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

Technical Field

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

Background Art

[0002] As a catalyst for the oxidation reaction occurring at the anode and the reduction reaction occurring at the cathode in a fuel cell, mainly platinum or a platinum alloy having high reaction activity, such as metals and alloys, is used. Further, as a carrier for supporting the catalyst metal, a carbon material, specifically porous carbon, is generally used.

[0003] For example, in Patent Document 1, as a catalyst support for a solid polymer fuel cell, the specific surface area of mesopores (pores having a pore diameter of 2 to 50 nm) is 600 to 1600 m 2 / g, and a porous carbon material having a specific intensity ratio between the G-band and the G'-band and a specific G'-band peak position in the Raman spectrum is described.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In fuel cells, the catalyst support on the cathode side experiences a phenomenon where water molecules moving from the anode side to the cathode side along with protons, and water vapor generated by the reduction reaction on the cathode side, accumulate in the catalyst layer. This results in insufficient diffusion and supply of reactants (i.e., oxygen gas) to the catalyst layer, leading to a decrease in fuel cell output. The voltage drop loss due to the above phenomenon is called concentration overpotential, and concentration overpotential causes a decrease in fuel cell output. Therefore, to improve the output performance of fuel cells, it is important to improve the gas diffusivity in the catalyst support, suppress the above phenomenon, and supply reactant gas to the electrodes to produce a low concentration overpotential. According to the inventors' studies, the porous carbon material for polymer electrolyte fuel cells described in Patent Document 1 has a dendritic structure to improve gas diffusivity and a relatively large specific surface area of ​​mesopores to efficiently support catalyst metal particles. However, there is always a need for further improvement in the output performance of fuel cells. The object of the present invention has been addressed in view of the above circumstances, and is to provide a porous carbon for fuel cell catalyst support that can suppress the above phenomenon and produce a low concentration overvoltage when used as a catalyst support in a fuel cell. [Means for solving the problem]

[0006] In order to solve the aforementioned problems, the inventors of the present invention have conducted detailed studies on porous carbon and have completed the present invention. That is, the present invention encompasses the following preferred embodiments. [1] In the measurement of nitrogen adsorption isotherms, the pressure change of nitrogen until adsorption equilibrium is reached at each predetermined relative pressure is converted to a mass transfer coefficient using the LDF approximation, and the relative pressure is 1.0 × 10 -4 The above 1.0 × 10 -3 The slope of the approximate straight line obtained by linearly approximating the relationship between relative pressure and mass transfer coefficient in the following case is 2.5 or greater, for porous carbon for fuel cell catalyst support. [2] The pore volume of pores with a diameter of 2 nm to 200 nm, calculated by the BJH method from nitrogen adsorption isotherms, was 0.8 cm³. 3 Porous carbon as described in [1], which is 1 / g or more. [3] In the approximate straight line, relative pressure 1.0 × 10 -3The mass transfer coefficient at this time is 3.0 × 10 -3 seconds -1 The porous carbon described above, as in [1] or [2]. [4] A porous carbon described in any of [1] to [3], having an average primary particle size of 500 nm to 5 μm. [5] The bulk density is 0.10 g / cm³. 3 A smaller porous carbon as described in any of [1] to [4]. A fuel cell catalyst comprising porous carbon and a catalytic metal as described in any of [6][1] to [5], wherein the porous carbon supports the catalytic metal. A fuel cell comprising the fuel cell catalyst described in [7] and [6]. [Effects of the Invention]

[0007] According to the present invention, a porous carbon material for fuel cell catalyst supports can be provided that, when used as a catalyst support in a fuel cell, can produce a low concentration overpotential. [Brief explanation of the drawing]

[0008] [Figure 1] This graph shows the relationship between relative pressure and mass transfer coefficient for the porous carbon of Example 1. [Figure 2] This figure shows the relationship between the IV characteristics and various overvoltages in a fuel cell. [Figure 3] This graph shows the IV curve in Example 1. [Figure 4] This graph shows the Tafel plot and its regression line in Example 1. [Modes for carrying out the invention]

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

[0010] [Porous carbon] The porous carbon for a fuel cell catalyst of the present invention, in the measurement of the nitrogen adsorption isotherm of the porous carbon, converts the pressure change of nitrogen until adsorption equilibrium is reached at each predetermined relative pressure (P / P0) into a mass transfer coefficient using the LDF (Linear Driving Force) approximation, and the relative pressure is 1.0×10 -4 or more and 1.0×10 -3 or less, the relationship between the relative pressure and the mass transfer coefficient is linearly approximated, and the slope of the approximated straight line obtained thereby is 2.5 or more. Here, P is the adsorption equilibrium pressure and P0 is the saturated vapor pressure.

[0011] <Relationship between mass transfer coefficient and relative pressure and mass transfer coefficient> The mass transfer coefficient of the porous carbon is an index representing the speed of movement of reactant gas (e.g., oxygen) and product gas (e.g., water vapor) when they move inside the porous carbon in the porous carbon for a fuel cell catalyst. The greater the change in the mass transfer coefficient with respect to the change in relative pressure, the smoother the reactant gas can move from the mesopores to the micropores, and the smoother the product gas can move from the micropores to the mesopores. Also, the greater the mass transfer coefficient, the faster the movement speed of the reactant gas and the product gas inside the pores of the porous carbon. Therefore, the change in the mass diffusivity in the porous carbon. Here, according to IUPAC, micropores refer to pores having a diameter of less than 2 nm, and mesopores refer to pores having a diameter of 2 to 50 nm. The relationship between the relative pressure and the mass transfer coefficient, and the mass transfer coefficient can be examined by adsorbing nitrogen gas on the porous carbon at -196 °C to create an adsorption isotherm and analyzing this adsorption isotherm. Specifically, for example, as described in the examples below, an adsorption isotherm of nitrogen for the porous carbon is created, and 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. Next, by creating a graph with the relative pressure on the x-axis and the mass transfer coefficient on the y-axis, the relationship between the relative pressure and the mass transfer coefficient can be represented.

[0012] The inventors of the present invention, in this graph, when the relative pressure is 1.0×10 -4 or more and 1.0×10-3 We found that porous carbon can exhibit excellent gas diffusivity when the slope of the approximate straight line drawn within the following range [in this specification, this slope may be referred to as (a)] is 2.5 or greater. Here, the relative pressure range is 1.0 × 10 -4 The above 1.0 × 10 -3 The reason for the settings is explained below. In this specification, pores with a diameter of approximately 2 to 200 nm are assumed as diffusion pathways for reactant gases and product gases, such as oxygen or water vapor, that diffuse into porous carbon. If the relative pressure is too low, that is, if the relative pressure is 1.0 × 10⁻⁶ -4 Below 1.0 × 10⁻¹⁰, the diffusion behavior in pores smaller than those that effectively act as pathways for the movement of reactant and product gases is thought to be reflected in the mass transfer coefficient. On the other hand, if the relative pressure is too high, i.e., the relative pressure is 1.0 × 10⁻¹⁰ -3 If the relative pressure is larger, it is thought that the diffusion behavior in pores larger than those that effectively function as pathways for the movement of such reactant and product gases will be reflected in the mass transfer coefficient. -4 The above 1.0 × 10 -3 By setting the parameters as follows, 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 the slope of the approximate straight line in this range can be considered as one of the indicators of the gas diffusivity.

[0013] A more specific method for determining (a) will be explained using the graph shown in Figure 1, which represents the relationship between relative pressure and mass transfer coefficient for the porous carbon in Example 1 described later. First, as described in the examples below, a graph showing the relationship between relative pressure and mass transfer coefficient was created, with a relative pressure of 1.0 × 10 -4 The above 1.0 × 10 -3 A straight line is drawn within the following range (in Figure 1, this range is represented by a dashed line and arrows). Next, the slope (a) of this straight line is calculated.

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

[0015] The relative pressure of the approximate straight line within the aforementioned range is 1.0 × 10 -3 The mass transfer coefficient at this time [this value may be referred to as (b) in this specification] is preferably 3.0 × 10 -3 seconds -1 The above is not an exhaustive list, but it is usually 1.0 × 10 -2 seconds -1 The following applies: (b) is more preferably 3.0 × 10 -3 ~1.0×10 -2 seconds -1 , 3.5×10 -3 ~8.0×10 -3 seconds -1 , 4.0×10 -3 ~6.0×10 -3 seconds -1 Therefore, if (b) is greater than or equal to the lower limit, porous carbon tends to have a pore structure suitable for the diffusion of reactant gases and the discharge of generated gases, and thus can exhibit excellent gas diffusivity. To explain using the graph shown in Figure 1, (b) is relative pressure 1.0 × 10 -3 This is the mass transfer coefficient when (the dashed line on the right or the right end of the arrow) is reached.

[0016] The aforementioned "pore structure suitable for the diffusion of reactant gases and the discharge of product gases" is considered to be a pore structure in which there are many pores that effectively function as pathways for the movement of reactant gases and product gases, the openings of such pores have dimensions suitable for the entry and exit of reactant gases and product gases, there is a high proportion of pores that are connected from one end to the other (pores that have an opening at one end and are not blocked at the other end), there are few structures that hinder the movement of reactant gases and product gases in the connecting parts (e.g., bottlenecks), and the gas movement pathways (movement pathways for discharging product gases that are unnecessary for the chemical reaction in a fuel cell and supplying reactant gases necessary for the chemical reaction onto the catalyst metal supported on porous carbon) are relatively short. Therefore, the porous structure of the porous carbon of the present invention is distinct from a "three-dimensional network structure" (e.g., Japanese Patent Application Publication No. 2011-1224) formed by mixing a carbon source and template particles (pore source) and carbonizing them, then removing the aggregated and connected template particles, resulting in voids (continuous pores created by removing the existing template particles) and the surrounding carbon skeleton. In such a structure, the length of the paths through which reactant gases and product gases move within the porous carbon is relatively long, and there are many structures (e.g., bottlenecks) that hinder the movement of reactant gases and product gases corresponding to the regions between existing template particles or the gaps between continuously existing template particles. Consequently, in the case of a "three-dimensional network structure," the numerical ranges of (a) and / or (b) for the porous carbon of the present invention are not satisfied. Furthermore, the pore structure of the porous carbon of the present invention is distinct from pore structures such as the "three-dimensional dendritic structure" (e.g., Japanese Patent Application Publication No. 2018-174078), which is formed by a three-dimensional branched structure of rod-shaped or ring-shaped bodies containing carbon. In such structures, the length of the paths through which reactant gases and product gases move within the porous carbon is relatively long, and there are many structures (e.g., bottlenecks) that hinder the movement of reactant gases and product gases in areas between existing template particles or gaps between continuously existing template particles. Therefore, in the case of the "three-dimensional dendritic structure," the numerical ranges of (a) and / or (b) for the porous carbon of the present invention are not satisfied. In one preferred embodiment of the present invention, all pores are considered to be in communication from the opening at one end to the opening at the other end. Furthermore, (a) above is thought to correspond to the length of the path through which reactant gases are supplied to the porous carbon and product gases are discharged from the porous carbon, and the larger (a) is, the shorter the length of the path. (b) above is thought to correspond to the amount of structure that hinders the movement of reactant gases and product gases in the connecting section, and the larger (b) is thought to correspond to the smaller the structure that hinders the movement of reactant gases and product gases.

[0017] Porous carbon in which (a) is 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 of high miscibility to separate the phases, then carbonizing it to fix the phase-separated structure, and removing the pore source from the resulting carbide. For example, in the method for producing porous carbon described later, (a) can be adjusted to be equal to or greater than the lower limit or within the range by appropriately adjusting the temperature and time when heat-treating the mixture of the carbon source and a calcium compound that serves as the pore source, or by changing the type of carbon source and appropriately adjusting the temperature and time when heat-treating the mixture. Similarly, (b) can also be adjusted to be equal to or greater than the lower limit or within the range by appropriately adjusting the same conditions.

[0018] <Pore volume of pores with a diameter of 2 nm or more and 200 nm or less> In porous carbon, the lower limit of the pore volume (hereinafter sometimes referred to as "pore volume of 2 nm to 200 nm") calculated from nitrogen adsorption isotherms by the BJH method is preferably 0.8 cm³. 3 The value must be at least / g, and there is no particular upper limit, but it is usually 4.00cm. 3 The density is less than or equal to / g. The pore volume between 2nm and 200nm is preferably 0.80 to 4.00 cm³. 3 / g, more preferably 0.90-3.90cm 3 / g, more preferably 1.00 to 3.80 cm 3 / g, more preferably 1.30-3.70 cm 3 / g, particularly preferably 1.60 to 3.60 cm 3 / g, more preferably 1.90-3.50 cm3 / g, most preferably 2.00~3.40cm 3 / g, more preferably 2.20-3.30 cm 3 The value is / g. Pores between 2nm and 200nm are thought to be involved in the diffusion of reactant gases and product gases, and in the highly dispersed support of catalyst metals on porous carbon. High dispersion refers to a state in which catalyst metals are dispersed and supported on porous carbon at appropriate distances. If the spatial distribution of catalyst metals within porous carbon is sufficiently sparse, 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, thus enabling efficient diffusion and supply of oxygen gas and discharge of water vapor. Generally, oxidation and reduction reactions in fuel cells occur on catalyst metals. Therefore, from the viewpoint of increasing the efficiency of catalyst utilization, catalyst metals generally have a size of several nanometers (for example, an average particle diameter of 2-3 nm) in order to increase the specific surface area per unit mass. Accordingly, if the pore volume between 2nm and 200nm is greater than or equal to the lower limit, catalyst metals can be supported on porous carbon in a highly dispersed state, achieving good discharge of product gases and smooth supply of reactant gases.

[0019] The pore volume between 2 nm and 200 nm can be adjusted to be above the lower limit, below the upper limit, or within the range by appropriately adjusting the type and / or ratio of the carbon source and calcium compound; the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process), etc., in the porous carbon manufacturing method described later. The pore volume between 2 nm and 200 nm can be calculated by analyzing the nitrogen adsorption isotherm of porous carbon using the BJH method, and can be determined, for example, by the method described in the examples below.

[0020] <Mode diameter> In porous carbon, the mode diameter of the pores calculated from nitrogen adsorption isotherms by the BJH method (hereinafter sometimes simply referred to as "mode diameter") may be, for example, 2-200 nm, 5-190 nm, or 10-180 nm, depending on the size of the supported catalyst 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 respect to the common logarithm of the pore diameter (D). When the mode diameter is less than or equal to the upper limit, good utilization efficiency of the supported catalyst can be achieved. The smaller the mode diameter, the more dispersed the catalyst metal can be supported, and the larger the mode diameter, the better the gas diffusion tends to be.

[0021] The mode diameter can be adjusted within the aforementioned range by appropriately adjusting, for example, the type and / or proportion of the carbon source and calcium compound; the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process) in the porous carbon manufacturing method described later. The mode diameter can be calculated by analyzing the nitrogen adsorption isotherm of porous carbon using the BJH method, and can be determined, for example, by the method described in the examples below.

[0022] <Bulk density> In porous carbon, bulk density indicates the degree of pore structure development. When the pore volume of pores with a diameter of 2 nm to 200 nm is the same, a lower bulk density indicates a larger pore volume of pores with a diameter greater than 200 nm. Pores with a diameter of 2 nm to 200 nm contribute to the movement of reactant gases and product gases, and to the highly dispersed support of catalyst metals in porous carbon, while pores with a diameter greater than 200 nm are thought to act as entry points or migration pathways for the smooth movement of reactant gases, product gases, and catalyst metals into pores with a diameter of 2 nm to 200 nm. Therefore, in addition to the pore volume of pores with a diameter of 2 nm to 200 nm, it is preferable to have a large pore volume of pores with a diameter of 200 nm or more, i.e., a low bulk density.

[0023] In one preferred embodiment, the bulk density of the porous carbon is preferably 0.10 g / cm³. 3 The lower limit is smaller, and is not particularly limited, but is usually 0.001 g / cm³. 3 That concludes the explanation. The bulk density of porous carbon is preferably 0.001 to 0.10 g / cm³. 3 Less than, more preferably 0.004 to 0.08 g / cm³ 3 More preferably 0.008 to 0.06 g / cm³ 3 Particularly preferred is 0.01 to 0.05 g / cm³. 3 When the bulk density is less than or equal to the upper limit, the reactant gas and product gas can move smoothly into the pores with a diameter of 2 nm to 200 nm, resulting in excellent gas diffusion. When the bulk density is equal to or greater than the lower limit, scattering can be suppressed when porous carbon is used as a catalyst support, improving handling. The bulk density can be adjusted to be below the upper limit or within the range by appropriately adjusting, for example, the type and / or proportion of the carbon source and calcium compound; the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process) in the porous carbon manufacturing method described later. The bulk density can be measured by the method described in the examples below.

[0024] <Pore volume of pores with a diameter of less than 2 nm> In one preferred embodiment, the pore volume of porous carbon with a pore diameter of less than 2 nm, calculated by DFT from the nitrogen adsorption isotherm, is preferably 0.35 cm³. 3 The value is less than or equal to / g, and the lower limit is not particularly limited, but preferably 0.01cm 3 The concentration is 1 / g or more. The pore volume of pores with a diameter of less than 2 nm is preferably 0.01 to 0.35 cm³. 3 / g, more preferably 0.02~0.32cm 3 / g, more preferably 0.04~0.30cm 3 / g, particularly preferably 0.06~0.28cm 3 / g, more preferably 0.08~0.25cm 3 / g, more preferably 0.10-0.23cm3 / g, and more 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 the generated gas (water vapor) is retained. When the pore volume of pores with a pore diameter of less than 2 nm is below the above upper limit value, it becomes difficult to retain the generated gas in the porous carbon, so that good discharge of the generated gas becomes possible. In addition, when the pore volume of pores with a pore diameter of less than 2 nm is below the above upper limit value, the pore volume of pores with a pore 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 pore diameter of less than 2 nm can be adjusted to be not less than the above lower limit value and not more than the above upper limit value, or within the above range, by appropriately adjusting, for example, the types and / or ratios of the carbon source and the 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 pore diameter of less than 2 nm can be determined by analyzing the adsorption isotherm obtained by nitrogen adsorption measurement of porous carbon by the QS-DFT method and calculating the volume of pores having a pore diameter of less than 2 nm.

[0025] <Specific surface area calculated by BET method> In a preferred embodiment, the specific surface area calculated by the BET method from the nitrogen adsorption isotherm of the porous carbon is preferably 300 to 1500 m 2 / g, more preferably 400 to 1400 m 2 / g, still more preferably 450 to 1300 m 2 / g, particularly preferably 500 to 1200 m 2The specific surface area is / g. When the specific surface area is within the above range, porous carbon tends to have many pores with a diameter of 2 to 200 nm, which are suitable for diffusing reactant gases and product gases and supporting catalyst metals in a highly dispersed state, and therefore excellent gas diffusion properties are easily exhibited. The specific surface area can be adjusted to within the above range, for example, in the porous carbon manufacturing method described later, by appropriately adjusting the type and / or ratio of the carbon source and calcium compound; the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process), etc. The specific surface area can be calculated by measuring the adsorption isotherm of porous carbon, performing a multipoint analysis of this adsorption isotherm using the BET formula, and approximating the curve obtained from the relative pressure range of 0.05 to 0.1.

[0026] <Calcium content> In one preferred embodiment, the calcium content in 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, it is easier to suppress an excessive increase in the mass of the porous carbon, and productivity tends to be excellent. The calcium content can be adjusted within the above range, for example, by appropriately adjusting the conditions of the step in removing calcium compounds in the porous carbon manufacturing method described later (e.g., the type and / or concentration of the acid used for acid washing, the time and / or temperature of acid washing, etc.).

[0027] <Sulfur content and silicon content> In a preferred embodiment, the sulfur and silicon content in porous carbon is preferably 1000 ppm or less, and their lower limits are not particularly limited, but are 0 ppm. The sulfur content in porous carbon is preferably 0 to 1000 ppm, more preferably 0 to 900 ppm, and even more preferably 0 to 800 ppm. Also in a preferred embodiment, the silicon content in 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 content is below or within the above upper limits, it is easier to suppress an excessive increase in the mass of porous carbon, and productivity tends to be excellent. The sulfur and silicon content can be adjusted to below or within the above upper limits, or within the above range, for example, in the method for producing porous carbon described later, by appropriately selecting the type of carbon source; and / or appropriately adjusting the conditions of the step for removing calcium compounds (for example, the type and / or concentration of the acid used for acid washing, the time and / or temperature of acid washing, etc.). The calcium, sulfur, and silicon content can be measured by X-ray fluorescence analysis.

[0028] <Average primary particle diameter> The shape of the porous carbon may be, for example, particulate, flaky, layered, or thin flake, but it is preferably particulate or thin flake. In one preferred embodiment, the porous carbon is preferably in particulate form. 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 decreases, and the conductivity of the electrode tends to be excellent. The average primary particle diameter can be adjusted within the aforementioned range, for example, by appropriately selecting the type of carbon source in the porous carbon manufacturing method described later, and / or by appropriately adjusting the conditions of the grinding process. In this specification, the average primary particle diameter is the particle diameter at which the cumulative volume measured by laser diffraction-scattering method reaches 50%. If measurement by laser diffraction-scattering method is not possible, the average primary particle diameter may refer to the average particle diameter obtained by measuring the particle diameter of primary particles displayed in an electron microscope image and calculating the average value.

[0029] [Method for producing porous carbon] The porous carbon of the present invention is, for example, (1) A step of obtaining a mixture containing a carbon source and a calcium compound, (2) A step of heat-treating the mixture in an inert gas atmosphere to separate the carbon source from the calcium compound. (3) A step of obtaining a carbide by heat-treating the phase-separated mixture in an inert gas atmosphere, (4) A step of removing calcium compounds from the carbide to obtain porous carbon, and (5) If necessary, the process of crushing the porous carbon. It can be manufactured by a method that includes [a specific component].

[0030] <Process (1)> The carbon source is not particularly limited. From the viewpoint of increasing compatibility with calcium compounds that serve as pore sources, 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 individually or in combination of two or more. Among these sugars, glucose and starch are preferred from the viewpoint of easily producing porous carbon that exhibits excellent gas diffusion properties and being readily available in large quantities.

[0031] The starch is not particularly limited, and for example, starch derived from corn, cassava, potato, sweet potato, tapioca, beans, wheat, rice, etc., can be used. The preferred amylose content of the starch is preferably 50% by mass or less, more preferably 30% by mass or less, relative to the mass of the starch. The lower the amylose content of the starch, the lower the gelatinization temperature tends to be. Therefore, it is preferable that the amylose content of the starch is below the above upper limit because it is easier to gelatinize at low temperatures and the compatibility with calcium compounds that serve as pore sources is easily increased. The above amylose content can be determined, for example, by the iodine colorimetric method. Furthermore, the starch may be modified starch. Examples of modified starches include etherified starch, esterified starch, cationized starch, and crosslinked starch. One type of starch may be used alone or two or more types may be used in combination. When two or more types 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 improving compatibility with the carbon source, the pore source is preferably a calcium compound. The calcium compound is not particularly limited, and for example, 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, calcium lactate, etc. can be used. Among these calcium compounds, from the viewpoint of easily producing porous carbon that exhibits excellent gas diffusion properties, a calcium compound with a melting point of 300°C or less (if the mixture contains water and / or polyhydric alcohols and / or carboxylic acids as described later, the melting point of the eutectic compound of the calcium compound, carbon source and water and / or polyhydric alcohols and / or carboxylic acids is 300°C or less) is preferred, and it is more preferable that it be at least one selected from the group consisting of calcium chloride hydrate, calcium hydroxide, calcium oxide, calcium carbonate, and calcium acetate. Calcium chloride hydrate exists in dihydrate, tetrahydrate, and hexahydrate forms, and any of these hydrates may be used, but the dihydrate is preferred because it has good reactivity with sugars.

[0033] The mixture containing the carbon source and the calcium compound may further contain at least one selected from the group consisting of water, polyhydric alcohols, and carboxylic acids. When water and / or polyhydric alcohols and / or carboxylic acids are present in the mixture, it is believed that the carbon source and the calcium compound become miscible via the water and / or polyhydric alcohols and / or carboxylic acids, forming a eutectic compound. Therefore, the melting point of the calcium compound, which has a melting point of 300°C or higher on its own, can be reduced to 300°C or lower as the melting point of the eutectic compound. Accordingly, in this specification, "the melting point of the calcium compound is 300°C or lower" also includes "the melting point of the eutectic compound of water and / or polyhydric alcohols and / or carboxylic acids, the carbon source, and the calcium compound is 300°C or lower." Water is preferred from the viewpoint of its easy availability. Examples of usable polyhydric alcohols include glycerin, ethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol. Among these polyhydric alcohols, glycerin and ethylene glycol are preferred from the viewpoint of easily dissolving calcium compounds and being readily available in large quantities. Examples of usable carboxylic acids 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 from the viewpoint of readily dissolving calcium compounds and being readily available in large quantities. When using a mixture of water, one or more polyhydric alcohols, and two or more carboxylic acids, the ratio of water, polyhydric alcohols, and two or more carboxylic acids can be appropriately changed according to the desired properties of the porous carbon.

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

[0035] The amount of 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, per 100 parts by mass of the carbon source. When the amount of calcium compound is within the above range, the relationship between the mass transfer coefficient and relative pressure, and the pore volume of the resulting porous carbon can be suitably determined.

[0036] If the mixture containing the carbon source and calcium compound further contains water and / or polyhydric alcohols and / or carboxylic acids, the amount of water and / or polyhydric alcohols and / or carboxylic acids (the total amount if two or more of water, one or more polyhydric alcohols, and one or more carboxylic acids are used) 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, if the mixture containing the carbon source and calcium compound contains water but no polyhydric alcohols and carboxylic acids, 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 polyhydric alcohols and / or carboxylic acids is within the above range, eutectic compounds are more likely to form, which allows for a favorable relationship between the mass transfer coefficient and relative pressure, and the pore volume of the resulting porous carbon.

[0037] <Process (2)> In step (2), the mixture obtained in step (1) is heat-treated in an inert gas atmosphere to separate the carbon source from the calcium compound. 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, especially oxygen, is usually 1% by volume or less, more preferably 0.1% by volume or less, relative to the volume of the gas used. When the concentration of the oxidizing gas is below the above upper limit, oxidation of the mixture is suppressed, making it easier to obtain a structure with the desired characteristics. In addition, oxidative decomposition of the generated structure can be suppressed. The supply rate (flow rate) of the inert gas is typically 200 to 7000 mL / min per gram of the mixture, preferably 500 to 6000 mL / min, and more preferably 1000 to 5000 mL / min.

[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 preferably greater than 300°C to 420°C, and even more preferably greater than 300°C to 400°C. The inventors have found that a characteristic pore structure of porous carbon can be obtained by heat-treating the mixture obtained in step (1) in an inert gas atmosphere, preferably at a temperature within the above range. The reason for this is not clear, but the following non-limiting mechanism of action is considered.

[0039] As a result of the above heat treatment, the carbon source and the calcium compound, which was miscible with the carbon source by hydrogen bonding, undergo phase separation in the mixture, starting with the dehydration of 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 it is thought that the structure in which the carbon source and the calcium compound have undergone phase separation is fixed. At this time, at least a portion of the phase-separated calcium compound (preferably 50% by mass or more of the phase-separated calcium compound, more preferably almost all or all of it) is continuously connected in three dimensions. In the subsequent carbonization step, the carbon source is made insoluble in hot water or hot acid by aromatization (carbonization), and in the subsequent calcium compound removal step, the calcium compound is removed from the carbide (preferably by washing with acid). Since the area where the calcium compound has been removed becomes a pore, it is thought that the characteristic pore structure of porous carbon is obtained by the phase separation of the calcium compound by the above heat treatment.

[0040] The heating rate during the above heat treatment process is preferably 2°C / min or higher, and while there is no particular upper limit, it is preferably 200°C / min or lower from the viewpoint of easily achieving uniform heat treatment. The heating rate during the above heat treatment process 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 heating rate is above the lower limit, it is easier to obtain the desired pore volume and / or the desired mass transfer coefficient.

[0041] The heat treatment time is appropriately selected according to 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. If the heat treatment time is above the lower limit, phase separation proceeds sufficiently. If the heat treatment time is below the upper limit, it is preferable from an economic standpoint as it is a suitable time.

[0042] Various types of furnaces can be used for heat treatment, including rotary kilns, fluidized bed furnaces, fixed bed furnaces, moving bed furnaces, and moving bed furnaces, as well as various types of dryers, such as spray dryers, mobile dryers, agitated dryers, roll dryers, and thin-film evaporators. Furthermore, either a continuous furnace or dryer, which continuously loads the material to be heat-treated and removes the material after heat treatment, or a batch furnace or batch dryer, which performs the process discontinuously, may be used. The heating means can be any means capable of heating to a predetermined temperature, such as electric heating, gas combustion heating, microwave heating, high-frequency induction heating, or electrostatic heating. These heating means may be used individually or in combination.

[0043] <Process (3)> In step (3), the mixture 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., 500-900°C) followed by a high-temperature heat treatment (e.g., 900-1300°C). The medium-temperature heat treatment temperature is preferably 500 to 900°C, more preferably over 550°C 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. By heat treatment at a medium temperature, the carbon source can be hardened without abrupt changes in the pore structure. By heat treatment at a high temperature thereafter, the crystallinity of the carbon can be improved, and highly conductive porous carbon can be obtained.

[0044] The heating rate during the above heat treatment process is preferably 2°C / min or higher, and while there is no particular upper limit, it is preferably 200°C / min or lower from the viewpoint of easily achieving uniform heat treatment. The heating rate during the above heat treatment process is preferably 2 to 200°C / min, more preferably 5 to 100°C / min, and even more preferably 10 to 50°C / min. If the heating rate is above the lower limit, it is easier to obtain the desired pore volume.

[0045] The heat treatment time is appropriately selected according to 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. If the heat treatment time is above the lower limit, carbonization proceeds sufficiently. If the heat treatment time is below the upper limit, it is preferable from an economic standpoint as it is a suitable time.

[0046] Furthermore, the phase separation step (2) and the medium-temperature heat treatment step (3) may be carried out simultaneously. Doing so can increase productivity. For example, the mixture obtained in step (1) may be subjected to medium-temperature heat treatment at 500 to 900°C. In this case, the desired pore structure can be obtained because the temperature rise in the medium-temperature heat treatment step passes through 150 to 500°C, which is the temperature at which phase separation progresses. To promote phase separation, it is necessary to slow down the rate of heating up to the medium temperature. Specifically, from the viewpoint of achieving both the promotion of the phase separation structure and the improvement of productivity, the heating 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 the phase separation structure, it is preferable to carry out the phase separation step (2) and the medium-temperature heat treatment step (3) independently.

[0047] The inert gas, its supply amount, and the furnace used for heat treatment can be the same as those used in process (2).

[0048] <Process (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 for acid washing include hydrochloric acid, sulfuric acid, and nitric acid. Hydrochloric acid is preferred from the viewpoint of easily dissolving metal compounds in the carbide, leaving fewer impurities such as sulfur behind, and suppressing oxidation of the carbide. The concentration of the acid used during acid washing may be appropriately changed depending on the type of acid used. For example, when using hydrochloric acid, 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 preferable because it is easy to remove metal compounds and less hydrochloric acid remains in the carbide.

[0049] The pH of the acid used during acid washing may be adjusted as appropriate depending on the type, concentration, and temperature of the acid used. The pH of the acid is preferably 3 or lower, more preferably 2.5 or lower. When the pH of the acid is below the aforementioned upper limit, metal compounds can be removed more efficiently.

[0050] Acid cleaning may be carried out, for example, by immersing the obtained carbide in the acid. When acid cleaning is carried out by immersion in acid, the mass ratio of acid to carbide may be appropriately adjusted according to the type, concentration, and temperature of the acid used. The mass of the carbide to be immersed relative to the mass of the acid is preferably 2 to 50% by mass, more preferably 5 to 30% by 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 cleaning effect is easily obtained.

[0051] The method of immersing the carbide in acid is not particularly limited. It may be a method in which the acid is continuously added, left for a predetermined time, and then removed while immersing, or a method in which the carbide is immersed in acid, left for a predetermined time, dehydrated, and then fresh acid is added and the immersion-dehydration process is repeated. It may also be a method in which all of the acid is replaced, or a method in which only part of the acid is replaced. Furthermore, the acid may be stirred during immersion.

[0052] The atmosphere in which acid cleaning is performed is not particularly limited and may be selected as appropriate depending on the cleaning method used. Acid cleaning is usually carried out in an air atmosphere.

[0053] The immersion time of the carbide in the acid can be adjusted as appropriate depending on the acid used, the processing temperature, etc. Preferably, the immersion time of the carbide in the acid is 5 to 60 minutes, more preferably 10 to 40 minutes, and even more preferably 15 to 35 minutes. If the above time is above the lower limit, metal compounds can be easily removed sufficiently, and if it is below the upper limit, good productivity can be ensured.

[0054] It is preferable to remove the acid from the porous carbon by acid washing the carbide and then rinsing with water. This acid washing and rinsing may be repeated until the calcium compounds in the porous carbon are removed to the desired extent. Furthermore, the temperature of the solution during acid washing and rinsing 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 perform acid washing between the medium-temperature heat treatment and the high-temperature heat treatment of the phase-separated mixture. For example, it is preferable to perform acid washing on the phase-separated mixture in step (2) after medium-temperature heat treatment (for example at 500 to 900°C), and then perform high-temperature heat treatment (for example at 900 to 1300°C). In one embodiment of the present invention, when the phase separation step (2) and the medium-temperature heat treatment step (3) are carried out simultaneously, it is preferable to perform acid washing between the phase separation step (2) and the medium-temperature heat treatment step (3), which are carried out simultaneously, and the high-temperature heat treatment. In one of these embodiments, the carbon source is rendered infusible by a medium-temperature heat treatment before washing to remove the calcium compound, so changes in the pore structure due to the dissolution of the carbon source by acid washing are less likely to occur.

[0056] After acid washing and water washing, the porous carbon may be subjected to drying using known dryers such as hot air dryers and vacuum dryers. Drying is preferably carried out at a temperature of 50 to 150°C. A drying temperature within this range is preferable because oxidation of the porous carbon is less likely 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 pulverizing, the shape and particle size of the porous carbon obtained in the end can be controlled to a desired shape and particle size. The pulverization method is not particularly limited. For example, known pulverizers such as ball mills, centrifugal roll mills, ring roll mills, centrifugal ball mills, jet mills, cone crushers, double roll crushers, disc crushers, and rotary crushers can be used individually or in combination.

[0058] The method for producing porous carbon may include a classification step after the grinding step. For example, porous carbon with a narrow particle size distribution can be obtained by removing particles that are extremely small or large compared to the desired particle size. The classification method is not particularly limited. Examples of classification methods include sieving, wet classification, and dry classification. Examples of wet classifiers include those that utilize principles such as gravity classification, inertial classification, hydraulic classification, and centrifugal classification. Examples of dry classifiers include those that utilize principles such as sedimentation classification, mechanical classification, and centrifugal classification. From an economic standpoint, it is preferable to use a dry classifier. To prevent surface oxidation during grinding, it is preferable to carry out the grinding and classification processes in an inert gas atmosphere.

[0059] Grinding and classification can also be performed using a single device. For example, grinding and classification can be performed using a jet mill equipped with a dry classification function. Furthermore, it is also possible to use a device in which the grinder and classifier are independent. In this case, grinding and classification can be performed continuously or discontinuously.

[0060] <Fuel cell catalyst> The porous carbon of the present invention, having a characteristic pore structure, can be suitably used as a support for fuel cell catalysts. The porous carbon can effectively discharge water vapor generated by the chemical reaction (reduction reaction) at the cathode and smoothly supply oxygen, a reactant gas, to the catalyst metal supported on the porous carbon. Therefore, a fuel cell catalyst comprising the porous carbon and catalyst metal of the present invention, wherein the porous carbon supports the catalyst metal, can achieve a low concentration overpotential in a fuel cell. Due to the low concentration overpotential resulting from the fuel cell catalyst, a fuel cell using this fuel ionization catalyst may have improved power characteristics.

[0061] As the catalytic metal in the anode, any known catalyst can be used without particular restrictions, as long as it catalytically reacts to the oxidation of hydrogen. Similarly, as the catalytic metal in the cathode, any known catalyst can be used without particular restrictions, as long as it catalytically reacts to the reduction 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 and alloys thereof, and one or more of these 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 catalyst metal are not particularly limited, and shapes and sizes similar to those of known catalyst components can be used. The shape may be particulate, flaky, layered, etc., but particulate is preferred. In this case, the average particle size (diameter) of the catalyst 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% by mass, more preferably 0.1 to 60% by mass, and more preferably 0.5 to 40% by mass, based on the total mass of the catalyst support and catalyst metal. When the amount of catalyst supported is within the above range, a sufficient amount of catalyst is supported, and a large amount of catalyst metal that functions as a catalyst is present, making it preferable to use it as a catalyst with high utilization efficiency.

[0062] The method for supporting the catalyst metal on the catalyst support is not particularly limited. Known preferred methods include the noble metal acetylacetonate method using noble metal acetylacetonate and the colloidal method using noble metal colloid, and either method is preferable. Preferably, a method is used in which catalyst metal is deposited on the surface of the catalyst support, and then heat-treated to increase the particle size of the catalyst metal.

[0063] <Fuel cell> Fuel cells are generally constructed as follows: A polymer electrolyte membrane with proton conductivity is flanked by catalyst layers containing an ion-conducting binder made of a carbon material supporting a platinum metal catalyst and the polymer electrolyte. Outside each catalyst layer, a gas diffusion layer is formed, which is a porous material that allows fuel gas and oxidizer gas to pass through, respectively. Carbon paper, carbon cloth, etc., are used as the gas diffusion layer. A structure in which a gas diffusion layer is placed on a catalyst layer is called a gas diffusion electrode, and a structure in which a pair of gas diffusion electrodes are joined to the electrolyte membrane so that the catalyst layers face the electrolyte membrane is called a membrane electrode assembly (MEA). Conductive and airtight separators are placed on both sides of this membrane electrode assembly. Gas channels that supply fuel gas or oxidizer gas (e.g., air) to the electrode surface are formed at the contact point between the membrane electrode assembly and the separator, or within the separator. Electricity is generated by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and an oxidizing gas containing oxygen, such as air, to the other electrode (oxygen electrode). Specifically, at the fuel electrode, the fuel is ionized to produce protons and electrons. The protons pass through an 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 oxidizing agent to produce water. In this way, the chemical energy of the fuel can be directly converted and extracted as electrical energy. The porous carbon of the present invention can effectively discharge water vapor generated by the chemical reaction (reduction reaction) at the cathode, and can smoothly supply oxygen, which is a reactant gas, to the catalyst metal supported on the porous carbon. Therefore, a fuel cell using the porous carbon of the present invention as the carbon material for the catalyst layer can achieve a low concentration overpotential. Due to the low concentration overpotential resulting from the fuel cell catalyst, a fuel cell using this fuel ionization catalyst may have improved power characteristics. A fuel cell equipped with the fuel cell catalyst of the present invention can be manufactured using known fuel cell materials and in accordance with known fuel cell manufacturing methods, except that the fuel cell catalyst of the present invention is used. [Examples]

[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] [Analysis methods for porous carbon] <Mass transfer coefficient by nitrogen adsorption method> Porous carbon, the sample to be measured, was packed into a sample tube. This sample tube was placed in a Cantachrome "Autosorb-iQ-MP" and cooled to -196°C. The pressure was then reduced, and nitrogen (99.999% purity) was adsorbed onto the sample at a predetermined relative pressure. The amount of nitrogen adsorbed onto the sample was measured when equilibrium pressure was reached at each predetermined relative pressure, and an adsorption isotherm was created. The mass transfer coefficient was determined by analyzing the data from the created nitrogen adsorption isotherms. 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. In this case, the relative pressure was 1.0 × 10⁻⁶. -4 The above 1.0 × 10 -3 The mass transfer coefficients were converted so that the number of plots within the following range was at least 5. A graph was created with relative pressure (P / P0) on the x-axis and mass transfer coefficient on the y-axis. Relative pressure 1.0 × 10 -4 The above 1.0 × 10 -3 Draw an approximate straight line within the following range, and determine its slope (a) and relative pressure 1.0 × 10 -3 The mass transfer coefficient (b) was calculated for this case.

[0066] <Pore volume and mode diameter of 2 nm to 200 nm using nitrogen adsorption method> The nitrogen adsorption isotherms obtained as described above were analyzed using the BJH method, and the volume of pores with a pore diameter of 2 nm to 200 nm was calculated. Furthermore, the nitrogen adsorption isotherms obtained as described above were analyzed using the BJH method to obtain the logarithmic derivative pore volume distribution (dV / d(log D)) by differentiating the cumulative pore volume (V) with respect to the common logarithm of the pore diameter (D). The pore diameter with the highest occurrence ratio was adopted as the mode diameter of the measurement sample.

[0067] <Specific surface area> As described above, the nitrogen adsorption isotherms were analyzed using the BET equation with a multipoint method. The specific surface area of ​​the measured sample was calculated from the approximate straight line in the relative pressure range of 0.05 to 0.1 of the obtained curves.

[0068] <Bulk density> The bulk density of the sample was measured using a Hosokawa Micron PT-X powder tester. Specifically, the sample was placed in an automatic tap density measurement unit, and the bulk density was calculated from the volume after 3000 taps.

[0069] <Average primary particle diameter> 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 with an ultrasonic cleaner for more than 10 minutes, and dispersed in the aqueous solution. The particle size distribution was measured using this dispersion with a particle size analyzer ("Microtrac MT3300EXII," manufactured by Microtrac-Bell Corporation). The particle size at which the cumulative volume reached 50% was defined as the average primary particle size of the porous carbon sample.

[0070] [Example 1] <Production of porous carbon> 1 g of glucose (distributed by Fujifilm Wako Pure Chemical Industries, Ltd.), 2 g of calcium chloride dihydrate (200 parts by mass per 100 parts by mass of glucose), and 1 g of deionized water (100 parts by mass per 100 parts by mass 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 1 g of mixture, and heat-treated at this temperature for 30 minutes (phase separation). 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 carbide. The carbide was washed by immersion in 0.2 L of 0.1 mol / L hydrochloric acid and stirring at 80°C for 30 minutes, and then removed onto a Buchner funnel. The filtrate was washed with water until the pH was in the range of 6 to 8. After repeating the acid washing and water washing three times, it was dried with hot air at 80°C. The carbonized material after hot air drying was mixed with 12 mL of deionized water, and the resulting mixture was subjected to ball mill grinding for 10 minutes. While circulating nitrogen gas at the same gas supply rate as above, the ground 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] <Fabrication of fuel cell catalysts> Pt catalyst particles, which are electrode catalyst particles, were supported on the prepared porous carbon by the platinum acetylacetonate method. Pt precursor (Pt(C5H7O2)2) was dissolved in dichloromethane (3 mL) so that the amount of Pt was 35% by mass per 100 mg of porous carbon. The resulting solution was added to a round-bottom flask containing 100 mg of porous carbon. Next, while cooling the round-bottom flask with ice, the porous carbon was dispersed in the solution using an ultrasonic stirrer for 30 minutes. Further heating to 50°C and stirring until all the solvent evaporated yielded a pale yellow powder. The obtained powder was subjected to a reduction treatment by heat treatment under a nitrogen atmosphere at 210°C for 3 hours and then at 240°C for 3 hours to obtain a fuel cell catalyst.

[0072] <Fabrication of Membrane-Electrode Assemblies (MEAs)> A Nafion membrane (thickness: 50 μm) was used as the electrolyte membrane. A dispersion for anode formation was prepared by dispersing Pt / C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E) in a mixed solvent containing Nafion dispersion solution (Fujifilm Wako Chemical Co., Ltd., 5% Nafion® dispersion solution DE521 CS type), water, and ethanol in a volume ratio of 100:53:480, based on the total volume of the dispersion, to contain 46% by mass of Pt / C. The amount of Pt at the anode was 0.3 mg / cm³. 2 To achieve this, the obtained dispersion was spray-printed onto the Nafion film, and the solvent was removed by drying it on a hot plate at 60°C, thereby creating an anode (electrode catalyst layer) on the Nafion film. Next, a cathode-forming dispersion was prepared in the same manner as the anode-forming dispersion, except that the fuel cell catalyst mentioned above was used instead of Pt / C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E). The amount of Pt in the cathode was 0.3 mg / cm³. 2 To achieve this, the obtained cathode-forming dispersion was spray-printed onto the opposite side of the Nafion film on which the anode was formed, and the solvent was removed by drying it on a hot plate at 60°C, thereby creating a cathode (electrode catalyst layer) on the Nafion film. Subsequently, the anode, Nafion film, and cathode were pressed together at 0.3kN and 130°C, and then sandwiched between two sheets of water-repellent carbon paper (Toray Industries, model number: EC-TP1-060T), which served as a gas diffusion layer, to obtain a MEA.

[0073] <Concentration overvoltage> As shown in Figure 2, the concentration overpotential (η mt ) is the theoretical electromotive force (E rev The difference between ) and the operating potential gives the ohmic overvoltage (η ohm The non-ohmic overvoltage (η) is calculated by subtracting the non-ohmic overvoltage (η) non-ohm ) from, activation overpotential (η a This is the overvoltage calculated by subtracting the IR loss (η). Here, ohmic overvoltage is defined as IR loss (η). IRAlso known as ) overpotential, this is a voltage drop caused by the electrical resistance of the electrodes and the resistance to the flow of electrolyte ions. Activation overpotential is the voltage drop that occurs when activation energy is consumed for the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode to proceed. Concentration overpotential is a voltage drop that occurs because the accumulation of water vapor, a reaction byproduct, prevents sufficient oxygen, a reactant, from being supplied to the electrode surface. Of these three overpotentials, concentration overpotential is the one that is affected by the properties of porous carbon for catalyst support, and therefore, concentration overpotential was used as an indicator for evaluating porous carbon for catalyst support. Furthermore, a higher current density requires a greater oxygen supply, resulting in increased water vapor emissions. This makes oxygen supply and water vapor emission more difficult, thus increasing the proportion of concentration overvoltage to the total overvoltage. For this reason, a relatively high current density of 700 [mA / cm²] is used. 2 The concentration overpotential value at [condition] was adopted as an indicator for evaluating porous carbon for catalyst support. Ohmic overpotential, activation overpotential, and concentration overpotential can each be separated according to the test name: IV Measurement Method / Overpotential Separation Analysis Method (sometimes referred to as the "NEDO Protocol Test Method" in this specification) 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 below in (1) to (4). 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. Step [4] in the NEDO protocol test method is the step for calculating activation overpotential and does not directly contribute to the calculation of concentration overpotential, so it is omitted here. (1) IV measurements were performed on MEA in accordance with the conditions described in the NEDO protocol test method. (2) The theoretical electromotive force was calculated in accordance with the NEDO protocol test method. (3) From the ohmic resistance obtained by the known AC impedance method, the ohmic overvoltage was calculated according to (v) below. (v) Current density (A / cm²) of the point to be analyzed 2 ) × internal resistance (Ω·cm) 2 ) was defined as resistive overvoltage (synonymous with ohmic overvoltage). (4) Using the ohmic overpotential calculated in (3), the concentration overpotential was calculated according to (i) to (iii) and (vi) below. (i) Based on the IV measurement data measured in (1), a Tafel plot was created with current density (logarithmic axis) on the horizontal axis and IR-free voltage (operating potential plus ohmic overvoltage) on the vertical axis. (ii) The equation of the regression line was obtained from the values ​​of 3 to 4 points that showed linearity on the low current density side. (iii) The current density value at the point to be analyzed was substituted as x in the equation, and the voltage y on the regression equation was found. (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 overpotential (synonymous with concentration overpotential). More detailed instructions are provided below.

[0074] <Step (1)> A single-cell power generation evaluation jig incorporating a MEA (manufactured by Nippon FC Kikaku Co., Ltd.) was placed in a constant temperature chamber set to 80°C, and IV measurements were performed under the following conditions. A fuel cell evaluation device (AutoPEM-KUG2, manufactured by Toyo Technica Co., Ltd.) and a potentiometer / galvanostat (SP-240, manufactured by BioLogic Inc.) were used for the IV measurements. As an example, Figure 3 shows the IV curve obtained by the above IV measurement of the MEA prepared in Example 1. Current density: 700 [mA / cm²] 2 The operating potential at [ ] was 0.514[V]. (Anode condition) Electrode area: 1cm 2 Supply gas type: 100% H2 Gas supply rate: 139 mL / min Supply gas humidification temperature: 80℃ (relative humidity: 98%) (Cathode condition) Electrode area: 1cm 2 Supply gas type: Air Gas supply rate: 332 mL / min Supply gas humidification temperature: 80 °C (relative humidity: 98%)

[0075] <Procedure (2)> Substitute the test conditions, T = 80 °C (353 K), (PH2 / PH * 2) = 1.0, (PO2 / PO * 2) = 0.21 into the following formula described in the NEDO protocol test method, and calculate the theoretical electromotive force E rev(PH2,PO2,T) [[ID=1�]]As a result, it was 1.17 [V].

Number

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

[0077] <Procedure (4)> (i) Based on the I-V measurement data measured in (1), a Tafel plot was created with the horizontal axis as the current density (logarithmic axis) and the vertical axis as the IR-free voltage. The Tafel plot of the MEA fabricated in Example 1 is shown in Fig. 4 (L1 in Fig. 4). (ii) Obtain the equation of the regression line from the values of 3 to 4 points with linearity on the low current density side. The equation of the regression line (L2 in Fig. 4) in Example 1 is​

number

[0078] <Step (5)> (vi) Current density x (700[mA / cm 2 The concentration overpotential was determined from the difference between the voltage y value on the regression equation (Y1 in Figure 4) and the IR-free voltage value (Y2 in Figure 4). In Example 1, the current density x = 700 [mA / cm²] 2 ] concentration overpotential (in Figure 4 (η mt The result was 0.672 - (0.514 + 0.0613) = 0.0967(V).

[0079] [Example 2] A fuel cell catalyst and MEA were prepared in the same manner as in Example 1, except that 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 MEA were produced in the same manner as in Example 1, except that the carbon source was changed to 0.5 g of glucose (distributed by Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.5 g of cellulose (distributed by Fujifilm Wako Pure Chemical Industries, Ltd.), 3 g of calcium chloride dihydrate (300 parts by mass per 100 parts by mass of carbon source), and 1.5 g of deionized water.

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

[0082] [Comparative Example 2] 1 g of polyvinyl alcohol (PVA) and 4 g of magnesium citrate (sold by Fujifilm Wako Pure Chemical Industries, Ltd.) (400 parts by mass per 100 parts by mass of PVA) were mixed. The resulting mixture was heated to 700°C in a nitrogen gas atmosphere at a heating rate of 10°C / min and heat-treated at this temperature for 60 minutes to obtain a carbide. The obtained carbide 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. The filtrate was washed with water until the pH was in the range of 6 to 8. After repeating the acid washing and water washing three times, it was hot-air dried at 80°C. The hot-air dried carbide was further heated to 1200°C in a nitrogen gas atmosphere and heat-treated (calcined) for 60 minutes to obtain porous carbon. The fuel cell catalyst and MEA were produced in the same manner as in Example 1.

[0083] Table 1 shows the physical properties of porous carbon and fuel cell catalysts in the examples and comparative examples. Note that the porous carbon in Examples 1-3 and Comparative Examples 1-2 was particulate.

[0084] [Table 1] [Industrial applicability]

[0085] The porous carbon of the present invention can produce a low concentration overpotential when used as a catalyst support in a fuel cell. Therefore, the porous carbon of the present invention can be suitably used as a fuel cell catalyst support.

Claims

1. In the measurement of nitrogen adsorption isotherms, 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 × 10⁻⁶. －４ The above 1.0 x 10 －３ Porous carbon for fuel cell catalyst support, wherein the slope of the approximate straight line obtained by linearly approximating the relationship between relative pressure and mass transfer coefficient in the following case is 2.5 or more and 10.0 or less, and in the said approximate straight line, the mass transfer coefficient at a relative pressure of 1.0 × 10⁻³ is 3.0 × 10⁻³ sec⁻¹ or more and 1.0 × 10⁻² sec⁻¹ or less.

2. The pore volume calculated from nitrogen adsorption isotherms using the BJH method for pores with a diameter of 2 nm to 200 nm was 0.8 cm³. ３ The porous carbon according to claim 1, wherein the amount is 1 / g or more.

3. In the aforementioned approximate straight line, the relative pressure is 1.0 × 10⁻⁶. －３ The mass transfer coefficient at this time is 3.5 × 10 －３ seconds －１ The porous carbon described in claim 1 is as described above.

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

5. The bulk density is 0.10 g / cm³. ３ A smaller porous carbon according to claim 1.

6. A fuel cell catalyst comprising porous carbon and a catalytic metal as described in claim 1, wherein the porous carbon supports the catalytic metal.

7. A fuel cell comprising the fuel cell catalyst described in claim 6.

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