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

Abstract

The present invention relates to a porous carbon for a fuel cell catalyst support, wherein a slope of an approximate straight line obtained by linearly approximating a relationship between a relative pressure and a mass transfer coefficient at a time when a relative pressure is 1.0 x 10 ‑4 or more and a relative pressure is 1.0 x 10 ‑3 or less in the above and the following is 2.5 or more.

Description

Technical Field

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

[0002] As catalysts for the oxidation reaction at the anode and the reduction reaction at the cathode in fuel cells, platinum or platinum alloys, which are primarily highly reactive metals and alloys, can be used. Furthermore, carbon materials, specifically porous carbon, are generally used as supports for the catalyst metal.

[0003] For example, Patent Document 1 describes porous carbon, in which, as a catalyst support for solid polymer fuel cells, the specific surface area of ​​the mesopores (pores with a diameter of 2-50 nm) is 600-1600 m². 2 / g has a specific intensity ratio of the G-band to the G'-band in the Raman spectrophotometer and a specific G'-band peak position.

[0004] Existing technical documents Patent documents Patent document 1: International application publication No. 2015 / 141810. Summary of the Invention

[0005] The problem that the invention aims to solve In the catalyst support on the cathode side of a fuel cell, the diffusion and supply of reactants (i.e., oxygen) to the catalyst layer become insufficient due to the phenomenon that water molecules moving from the anode side with protons to the cathode side and water vapor generated by the reduction reaction on the cathode side remain in the catalyst layer, resulting in a decrease in the fuel cell output. The voltage drop caused by the above phenomenon is called concentration overvoltage, which causes a decrease in fuel cell output. Therefore, in order to improve the output performance of the fuel cell, it is important to improve the gas diffusivity of the catalyst support, suppress the above phenomenon, and supply reactant gases to the electrodes, thereby achieving a low concentration overvoltage. According to the research of the inventors, the porous carbon material for solid polymer fuel cells described in Patent Document 1 has a dendritic structure to improve gas diffusivity. Although the specific surface area of ​​the mesopores is relatively large in order to efficiently support catalyst metal particles, further improvement in the output performance of the fuel cell is always required.

[0006] The present invention was made in view of the above circumstances, and provides a porous carbon for a fuel cell catalyst support that, when used as a catalyst support in a fuel cell, can suppress the above-mentioned phenomena and result in a low concentration of overvoltage.

[0007] means for solving problems In order to solve the aforementioned problems, the inventors conducted repeated and detailed studies on porous carbon, resulting in the present invention. Specifically, the present invention includes the following suitable embodiments.

[0008] [1] Porous carbon is used as the catalyst support for fuel cells. The pressure change of nitrogen gas until adsorption equilibrium is reached at specified relative pressures during the determination of nitrogen adsorption isotherms is converted into a mass transfer coefficient using the LDF approximation. This is applied to a relative pressure of 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 The slope of the approximate straight line obtained by linearly approximating the relationship between relative pressure and mass transfer coefficient is greater than 2.5.

[0009] [2] According to the porous carbon described in [1], the pore volume of the pores with a diameter of 2 nm or more and less than 200 nm, calculated by the BJH method based on the nitrogen adsorption isotherm, is 0.8 cm³. 3 / g or more.

[0010] [3] According to the porous carbon described in [1] or [2], wherein the relative pressure in the aforementioned approximate straight line is 1.0 × 10⁻⁶. -3 The mass transfer coefficient at that time was 3.0 × 10⁻⁶. -3 Second -1 above.

[0011] [4] Porous carbon according to any one of [1] to [3], wherein the average primary particle size is 500 nm to 5 μm.

[0012] [5] Porous carbon according to any one of [1] to [4], wherein the bulk density is less than 0.10 g / cm³. 3 .

[0013] [6] A fuel cell catalyst comprising any one of [1] to [5] porous carbon and a catalyst metal, wherein the porous carbon supports the catalyst metal.

[0014] [7] A fuel cell comprising the fuel cell catalyst described in [6].

[0015] Invention Effects According to the present invention, porous carbon for fuel cell catalyst support can be provided, which can provide low concentration overvoltage when used as a catalyst support in a fuel cell. Attached Figure Description

[0016] [ Figure 1 The graph shows the relationship between relative pressure and mass transfer coefficient for the porous carbon in Example 1.

[0017] [ Figure 2The graph shows the relationship between the IV characteristics of a fuel cell and various overvoltages.

[0018] [ Figure 3 [] represents a graph of the IV curve in Example 1.

[0019] [ Figure 4 [] represents the Tafel plot and its regression line in Example 1. Detailed Implementation

[0020] The embodiments of the present invention will now be described in detail. It should be noted that the following description is illustrative of embodiments of the present invention and does not imply that the present invention is limited to these embodiments.

[0021] Porous carbon The porous carbon used as a catalyst support for fuel cells in this invention is characterized in that, in the determination of the nitrogen adsorption isotherm of this porous carbon, the pressure change of nitrogen gas until adsorption equilibrium is reached at various specified relative pressures (P / P0) is approximated using LDF (Linear Driving Force) to convert it into a mass transfer coefficient. For a relative pressure of 1.0 × 10⁻⁶, this approximation is used. -4 Above and 1.0×10 -3 The relationship between relative pressure and mass transfer coefficient is approximated linearly below, resulting in an approximate line with a slope greater than 2.5. Here, P is the adsorption equilibrium pressure, and P0 is the saturated vapor pressure.

[0022] <Relationship between mass transfer coefficient and relative pressure, and mass transfer coefficient> The mass transfer coefficient of porous carbon is an indicator of the migration speed of reactant gases (e.g., oxygen) and product gases (e.g., water vapor) within the porous carbon used as fuel cell catalysts. A greater change in mass transfer coefficient relative to a change in relative pressure indicates that reactant gases can move more smoothly from mesopores to micropores, and product gases can move more smoothly from micropores to mesopores. Furthermore, a higher mass transfer coefficient indicates faster migration speeds of reactant and product gases within the micropores of the porous carbon. Therefore, the change in mass transfer coefficient relative to a change in relative pressure and the mass transfer coefficient itself become indicators of the gas diffusivity of porous carbon. Here, according to IUPAC, micropores refer to pores with a diameter less than 2 nm, and mesopores refer to pores with a diameter of 2–50 nm.

[0023] The relationship between relative pressure and mass transfer coefficient, and the mass transfer coefficient itself, can be investigated by creating an adsorption isotherm at -196°C and analyzing it. Specifically, as described in the examples described later, a nitrogen adsorption isotherm of porous carbon is created, and the pressure change of nitrogen until adsorption equilibrium is reached at various specified relative pressures is converted into the mass transfer coefficient using the LDF approximation. Then, by creating a graph with relative pressure as the x-axis and mass transfer coefficient as the y-axis, the relationship between relative pressure and mass transfer coefficient can be represented.

[0024] The inventors discovered that: in this graph, if the relative pressure is 1.0 × 10⁻⁶, -4 Above and 1.0×10 -3 Porous carbon exhibits excellent diffusivity when the slope of an approximate straight line drawn within the following range (sometimes referred to as (a) in this specification) is 2.5 or higher.

[0025] Here, it is explained that the range of relative pressure is set to 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 The following is the rationale. In this specification, it is assumed that fine pores with a diameter of approximately 2–200 nm serve as the diffusion pathway for reactant and product gases, such as oxygen or water vapor, diffusing into porous carbon. If the relative pressure is too low, i.e., less than 1.0 × 10⁻⁶, it will affect the diffusion path. -4 At that time, it was assumed that diffusion behavior in pores smaller than the pores that effectively function as the movement path of such reactant and product gases was reflected in the mass transfer coefficient. On the other hand, if the relative pressure is too high, i.e., the relative pressure is greater than 1.0 × 10⁻⁶, the diffusion behavior in such pores is reflected in the mass transfer coefficient. -3 When this is the case, diffusion behavior in pores larger than those that effectively act as the movement path for both reactant and product gases is considered to be reflected in the mass transfer coefficient. This is achieved by setting the relative pressure to 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 The following can be used to evaluate the region in which the diffusion behavior of the reactant gas and the generated gas, as envisioned in this specification, is reflected in the mass transfer coefficient. The slope of the approximate straight line within this range can be considered as one of the indicators of the gas's diffusivity.

[0026] For a more specific calculation method in (a), use Figure 1 The graph shown below illustrates the relationship between relative pressure and mass transfer coefficient related to porous carbon in Example 1 described later.

[0027] First, the relative pressure in the graph showing the relationship between relative pressure and mass transfer coefficient, prepared as described in the embodiments below, is 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 The following range (in) Figure 1 Within the range indicated by dashed lines and arrows, draw an approximate straight line. Then, calculate the slope (a) of this approximate straight line.

[0028] (a) is 2.5 or higher, not limited, but usually 10.0 or lower. (a) is preferably 2.5 to 10.0, 3.0 to 7.0, or 3.5 to 5.0. If (a) is 2.5 or higher than the aforementioned lower limit, when used as a catalyst support in a fuel cell, porous carbon tends to have a fine pore structure suitable for the diffusion of reactant gases and the discharge of generated gases, thus exhibiting excellent gas diffusivity. Furthermore, if (a) is 2.5 or higher than the aforementioned lower limit, porous carbon can have a greater number of mesopores. When (a) is less than 2.5, porous carbon is unlikely to exhibit excellent gas diffusivity.

[0029] The relative pressure of the approximate straight line within the aforementioned range is 1.0 × 10⁻⁶. -3 The mass transfer coefficient at that time (in this specification, this value is sometimes referred to as (b)) is preferably 3.0 × 10⁻⁶. -3 Second -1 The above is not limited, but is usually 1.0 × 10. -2 Second -1 Below. (b) More preferably, 3.0 × 10 -3 ~1.0×10 -2 Second -1 3.5×10 -3 ~8.0×10 -3 Second -1 4.0×10 -3 ~6.0×10 -3 Second -1 If (b) is above the aforementioned lower limit, porous carbon tends to have a fine porous structure suitable for the diffusion of reactant gases and the expulsion of generated gases, thus exhibiting excellent gas diffusivity. Figure 1 The chart shown illustrates this; (b) represents a relative pressure of 1.0 × 10⁻⁶. -3 The mass transfer coefficient at the point indicated by the dashed line or the right end of the arrow on the right.

[0030] It can be considered that the above-mentioned "microporous structure suitable for the diffusion of reactant gases and the discharge of generated gases" refers to: a large number of micropores that effectively function as the movement path of reactant gases and generated gases; the opening of such micropores having a suitable size for the entry and exit of reactant gases and generated gases; a high proportion of micropores that connect from one end of the opening to the other end (micropores with an opening at one end and not blocked at the other end); few structures (such as bottlenecks) that hinder the movement of reactant gases and generated gases in their connecting parts; and a relatively short microporous structure for the movement path of the gas (the movement path used to discharge generated gases not needed in the chemical reaction in the fuel cell and to supply reactant gases needed for the chemical reaction to the catalyst metal supported on porous carbon). Therefore, the fine-pore structure of the porous carbon of the present invention differs from the fine-pore structure of a "three-dimensional network structure" (e.g., Japanese Patent Application Publication No. 2011-1224) formed by mixing and carbonizing a carbon source with molded particles (fine-pore source) and removing the aggregated / linked molded particles (continuous pores formed by removing existing molded particles) and the surrounding carbon skeleton. In such cases, the path length for the reactant gas and the generated gas to move within the porous carbon is relatively long, and multiple structures (e.g., bottlenecks) exist that impede the movement of reactant gas and generated gas, corresponding to the regions between the existing molded particles or the gaps between continuously existing molded particles. Therefore, in the case of a "three-dimensional network structure," the numerical ranges of (a) and / or (b) in the porous carbon of the present invention are not satisfied. Furthermore, the porous carbon of the present invention has a fine pore structure that differs from a "three-dimensional tree structure" (e.g., Japanese Patent Application Publication No. 2018-174078) formed by a tree-like structure of three-dimensional branches of rod-shaped or ring-shaped carbon containing carbon. In such cases, the path length for the reactant gas and the generated gas to move within the porous carbon is relatively long, and there are multiple structures (e.g., bottlenecks) that impede the movement of the reactant gas and the generated gas, corresponding to the regions between the existing mold particles or the gaps between continuously existing mold particles. Therefore, in the case of the "three-dimensional tree structure," the numerical ranges of (a) and / or (b) in the porous carbon of the present invention are not satisfied. In a preferred embodiment of the present invention, it can be considered that all the pores are connected from the opening at one end to the opening at the other end.

[0031] Furthermore, considering that (a) above corresponds to the length of the path for supplying reactant gas to porous carbon and for discharging generated gas from porous carbon, it can be assumed that the larger (a) is, the shorter the length of the path. It can also be assumed that (b) above corresponds to the amount of structures in the connecting portion that obstruct the movement of reactant gas and generated gas, and the larger (b) is, the fewer structures that obstruct the movement of reactant gas and generated gas.

[0032] (a) Porous carbon exceeding the aforementioned lower limit can be manufactured as follows: for example, a carbon source and a microporous source are mixed in a highly compatible state, and the resulting mixture is subjected to phase separation by heat treatment. Then, the phase-separated structure is immobilized by carbonization, and the microporous source is removed from the resulting carbide. For example, in the method for manufacturing porous carbon described later, by appropriately adjusting the temperature and time during heat treatment of the mixture of carbon source and calcium compound that serves as the microporous source, or by changing the type of carbon source and appropriately adjusting the temperature and time during heat treatment of the aforementioned mixture, (a) can be adjusted to exceed the aforementioned lower limit or within the aforementioned range. Furthermore, (b) can also be adjusted to exceed the aforementioned lower limit or within the aforementioned range by appropriately adjusting the same conditions.

[0033] <Volume of pores with a diameter of 2 nm or more but less than 200 nm> In porous carbon, the lower limit of the pore volume (hereinafter sometimes referred to as "pore volume of 2 nm or more and 200 nm or less") with a pore diameter calculated based on the nitrogen adsorption isotherm and using the BJH method is preferably 0.8 cm. 3 Above / g, there is no specific upper limit, usually 4.00cm 3 / g or less. The preferred pore volume is 0.80~4.00 cm³, with a pore size of 2nm or more and 200nm or less. 3 / g, more preferably 0.90~3.90cm 3 / g, further preferably 1.00~3.80cm 3 / g, more preferably 1.30~3.70cm 3 / g, preferably 1.60~3.60cm 3 / g, more preferably 1.90~3.50cm 3 / g, with an optimal value of 2.00~3.40cm 3 / g, and more preferably 2.20~3.30cm 3 / g. Pores larger than 2 nm and smaller than 200 nm can be considered to participate in the diffusion of reactant and product gases, as well as the support of the catalyst metal on porous carbon in a highly dispersed state. A highly dispersed state refers to the state in which the catalyst metal is dispersed and supported on porous carbon at an appropriate distance. If the spatial distribution of the catalyst metal within the porous carbon is relatively 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 and removal of water vapor. Typically, oxidation and reduction reactions in fuel cells occur on the catalyst metal. Therefore, from the viewpoint of improving catalyst utilization efficiency, catalyst metals generally have a size on the order of several nm (e.g., an average particle size of 2–3 nm) to increase the specific surface area per mass. Therefore, if the pore volume of 2 nm to 200 nm is above the aforementioned lower limit, the catalyst metal can be supported on porous carbon in a highly dispersed state, achieving good removal of product gases and smooth supply of reactant gases.

[0034] The pore volume of 2 nm or more and 200 nm or less can be adjusted, for example, by appropriately adjusting the type and / or ratio of the carbon source and calcium compound in the porous carbon manufacturing method described later; and by adjusting the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process), to be above the aforementioned lower limit and below the aforementioned upper limit or within the aforementioned range. The pore volume of 2 nm or more and 200 nm or less can be calculated by analyzing the nitrogen adsorption isotherm of porous carbon using the BJH method, and can be determined by, for example, the method described in the examples described later.

[0035] <Mode Diameter> In porous carbon, the mode diameter of the pores (hereinafter sometimes simply referred to as "mode diameter"), calculated from the nitrogen adsorption isotherm using the BJH method, varies depending on the size of the supported catalyst metal, and can be, for example, 2~200 nm, 5~190 nm, or 10~180 nm. Here, "mode diameter" refers to the pore diameter with the largest 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). If the aforementioned mode diameter is below the aforementioned upper limit, a good utilization efficiency of the supported catalyst can be achieved. The smaller the aforementioned mode diameter, the more readily the catalyst metal can be supported in a highly dispersed state; there is a tendency for the aforementioned mode diameter to be larger, resulting in better gas diffusion.

[0036] The mode diameter can be adjusted to the aforementioned range, for example, by appropriately adjusting the type and / or ratio of the carbon source and calcium compound in the porous carbon manufacturing method described later; the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process), etc. The mode diameter can be calculated by analyzing the nitrogen adsorption isotherm of porous carbon using the BJH method, and can be determined by, for example, the method described in the examples described later.

[0037] <Bulk density> The bulk density of porous carbon indicates the degree of development of its microporous structure. When the volume of micropores with diameters greater than 2 nm and less than 200 nm is equal, a lower bulk density indicates a larger volume of micropores with diameters exceeding 200 nm. It can be considered that micropores with diameters greater than 2 nm and less than 200 nm facilitate the movement of reactant and product gases, as well as the loading of catalyst metals in a highly dispersed state within the porous carbon. Conversely, micropores with diameters exceeding 200 nm act as inlets or pathways for the smooth movement of reactant and product gases, as well as catalyst metals, into micropores with diameters greater than 2 nm and less than 200 nm. Therefore, it is preferable that, based on the volume of micropores with diameters greater than 2 nm and less than 200 nm, the volume of micropores with diameters greater than 200 nm is larger, i.e., the bulk density is lower.

[0038] In a preferred embodiment, the bulk density of the porous carbon is preferably less than 0.10 g / cm³. 3 The lower limit is not specifically defined, but is usually 0.001 g / cm³. 3 The above. The preferred bulk density of porous carbon is 0.001 to less than 0.10 g / cm³. 3 More preferably, it is 0.004~0.08 g / cm³. 3 Further preferably, it is 0.008~0.06 g / cm³. 3 The preferred concentration is 0.01~0.05 g / cm³. 3 If the bulk density is less than or below the aforementioned upper limit, the reactant and product gases can move smoothly into the interior of pores with a diameter of 2 nm or more but less than 200 nm, resulting in excellent gas diffusion. If the bulk density is above the aforementioned lower limit, using porous carbon as a catalyst support can suppress dispersion and improve processability.

[0039] The bulk density can be adjusted to be less than the aforementioned upper limit or within the aforementioned range, for example, 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) in the porous carbon manufacturing method described later.

[0040] Bulk density can be determined by the methods described in the examples below.

[0041] <Volume of pores with a diameter of less than 2 nm> In a preferred embodiment, the pore volume, calculated using the DFT method based on the nitrogen adsorption isotherm of porous carbon and having a pore diameter of less than 2 nm, is preferably 0.35 cm³. 3 For values ​​below / g, the lower limit is not particularly limited, but is preferably 0.01cm. 3 / g or more. The preferred pore volume for pores with a diameter less than 2nm is 0.01~0.35cm³. 3 / g, more preferably 0.02~0.32cm 3 / g, further preferably 0.04~0.30cm 3 / g, preferably 0.06~0.28cm 3 / g, more preferably 0.08~0.25cm 3 / g, more preferably 0.10~0.23cm 3 / g, further preferably 0.12~0.21cm 3 / g. Pores with a diameter less than 2 nm are those where generated gases (water vapor) are difficult to expel once retained. If the volume of pores with a diameter less than 2 nm is below the aforementioned upper limit, generated gases are difficult to retain within the porous carbon, thus enabling good expulsion of the generated gases. Furthermore, if the volume of pores with a diameter less than 2 nm is below the aforementioned upper limit, there is a tendency for high volumes of pores with diameters between 2 and 200 nm, thus easily exhibiting excellent gas diffusivity. The volume of pores with a diameter less than 2 nm can be adjusted, for example, by appropriately adjusting the type and / or ratio of the carbon source and calcium compound in the porous carbon manufacturing method described later; and by adjusting the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process), to be above the aforementioned lower limit and below the aforementioned upper limit or within the aforementioned range. The volume of pores with a diameter of less than 2 nm can be calculated by analyzing the adsorption isotherm obtained from nitrogen adsorption measurements on porous carbon using the QS-DFT method.

[0042] Specific surface area calculated using the BET method In a preferred embodiment, the specific surface area calculated from the nitrogen adsorption isotherm of porous carbon by the BET method is preferably 300~1500 m². 2 / g, more preferably 400~1400m 2 / g, further preferably 450~1300m 2 / g, preferably 500~1200m 2 / g. If the specific surface area is within the aforementioned range, porous carbon allows for the diffusion of reactant and product gases. Porous carbon tends to have multiple pores with diameters of 2-200 nm suitable for supporting catalyst metals in a highly dispersed state, thus exhibiting excellent gas diffusivity. The specific surface area can be adjusted to the aforementioned range, for example, by appropriately adjusting the type and / or ratio of the carbon source and calcium compound in the porous carbon manufacturing method described later; and by adjusting the temperature and / or time of the heat treatment process (phase separation process and / or carbonization process). The specific surface area can be calculated as follows: The adsorption isotherm of porous carbon is measured, and for this adsorption isotherm, analysis is performed using the BET formula and a multi-point method, based on an approximate straight line within the range of relative pressure above 0.05 and below 0.1.

[0043] <Calcium content> In a preferred embodiment, the calcium content in the porous carbon is preferably 20-2000 ppm, more preferably 50-1500 ppm, and even more preferably 100-1000 ppm. If the calcium content is within the aforementioned range, there is a tendency to easily suppress excessive increases in the mass of the porous carbon and to achieve excellent productivity. The calcium content can be adjusted to the aforementioned range, for example, by appropriately adjusting the conditions of the process for removing calcium compounds (e.g., the type and / or concentration of acid used in acid cleaning, the time and / or temperature of acid cleaning, etc.) in the porous carbon manufacturing method described later.

[0044] <Sulfur content and silicon content> In a preferred embodiment, the sulfur and silicon contents in the porous carbon are preferably 1000 ppm or less, with no particular limitation on their lower limit, which is 0 ppm. The sulfur content in the porous carbon is preferably 0-1000 ppm, more preferably 0-900 ppm, and even more preferably 0-800 ppm. Additionally, in a preferred embodiment, the silicon content in the porous carbon is preferably 0-1000 ppm, more preferably 0-900 ppm, and even more preferably 0-800 ppm. If the sulfur and silicon contents are below the aforementioned upper limit or within the aforementioned range, there is a tendency to easily suppress excessive increases in the quality of the porous carbon and achieve excellent productivity. The sulfur and silicon contents can be adjusted to below the aforementioned upper limit or within the aforementioned range, for example, by appropriately selecting the type of carbon source in the porous carbon manufacturing method described later; and / or appropriately adjusting the conditions of the process for removing calcium compounds (e.g., the type and / or concentration of the acid used in acid cleaning, the acid cleaning time and / or temperature, etc.).

[0045] The contents of calcium, sulfur, and silicon can be determined by fluorescence X-ray analysis.

[0046] <Average primary particle size> The porous carbon can be in the form of granules, flakes, layers, or flakes, with granules or flakes being the most preferred.

[0047] In a preferred embodiment, the porous carbon is preferably in particulate form.

[0048] In this embodiment, the average primary particle size 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. If the average primary particle size is within the aforementioned range, there is a tendency for reduced contact resistance between particles and excellent conductivity of the electrode.

[0049] The average primary particle size can be adjusted to the aforementioned range, for example, by appropriately selecting the type of carbon source in the porous carbon manufacturing method described later; and / or appropriately adjusting the conditions of the pulverizing process. In this specification, the average primary particle size refers to the particle size at which the cumulative volume reaches 50%, as measured by laser diffraction and scattering. When laser diffraction and scattering cannot be used for measurement, the average primary particle size sometimes refers to the average particle size obtained by measuring the particle size of the primary particles appearing in an electron microscope image and calculating the average value.

[0050] Methods for manufacturing porous carbon The porous carbon of the present invention can be manufactured by, for example, a method including the following steps.

[0051] (1) A process for obtaining a mixture containing a carbon source and a calcium compound; (2) A process of heat-treating the aforementioned mixture in an inactive gas atmosphere to cause phase separation between the carbon source and the calcium compound; (3) The process of heat-treating the aforementioned mixture that has been phase-separated in an inactive gas atmosphere to obtain carbides; (4) The process of removing calcium compounds from the aforementioned carbides to obtain porous carbon; and (5) The process of pulverizing the aforementioned porous carbon as needed.

[0052] <Process (1)> The carbon source is not particularly limited. From the viewpoint of improving compatibility with calcium compounds that can serve as microporous sources, carbohydrates are preferred. Examples of carbohydrates 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, gelling polysaccharides, and guar gum. One type of carbohydrate can be used alone or in combination of two or more. Among these carbohydrates, glucose and starch are preferred from the viewpoint of easily producing porous carbon exhibiting excellent gas diffusion properties and being readily available in large quantities.

[0053] The starch used is not particularly limited and can be derived from sources such as corn, cassava, potato, sweet potato, tapioca, beans, wheat, and rice. The preferred amylose content of the starch is 50% by mass or less, more preferably 30% by mass or less relative to the starch mass. There is a tendency for the gelatinization temperature to decrease as the amylose content of the starch decreases. Therefore, if the amylose content of the starch is below the aforementioned upper limit, it gelatinizes easily at low temperatures, and its compatibility with calcium compounds that serve as a source of pores is easily improved, which is preferred. The aforementioned amylose content can be determined by, for example, an iodine colorimetric method. Furthermore, the starch can be a modified starch. Examples of modified starches include etherified starch, esterified starch, cationic starch, and cross-linked starch. One type of starch can be used alone or in combination of two or more types. When two or more types are used in combination, the aforementioned amylose content refers to the average amylose content of the combined starches.

[0054] The microporous source is not particularly limited. From the viewpoint of easily improving compatibility with the carbon source, the microporous source is preferably a calcium compound. As the calcium compound, there are no particular limitations; examples 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, calcium lactate, etc. Among these calcium compounds, from the viewpoint of easily producing porous carbon exhibiting excellent gas diffusion, a calcium compound with a melting point of 300°C or less is preferred (in the case where the mixture contains water and / or polyols and / or carboxylic acids, the melting point of the eutectic compound of the calcium compound and the carbon source with water and / or polyols and / or carboxylic acids is 300°C or less), more preferably at least one selected from calcium chloride hydrate, calcium hydroxide, calcium oxide, calcium carbonate, and calcium acetate. Calcium chloride hydrate includes dihydrate, tetrahydrate, and hexahydrate; any hydrate can be used, but dihydrate is preferred from the viewpoint of good reactivity with sugars.

[0055] The mixture containing a carbon source and a calcium compound may further contain at least one selected from water, polyols, and carboxylic acids. It can be considered that if the mixture contains water and / or polyols and / or carboxylic acids, the carbon source and calcium compound are compatible with each other via water and / or polyols and / or carboxylic acids, forming a eutectic compound. Therefore, the melting point of the calcium compound, which individually has a melting point of 300°C or higher, can be made to be 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 includes "the melting point of the eutectic compound of water and / or polyols and / or carboxylic acids with the carbon source and calcium compound is 300°C or lower." From the viewpoint of easy availability, water is preferred. Examples of usable polyols include glycerol, ethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol. Among these polyols, glycerol and ethylene glycol are preferred from the viewpoint of easy dissolution of the calcium compound and easy availability in large quantities. Examples of usable carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic 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 water, one or more polyols, and two or more carboxylic acids are used in combination, the ratio of water, polyols, and carboxylic acids can be appropriately varied according to the desired properties of the porous carbon.

[0056] There are no particular limitations on the method of mixing carbon sources and calcium compounds, as well as water, polyols and / or carboxylic acids as appropriate, and any mixing method can be used.

[0057] The amount of calcium compound mixed with the carbon source is preferably 80 to 500 parts by mass relative to 100 parts by mass of the carbon source, more preferably 130 to 400 parts by mass, and even more preferably 180 to 300 parts by mass. If the amount of calcium compound is within the aforementioned range, the mass transfer coefficient of the obtained porous carbon can be suitably correlated with the relative pressure, as well as the pore volume.

[0058] When the mixture containing a carbon source and a calcium compound also contains water and / or a polyol and / or a carboxylic acid, the amount of water and / or the polyol and / or the carboxylic acid (the total amount when using two or more of the following: water, one or more polyols, and one or more carboxylic acids) 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 relative to 100 parts by mass of the carbon source. Where the mixture containing a carbon source and a calcium compound contains water but not polyols or 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 relative to 100 parts by mass of the carbon source. If the amount of water and / or the polyol and / or the carboxylic acid is within the aforementioned range, a eutectic compound is easily formed, thus enabling the obtained porous carbon to have a suitable mass transfer coefficient relative to relative pressure and a suitable pore volume.

[0059] <Process (2)> In step (2), the mixture obtained in step (1) is heat-treated in an inactive gas atmosphere to separate the carbon source from the calcium compound.

[0060] Examples of inert gases include nitrogen, argon, and mixtures thereof. A lower concentration of the oxidizing gas in the gas used is preferred. The concentration of the oxidizing gas, especially oxygen, is typically 1% by volume or less, more preferably 0.1% by volume or less, relative to the volume of the gas used. If the concentration of the oxidizing gas is below the aforementioned upper limit, oxidation of the mixture is suppressed, and a structure with the desired characteristics is easily obtained. Furthermore, oxidative decomposition of the generated structure can be suppressed.

[0061] The supply rate (flow rate) of the inactive gas is typically 200 to 7000 mL / min, preferably 500 to 6000 mL / min, and more preferably 1000 to 5000 mL / min per 1g of mixture.

[0062] The heat treatment temperature is preferably 150~500℃, more preferably 200℃ or higher and less than 500℃, further preferably 240~470℃, particularly preferably 250~450℃, even more particularly preferably more than 300℃ and less than 420℃, and even more particularly preferably more than 300℃ and less than 400℃.

[0063] The inventors have discovered that by heat-treating the mixture obtained through step (1) in an inert gas atmosphere at a temperature preferably within the aforementioned range, it is possible to obtain a characteristic microporous structure of porous carbon. The reason for this is not yet clear, but the following non-limiting mechanism of action can be considered.

[0064] Through the aforementioned heat treatment, in the mixture, the carbon source and the calcium compound compatible with the carbon source via hydrogen bonding undergo phase separation, starting with the detachment of water of crystallization from the mixture or the dehydration reaction of the carbon source. Furthermore, it can be considered that the structure of the phase-separated carbon source and calcium compound is fixed by solidifying the carbon source through a dehydration reaction. At this point, at least a portion (preferably 50% by mass or more, more preferably almost all or all) of the phase-separated calcium compound is three-dimensionally continuous. In the subsequent carbonization process, the carbon source is rendered insoluble in hot water or hot acid through aromatication (carbonization), and in the subsequent calcium compound removal process, the calcium compound is removed from the carbide (preferably by acid-based washing). It can be considered that since the traces left after calcium compound removal become pores, the phase separation of the calcium compound achieved by the aforementioned heat treatment yields a characteristic microporous structure of porous carbon.

[0065] The heating rate during the above-mentioned heat treatment process is preferably 2°C / min or higher, and its upper limit is not particularly limited. From the viewpoint of easily achieving uniform heat treatment, it is preferably 200°C / min or lower. The heating rate during the above-mentioned heat treatment process is preferably 2~200°C / min, more preferably 5~100°C / min, and even more preferably 10~50°C / min. If the heating rate is above the aforementioned lower limit, it is easy to obtain the desired pore volume and / or the desired mass transfer coefficient.

[0066] The heat treatment time is appropriately selected based on the heat treatment temperature, the supply amount of inactive gas, 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 aforementioned lower limit, phase separation can be easily and sufficiently achieved. If the heat treatment time is below the aforementioned upper limit, it is a suitable time from an economic point of view, and is therefore preferred.

[0067] As apparatus for heat treatment, various furnaces such as converters, fluidized bed furnaces, fixed-bed furnaces, moving-bed furnaces, and moving-bed furnaces can be used; as well as various dryers such as spray dryers, moving dryers, stirred dryers, roller dryers, and thin-film evaporators. Furthermore, either a continuous furnace or a continuous dryer that continuously feeds in the material to be heat-treated and removes the material after heat treatment, or a batch furnace or a batch dryer that performs the above operations discontinuously, can be used. The heating method can be any means capable of heating to a specified temperature, such as electric heating, gas combustion heating, microwave heating, high-frequency induction heating, or electric heating. Moreover, these heating methods can be used individually or in combination.

[0068] <Process (3)> In step (3), the mixture that has undergone phase separation 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) and a subsequent high-temperature heat treatment (e.g., 900–1300 °C).

[0069] The preferred medium-temperature heat treatment temperature is 500–900°C, more preferably above 550°C–800°C, further preferably 600–750°C, and particularly preferably 650–700°C. The preferred high-temperature heat treatment temperature is 900–1300°C, more preferably 950°C–1280°C, further preferably 1000–1250°C, and particularly preferably 1050–1200°C. By performing heat treatment at a medium temperature, the carbon source can be solidified without drastic changes in the microporous structure. Then, by performing heat treatment at a high temperature, the crystallinity of the carbon can be improved, resulting in porous carbon with high electrical conductivity.

[0070] The heating rate during the above-mentioned heat treatment process is preferably 2°C / min or higher, and its upper limit is not particularly limited. From the viewpoint of easily achieving uniform heat treatment, it is preferably 200°C / min or lower. The heating rate during the above-mentioned heat treatment process is preferably 2~200°C / min, more preferably 5~100°C / min, and even more preferably 10~50°C / min. If the heating rate is above the aforementioned lower limit, the desired pore volume can be easily obtained.

[0071] The heat treatment time is appropriately selected based on the heat treatment temperature, the supply of inactive gas, etc. For example, the medium-temperature heat treatment time is 0.5 to 12 hours, more preferably 0.7 to 6 hours, and even more preferably 0.8 to 4 hours. For example, the high-temperature heat treatment time is 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 above the aforementioned lower limit, carbonization is easily and fully carried out. When the heat treatment time is below the aforementioned upper limit, it is a suitable time from an economic point of view, and is therefore preferred.

[0072] Alternatively, the phase separation process of step (2) and the intermediate-temperature heat treatment process of step (3) can be performed simultaneously. This can improve productivity. For example, the mixture obtained in step (1) can be subjected to intermediate-temperature heat treatment at 500 to 900°C. In this case, during the heating of the intermediate-temperature heat treatment process, since the temperature of 150 to 500°C, which is the temperature for phase separation, is reached, the desired fine-porous structure can be obtained. In order to promote phase separation, the heating rate is required to be delayed until the intermediate temperature. Specifically, from the viewpoint of balancing the promotion of 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. The heat treatment time at this 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 phase separation structure, it is preferable that the phase separation process of step (2) and the intermediate-temperature heat treatment process of step (3) are performed independently.

[0073] For inactive gases, their supply, and the furnace used in heat treatment, the same furnace as in process (2) can be used.

[0074] <Process (4)> In step (4), calcium compounds are removed from the resulting carbides. Porous carbon is thus obtained.

[0075] The removal of calcium compounds is preferably performed by acid cleaning. Examples of acids used in acid cleaning include hydrochloric acid, sulfuric acid, and nitric acid. Hydrochloric acid is preferred from the viewpoint that it readily dissolves metal compounds in carbides, minimizes the residue of impurities such as sulfur, and effectively inhibits the oxidation of carbides. The acid concentration during acid cleaning can be appropriately varied depending on the type of acid used. For example, when using hydrochloric acid, the concentration is preferably 0.01 to 1.0 mol / L, more preferably 0.05 to 0.5 mol / L. If the concentration of hydrochloric acid is within the aforementioned range, metal compounds are easily removed and hydrochloric acid is less likely to remain in the carbides, thus this is preferable.

[0076] The pH of the acid used in acid cleaning can be adjusted appropriately depending on the type, concentration, and temperature of the acid. Preferably, the pH is 3 or lower, more preferably 2.5 or lower. If the pH is below the aforementioned upper limit, metal compounds can be removed easily and efficiently.

[0077] Acid cleaning can be performed, for example, by immersing the obtained carbide in the aforementioned acid. When acid cleaning is performed by immersion in acid, the mass ratio of acid to carbide can 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. If the mass ratio of the carbide to the acid is within the aforementioned range, a sufficient cleaning effect is easily obtained.

[0078] There are no particular limitations on the method of impregnating carbides in acid. It can be a method of continuously adding acid and holding it for a specified time, then removing it while impregnating; or it can be a method of impregnating the carbides in acid and holding it for a specified time, then adding acid again after dehydration and repeating the impregnation-dehydration process. Alternatively, it can be a method of replacing all the acid or replacing only a portion of the acid. Furthermore, the acid can be stirred during impregnation.

[0079] There are no particular restrictions on the atmosphere used for acid cleaning; the appropriate atmosphere can be selected based on the cleaning method employed. Acid cleaning is typically carried out in an atmospheric atmosphere.

[0080] The immersion time of the carbide in acid can be adjusted appropriately according to the acid used and the processing temperature. The immersion time is preferably 5 to 60 minutes, more preferably 10 to 40 minutes, and even more preferably 15 to 35 minutes. If the aforementioned time is above the lower limit, the metal compounds are easily and thoroughly removed; if it is below the upper limit, good productivity can be ensured.

[0081] Preferably, after acid cleaning of the carbides, the acid in the porous carbon is removed by water washing. This acid cleaning and water washing can be repeated until the calcium compounds in the porous carbon are removed to the desired degree. Furthermore, from the viewpoint of efficiency in removing calcium compounds and residual acid, the solution temperature during acid cleaning and water washing is preferably relatively high, typically above 60°C.

[0082] As one embodiment of the present invention, acid cleaning is preferably performed between the intermediate-temperature heat treatment and the high-temperature heat treatment of the phase-separated mixture. For example, it is preferable to perform acid cleaning after the phase-separated mixture in step (2) (e.g., at 500 to 900°C) is subjected to intermediate-temperature heat treatment, and then perform high-temperature heat treatment (e.g., at 900 to 1300°C).

[0083] As an embodiment of the present invention, when the phase separation process of process (2) and the medium-temperature heat treatment process of process (3) are performed simultaneously, it is preferable to perform acid cleaning between the phase separation process of process (2) and the medium-temperature heat treatment process and the high-temperature heat treatment process of process (3) performed simultaneously.

[0084] In one embodiment of these, since the carbon source is prevented from melting by medium-temperature heat treatment before cleaning to remove the calcium compounds, changes in the microporous structure that accompany the dissolution of the carbon source by acid cleaning are unlikely to occur.

[0085] Known dryers such as hot air dryers and vacuum dryers can be used to dry the porous carbon after acid cleaning and water washing. Drying is preferably carried out at a temperature of 50–150°C. If the drying temperature is within the aforementioned range, oxidation of the porous carbon is less likely to occur, and drying is achieved appropriately, which is therefore preferred.

[0086] As needed, the porous carbon after step (4) can be pulverized, preferably in one method. By pulverizing, the shape and particle size of the final porous carbon can be controlled to the desired shape and particle size. There are no particular limitations on the pulverizing method. For example, well-known pulverizers such as ball mills, centrifugal roller mills, ring roller mills, centrifugal ball mills, jet mills, cone crushers, double roller crushers, disc crushers, and rotary crushers can be used alone or in combination.

[0087] In methods for manufacturing porous carbon, a classification process may be included after the pulverization step. For example, by removing particles that are extremely small or large compared to the desired particle size, porous carbon with a narrow particle size distribution can be obtained. The classification method is not particularly limited. Examples of classification methods include classification using sieves, wet classification, and dry classification. Examples of wet classifiers include those utilizing principles such as gravity classification, inertial classification, hydraulic classification, and centrifugal classification. Examples of dry classifiers include those utilizing principles such as sedimentation classification, mechanical classification, and centrifugal classification. From an economic point of view, dry classifiers are preferred.

[0088] To prevent surface oxidation during pulverization, the pulverization and classification processes are preferably carried out in an inert gas atmosphere.

[0089] Crushing and classifying can also be carried out using a single device. For example, a jet mill with dry classification capabilities can be used for crushing and classification. Furthermore, a device in which the crusher and classifier are separate can also be used. In this case, crushing and classification can be performed continuously or discontinuously.

[0090] <Fuel Cell Catalysts> The porous carbon of this invention, with its characteristic microporous structure, is suitable as a support for fuel cell catalysts. This porous carbon effectively dissipates water vapor generated by the chemical reaction (reduction reaction) at the cathode, and smoothly supplies oxygen, as a reactant gas, to the catalyst metal supported on the porous carbon. Therefore, fuel cell catalysts comprising the porous carbon and catalyst metal of this invention, with the catalyst metal supported on the porous carbon, can achieve low-concentration overvoltages in fuel cells. Because this fuel cell catalyst provides low-concentration overvoltages, fuel cells using this fuel ionization catalyst can exhibit improved output characteristics.

[0091] As for the catalyst metal in the anode, there are no particular restrictions as long as it catalyzes the oxidation of hydrogen; any known catalyst can be used. Similarly, as for the catalyst metal in the cathode, there are no particular restrictions as long as it catalyzes the reduction of oxygen; any known catalyst can be used. Specific catalyst metals include platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and their alloys; one or more of these can be used. Due to their high catalytic activity, platinum or platinum-containing alloys are preferred as the catalyst metal.

[0092] There are no particular limitations on the shape and size of the catalyst metal, and it can adopt the same shape and size as known catalyst components. The shape can be, for example, granular, flake-like, layered, etc., preferably granular. 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.

[0093] The catalyst loading in the catalyst support, based on the total mass of the catalyst support and the catalyst metal, is preferably 0.05 to 80% by mass, more preferably 0.1 to 60% by mass, and even more preferably 0.5 to 40% by mass. If the catalyst loading is within the aforementioned range, a sufficient amount of catalyst can be loaded. In addition, since there are multiple catalyst metals that function as catalysts, a catalyst with high utilization efficiency can be used, which is therefore preferred.

[0094] There are no particular limitations on the method of supporting catalyst metals on catalyst supports. As well-known suitable methods, the noble metal acetylacetone method using noble metal acetylacetone and the colloidal method using noble metal colloids can be cited, and either method is preferred.

[0095] Preferably, after the catalyst metal is deposited on the surface of the catalyst support, heat treatment is performed, preferably by a method that increases the particle size of the catalyst metal.

[0096] <Fuel Cells> Fuel cells are typically constructed as follows: On either side of a proton-conducting polymer electrolyte membrane, catalyst layers are formed, each containing a carbon material with a platinum-supported metal catalyst and an ion-conducting binder containing the polymer electrolyte. On the outer side of each catalyst layer, porous materials, known as gas diffusion layers, are formed for venting fuel gas and oxidant gas, respectively. Carbon paper, carbon cloth, etc., can be used as gas diffusion layers. The structure in which the gas diffusion layers are arranged within the catalyst layers is called a gas diffusion electrode. Furthermore, the structure in which a pair of gas diffusion electrodes are joined to the electrolyte membrane, with the catalyst layer facing the electrolyte membrane, is called a membrane electrode assembly (MEA). Conductive and airtight separators are arranged on both sides of the MEA. Gas flow paths for supplying fuel gas or oxidant gas (e.g., air) to the electrode surfaces are formed at the contact points between the MEA and the separators or within the separators. Power is generated 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). In other words, at the fuel electrode, the fuel is ionized to produce protons and electrons. The protons pass through the electrolyte membrane, while the electrons move through an external circuit connecting the two electrodes and are transported to the oxygen electrode, where they react with an oxidant to produce water. In this way, the chemical energy of the fuel can be directly converted into electrical energy and extracted.

[0097] The porous carbon of this invention can effectively expel water vapor generated by the chemical reaction (reduction reaction) at the cathode, and can smoothly supply oxygen, as a reactant gas, to the catalyst metal supported on the porous carbon. Therefore, fuel cells using the porous carbon of this invention as a catalyst layer can achieve low-concentration overvoltage. Because this fuel cell catalyst introduces low-concentration overvoltage, fuel cells using this fuel ionization catalyst can have improved output characteristics.

[0098] A fuel cell equipped with the fuel cell catalyst of the present invention can be manufactured using known fuel cell materials and according to known fuel cell manufacturing methods, in addition to using the fuel cell catalyst of the present invention. Example

[0099] The present invention will be specifically described below through examples, but these examples do not limit the scope of the invention.

[0100] [Analytical methods for porous carbon] Mass transfer coefficient based on nitrogen adsorption method Porous carbon, serving as the test sample, was filled into a test tube. The test tube was then placed in an "Autosorb-iQ-MP" thermometer (Quantachrome), cooled to -196°C, and temporarily depressurized. Subsequently, nitrogen gas (99.999% purity) was adsorbed onto the test sample at a specified relative pressure. The amount of nitrogen adsorbed onto the sample at the equilibrium pressure was measured at each specified relative pressure, and adsorption isotherms were constructed.

[0101] The mass transfer coefficient was determined by analyzing the data from the constructed nitrogen adsorption isotherms. Specifically, the LDF approximation was used to convert the pressure change of nitrogen until adsorption equilibrium was reached at various specified relative pressures into the mass transfer coefficient. Here, a relative pressure of 1.0 × 10⁻⁶ was used. -4 Above and 1.0×10 -3 The following range of plotted numbers are converted into mass transfer coefficients in a way that is at least 5.

[0102] Create a graph with relative pressure (P / P0) on the x-axis and mass transfer coefficient on the y-axis. The relative pressure is 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 Draw an approximate straight line within the following range, and determine its slope (a) and relative pressure of 1.0 × 10⁻⁶. -3 The mass transfer coefficient (b) at that time.

[0103] <Pore volume and mode diameter above 2nm and below 200nm based on nitrogen adsorption> The nitrogen adsorption isotherms obtained as described above were analyzed using the BJH method, and the volume of the pores with a diameter of 2 nm or more and 200 nm or less was calculated.

[0104] In addition, the nitrogen adsorption isotherms obtained as described above were analyzed by the BJH method to obtain the logarithmic differential pore volume distribution (dV / d(logD)) obtained by differentiating the cumulative pore volume (V) with the common logarithm of the pore diameter (D). The pore diameter with the largest occurrence ratio was used as the mode diameter of the test sample.

[0105] Specific surface area The nitrogen adsorption isotherms obtained as described above were analyzed using the BET method and a multi-point approach. The specific surface area of ​​the sample was calculated based on the approximate straight line of the curve within the range of relative pressure above 0.05 and below 0.1.

[0106] <Bulk density> The bulk density of the samples was determined using a Hosokawa Micron PT-X powder analyzer. Specifically, the sample was placed into the automatic tap density measuring unit, and the bulk density was calculated based on the volume after 3000 taps.

[0107] <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.) and treated with an ultrasonic cleaner for at least 10 minutes to disperse it in the aqueous solution. Using this dispersion, the particle size distribution was determined using a particle size distribution measuring device (MICROTRAC MT3300EXII, manufactured by MICROTRAC BEL). The particle size at which the cumulative volume reaches 50% was set as the average primary particle size of the porous carbon in the sample.

[0108] [Example 1] <The manufacture of porous carbon> 1 g of glucose (sold by Fujifilm and Kojun Pharmaceutical Co., Ltd.), 2 g of calcium chloride dihydrate (200 parts by weight relative to 100 parts by weight of glucose), and 1 g of ion-exchanged water (100 parts by weight relative to 100 parts by weight of glucose) were mixed. The resulting mixture was heated to 350°C at a rate of 20°C / min under a nitrogen gas flow of 1250 mL / min relative to 1 g of the mixture, and heat-treated (phase separation) at this temperature for 30 minutes. Then, while continuing to flow with nitrogen at the same gas supply, the mixture was heated to 700°C at a rate of 10°C / min, and heat-treated at this temperature for 60 minutes to obtain a carbide. The carbide was immersed in 0.2 L of 0.1 mol / L hydrochloric acid and stirred at 80°C for 30 minutes for washing, and then transferred to a Buchner funnel. Washing with water was performed until the pH of the filtrate was in the range of 6–8. After repeating the acid washing and water washing three times, the mixture was dried with hot air at 80°C. The hot-air dried carbide was mixed with 12 mL of ion-exchanged water, and the resulting mixture was ball-milled for 10 minutes. While supplying nitrogen gas at the same rate as above, the pulverized carbide was heated to 1200°C at a heating rate of 10°C / min for 60 minutes to obtain porous carbon.

[0109] <Fabrication of Fuel Cell Catalysts> Pt catalyst particles, serving as electrode catalyst particles, were supported on the prepared porous carbon using the platinum acetylacetone method. A Pt precursor (Pt(C5H7O2)2) was dissolved in dichloromethane (3 mL) to achieve a Pt content of 35% by mass relative to 100 mg of porous carbon. The resulting solution was added to a flask containing 100 mg of porous carbon. The porous carbon was then dispersed in the solution for 30 minutes using an ultrasonic stirrer while the flask was ice-cooled. The solution was then heated to 50 °C and stirred until all the solvent evaporated, yielding a pale yellow powder. The powder was then heat-treated at 210 °C for 3 hours under a nitrogen atmosphere, followed by heat-treated at 240 °C for 3 hours to perform a reduction treatment, thus obtaining the fuel cell catalyst.

[0110] <Fabrication of Membrane-Electrode Assembly (MEA)> A Nafion membrane (thickness: 50 μm) was used as the electrolyte membrane. Pt / C (Tanaka Precious Metals Industry Co., Ltd., TEC10E50E) was dispersed in a mixed solvent containing a Nafion dispersion (Fujifilm Waco Chemical Co., Ltd., 5% Nafion (registered trademark) dispersion DE521 CS type), water, and ethanol at a volume ratio of 100:53:480. Based on the total amount of the dispersion, a dispersion containing 46% by mass of Pt / C for anode formation was prepared. The obtained dispersion was spray-printed onto the Nafion membrane to achieve a Pt content of 0.3 mg / cm³ in the anode. 2 The solvent is removed by drying on a hot plate at 60°C, thereby creating an anode (electrode catalyst layer) on the Nafion membrane.

[0111] Next, except that the aforementioned fuel cell catalyst was used instead of Pt / C (Tanaka Precious Metals Industry Co., Ltd., TEC10E50E), a cathode-forming dispersion was prepared using the same method as the anode-forming dispersion. The obtained cathode-forming dispersion was spray-printed onto the opposite side of the Nafion film that formed the anode, and dried on a hot plate at 60°C to achieve a Pt content of 0.3 mg / cm³ in the cathode. 2 The solvent is removed, thereby creating a cathode (electrode catalyst layer) on the Nafion membrane. Then, the anode, Nafion membrane, and cathode are pressed together at 0.3 kN and 130 °C, and clamped together with two sheets of hydrophobic carbon paper (manufactured by Toray Industries, model: EC-TP1-060T) as a gas diffusion layer, thus obtaining the MEA.

[0112] <Concentration Overvoltage> like Figure 2 As shown, concentration overvoltage (η) mt ) is obtained from the theoretical electromotive force (E) rev Subtract the ohmic overvoltage (η) from the difference between the action potential and the action potential.ohm The non-ohmic overvoltage (η) calculated accordingly. non-ohm Subtract the activation overvoltage (η) from the value. a The overvoltage is calculated based on this. Furthermore, the so-called ohmic overvoltage is also known as IR loss (η). IR The voltage drop is caused by the resistance of the electrodes and the resistance to the flow of electrolyte ions. Activation overvoltage refers to the voltage drop caused by the consumption of activation energy due to the hydroxide reaction at the anode and the oxygen reduction reaction at the cathode. Concentration overvoltage is the voltage drop caused by the retention of water vapor, a reaction byproduct, which prevents sufficient oxygen supply to the electrode surface. Among these three overvoltages, the properties of porous carbon used as a catalyst support affect the concentration overvoltage; therefore, the concentration overvoltage is used as an evaluation index for porous carbon used as a catalyst support.

[0113] It should be noted that a higher current density requires a greater oxygen supply. Consequently, due to increased water vapor emission, both oxygen supply and water vapor emission become difficult, thus increasing the proportion of concentration overvoltage relative to the overall overvoltage. Therefore, a higher current density of 700 mA / cm² is used. 2 The concentration overvoltage value at a given time is used as an evaluation index for porous carbon used as a catalyst support.

[0114] The ohmic overvoltage, activation overvoltage, and concentration overvoltage can be separated according to the test name recorded in the New Energy and Industrial Technology Development Organization (NEDO) Battery Evaluation Procedure (NEDO PEFC Battery Evaluation and Analysis Procedure 2023 Edition): IV Measurement Method - Overvoltage Separation Analysis Method (sometimes referred to as "NEDO Procedure Test Method" in this specification).

[0115] The specific procedures are shown below (1) to (4). It should be noted that (i) to (iii) and (v) to (vi) are the steps corresponding to steps [1] to [3] and [5] to [6] as described in the NEDO Procedure Test Method. It should be noted that step [4] in the NEDO Procedure Test Method is the calculation step for activation overvoltage, which does not directly contribute to the calculation of concentration overvoltage, and is therefore omitted here.

[0116] (1) Perform IV determination on MEA according to the conditions described in the NEDO procedure test method.

[0117] (2) Calculate the theoretical electromotive force according to the NEDO procedure test method.

[0118] (3) Calculate the ohmic overvoltage according to the following (v) based on the ohmic resistance obtained by the known AC impedance method.

[0119] (v) Current density at the point to be analyzed (A / cm²)2 ) × Internal resistance value (Ω·cm 2 ) is set as resistive overvoltage (synonymous with ohmic overvoltage).

[0120] (4) Using the ohmic overvoltage calculated in (3), calculate the concentration overvoltage according to (i) to (iii) and (vi) below.

[0121] (i) Based on the IV measurement data measured in (1), construct a Tafel plot with the horizontal axis current density (logarithmic axis) and the vertical axis IR-free voltage (the value of action potential plus ohmic overvoltage).

[0122] (ii) Determine the formula for the regression line based on the values ​​of 3 to 4 points that exhibit linearity on the low current density side.

[0123] (iii) Substitute the current density value of the point to be analyzed into the equation as x, and find the voltage y in the regression equation.

[0124] (vi) The difference between the voltage y value on the regression equation at current density x and the IR-free voltage value is taken as the diffusion overvoltage (synonymous with concentration overvoltage).

[0125] The more detailed process is shown below.

[0126] <Process (1)> The single-cell power generation evaluation fixture (manufactured by FC Planning Co., Ltd., Japan) with MEA assembled was placed in a constant temperature bath set at 80°C, and IV measurements were performed under the following conditions. The IV measurements used a fuel cell evaluation device (manufactured by Toyo Technica Co., Ltd., AutoPEM-KUG2) and a potentiometer / ammeter (manufactured by BioLogic Inc., SP-240).

[0127] As an example, Figure 3 The IV curves obtained by the above IV measurements using the MEA prepared in Example 1 are shown. At 700 [mA / cm] 2 The operating potential at the current density is 0.514 [V].

[0128] (Anode conditions) Electrode area: 1cm 2 Gas supplied: 100% H2 Gas supply rate: 139 mL / min Humidification temperature of supplied gas: 80℃ (relative humidity: 98%).

[0129] (Cathode conditions) Electrode area: 1cm 2 Type of gas supplied: Air Gas supply rate: 332 mL / min Humidification temperature of supplied gas: 80℃ (relative humidity: 98%).

[0130] <Process (2)> Substitute the following formula, as described in the NEDO test method, into the test conditions T = 80℃ (353K) and (PH2 / PH2) of this test. * 2) = 1.0, (PO2 / PO * 2) = 0.21, calculate the theoretical electromotive force E rev(PH2,PO2,T) The result is 1.17[V].

[0131] [Number 1] .

[0132] <Process (3)> Under the following conditions, perform AC impedance measurement to determine the ohmic resistance.

[0133] (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 Based on the obtained ohmic resistance, calculate the ohmic overvoltage according to the NEDO test method (V).

[0134] By setting the current density at the point to be analyzed to 0.700 [A / cm²] 2 Multiply by the internal resistance value of 0.0875 [Ω·cm] 2 To calculate the ohmic overvoltage (0.0613 [V]).

[0135] <Process (4)> (i) Tafel plots of current density (logarithmic axis) and IR-free voltage (vertical axis) were created based on the IV measurement data measured in (1). Figure 4 Tafel plot of the MEA manufactured in Example 1 is shown. Figure 4 L1 in the middle.

[0136] (ii) Determine the formula for the regression line based on the values ​​at 3-4 points exhibiting linearity on the low current density side. The regression line in Example 1 ( Figure 4 The formula for L2 in the middle is: [Number 2] [In the formula, A = -0.0360, B = 0.908].

[0137] (iii) Set the current density to 700 mA / cm² 2 Substituting x into the equation, we can find the voltage y in the regression equation. In Example 1, the current density x = 700 mA / cm². 2 The value of voltage y under ] Figure 4 Y1) is 0.672 [V].

[0138] <Process (5)> (vi) Based on the current density x (700 mA / cm²) 2 The value of voltage y under the regression equation () Figure 4 Y1 in the figure) and IR-free voltage value ( Figure 4 The concentration overvoltage is calculated from the difference between Y2 and Y2. In Example 1, the current density x = 700 mA / cm². 2 Concentration overvoltage under ] Figure 4 (η) mt The value is 0.672 - (0.514 + 0.0613) = 0.0967 (V).

[0139] [Example 2] The calcium chloride dihydrate was changed to 1g (100 parts by mass relative to 100 parts by mass of glucose), and the fuel cell catalyst and MEA were prepared in the same manner as in Example 1.

[0140] [Example 3] The carbon source was changed to 0.5g of glucose (sold by Fujifilm and Koichi Chemical Co., Ltd.) and 0.5g of cellulose (sold by Fujifilm and Koichi Chemical Co., Ltd.), the calcium chloride dihydrate was changed to 3g (300 parts by mass relative to 100 parts by mass of carbon source), and the deionized water was changed to 1.5g. Otherwise, the fuel cell catalyst and MEA were manufactured in the same manner as in Example 1.

[0141] [Comparative Example 1] Ketjen Black, manufactured by Lion Specialty Chemicals Co., Ltd. of Japan, was used as the catalyst carrier. Otherwise, the fuel cell catalyst and MEA were manufactured in the same manner as in Example 1.

[0142] [Comparative Example 2] 1 g of polyvinyl alcohol (PVA) was mixed with 4 g of magnesium citrate granules (sold by Fujifilm and Kojun Pharmaceutical Co., Ltd.) (400 parts by weight relative to 100 parts by weight of PVA). The resulting mixture was heated to 700°C at a rate of 10°C / min under a nitrogen atmosphere 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 stirred at 80°C for 30 minutes to clean it, and then removed onto a Buchner funnel. Water washing was performed until the pH of the filtrate reached the range of 6-8. After repeating the acid washing and water washing three times, the carbide was hot-air dried at 80°C. The hot-air dried carbide was further heated to 1200°C under a nitrogen atmosphere for heat treatment (calcination) for 60 minutes to obtain porous carbon. Otherwise, the fuel cell catalyst and MEA were manufactured in the same manner as in Example 1.

[0143] The physical properties of the porous carbon and fuel cell catalysts in the examples and comparative examples are shown in Table 1. It should be noted that the porous carbon in Examples 1-3 and Comparative Examples 1-2 is in particulate form.

[0144] [Table 1] .

[0145] Industrial applicability The porous carbon of the present invention can provide a low concentration of overvoltage when used as a catalyst support in a fuel cell. Therefore, the porous carbon of the present invention is suitable for use as a fuel cell catalyst support.

Claims

1. Porous carbon is used as a catalyst support for fuel cells, wherein, By using the LDF approximation, the pressure change of nitrogen gas until adsorption equilibrium is reached at various specified relative pressures during the determination of nitrogen adsorption isotherms is converted into a mass transfer coefficient, and this is applied to a relative pressure of 1.0 × 10⁻⁶. -4 Above and 1.0×10 -3 The slope of the approximate straight line obtained by linearly approximating the relationship between relative pressure and mass transfer coefficient is greater than 2.

5.

2. The porous carbon according to claim 1, wherein, Based on the nitrogen adsorption isotherm and calculated using the BJH method, the pore volume for pores with a diameter greater than 2 nm and less than 200 nm is 0.8 cm³. 3 / g or more.

3. The porous carbon according to claim 1, wherein, In the aforementioned approximate straight line, the relative pressure is 1.0 × 10⁻⁶. -3 The mass transfer coefficient at that time was 3.0 × 10⁻⁶. -3 Second -1 above.

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

5. The porous carbon according to claim 1, wherein, Bulk density less than 0.10 g / cm³ 3 .

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

7. A fuel cell comprising the fuel cell catalyst of claim 6.

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