C) and a subsequent high-temperature heat treatment (e.g., 900 to 1300°C). The medium-temperature heat treatment temperature is preferably 500 to 900°C, more preferably 550 to 800°C, even more preferably 600 to 750°C, and particularly preferably 650 to 700°C. The high-temperature heat treatment temperature is preferably 900 to 1300°C, more preferably 950 to 1280°C, even more preferably 1000 to 1250°C, and particularly preferably 1050 to 1200°C. The medium-temperature heat treatment hardens the carbon source without causing a sudden change in the pore structure. Subsequent high-temperature heat treatment improves the crystallinity of the carbon, resulting in highly conductive porous carbon.

[0044] The temperature rise rate during the heat treatment step is preferably 2°C / min or more, and although there is no particular upper limit, it is preferably 200°C / min or less from the viewpoint of easily achieving uniform heat treatment. The temperature rise rate during the heat treatment step is preferably 2 to 200°C / min, more preferably 5 to 100°C / min, and even more preferably 10 to 50°C / min. When the temperature rise rate is equal to or greater than the lower limit, it is easy to obtain the desired pore volume.

[0045] The heat treatment time is appropriately selected depending on the heat treatment temperature, the amount of inert gas supplied, etc. The medium-temperature heat treatment time is, for example, 0.5 to 12 hours, more preferably 0.7 to 6 hours, and even more preferably 0.8 to 4 hours. The high-temperature heat treatment time is, for example, 0.5 to 12 hours, more preferably 0.7 to 6 hours, and even more preferably 0.8 to 4 hours. When the heat treatment time is equal to or greater than the lower limit, carbonization tends to proceed sufficiently. When the heat treatment time is equal to or less than the upper limit, this is an appropriate time from the viewpoint of economy, and is therefore preferred.

[0046] Alternatively, the phase separation step (2) and the intermediate-temperature heat treatment step (3) may be performed simultaneously. This can improve productivity. For example, the mixture obtained in step (1) may be subjected to intermediate-temperature heat treatment at 500 to 900°C. In this case, the temperature rise during the intermediate-temperature heat treatment step passes through 150 to 500°C, the temperature at which phase separation progresses, thereby achieving the desired pore structure. To promote phase separation, it is necessary to slow the rate of temperature rise until the intermediate temperature is reached. Specifically, from the viewpoint of achieving both the promotion of a phase-separated structure and improved productivity, the temperature rise rate is preferably 2 to 100°C / min, more preferably 5 to 50°C / min, and even more preferably 10 to 30°C / min. In this case, the heat treatment time is, for example, 0.5 to 12 hours, more preferably 0.7 to 6 hours, and even more preferably 0.8 to 4 hours. However, from the viewpoint of promoting a phase-separated structure, it is preferable to perform the phase separation step (2) and the intermediate-temperature heat treatment step (3) independently.

[0047] The inert gas, the amount of supply thereof, and the furnace used for the heat treatment may be the same as those used in step (2).

[0048] <Step (4)> In step (4), calcium compounds are removed from the obtained carbide. This allows porous carbon to be obtained. The removal of calcium compounds is preferably carried out by acid washing. Examples of acids used in acid washing include hydrochloric acid, sulfuric acid, and nitric acid. Hydrochloric acid is preferred from the viewpoints that it easily dissolves metal compounds in the carbide, impurities such as sulfur are less likely to remain, and oxidation of the carbide is easily suppressed. The concentration of the acid used in acid washing may be changed appropriately depending on the type of acid used. For example, when hydrochloric acid is used, the concentration of hydrochloric acid is preferably 0.01 to 1.0 mol / L, more preferably 0.05 to 0.5 mol / L. A hydrochloric acid concentration within the above range is preferred because it makes it easy to remove metal compounds and is less likely to leave hydrochloric acid in the carbide.

[0049] The pH of the acid used during acid washing may be appropriately changed depending on the type, concentration, temperature, etc. of the acid used. The pH of the acid is preferably 3 or less, more preferably 2.5 or less. When the pH of the acid is the above upper limit or less, metal compounds can be easily and efficiently removed.

[0050] The acid washing may be carried out, for example, by immersing the obtained carbide in the acid. When the acid washing is carried out by immersion in acid, the mass ratio of the acid to the carbide may be adjusted appropriately depending on the type, concentration, temperature, etc. of the acid used. The mass of the carbide to be immersed relative to the mass of the acid is preferably 2 to 50 mass%, more preferably 5 to 30 mass%. When the mass ratio of the carbide to be immersed relative to the mass of the acid is within the above range, a sufficient washing effect is likely to be obtained.

[0051] The method for immersing the carbide in acid is not particularly limited. It may be a method in which acid is continuously added, retained for a predetermined time, and immersed while removing the acid, or a method in which the carbide is immersed in acid, retained for a predetermined time, drained, and then new acid is added, and the immersion-draining process is repeated. It may also be a method in which all or part of the acid is renewed. It may also be a method in which the acid is stirred during immersion.

[0052] The atmosphere in which the acid washing is carried out is not particularly limited and may be appropriately selected depending on the method used for washing. The acid washing is usually carried out in an air atmosphere.

[0053] The time for immersing the carbide in acid can be adjusted appropriately depending on the acid used, the treatment temperature, etc. The time for immersing the carbide in acid is preferably 5 to 60 minutes, more preferably 10 to 40 minutes, and even more preferably 15 to 35 minutes. When the time is equal to or greater than the lower limit, the metal compound can be easily removed sufficiently, and when the time is equal to or less than the upper limit, good productivity can be ensured.

[0054] After the carbonized product is acid-washed, it is preferable to remove the acid in the porous carbon by washing with water. This acid washing and water washing may be repeated until the calcium compounds in the porous carbon are removed to a desired extent. Furthermore, the temperature of the solution used in the acid washing and water washing is preferably high from the viewpoint of the efficiency of removing calcium compounds and residual acid, and is usually 60°C or higher.

[0055] In one embodiment of the present invention, it is preferable to carry out acid washing between the intermediate-temperature heat treatment and the high-temperature heat treatment of the phase-separated mixture. For example, it is preferable to subject the mixture that has undergone phase separation in step (2) to intermediate-temperature heat treatment (e.g., at 500 to 900°C), followed by acid washing, and then to high-temperature heat treatment (e.g., at 900 to 1300°C). In one embodiment of the present invention, when the phase separation step (2) and the intermediate-temperature heat treatment step (3) are carried out simultaneously, it is preferable to carry out acid washing between the simultaneously performed phase separation step (2) and the intermediate-temperature heat treatment step (3) and the high-temperature heat treatment. In these embodiments, the carbon source is infusible by the intermediate-temperature heat treatment before washing to remove the calcium compound, and therefore changes in the pore structure due to dissolution of the carbon source by acid washing are unlikely to occur.

[0056] After acid washing and water rinsing, the porous carbon may be subjected to a drying treatment using a known dryer such as a hot air dryer or a reduced pressure dryer. Drying is preferably carried out at a temperature of 50 to 150° C. A drying temperature within the above range is preferred because oxidation of the porous carbon is unlikely to occur and drying proceeds appropriately.

[0057] If necessary, the porous carbon after step (4) may be pulverized, and in one embodiment, pulverization is preferred. By pulverization, the shape and particle size of the finally obtained porous carbon can be controlled to the desired shape and particle size. The pulverization method is not particularly limited. For example, known pulverizers such as a ball mill, centrifugal roll mill, ring roll mill, centrifugal ball mill, jet mill, cone crusher, double roll crusher, disc crusher, and rotary crusher can be used alone or in combination.

[0058] The method for producing porous carbon may further include a classification step after the pulverization step. For example, porous carbon with a narrow particle size distribution can be obtained by removing particles that are significantly smaller or larger than the desired particle size. The classification method is not particularly limited. Examples of classification methods include classification using a sieve, wet classification, and dry classification. Examples of wet classifiers include classifiers that utilize the principles of gravity classification, inertia classification, hydraulic classification, centrifugal classification, etc. Examples of dry classifiers include classifiers that utilize the principles of sedimentation classification, mechanical classification, centrifugal classification, etc. From an economical standpoint, it is preferable to use a dry classifier. To prevent surface oxidation during pulverization, it is preferable to perform the pulverization step and classification step in an inert gas atmosphere.

[0059] Pulverization and classification can also be performed using a single device. For example, pulverization and classification can be performed using a jet mill equipped with a dry classification function. Furthermore, devices having a pulverizer and a classifier independent from each other can also be used. In this case, pulverization and classification can be performed continuously, or can be performed discontinuously.

[0060] <Fuel Cell Catalyst> The porous carbon of the present invention has a distinctive pore structure, making it suitable for use as a support for a fuel cell catalyst. The porous carbon can efficiently discharge water vapor generated by a chemical reaction (reduction reaction) at the cathode and smoothly supply oxygen, a reactant gas, to a catalytic metal supported on the porous carbon. Therefore, a fuel cell catalyst comprising the porous carbon of the present invention and a catalytic metal, in which the porous carbon supports the catalytic metal, can achieve a low concentration overvoltage in a fuel cell. Because the fuel cell catalyst provides a low concentration overvoltage, a fuel cell using the fuel dissociation catalyst can have improved output characteristics.

[0061] The catalytic metal in the anode can be any known catalyst, as long as it catalyzes the oxidation reaction of hydrogen. The catalytic metal in the cathode can be any known catalyst, as long as it catalyzes the reduction reaction of oxygen. Specific catalytic metals include platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and other metals, as well as alloys thereof. One or more of these metals can be used. Due to their high catalytic activity, the catalytic metal is preferably platinum or a platinum-containing alloy. The shape and size of the catalytic metal are not particularly limited, and shapes and sizes similar to those of known catalytic components can be adopted. The shape may be, for example, particulate, flake, or layered, with particulate being preferred. In this case, the average particle size (diameter) of the catalytic metal is preferably 0.3 to 30 nm, more preferably 1 to 10 nm. The amount of catalyst supported on the catalyst support is preferably 0.05 to 80 mass %, more preferably 0.1 to 60 mass %, and more preferably 0.5 to 40 mass %, based on the total mass of the catalyst support and the catalytic metal. If the catalyst support amount is within the above range, a sufficient amount of catalyst is supported, and there is a large amount of catalytic metal that functions as a catalyst, so that the catalyst can be used with high utilization efficiency, which is preferable.

[0062] The method for supporting the catalytic metal on the catalyst carrier is not particularly limited. Suitable known methods include the noble metal acetylacetonate method using a noble metal acetylacetonate and the colloid method using a noble metal colloid, and either method is preferred. A preferred method is to deposit the catalytic metal on the surface of the catalyst carrier, and then perform heat treatment to increase the particle size of the catalytic metal.

[0063] <Fuel Cells> Fuel cells are generally constructed as follows: On both sides of a proton-conductive polymer electrolyte membrane, catalyst layers containing a carbon material supporting a platinum group metal catalyst and an ion-conductive binder made of a polymer electrolyte are formed. On the outside of each catalyst layer, a gas diffusion layer, a porous material that allows the fuel gas and oxidant gas to pass through, is formed. Examples of gas diffusion layers include carbon paper and carbon cloth. A catalyst layer with a gas diffusion layer disposed thereon is called a gas diffusion electrode, and a structure in which a pair of gas diffusion electrodes are joined to an electrolyte membrane with the catalyst layer facing the electrolyte membrane is called a membrane electrode assembly (MEA). Conductive and airtight separators are disposed on both sides of this membrane electrode assembly. Gas flow channels for supplying fuel gas or oxidant gas (e.g., air) to the electrode surfaces are formed at the contact points between the membrane electrode assembly and the separators or within the separators. Power generation is achieved by supplying a fuel gas, such as hydrogen or methanol, to one electrode (fuel electrode) and an oxygen-containing oxidant gas, such as air, to the other electrode (oxygen electrode). At the fuel electrode, the fuel is ionized, generating protons and electrons. The protons pass through the electrolyte membrane, and the electrons travel through an external electrical circuit formed by connecting the two electrodes to the oxygen electrode, where they react with the oxidant to produce water. In this way, the chemical energy of the fuel can be directly converted into electrical energy and extracted. The porous carbon of the present invention can efficiently exhaust water vapor generated by the chemical reaction (reduction reaction) at the cathode and smoothly supply oxygen, the reactant gas, to the catalytic metal supported on the porous carbon. Therefore, fuel cells using the porous carbon of the present invention as the carbon material for the catalyst layer can achieve low concentration overvoltage. Due to the low concentration overvoltage achieved by the fuel cell catalyst, fuel cells using the fuel ionization catalyst can exhibit improved output characteristics. A fuel cell equipped with the fuel cell catalyst of the present invention can be manufactured using known fuel cell materials and following known fuel cell manufacturing methods, except for the fuel cell catalyst of the present invention.

[0064] The present invention will be specifically described below with reference to examples, but these examples are not intended to limit the scope of the present invention.

[0065] [Method for Analyzing Porous Carbon] <Mass Transfer Coefficient by Nitrogen Adsorption Method> A sample tube was filled with porous carbon as a measurement sample. This sample tube was placed in an "Autosorb-iQ-MP" manufactured by Quantachrome, and the pressure was reduced once while cooled to -196°C. Nitrogen (purity 99.999%) was then adsorbed onto the measurement sample at a predetermined relative pressure. The amount of nitrogen adsorbed onto the sample when equilibrium pressure was reached at each predetermined relative pressure was measured, and an adsorption isotherm was created. The mass transfer coefficient was determined by analyzing the data of the created nitrogen adsorption isotherm. Specifically, the pressure change of nitrogen until adsorption equilibrium was reached at each predetermined relative pressure was converted into a mass transfer coefficient using the LDF approximation. At this time, the relative pressure was 1.0 x 10 －４ Above 1.0 x 10 －３ The following ranges were converted to mass transfer coefficients so that the number of plots was at least 5: Relative pressure (P / P ０ A graph was created with the relative pressure 1.0 × 10) on the x-axis and the mass transfer coefficient on the y-axis. －４ Above 1.0 x 10 －３ Draw an approximate straight line in the following range, and calculate the slope (a) and relative pressure 1.0 x 10 －３ The mass transfer coefficient (b) was calculated.

[0066] <Pore Volume and Mode Diameter of 2 nm or More and 200 nm or Less by Nitrogen Adsorption Method> The nitrogen adsorption isotherm obtained as described above was analyzed by the BJH method, and the volume of pores having a pore diameter of 2 nm or more and 200 nm or less was calculated. The nitrogen adsorption isotherm obtained as described above was also analyzed by the BJH method, and the logarithmic differential pore volume distribution (dV / d(log D)) was determined by differentiating the cumulative pore volume (V) with the common logarithm of the pore diameter (D), and the pore diameter with the largest appearance ratio was adopted as the mode diameter of the measurement sample.

[0067] <Specific Surface Area> The nitrogen adsorption isotherm obtained as described above was analyzed by a multipoint method using the BET equation. The specific surface area of ​​the measurement sample was calculated from the approximation line of the obtained curve in the relative pressure range of 0.05 to 0.1.

[0068] <Bulk Density> The bulk density of the measurement sample was measured using a Powder Tester PT-X manufactured by Hosokawa Micron Corp. Specifically, the measurement sample was placed in an automatic tap density measurement unit, and the bulk density was calculated from the volume after tapping 3,000 times.

[0069] <Average primary particle size> The sample was placed in an aqueous solution containing 5% by mass of a surfactant ("Toriton X100" sold by Wako Pure Chemical Industries, Ltd.), treated in an ultrasonic cleaner for 10 minutes or more, and dispersed in the aqueous solution. Using this dispersion, the particle size distribution was measured using a particle size / particle size distribution measuring device ("Microtrac MT3300EXII" manufactured by Microtrac-Bell Corporation). The particle size at which the cumulative volume reached 50% was taken as the average primary particle size of the porous carbon sample.

[0070] Example 1 Production of Porous Carbon One gram of glucose (available from Fujifilm Wako Pure Chemical Industries, Ltd.), 2 grams of calcium chloride dihydrate (200 parts by weight per 100 parts by weight of glucose), and 1 gram of ion-exchanged water (100 parts by weight per 100 parts by weight of glucose) were mixed. The resulting mixture was heated to 350°C at a heating rate of 20°C / min under a nitrogen gas flow of 1250 mL / min per gram of mixture and heat-treated (phase-separated) at this temperature for 30 minutes. Next, while flowing nitrogen gas at the same gas supply rate, the mixture was heated to 700°C at a heating rate of 10°C / min and heat-treated at this temperature for 60 minutes to obtain a carbonized product. The carbonized product was immersed in 0.2 L of 0.1 mol / L hydrochloric acid and washed by stirring at 80°C for 30 minutes, then transferred to a Buchner funnel. The filtrate was washed with water until the pH reached a range of 6-8. After acid washing and water washing were repeated three times, the carbonized material was dried with hot air at 80°C. The carbonized material after hot air drying was mixed with 12 mL of ion-exchanged water, and the resulting mixture was pulverized in a ball mill for 10 minutes. While flowing nitrogen gas at the same gas supply rate as above, the pulverized carbonized material was heated to 1200°C at a heating rate of 10°C / min and heat-treated for 60 minutes to obtain porous carbon.

[0071] <Preparation of fuel cell catalyst> Pt catalyst particles, which are electrode catalyst particles, were supported on the produced porous carbon by a platinum acetylacetonate method. A Pt precursor (Pt(C ５ H ７ O ２ ) ２ ) was dissolved in dichloromethane (3 mL). The resulting solution was added to a recovery flask containing 100 mg of porous carbon. Next, while the recovery flask was ice-cooled, the porous carbon was dispersed in the solution using an ultrasonic stirrer for 30 minutes. The mixture was further heated to 50°C and stirred until all of the solvent had evaporated, yielding a pale yellow powder. The resulting powder was reduced by heat treatment in a nitrogen atmosphere at 210°C for 3 hours and then at 240°C for 3 hours, yielding a fuel cell catalyst.

[0072] <Preparation of Membrane-Electrode Assembly (MEA)> A Nafion membrane (thickness: 50 μm) was used as the electrolyte membrane. Pt / C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E) was dispersed in a mixed solvent containing a Nafion dispersion (5% Nafion (registered trademark) dispersion DE521 CS type, manufactured by Fujifilm Wako Chemical Co., Ltd.), water, and ethanol in a volume ratio of 100:53:480 to prepare an anode-forming dispersion containing 46 mass % Pt / C based on the total amount of the dispersion. The amount of Pt in the anode was 0.3 mg / cm. ２ The resulting dispersion was spray-printed onto a Nafion membrane so that the Pt content in the cathode was 0.3 mg / cm. The solvent was removed by drying on a hot plate at 60°C, forming an anode (electrode catalyst layer) on the Nafion membrane. Next, a cathode-forming dispersion was prepared in the same manner as the anode-forming dispersion, except that the fuel cell catalyst described above was used instead of Pt / C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E). 2The resulting cathode-forming dispersion was spray-printed on the opposite side of the Nafion membrane on which the anode had been formed, and the solvent was removed by drying on a hot plate at 60°C, thereby forming a cathode (electrode catalyst layer) on the Nafion membrane. The anode, Nafion membrane, and cathode were then pressed together at 0.3 kN and 130°C, and sandwiched between two sheets of water-repellent carbon paper (manufactured by Toray Industries, Inc., model number EC-TP1-060T) as gas diffusion layers, to obtain an MEA.

[0073] <Concentration overvoltage> As shown in FIG. 2, the concentration overvoltage (η mt ) is the theoretical electromotive force (E rev ) and the operating potential, the ohmic overvoltage (η ohm ) and the non-ohmic overpotential (η non-ohm ) to obtain the activation overvoltage (η a Here, the ohmic overvoltage is calculated by subtracting the IR loss (η IR ), which is a voltage drop caused by the electrical resistance of the electrodes and resistance to the flow of electrolyte ions. Activation overpotential is the voltage drop that occurs when activation energy is consumed to promote the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode. Concentration overpotential is the voltage drop that occurs when oxygen, a reactant, is not sufficiently supplied to the electrode surface due to the accumulation of water vapor, a by-product of the reaction. Of these three overpotentials, the concentration overpotential is the one that is most affected by the properties of the porous carbon for catalyst supports, and therefore, the concentration overpotential was used as an index for evaluating the porous carbon for catalyst supports. Note that a higher current density requires a larger oxygen supply, which results in a larger amount of water vapor discharge, making it difficult to supply oxygen and discharge water vapor, and therefore the proportion of the concentration overpotential to the total overpotential increases. For this reason, a current density of 700 [mA / cm], which is relatively high, is considered to be a significant overpotential. 2The concentration overpotential value at this time was used as an index for evaluating porous carbon for catalyst supports. The ohmic overpotential, activation overpotential, and concentration overpotential can be separated according to the test title: IV measurement method / overpotential separation analysis method (hereinafter sometimes referred to as the "NEDO protocol test method") described in the New Energy and Industrial Technology Development Organization (NEDO) cell evaluation protocol (NEDO PEFC Cell Evaluation and Analysis Protocol 2023 Edition). The specific procedures are shown in (1) to (4) below. Note that steps (i) to (iii) and (v) to (vi) correspond to steps [1] to [3] and [5] to [6] described in the NEDO protocol test method. Note that step [4] in the NEDO protocol test method is a calculation step for activation overpotential, but does not directly contribute to the calculation of concentration overpotential, so it is omitted here. (1) IV measurement was performed on the MEA according to the conditions described in the NEDO protocol test method. (2) The theoretical electromotive force was calculated according to the NEDO protocol test method. (3) The ohmic overvoltage was calculated from the ohmic resistance obtained by the known AC impedance method according to (v) below. (v) The current density (A / cm) at the point to be analyzed was calculated. 2 ) x internal resistance value (Ω cm 2 ) was defined as the resistance overvoltage (synonymous with ohmic overvoltage). (4) Using the ohmic overvoltage calculated in (3), the concentration overvoltage was calculated according to the following steps (i) to (iii) and (vi). (i) Based on the IV measurement data obtained in (1), a Tafel plot was created with the horizontal axis representing current density (logarithmic axis) and the vertical axis representing IR-free voltage (working potential plus ohmic overvoltage). (ii) The regression line was calculated from three to four linear points on the low current density side. (iii) The current density value at the point to be analyzed was substituted for x in the equation to determine the voltage y on the regression equation. (vi) The difference between the voltage y value on the regression equation at current density x and the IR-free voltage value was defined as the diffusion overvoltage (synonymous with concentration overvoltage). A more detailed procedure is described below.

[0074] <Procedure (1)> A single cell power generation evaluation jig (manufactured by Nippon FC Kikaku Co., Ltd.) incorporating an MEA was placed in a thermostatic chamber set at 80°C, and IV measurements were carried out under the following conditions. For the IV measurements, a fuel cell evaluation device (AutoPEM-KUG2, manufactured by Toyo Corporation) and a potentio / galvanostat (SP-240, manufactured by BioLogic Inc.) were used. As an example, the IV curve obtained by the above IV measurement of the MEA produced in Example 1 is shown in Figure 3. Current density 700 [mA / cm 2 The operating potential at 0.514 V was 0.514 V. (Anode conditions) Electrode area: 1 cm 2 Supply gas type: 100% H2 Gas supply rate: 139 mL / min Supply gas humidification temperature: 80°C (relative humidity: 98%) (cathode conditions) Electrode area: 1 cm 2 Supply gas type: Air Gas supply rate: 332 mL / min Supply gas humidification temperature: 80°C (relative humidity: 98%)

[0075] <Procedure (2)> The following formula, as described in the NEDO protocol test method, is used to calculate the test conditions: T = 80°C (353K), (PH ２ / PH * 2) = 1.0, (PO ２ / PO * 2) = 0.21, and calculate the theoretical electromotive force E rev(PH2,PO2,T) The calculated value was 1.17 [V].

[0076] <Procedure (3)> AC impedance measurement was carried out under the following conditions to measure the ohmic resistance. (Conditions for AC impedance measurement) Applied current density: 20±2 mA / cm 2 ] Frequency: 100 [kHz] to 100 [MHz] The ohmic resistance in Example 1 is 0.0875 [Ω cm 2 From the obtained ohmic resistance, the ohmic overvoltage was calculated according to (v) of the NEDO protocol test method. The current density at the point to be analyzed was 0.700 [A / cm 2 ] and internal resistance value 0.0875 [Ω cm 2 ] was multiplied by 0.0613 [V] to calculate the ohmic overvoltage (0.0613 [V]).

[0077] <Procedure (4)> (i) Based on the IV measurement data obtained in (1), a Tafel plot was created with the horizontal axis representing current density (logarithmic axis) and the vertical axis representing IR-free voltage. The Tafel plot of the MEA produced in Example 1 is shown in Figure 4 (L in Figure 4). １ (ii) The regression line equation was determined from three to four linear points on the low current density side. ２ ) is [wherein A = -0.0360, B = 0.908]. (iii) Current density 700 [mA / cm 2 In Example 1, the current density x was set to 700 [mA / cm 2 ] and the voltage y was calculated. 2 ] (Y in FIG. 4) １ ) was 0.672 [V].

[0078] <Procedure (5)> (vi) Current density x (700 [mA / cm 2 ]) the value of voltage y on the regression equation (Y in Figure 4 １ ) and IR-free voltage value (Y in Figure 4 ２ The concentration overpotential was calculated from the difference between the current density x and the current density x. 2 ] (η in Figure 4) mt )) was 0.672-(0.514+0.0613)=0.0967(V).

[0079] Example 2 A fuel cell catalyst and an MEA were produced in the same manner as in Example 1, except that the amount of calcium chloride dihydrate was changed to 1 g (100 parts by mass per 100 parts by mass of glucose).

[0080] Example 3 A fuel cell catalyst and an MEA were produced in the same manner as in Example 1, except that the carbon source was changed to 0.5 g of glucose (available from Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.5 g of cellulose (available from Fujifilm Wako Pure Chemical Industries, Ltd.), the amount of calcium chloride dihydrate was changed to 3 g (300 parts by mass per 100 parts by mass of the carbon source), and the amount of ion-exchanged water was changed to 1.5 g.

[0081] Comparative Example 1 A fuel cell catalyst and MEA were produced in the same manner as in Example 1, except that Ketjen Black manufactured by Lion Specialty Chemicals Co., Ltd. was used as the catalyst support.

[0082] Comparative Example 2: 1 g of polyvinyl alcohol (PVA) and 4 g of magnesium citrate (available from Fujifilm Wako Pure Chemical Industries, Ltd.) (400 parts by weight per 100 parts by weight of PVA) were mixed. The resulting mixture was heated to 700°C at a rate of 10°C / min in a nitrogen gas atmosphere and heat-treated at this temperature for 60 minutes to obtain a carbonized product. The carbonized product was then immersed in 0.2 L of 1 mol / L sulfuric acid and washed by stirring at 80°C for 30 minutes, after which it was removed onto a Buchner funnel. Water washing was performed until the pH of the filtrate reached a range of 6 to 8. Acid washing and water washing were repeated three times, followed by hot air drying at 80°C. After hot air drying, the carbonized product was further heated to 1200°C in a nitrogen gas atmosphere and heat-treated (calcined) for 60 minutes to obtain porous carbon. A fuel cell catalyst and MEA were otherwise manufactured in the same manner as in Example 1.

[0083] The physical properties of the porous carbon and fuel cell catalyst in the examples and comparative examples are shown in Table 1. The porous carbon in examples 1 to 3 and comparative examples 1 and 2 was in a particulate form.

[0084]

[0085] The porous carbon of the present invention can provide a low concentration overvoltage when used as a catalyst support in a fuel cell, and therefore can be suitably used as a fuel cell catalyst support.

Claims

1. In the measurement of the nitrogen adsorption isotherm, the pressure change of nitrogen until adsorption equilibrium is reached at each predetermined relative pressure is converted into a mass transfer coefficient using the LDF approximation, and the relative pressure is 1.0 × 10 －４ or more and 1.0 × 10 －３ or less. The slope of the approximate straight line obtained by linearly approximating the relationship between the relative pressure and the mass transfer coefficient is 2.5 or more. The porous carbon for a fuel cell catalyst support.

2. The pore volume of pores with a pore diameter of 2 nm or more and 200 nm or less calculated by the BJH method from the nitrogen adsorption isotherm is 0.8 cm ３ / g or more, and the porous carbon according to claim 1.

3. In the approximate straight line, the mass transfer coefficient when the relative pressure is 1.0 × 10 －３ is 3.0 × 10 －３ seconds －１ or more. The porous carbon according to claim 1.

4. The porous carbon according to claim 1, wherein the average primary particle diameter is 500 nm to 5 μm.

5. The bulk density is less than 0.10 g / cm ３ The porous carbon according to claim 1.

6. A fuel cell catalyst comprising the porous carbon according to claim 1 and a catalyst metal, wherein the porous carbon supports the catalyst metal.

7. A fuel cell comprising the fuel cell catalyst according to claim 6.

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