Catalysts for fuel cells

Abstract

To provide a catalyst for a fuel cell that can improve durability and suppress deterioration of catalyst performance.SOLUTION: A catalyst for a fuel cell includes a carbon carrier having micropores, and a metal supported on the carbon carrier, and the metal is at least one of platinum and a platinum alloy, the specific surface area of the carbon carrier is 250 m2 / g-carbon or more and 338 m2 / g-carbon or less, and the area of the micropores of the carbon carrier is greater than or equal to 48 m2 / g-carbon and less than or equal to 74 m2 / g-carbon.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to catalysts for fuel cells. [Background technology]

[0002] Various studies have been conducted on catalysts for fuel cells. For example, in Patent Document 1, (a) 100 to 600 m 2 / g specific surface area (BET), (b) 10 to 90 m 2 A catalyst comprising a carbon support material having a micropore area of ​​1 / g is disclosed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2014-534052 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to commercialize fuel cell-equipped commercial vehicles, further improvements in the durability of fuel cell catalysts are required.

[0005] The present disclosure has been made in view of the above circumstances, and has as its main object to provide a catalyst for a fuel cell that can improve durability and suppress deterioration of catalytic performance. [Means for solving the problem]

[0006] The catalyst of the present disclosure is a catalyst for a fuel cell, comprising a carbon support having micropores and a metal supported on the carbon support, the metal is at least one of platinum and a platinum alloy; The specific surface area of ​​the carbon support is 250 m 2 / g-carbon or more 338m 2 / g-carbon or less, The area of ​​the micropores of the carbon support is 48 m 2 / g-carbon or more 74m 2 / g-carbon or less.

[0007] In the catalyst of the present disclosure, the carbon support may be acetylene black.

[0008] In the catalyst of the present disclosure, the gas diffusion limiting current value observed by IV measurement of the fuel cell is 0.15 A / cm 2 It may be more than that. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to improve the durability of a catalyst for a fuel cell and suppress a decrease in catalyst performance. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the relationship between the specific surface area (SSA) and the micropore area of ​​the carbon support used in each example and each comparative example. [Figure 2] FIG. 2 is a graph showing the relationship between the cell voltage and the current density in the range of 0 to 3.0 A / cm 2 for Examples 1 and 2 and Comparative Examples 1 and 6. [Figure 3] FIG. 3 is a graph showing the relationship between the cell voltage and the current density in the range of 0 to 0.2 A / cm 2 for Examples 1 and 2 and Comparative Examples 1 and 6. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the present disclosure (for example, the general configuration and manufacturing process of a fuel cell catalyst or the like that do not characterize the present disclosure) can be understood as design matters for those skilled in the art based on prior art in the relevant field. The present disclosure can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. Furthermore, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships. In this specification, the use of "to" to indicate a range of values ​​means that the values ​​before and after it are included as the lower and upper limits. Any combination of upper and lower limits in the numerical range can be adopted.

[0012] <Fuel cell catalyst> The catalyst of the present disclosure is a catalyst for a fuel cell, comprising a carbon support having micropores and a metal supported on the carbon support, the metal is at least one of platinum and a platinum alloy; The specific surface area of ​​the carbon support is 250 m 2 / g-carbon or more 338m 2 / g-carbon or less, The area of ​​the micropores of the carbon support is 48 m 2 / g-carbon or more 74m 2 / g-carbon or less.

[0013] High durability is a primary requirement for catalysts used in commercial vehicles equipped with fuel cells that are expected to be driven long distances. In addition, fuel cell stacks must be made smaller to accommodate larger load capacity, and high performance catalysts are also required. In conventional technology, catalyst performance and durability are in a trade-off relationship, making it difficult to achieve both. This is because catalyst durability and performance are correlated with the specific surface area of ​​the carbon support, and improving durability requires reducing the specific surface area, while improving catalyst performance requires increasing the specific surface area, resulting in a trade-off between the two. The present researchers discovered that the desired catalytic performance of a catalyst can be ensured by increasing the area of ​​the micropores in the carbon support within a predetermined range, while the desired durability can be ensured by keeping the specific surface area of ​​the carbon support, which is a factor in reducing durability, small within a predetermined range. Therefore, according to the present disclosure, by controlling the specific surface area and micropore area of ​​the carbon support within a predetermined range, the durability of the catalyst can be improved and the deterioration of catalytic performance can be suppressed. This is because reducing the specific surface area reduces the number of degradation points in the carbon support, while increasing the micropore area ensures gas diffusion paths and reduces mass transfer overvoltage, which causes loss of catalytic performance.

[0014] The catalyst of the present disclosure includes a carbon support and a metal supported on the carbon support. In the catalyst of the present disclosure, the gas diffusion limiting current value observed by IV measurement of a fuel cell is 0.15 A / cm 2 This allows the catalyst to have the desired durability and ensures catalytic performance equal to or greater than that of conventional high specific surface area products. In this disclosure, the gas diffusion limiting current value is the average value of the current observed between 0.1 and 0.6 V in IV measurement under specified conditions in the low current density region (cell temperature: 45°C, relative humidity of both electrodes: 165%).

[0015] The metal is supported on a carbon support. The metal is at least one of platinum and a platinum alloy. The platinum alloy may be an alloy containing platinum and one or more metals selected from the group consisting of cobalt, nickel, iron, manganese, copper, titanium, tungsten, tin, gallium, zirconium, chromium, gadolinium, terbium, ytterbium, hafnium, and osmium. Among these, platinum, platinum-cobalt alloys, and platinum-nickel alloys may be used, and particularly platinum-cobalt alloys may be used. The metal may be metal particles that are particulate in shape. The particle size of the metal particles is not particularly limited, but may be 1 nm or more and 10 nm or less. The particle size of the metal particles can be measured by 3D-TEM or the like.

[0016] The carbon support supports the metal. The carbon support has micropores. The average pore diameter of the micropores needs to be less than 2 nm. The average pore diameter of the micropores can be obtained by measuring the pore diameters of multiple randomly selected micropores and calculating the average value. The average pore diameter can be measured using a 3D-TEM (transmission electron microscope) or the like. The carbon support may be a porous body. The carbon support may be carbon support particles that are particulate in shape. The particle size of the carbon support particles is not particularly limited as long as it is larger than the pore size of the micropores, and may be, for example, 2 nm or more and 100 nm or less. The particle size of the carbon support particles can be measured by 3D-TEM or the like.

[0017] The specific surface area of ​​the carbon support is 250m 2 / g-carbon or more 338m 2 By controlling the specific surface area within this range, the durability of the catalyst can be improved. The specific surface area is measured by the BET method. The measurement of the specific surface area by the BET method is carried out by the following process. After degassing the carbon support to form a clean solid surface, a nitrogen adsorption isotherm is obtained, measuring the amount of gas adsorbed as a function of gas pressure at a constant temperature (usually the temperature of liquid nitrogen at its boiling point at 1 atmosphere). Then, for P / P values ​​in the range of 0.05 to 0.3 (or sometimes as low as 0.2), 1 / [V a ((P / P)−1)] to P / P (where V a is the amount of gas adsorbed at pressure P, where P0 is the saturation pressure of the gas. Fit the plot with a line, intercept 1 / V m C and slope (C-1) / V mC (where C is a constant) to the monolayer volume (V m The surface area of ​​the sample can be determined from the monolayer volume by correcting for the area occupied by a single adsorbate molecule.

[0018] The area of ​​the micropores in the carbon support is 48m 2 / g-carbon or more 74m 2 By ensuring the area of ​​the micropores in this range, the desired performance of the catalyst can be ensured. Micropore area refers to the surface area associated with the micropores, where micropores are defined as pores with an internal width of less than 2 nm. Micropore area is determined by using a t-plot generated from the nitrogen adsorption isotherm as described above. The t-plot has the volume of adsorbed gas plotted as a function of standard multilayer thickness t, where the t value is calculated using the pressure value from the adsorption isotherm in a thickness equation, in this case the Harkins-Jura equation. The slope of the linear portion of the t-plot at thickness values ​​between 0.35 nm and 0.5 nm is used to calculate the external surface area, i.e., the surface area associated with all pores excluding the micropores. The micropore surface area is then calculated by subtracting the external surface area from the BET surface area.

[0019] The carbon support may be a conductive carbon support having electrical conductivity. Conductive carbon supports can be obtained by functionalization of existing carbon materials. Carbon functionalization or activation can be understood as post-treatment of the carbon with gases such as oxygen, air, carbon dioxide, water vapor, ozone, or nitrogen oxides, in the case of physical activation, or as reaction of a carbon precursor with solid or liquid reagents such as KOH, ZnCl2, or H3PO4 at high temperatures, in the case of chemical activation. During the activation process, part of the carbon is lost through chemical reactions or combustion. Activation of carbon supports is typically carried out using oxidizing gases such as oxygen, ozone, hydrogen peroxide, or nitrogen dioxide, which increases the specific surface area and also the amount of surface groups. Activation can also be carried out by treatment with air, carbon dioxide, or water vapor. The carbon support may be acetylene black. The acetylene black may be air-activated acetylene black that has been activated with air.

[0020] The carbon support may also have a loss (absolute corrosion amount, unit: wt%) of less than 9.1%, or even 8.0% or less, or even 7.9% or less, in an accelerated test including a 1.2 V potential hold for 24 hours at 80° C. Carbon loss may be determined by the following generally accepted test. Electrodes of selected catalysts or carbons are held at 1.2 V (vs. reversible hydrogen electrode (RHE)) in 1 M H2SO4 liquid electrolyte at 80 °C, and the corrosion current is monitored over a 24-hour period. The charge passed during the experiment is integrated and used to calculate the carbon removed, assuming a four-electron process converting the carbon to CO2 gas. The first minute of the test is not included, as the charge passed during this period is due to charging of the electrochemical double layer and therefore not due to the corrosion process. The mass of carbon lost during the 24-hour test is expressed as a percentage of the electrode's initial carbon content.

[0021] The specific corrosion rate of the carbon support may be less than 56%, or even less than 34%, or even less than 31%. The specific corrosion rate is determined by expressing the amount of corroded carbon as a percentage of the number of surface carbon atoms: 3.79 x 10 19 atom m -2 of carbon, and assuming a four-electron process, the maximum charge required to remove one monolayer of carbon is determined. The experimentally determined charge associated with carbon corrosion is expressed as a percentage of the monolayer to obtain the specific corrosion rate.

[0022] The catalyst of the present disclosure is for use in fuel cells. Examples of types of fuel cells include polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), alkaline electrolyte fuel cells (AFCs), and direct current fuel cells (DFCs), and among these, polymer electrolyte fuel cells may be used. The fuel cell may have only one unit cell, or may be a fuel cell stack, which is a stack of a plurality of unit cells. The number of stacked unit cells is not particularly limited, and may be, for example, from 2 to several hundred.

[0023] A single cell of the fuel cell comprises at least a membrane electrode gas diffusion layer assembly. The membrane electrode gas diffusion layer assembly has, in this order, an anode gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode gas diffusion layer.

[0024] The cathode (oxidant electrode) includes a cathode catalyst layer, and optionally a cathode-side gas diffusion layer. The anode (fuel electrode) includes an anode catalyst layer, and optionally an anode-side gas diffusion layer. The cathode catalyst layer and the anode catalyst layer are collectively referred to as catalyst layers. The catalyst layer contains the above-described fuel cell catalyst and usually further contains an electrolyte. The electrolyte may be a proton-conductive material, such as a fluorine-based resin, etc. As the fluorine-based resin, for example, a perfluorosulfonic acid-based resin such as Nafion (registered trademark) may be used.

[0025] The cathode side gas diffusion layer and the anode side gas diffusion layer are collectively referred to as gas diffusion layers. The gas diffusion layer may be a gas-permeable conductive material or the like. Examples of the conductive member include porous carbon materials such as carbon cloth and carbon paper, and porous metal materials such as metal mesh and foam metal.

[0026] The electrolyte membrane may be a solid polymer electrolyte membrane. Examples of the solid polymer electrolyte membrane include a fluorine-based electrolyte membrane such as a thin film of perfluorosulfonic acid containing water, and a hydrocarbon-based electrolyte membrane. The electrolyte membrane may be, for example, a Nafion membrane (manufactured by DuPont).

[0027] The single cell may optionally include two separators sandwiching both sides of the membrane electrode gas diffusion layer assembly. One of the two separators is an anode-side separator, and the other is a cathode-side separator. In the present disclosure, the anode-side separator and the cathode-side separator are collectively referred to as separators. The separator may have holes such as supply holes and discharge holes for allowing the reaction gas, coolant, etc. to flow in the stacking direction of the unit cells. As the coolant, for example, a mixed solution of ethylene glycol and water can be used to prevent freezing at low temperatures. In this disclosure, the fuel gas and the oxidant gas are collectively referred to as reactant gases. The reactant gas supplied to the anode is the fuel gas, and the reactant gas supplied to the cathode is the oxidant gas. The fuel gas is a gas that mainly contains hydrogen and may be hydrogen. The oxidant gas may be oxygen, air, dry air, or the like. The separator may have a reactant gas flow path on the surface in contact with the gas diffusion layer, and may have a coolant flow path on the surface opposite to the surface in contact with the gas diffusion layer to maintain a constant temperature of the fuel cell. The separator may be a gas-impermeable conductive material. Examples of the conductive material include dense carbon made by compressing carbon to make it gas-impermeable, and press-formed metal (e.g., iron, aluminum, stainless steel, etc.) plates. The separator may also have a current collecting function.

[0028] <Method of manufacturing a catalyst for fuel cells> The method for producing a catalyst according to the present disclosure includes (1) a supporting step of supporting Pt or Pt and a metal M other than Pt on a carbon support, and may optionally include (2) an alloying step of alloying the Pt supported on the carbon support with a metal other than Pt (e.g., Co), and (3) an acid treatment step of acid treating the Pt or Pt alloy supported on the carbon support. Examples of the metal M other than Pt include one or more metals selected from the group consisting of cobalt, nickel, iron, manganese, copper, titanium, tungsten, tin, gallium, zirconium, chromium, gadolinium, terbium, ytterbium, hafnium, and osmium.

[0029] (1) Supporting process In the supporting step, Pt or Pt and a metal M other than Pt are supported on the carbon support in a molar ratio of Pt:M=3:1 to 7:1, for example. Since a portion of the metal M other than Pt is removed in the acid treatment step described below, a larger amount of the metal M other than Pt is supported in the supporting step compared to the predetermined molar ratio of Pt to the metal M other than Pt in the finished catalyst. By using a catalyst manufactured using such a molar ratio, it is possible to further improve the initial power generation performance of the fuel cell and the durability of the fuel cell.

[0030] (2) Alloying process In the alloying step, Pt and a metal M other than Pt are alloyed at 700 to 900°C or 750 to 850°C. By using a catalyst manufactured using such alloying temperatures, the initial power generation performance of the fuel cell and the durability of the fuel cell can be further improved.

[0031] (3) Acid treatment process In the acid treatment step, Pt or a Pt alloy supported on a carbon support is subjected to an acid treatment at, for example, 70 to 90°C or 75 to 85°C. By performing the acid treatment at such a temperature, metals M other than Pt that do not contribute to the reaction can be sufficiently removed, thereby suppressing the elution of metals M other than Pt. Examples of acids used in the acid treatment step include inorganic acids (nitric acid, phosphoric acid, permanganic acid, sulfuric acid, hydrochloric acid, etc.) and organic acids (acetic acid, malonic acid, oxalic acid, formic acid, citric acid, lactic acid, etc.).

[0032] The materials, products, and their characteristics in the catalyst manufacturing method of the present disclosure have already been explained in the section <Catalyst for fuel cells>. The matters explained in the above section shall be taken into consideration as appropriate in this section. [Example]

[0033] Hereinafter, the present disclosure will be described in more detail using examples and comparative examples, but the technical scope of the present disclosure is not limited thereto. Note that examples and comparative examples are not distinguished by whether they are included in the scope of the claims. Embodiments that obtained particularly good results are designated as examples, and other embodiments are designated as comparative examples.

[0034] <Production of electrode catalyst> Example 1 [Supporting process] A microporous carbon support (1.0 g, manufactured by Denki Kagaku Kogyo Co., Ltd.) was dispersed in pure water (41.6 mL). A dinitrodiamine platinum nitrate solution (Patent No. 4315857, manufactured by Cataler Corporation) containing platinum (1.0 g) was added dropwise and allowed to fully blend with the carbon support. Ethanol (3.2 g) was added as a reducing agent, and platinum was supported on the carbon support via a reduction reaction. The dispersion was filtered and washed, and the resulting powder was dried to obtain a platinum-supported carbon support. Next, the oxygen content on the surface of the platinum-supported carbon support was reduced to 4 wt% or less, and cobalt (0.03 g) was supported on the platinum-supported carbon support so that the product ratio (molar ratio) was Pt:Co was 7:1. The carbon support used in this example was acetylene black. The BET specific surface area (m) of the carbon support determined by N adsorption was 2 / g-carbon), and micropore area (m 2 The results of the measurement of carbon dioxide (CO₂ / g-carbon) are shown in Table 1. [Alloying process] The platinum-cobalt supported carbon support was alloyed at 800°C under an argon atmosphere to obtain a platinum-cobalt alloy supported carbon support. [Acid treatment process] The platinum-cobalt alloy-supported carbon support was acid-treated with 0.5N nitric acid at 80°C to obtain a catalyst.

[0035] (Examples 2 to 6, Comparative Examples 1 to 6) A catalyst was produced in the same manner as in Example 1, except that a carbon support having the specific surface area and micropore area shown in Table 1 was used.

[0036] <Corrosion test of carbon support> The corrosion test was carried out at a single cell temperature of 80°C, a bubbler temperature of 80°C (100% RH), and a potential of 1.2 V for 24 hours. After the corrosion test, the loss of the carbon support (absolute corrosion amount, unit: wt%) and the specific corrosion rate (%) were calculated. The results are shown in Table 1.

[0037] <Evaluation of power generation performance> The catalysts produced in each example and comparative example were dispersed in an organic solvent, and the dispersion was applied to a Teflon (registered trademark) sheet to form two electrodes. A polymer electrolyte membrane was sandwiched between the two electrodes and bonded together by hot pressing to obtain a membrane electrode assembly. The membrane electrode assembly was sandwiched between two gas diffusion layers to produce a single cell for a polymer electrolyte fuel cell. The cell temperature was set to 60°C, and the relative humidity of both electrodes was set to 80%, and IV measurements were performed using a small single cell evaluation device system (manufactured by Toyo Corporation). For IV measurement, 0.01~4.0A / cm 2 The current was controlled arbitrarily within the range of 0.2 A / cm 2 The voltage value at this time was defined as activity. In addition, the gas diffusion limiting current value (A / cm 2 ) was measured as the average value of the current observed between 0.1 and 0.6 V in IV measurements in the low current density region when the cell temperature was 45°C and the relative humidity of both electrodes was 165%. The measurement results for Examples 1 to 6 and Comparative Examples 1 and 6 are shown in Table 1.

[0038] [Table 1]

[0039] FIG. 1 is a diagram showing the relationship between the specific surface area (SSA) and the micropore area of ​​the carbon support used in each example and each comparative example. FIG. 2 shows the 0 to 3.0 A / cm of Examples 1 and 2 and Comparative Examples 1 and 6. 2 1 is a graph showing the relationship between the cell voltage and the current density in the range of 1000 s.c. FIG. 3 shows the current density of 0 to 0.2 A / cm for Examples 1 and 2 and Comparative Examples 1 and 6. 2 1 is a graph showing the relationship between the cell voltage and the current density in the range of 1000 s.c. As shown in Table 1, Comparative Example 1 has high durability due to a small absolute amount of corrosion, but has low catalytic performance due to a low gas diffusion limiting current value. Comparative Example 5 has a large absolute amount of corrosion and therefore low durability. Comparative Example 6 has a high gas diffusibility limiting current value and therefore high catalytic performance, but has a large absolute amount of corrosion and therefore low durability. Examples 1, 4 and 6 exhibit high durability due to small absolute corrosion amounts, and also exhibit high gas diffusibility limiting current values, demonstrating high performance equivalent to that of Comparative Example 6. Among Examples 1 to 6, Examples 2 and 3 exhibited relatively high durability due to a relatively small absolute amount of corrosion, and although the catalytic performance was inferior to Examples 1, 4 and 6, they still exhibited the desired catalytic performance. From the above results, it can be seen that by reducing the specific surface area of ​​the carbon support, the number of degradation points of the carbon support can be reduced and the durability of the catalyst can be improved, and by increasing the micropore area of ​​the carbon support, the desired gas diffusibility can be ensured, thereby suppressing the deterioration of catalyst performance.

Claims

1. A catalyst for a fuel cell comprising a carbon support having micropores and a metal supported on the carbon support, the metal is at least one of platinum and a platinum alloy; The specific surface area of ​​the carbon support is 250 m ２ / g-carbon or more 338m ２ / g-carbon or less, The area of ​​the micropores of the carbon support is 48 m ２ / g-carbon or more 74m ２ / g-carbon or less.

2. 2. The catalyst for a fuel cell according to claim 1, wherein the carbon support is acetylene black.

3. The gas diffusion limiting current value observed by IV measurement of the fuel cell was 0.15 A / cm ２ 3. The catalyst for a fuel cell according to claim 1 or 2, wherein:

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