Catalyst for oxygen reduction reaction and method for producing the catalyst for oxygen reduction reaction

Abstract

To provide a highly durable catalyst for oxygen reduction reactions and a method for producing such a catalyst. [Solution] The catalyst for the oxygen reduction reaction comprises conductive diamond particles and platinum supported on the conductive diamond particles, wherein the conductive diamond particles have a total acid content of 1.40 mmol / g or more. The conductive diamond particles have a specific surface area of ​​400 m². 2 It is preferable that the amount is 0.90 mmol / g or more, the amount of carboxyl groups is preferably 0.90 mmol / g or more, and the amount of phenolic hydroxyl groups is preferably 0.45 mmol / g or less.

Description

[Technical Field]

[0001] The present invention relates to a catalyst for oxygen reduction reactions and a method for producing a catalyst for oxygen reduction reactions. [Background technology]

[0002] Polymer electrolyte fuel cells (PEFCs) are attracting attention as a clean energy conversion device that uses hydrogen as an energy source. In polymer electrolyte fuel cells, hydrogen oxidation occurs at the anode and oxygen reduction occurs at the cathode. Catalysts are used to accelerate the reactions at the anode and cathode. As a catalyst for the cathode, for example, a catalyst in which platinum is supported on carbon or nanodiamond is known (see Patent Document 1, etc.). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-128418 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, conventional cathode catalysts suffer from insufficient durability, for example, a decrease in catalytic activity after prolonged use. Therefore, there is a need for a cathode catalyst with superior durability, i.e., a highly durable catalyst for oxygen reduction reactions.

[0005] Therefore, the object of the present invention is to provide a catalyst for oxygen reduction reactions with excellent durability and a method for producing said catalyst for oxygen reduction reactions. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by a catalyst for oxygen reduction reaction comprising conductive diamond particles and platinum supported on the conductive diamond particles, wherein the total acid content of the conductive diamond particles is 1.40 mmol / g or more, and have accomplished the present invention. More specifically, the present invention is as follows.

[0007] (1) A catalyst for oxygen reduction reaction comprising conductive diamond particles and platinum supported on the conductive diamond particles, wherein the total acid content of the conductive diamond particles is 1.40 mmol / g or more.

[0008] (2) The catalyst for oxygen reduction reaction according to (1) above, wherein the specific surface area of the conductive diamond particles is 400 m 2 / g or more.

[0009] (3) The catalyst for oxygen reduction reaction according to (1) or (2) above, wherein the amount of carboxy groups in the conductive diamond particles is 0.90 mmol / g or more.

[0010] (4) The catalyst for oxygen reduction reaction according to any one of (1) to (3) above, wherein the amount of phenolic hydroxyl groups in the conductive diamond particles is 0.45 mmol / g or less.

[0011] (5) The catalyst for oxygen reduction reaction according to any one of (1) to (4) above, wherein the ratio of the molar amount of the total acid content to the molar amount of platinum is 0.38 or more.

[0012] (6) The catalyst for oxygen reduction reaction according to (3) above, wherein the ratio of the molar amount of the carboxy groups to the molar amount of platinum is 0.26 or more.

[0013] (7) The catalyst for oxygen reduction reaction according to (4) above, wherein the ratio of the molar amount of the phenolic hydroxyl groups to the molar amount of platinum is 0.15 or less.

[0014] (8) The surface area of platinum is 40 m 2The catalyst for oxygen reduction reaction according to any one of (1) to (7) above, which is / g or more.

[0015] (9) An oxygen reduction electrode comprising an electrode substrate and the catalyst for oxygen reduction reaction according to any one of (1) to (8) above.

[0016] (10) A method for producing the catalyst for oxygen reduction reaction according to any one of (1) to (8) above, comprising: a surface oxidation step of oxidizing a surface of conductive diamond particles by heating the conductive diamond particles in the presence of an acid; a supporting step of supporting a platinum-containing complex on the conductive diamond particles after the surface oxidation step; a platinum reduction step of reducing platinum ions in the complex to platinum by heating the conductive diamond particles after the supporting step in an atmosphere containing hydrogen; the method for producing a catalyst for oxygen reduction reaction comprising the above steps.

[0017] (11) The method for producing a catalyst for oxygen reduction reaction according to (10) above, wherein a heating temperature in the platinum reduction step is 100°C or higher and 200°C or lower.

[0018] (12) The method for producing a catalyst for oxygen reduction reaction according to (10) or (11) above, wherein the acid is at least one acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid.

Effects of the Invention

[0019] According to the present invention, a catalyst for oxygen reduction reaction having excellent durability and a method for producing the catalyst for oxygen reduction reaction can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] [Figure 1] These are TEM observation results of Example 1 and Comparative Examples 1 to 2. [Figure 2] These are cyclic voltammetry (CV) measurement results of Example 1 and Comparative Examples 1 to 2. [Figure 3]These are the linear sweep voltameometry (LSV) measurement results for Example 1 and Comparative Examples 1-2. [Figure 4] This figure shows the mass activity of Example 1 and Comparative Examples 1-2. [Figure 5] This figure shows the electrochemical effective surface area (ECA) retention rate of platinum in Example 1 and Comparative Examples 1-2. [Figure 6] These are the CV measurement results after multiplying each cycle number in Example 1 and Comparative Examples 1-2. [Modes for carrying out the invention]

[0021] ≪Catalyst for oxygen reduction reaction≫ The catalyst for the oxygen reduction reaction comprises conductive diamond particles and platinum supported on the conductive diamond particles. The conductive diamond particles have a total acid content of 1.40 mmol / g or more.

[0022] The above-mentioned catalyst for the oxygen reduction reaction exhibits excellent durability, as shown in the examples described later. When this catalyst is used, for example, as a catalyst in the cathode of a fuel cell such as a polymer electrolyte fuel cell (PEFC), it results in a fuel cell that can perform the oxygen reduction reaction with high efficiency for a long period of time. On the other hand, if the catalyst is not for the oxygen reduction reaction described above (for example, if the total acid content is less than 1.40 mmol / g, or if platinum is supported on particles other than conductive diamond particles (e.g., carbon) instead of conductive diamond particles), the durability will be inferior.

[0023] Furthermore, the above-mentioned catalyst for oxygen reduction reaction exhibits excellent initial mass activity. For example, the above-mentioned catalyst for oxygen reduction reaction has mass activity equivalent to or greater than that of conventional catalysts for oxygen reduction reaction in which the total acid content of conductive diamond particles is less than 1.40 mmol / g.

[0024] Examples of conductive diamond particles included in the catalyst for oxygen reduction include diamond doped with group 13 or group 15 elements to impart conductivity. The conductivity of the conductive diamond particles is, for example, 0.01 S / cm or higher. Examples of group 13 or group 15 elements include boron, nitrogen, and phosphorus. Among conductive diamond particles, boron-doped diamond particles (BDDP) are preferred, and boron-doped nanodiamond (BDND) particles are more preferred, due to their low electrical resistance.

[0025] The method for producing BDDP is not particularly limited, and known production methods described in Japanese Patent Publication No. 2008-36631, Japanese Patent Publication No. 2018-76216, etc., can be employed. A preferred production method is one in which a boron-doped diamond layer (BDD layer) is formed on the surface of a particulate substrate. This production method will be described below.

[0026] The particulate substrate is not particularly limited as long as it does not melt or deform during the formation of the BDD layer, and can be appropriately selected according to the purpose. Examples of particulate substrates include natural or artificial diamond particles; silicon particles; metal particles such as molybdenum particles; metal oxide particles such as alumina particles; boron nitride particles, quartz particles, etc. Among these, natural or artificial diamond particles are preferred.

[0027] The method for forming the BDD layer is not particularly limited and can be appropriately selected depending on the purpose. Examples of BDD layer formation methods include CVD methods such as microwave plasma CVD (MPCVD) and thermal filament CVD; physical vapor deposition (PVD) methods such as ion beam and ionization deposition; and high-temperature, high-pressure methods. Among these, microwave plasma CVD is preferred.

[0028] In the CVD method described above, the carbon source and boron source used as raw materials for the BDD layer are not particularly limited. Examples of carbon sources include aliphatic hydrocarbons such as methane, ethane, propane, butane, pentane, hexane, heptane, and octane; alicyclic hydrocarbons such as cyclohexane; aromatic hydrocarbons such as benzene, toluene, xylene, and ethylbenzene; alcohols such as methanol, ethanol, isopropyl alcohol, and butanol; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ethers such as diethyl ether, dimethoxyethane, tetrahydrofuran, and dioxane; and esters such as methyl acetate, ethyl acetate, isopropyl acetate, and butyl acetate. Examples of boron sources include boron oxide, boron carbide, boron nitride, boric acid, diborane, triethylborane, trimethoxyborane, triethoxyborane, tripropoxyborane, and tri(1,1-dimethylethoxy)borane.

[0029] In the BDD layer, the number of boron atoms doped into the diamond is preferably 10 to 100,000 ppm, more preferably 100 to 50,000 ppm, and even more preferably 1,000 to 30,000 ppm, relative to the number of carbon atoms constituting the diamond.

[0030] The shape of conductive diamond particles is not particularly limited and can be spherical, polyhedral, needle-shaped, spindle-shaped, etc.

[0031] The average particle diameter of the conductive diamond particles is preferably 1 nm to 100 μm, and more preferably 5 nm to 500 nm. The average particle diameter of the conductive diamond particles is the volume-average particle diameter measured by dynamic light scattering (DLS).

[0032] The specific surface area of ​​conductive diamond particles is 400 m². 2 It is preferable that the concentration is 1 / g or more. The specific surface area of ​​the conductive diamond particles is 450 m². 2 Preferably 1 / g or more, 500m 2A value of 1000 m² or more is more preferable. A larger specific surface area of ​​conductive diamond particles tends to improve the activity of the catalyst for the oxygen reduction reaction. The specific surface area of ​​conductive diamond particles is, for example, 1000 m². 2 It is less than / g and 900m 2 / g or less, or 800m 2 It is also acceptable to use less than / g.

[0033] The specific surface area of ​​conductive diamond particles can be measured using a nitrogen gas adsorption specific surface area analyzer (for example, the BELSORP MaxII manufactured by Microtrac-Bel).

[0034] The total acid content of the conductive diamond particles may be 1.40 mmol / g or more, but is preferably 1.45 mmol / g or more, and more preferably 1.50 mmol / g or more. Furthermore, the total acid content of the conductive diamond particles is preferably 2.00 mmol / g or less, and more preferably 1.80 mmol / g or less. The total acid content of conductive diamond particles is the sum of the acidic functional groups, such as carboxyl groups and phenolic hydroxyl groups.

[0035] The conductive diamond particles preferably have a carboxyl group content of 0.90 mmol / g or more, and more preferably 1.00 mmol / g or more. Furthermore, the carboxyl group content of the conductive diamond particles is preferably 1.80 mmol / g or less, and more preferably 1.70 mmol / g or less. When the amount of carboxyl groups is within the above range, the interaction between the supported platinum and the support (conductive diamond particles) becomes stronger, and the material tends to have better durability.

[0036] The conductive diamond particles preferably have a phenolic hydroxyl group content of 0.45 mmol / g or less, and more preferably 0.44 mmol / g or less. Furthermore, the conductive diamond particles preferably have a carboxyl group content of 0.20 mmol / g or more, and more preferably 0.30 mmol / g or more. When the amount of phenolic hydroxyl groups is within the above range, the interaction between the supported platinum and the support becomes stronger, and the material tends to have better durability.

[0037] The total amount of acid, the amount of carboxyl groups, and the amount of phenolic hydroxyl groups in conductive diamond particles can be determined by the Boehm method (acid-base titration). The Boehm method is a method for quantifying the amount of acidic functional groups present on the carbon surface by adding various alkalis to a carbon material, reacting them, and then back-titrating the concentration of the alkali after the reaction with an acid. Specifically, the total acid content, the amount of carboxyl groups, and the amount of phenolic hydroxyl groups in conductive diamond particles can be determined by the methods described in the examples below.

[0038] For the oxygen reduction reaction catalyst, the ratio of the molar amount of total acid to the molar amount of platinum (moles of total acid / molar amount of platinum) is preferably 0.38 or higher, and more preferably 0.43 or higher. The ratio of the molar amount of total acid to the molar amount of platinum (moles of total acid / molar amount of platinum) is preferably 0.55 or lower, and more preferably 0.50 or lower. When the ratio of the molar amount of total acid to the molar amount of platinum is within the above range, the interaction between the supported platinum and the support becomes stronger, and the durability tends to be superior.

[0039] For the oxygen reduction reaction catalyst, the ratio of the molar amount of carboxyl groups to the molar amount of platinum (moles of carboxyl groups / molar amount of platinum) is preferably 0.26 or higher, and more preferably 0.30 or higher. The ratio of the molar amount of carboxyl groups to the molar amount of platinum (moles of carboxyl groups / molar amount of platinum) is preferably 0.38 or lower, and more preferably 0.36 or lower. When the ratio of the molar amount of carboxyl groups to the molar amount of platinum is within the above range, the interaction between the supported platinum and the support becomes stronger, and the durability tends to be superior.

[0040] Preferably, the ratio of the molar amount of phenolic hydroxyl groups to the molar amount of platinum (molar amount of phenolic hydroxyl groups / molar amount of platinum) in the oxygen reduction reaction catalyst is 0.15 or less, more preferably 0.14 or less. Preferably, the ratio of the molar amount of phenolic hydroxyl groups to the molar amount of platinum (molar amount of phenolic hydroxyl groups / molar amount of platinum) is 0.10 or more, more preferably 0.11 or more. When the ratio of the molar amount of phenolic hydroxyl groups to the molar amount of platinum falls within the above range, the interaction between the supported platinum and the carrier is enhanced, which tends to result in more excellent durability.

[0041] In the oxygen reduction reaction catalyst, the surface area of platinum is 40 m 2 / g or more, preferably 45 m 2 / g or more, more preferably 50 m 2 / g or more. A large platinum surface area tends to improve the activity of the oxygen reduction reaction catalyst. In the oxygen reduction reaction catalyst, the surface area of platinum is, for example, 200 m 2 / g or less, 150 m 2 / g or less, 100 m 2 / g or less may also be employed. The platinum surface area can be calculated from the carbon monoxide adsorption amount.

[0042] <<Method for Producing Oxygen Reduction Reaction Catalyst>> The method for producing the above oxygen reduction reaction catalyst is not particularly limited. For example, the oxygen reduction reaction catalyst can be produced by a production method comprising: a surface oxidation step of oxidizing the surface of conductive diamond particles by heating the conductive diamond particles in the presence of an acid; a supporting step of supporting a platinum-containing complex on the conductive diamond particles after the surface oxidation step; and a platinum reduction step of reducing platinum ions in the complex to platinum by heating the conductive diamond particles after the supporting step in an atmosphere containing hydrogen. Each step will be described below.

[0043] (Surface Oxidation Step) In the surface oxidation process, the conductive diamond particles are heated in the presence of an acid to oxidize the surface of the conductive diamond particles.

[0044] The conductive diamond particles are as described above in the section on "Catalysts for Oxygen Reduction Reactions."

[0045] The acid is not particularly limited, but examples include nitric acid, sulfuric acid, and hydrochloric acid. The acid can be one type or two or more types.

[0046] The temperature and time for heating the conductive diamond particles in the presence of acid are not particularly limited. The heating temperature is, for example, 60°C to 120°C, and preferably 70°C to 110°C. The heating time is, for example, between 1 hour and 10 hours, and preferably between 2 hours and 8 hours.

[0047] In the surface oxidation process, oxidizing the surface of conductive diamond particles increases the total acid content, carboxyl groups, and oxygen concentration of the conductive diamond particles.

[0048] (Supporting process) In the loading process, a platinum-containing complex is loaded onto conductive diamond particles after the surface oxidation process.

[0049] The platinum-containing complexes are not particularly limited, but examples include dinitrodiamine platinum(II) nitric acid solution ([Pt(NO2)2)2(NH3)2](HNO3sol.), chloroplatinic acid solution (H2[PtCl6]n·H2O), and hexaammineplatinum chloride solution ([Pt(NH3)6]Cl4n·H2O).

[0050] (Platinum reduction process) In the platinum reduction process, the conductive diamond particles, after the loading process, are heated in a hydrogen-containing atmosphere to reduce the platinum ions in the complex back to platinum. This allows for the production of a catalyst for the oxygen reduction reaction. The resulting catalyst can suppress the aggregation of platinum.

[0051] The temperature and time for heating conductive diamond particles in a hydrogen-containing atmosphere are not particularly limited, as long as they are sufficient to reduce platinum ions to platinum. The heating temperature is preferably between 100°C and 200°C. The heating time is, for example, between 1 minute and 10 hours, and preferably between 3 minutes and 5 hours.

[0052] In the above-mentioned method for producing the catalyst for the oxygen reduction reaction, the reduction temperature of platinum can be lowered (for example, to 100°C or higher and 200°C or lower). This is presumed to be because the surface oxidation process increases the total amount of acid (especially the amount of carboxyl groups) on the surface of the conductive diamond particles, and the interaction between the functional groups (especially carboxyl groups) on the surface of the conductive particles and platinum or platinum ions causes the platinum precursor to be highly dispersed and supported on the surface of the conductive diamond particles, making the platinum easier to reduce.

[0053] If the reduction temperature of platinum is low, the heating temperature in the platinum reduction process can be lowered (for example, to 100°C to 200°C) or the heating time in the platinum reduction process can be shortened (for example, to 1 minute to 30 minutes). Lowering the heating temperature or shortening the heating time in the platinum reduction process can increase the surface area of ​​platinum in the oxygen reduction catalyst, thereby improving the activity of the oxygen reduction catalyst.

[0054] In a method for producing a catalyst for oxygen reduction reactions, a heat treatment step may be included after the platinum reduction step, in which the conductive diamond particles after the platinum reduction step are heated in an inert gas.

[0055] ≪Oxygen Reduction Electrode≫ The above-mentioned catalyst for the oxygen reduction reaction can be used as a catalyst for the oxygen reduction electrode. In other words, the oxygen reduction electrode includes an electrode substrate and the catalyst for the oxygen reduction reaction. Since the oxygen reduction reaction catalyst described above has excellent durability, it can be formed into an oxygen reduction electrode with excellent durability.

[0056] The electrode substrate of the oxygen reduction electrode is not particularly limited, and known conductive substrates can be used. Examples of the electrode substrate include metal substrates, carbon substrates, glass substrates, and the like.

[0057] <<Fuel Cell>> The oxygen reduction electrode described above can be used as a cathode in fuel cells such as polymer electrolyte fuel cells. That is, the fuel cell has a cathode composed of the oxygen reduction electrode described above. For example, a polymer electrolyte fuel cell has a solid polymer serving as an electrolyte, and an anode and a cathode provided on both sides of the solid polymer, wherein the cathode is composed of the oxygen reduction electrode described above. Since the oxygen reduction reaction catalyst described above has excellent durability, a fuel cell formed therefrom has excellent durability.

[0058] The electrolyte and anode of the fuel cell are not particularly limited, and known electrolytes and anodes for fuel cells can be used. EXAMPLES

[0059] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[0060] Example 1 <Preparation of boron-doped nanodiamond particles (BDND particles)> Nanodiamond particles having an average particle diameter of 5 nm (specific surface area: 290 m 2 / g, conductivity: 10 -8 ~10 -9 S / cm) was treated under the following conditions for 8 hours using a microwave plasma chemical vapor deposition (MPCVD) apparatus, then heat-treated in air at 425°C for 8 hours to dope boron, thereby obtaining boron-doped nanodiamond particles (also referred to as "BDND particles"). <MPCVD Conditions> Microwave output: 1300 W Stage temperature: 800℃ Carrier gas flow rate: 400 sccm Bubbling gas flow rate: 0.5 sccm Carbon sources: acetone, methanol Boron source: Trimethoxyborane Boron / carbon ratio: 20,000 ppm Chamber pressure: 50 torr

[0061] (Measurement of specific surface area and conductivity of BDND particles) Approximately 20 mg of the obtained BDND particles were measured and taken, then vacuum-dried at 300°C for 3 hours. The resulting dried sample was then analyzed using an automated specific surface area analyzer (BELSORP-MAXII, Microtrac-Bel) to obtain nitrogen adsorption isotherms using a gas adsorption method with nitrogen gas, and the specific surface area was determined using an analytical method based on the BET method. As a result, the specific surface area of ​​BDND particles is 675 m². 2 It was / g. Furthermore, the conductivity of the obtained BDND particles was 0.19 S / cm.

[0062] <Preparation of a carrier on which platinum is supported> 0.2 g of the BDND particles obtained above were added to 20 mL of 60% concentrated nitric acid, heated at 90°C for 5 hours, and then filtered and dried to obtain surface-oxidized BDND particles (also referred to as "O-BDND particles"). The O-BDND particles are a carrier on which platinum is supported.

[0063] <Manufacturing of catalysts for oxygen reduction reactions> 0.10 g of the O-BDND particles obtained above were added to 10 g of ultrapure water, and after sonication (10 minutes) and treatment with a vortex mixer (10 minutes) in that order, the mixture was stirred until suspended, and then sonication (10 minutes) was performed again. Next, 0.81 g of Pt(NO2)2(NH3)2 was added, followed by sonication (10 minutes), stirring for 60 minutes, and then evaporation to dryness by heating at 60°C. The resulting dry material was crushed and then dried in argon at 80°C for 8 hours. Next, the mixture of hydrogen and nitrogen gas (H2 / N2 (volume ratio) = 4 / 96) was heated at 200°C for 2 hours, and then heated in argon at 200°C for 1 hour to obtain an oxygen reduction reaction catalyst (also referred to as "Pt / O-BDND particles") containing O-BDND particles (conductive diamond particles) and platinum supported on the O-BDND particles.

[0064] [Comparative Example 1] The BDND particles obtained in <Preparation of Boron-Doped Nanodiamond (BDND) Particles> of Example 1 were used as the carrier in Comparative Example 1. Except for using BDND particles instead of O-BDND particles, an oxygen reduction reaction catalyst (also referred to as "Pt / BDND particles") was obtained in the same manner as in <Production of Catalyst for Oxygen Reduction Reaction> in [Example 1].

[0065] [Comparative Example 2] 0.1 g of BDND particles obtained in <Preparation of Boron-Doped Nanodiamond (BDND) Particles> of Example 1 was heated at 600°C for 1.5 hours in a mixture of hydrogen and nitrogen gas (H2 / N2 (volume ratio) = 4 / 96) to obtain BDND particles with a hydrogenated surface (also referred to as "H-BDND particles"), which were used as the support in Comparative Example 2. Except for using H-BDND particles instead of O-BDND particles, an oxygen reduction catalyst (also referred to as "Pt / H-BDND particles") was obtained in the same manner as in <Production of Catalyst for Oxygen Reduction Reaction> in [Example 1].

[0066] <Measurement of oxygen concentration on the surface of carriers (O-BDND particles, BDND particles, H-BDND particles)> For each support material (O-BDND particles, BDND particles, H-BDND particles) in Example 1 and Comparative Examples 1-2, the surface oxygen concentration was measured by X-ray photoelectron spectroscopy (XPS). The results are shown in Table 1. As shown in Example 1 and Comparative Example 1, oxidation increased the surface oxygen concentration, and as shown in Comparative Example 1 and Comparative Example 2, reduction decreased the surface oxygen concentration.

[0067] <Measurement of total acid content, carboxyl group content, and phenolic hydroxyl group content of carriers (O-BDND particles, BDND particles, H-BDND particles)> For each support (O-BDND particles, BDND particles, H-BDND particles) in Example 1 and Comparative Examples 1-2, the total acid content, the amount of carboxyl groups, and the amount of phenolic hydroxyl groups were determined by the Boehm method (acid-base titration). Specifically, 0.05 g of the support was added to 2 mL of 0.1 mol / L NaOH aqueous solution, stirred for 3 days, filtered, and the filtrate was collected. The collected filtrate was titrated with 0.05 mol / L HCl aqueous solution to determine the total amount of acid (total amount of acidic functional groups). Furthermore, 0.05 g of the support was added to 2 mL of 0.1 mol / L NaHCO3 aqueous solution, stirred for 3 days, filtered, and the filtrate was collected. The collected filtrate was titrated with 0.05 mol / L HCl aqueous solution to determine the amount of carboxyl groups. Furthermore, the amount of phenolic hydroxyl groups was determined by subtracting the amount of carboxyl groups from the total amount of acid. The results are shown in Table 1. As shown in Example 1 and Comparative Example 1, it was confirmed that oxidation modified the surface and increased the total acid content, and as shown in Comparative Example 1 and Comparative Example 2, reduction modified the surface and decreased the total acid content.

[0068] <Evaluation of catalysts for oxygen reduction reactions (Pt / O-BDND particles, Pt / BDND particles, Pt / H-BDND particles)> The oxygen reduction reaction catalysts obtained in Example 1 and Comparative Examples 1-2 were evaluated by the following method.

[0069] (XRD measurement) XRD measurements were performed on the oxygen reduction reaction catalysts of Example 1 and Comparative Examples 1-2. As a result, a peak characteristic of Pt was observed in all cases. Therefore, in all cases, the added Pt 2+It was confirmed that the complex was reduced to Pt (metal).

[0070] (TEM observation) Figure 1 shows the results of TEM observation of the oxygen reduction reaction catalysts of Example 1 and Comparative Examples 1 and 2. Figure 1(a) is the oxygen reduction reaction catalyst of Example 1, Figure 1(b) is the oxygen reduction reaction catalyst of Comparative Example 1, and Figure 1(c) is the oxygen reduction reaction catalyst of Comparative Example 2. As shown in Figures 1(a) and 1(b), it can be seen that platinum is uniformly dispersed and supported in Example 1 and Comparative Example 1. On the other hand, as shown in Figure 1(c), in Comparative Example 2, the platinum is aggregated and not uniformly dispersed or supported.

[0071] (ICP measurement) 10 mg each of the oxygen reduction reaction catalysts from Example 1 and Comparative Examples 1-2 were placed in a weighing bottle and heated at 100°C overnight, after which the mass was measured. After cooling to room temperature, 40 mL of aqua regia was added, the bottle was covered with a watch glass, and stirred at 100°C for 1 hour to evaporate the aqua regia. Ultrapure water was then added, followed by sonication and suction filtration, and finally the volume was increased to 100 mL to obtain the sample solution. The obtained sample solutions were subjected to ICP measurements, and the platinum loading rate (the percentage of platinum content in the oxygen reduction catalyst) was determined from the measured platinum concentration. The platinum loading rate (%) was calculated as: mass of platinum / mass of oxygen reduction catalyst × 100. The results are shown in Table 1. As shown in Table 1, the platinum loading rate was close to the initial charge (40.0%) and was comparable to that of commercially available catalysts (38.6%), indicating a sufficient loading rate for use as a cathode catalyst in fuel cells, for example.

[0072] (Surface area and particle size of platinum) 20 mg of the oxygen reduction catalyst was pretreated at 130°C for 1 hour under a hydrogen gas flow, and then measured using a metal dispersion analyzer (Microtrac-Bel, BELMETAL3) with a pulse method using carbon monoxide gas. The surface area and particle size of platinum in the oxygen reduction catalysts of Example 1 and Comparative Examples 1-2 were calculated from the amount of carbon monoxide adsorbed. The results are shown in Table 1.

[0073] [Table 1]

[0074] [Electrode fabrication] In each sample bottle, 4.81 mg of Pt / O-BDND particles (Example 1), 5.09 mg of Pt / BDND particles (Comparative Example 1), and 4.93 mg of Pt / H-BDND particles (Comparative Example 2) were added along with a mixture of 3.8 mL of ultrapure water and 1.2 mL of 2-propanol, and 20 μL of a 5% by mass Nafion® dispersion solution of polymer electrolyte. The mixture was then stirred ultrasonically for 1 hour to prepare a catalyst ink. In Example 1 and Comparative Examples 1-2, the amounts of Pt / O-BDND particles, Pt / BDND particles, and Pt / H-BDND particles used were set to ensure a constant amount of platinum. 10 μL of catalyst ink, 0.196 cm 2 The electrode was fabricated by applying the material evenly to the entire surface of the glassy carbon (GC) electrode and drying it at 60°C for 15 minutes (17.3 μg). Pt / cm 2 geo ))

[0075] [Evaluation of electrodes] The electrodes of Example 1 and Comparative Examples 1-2 were evaluated using the following method. (Electrochemical Effective Surface Area (ECA) of Platinum) Cyclic voltammetry (CV) measurements were performed using the electrodes prepared for Example 1 and Comparative Examples 1-2, and the electrochemical effective surface area (ECA) of platinum was determined using the following formula. The fabricated electrode was used as the working electrode, and a three-electrode system was constructed using a platinum (Pt) coil counter electrode and a silver-silver chloride (Ag / AgCl) reference electrode. Electrochemical measurements were performed at room temperature in a 0.1 M perchloric acid (HClO4) aqueous solution (pH: 1.27) using a potentiometer / galvanostat. The scanning speed was 50 mV / s, and the scanning potential was -0.23 to 0.92 V, with five measurements performed. A s [m 2 / g]=Q H [C] / (2.1[C / m 2 ]·L Pt [g]) In the formula, Q H This is the amount of hydrogen adsorption electricity, L Pt This represents the amount of platinum loaded. Furthermore, a commercially available electrode (an electrode with platinum supported on Ketjenblack) was designated as Comparative Example 3, and the electrochemical effective surface area (ECA) of platinum was determined in the same manner as described above. The results are shown in Figure 2 and Table 2. As shown in Figure 2, both Example 1 and Comparative Examples 1-3 exhibited hydrogen adsorption / desorption peaks characteristic of platinum, confirming catalytic activity.

[0076] (Oxygen reduction (ORR) activity) Linear sweep voltometry (LSV) was performed using a rotating disk electrode at a temperature of 25°C, with a 0.1 M perchloric acid (HClO4) aqueous solution (pH: 1.27) as the electrolyte, and under oxygen-saturated conditions. The scanning speed was 50 mV / s, and the scanning potential was -0.2 to 1.2 V. LSV was performed at each electrode rotation speed (100 rpm, 400 rpm, 900 rpm, 1600 rpm, and 2500 rpm). The results at a rotation speed of 1600 rpm are shown in Figure 3. Furthermore, the Koutecky-Levich equation was used to correct for mass transfer (oxygen diffusion), and the oxygen reduction activity was evaluated using the mass activity at 0.85 V. The results are shown in Figure 4. As shown in Figure 3, both Example 1 and Comparative Examples 1-3 showed a rising potential around 0.7-0.8V, confirming that they exhibited oxygen reduction activity. Furthermore, as shown in Figure 4, Example 1 was found to have higher mass activity and higher oxygen reduction activity compared to Comparative Examples 1 and 2. This is thought to be due to the good dispersibility of platinum in Example 1 and the interaction between platinum and the functional groups on the support surface.

[0077] (Durability test) The coefficient of variation (CV) was measured after multiplying by each cycle number, and the electrochemical effective surface area (ECA) retention rate (%) of platinum was determined using the following formula. The results are shown in Figure 5. Furthermore, the CV results after multiplying by each cycle number are shown in Figure 6(a) for Comparative Example 3, Figure 6(b) for Comparative Example 2, Figure 6(c) for Comparative Example 1, and Figure 6(d) for Example 1. Durability was evaluated by defining the lifespan as the number of cycles at which the ECA (Electronic Computational Ability) reaches 50% of its initial value (ECA maintenance rate (%) = 50). The results are shown in Table 2. This durability test simulates the startup and shutdown operations of a polymer electrolyte fuel cell. ECA maintenance rate (%) = ECA after multiplying by number of cycles / Initial (0 cycles) ECA × 100 As shown in Figure 5 and Table 2, Example 1 was confirmed to have significantly higher durability compared to Comparative Examples 1-3. Furthermore, as shown in Figure 6, Comparative Example 3 showed an increase in the current value in the electrical double layer region due to deterioration of the carrier, while Example 1 and Comparative Examples 1-2 did not show an increase in the current value in the electrical double layer region. From these findings, it is considered that Example 1 exhibits extremely high durability due to its high durability derived from BDND and its interaction with the functional groups on the support surface.

[0078] [Table 2]

[0079] [Example 2] <Manufacturing of a platinum-supported carrier> 0.2 g of boron-doped nanodiamond particles (also referred to as "BDND particles-A"), prepared in the same manner as in Example 1, were added to 20 mL of 60% concentrated nitric acid, heated at 90°C for 5 hours, and then filtered and dried to obtain surface-oxidized BDND particles (also referred to as "O-BDND particles-A"). O-BDND particles-A are a carrier on which platinum is supported.

[0080] <Manufacturing of catalysts for oxygen reduction reactions> 0.6 g of support O-BDND particles-A were dispersed in 40 mL of 0.1 mol / L aqueous nitric acid solution, and 4.8 g of 8.3 wt% dinitrodiamine platinum nitric acid aqueous solution was added. The mixture was stirred at room temperature for 20 minutes to obtain a dispersion of the support and platinum precursor. The obtained dispersion was evaporated using a rotary evaporator at 33 hPa, 40°C for 12 hours, 60°C for 2 hours, and 80°C for 2 hours to remove the solvent and obtain a solid with platinum supported on the support. Subsequently, the solid was heated (calcined) at 200°C for 3 hours in a mixture of hydrogen and nitrogen gas (H2 / N2 (volume ratio) = 4 / 96) to reduce the platinum and obtain an oxygen reduction reaction catalyst (also referred to as "Pt / O-BDND particles-A") containing O-BDND particles-A (conductive diamond particles) and the platinum supported on the O-BDND particles-A.

[0081] [Example 3] Except for heating at 200°C for 5 minutes instead of 3 hours, an oxygen reduction catalyst (also referred to as "Pt / O-BDND particle-B") containing O-BDND particles-A (conductive diamond particles) and platinum supported on the O-BDND particles-A was obtained in the same manner as in Example 2.

[0082] [Comparative Example 4] The BDND particles-A used in Example 2 were used as the carrier in Comparative Example 4. Except for using BDND particles-A instead of O-BDND particles-A, an oxygen reduction reaction catalyst (also referred to as "Pt / BDND particles-A") was obtained in the same manner as in <Production of Catalyst for Oxygen Reduction Reaction> in [Example 2].

[0083] <Evaluation of catalysts for oxygen reduction reactions (Pt / O-BDND particles-A, Pt / O-BDND particles-B, Pt / BDND particles-A)> The oxygen reduction reaction catalysts obtained in Examples 2-3 and Comparative Example 4 were evaluated.

[0084] (XRD measurement) XRD measurements were performed on the oxygen reduction reaction catalysts obtained in Examples 2-3 and Comparative Example 4 using the same method as described above. As a result, a peak characteristic of Pt was confirmed in all cases. Therefore, the added Pt 2+ It was confirmed that the complex was reduced to Pt (metal). It was also confirmed that Pt could be sufficiently reduced even in Example 3, where the heating time was shortened.

[0085] (ICP measurement) The platinum loading rate (the percentage of platinum content in the oxygen reduction catalyst) was determined for the oxygen reduction catalysts obtained in Examples 2-3 and Comparative Example 4 using the same method as described above. The results are shown in Table 3.

[0086] (Surface area of ​​platinum) Using the same method as described above, the surface area and particle size of platinum in the oxygen reduction reaction catalysts obtained in Examples 2-3 and Comparative Example 4 were calculated. The results are shown in Table 3.

[0087] [Electrode fabrication] In each sample bottle, 4.36 mg of Pt / O-BDND particles-A (Example 2), 4.32 mg of Pt / O-BDND particles-B (Example 3), and 4.54 mg of Pt / BDND particles-A (Comparative Example 4) were added along with a mixture of 3.8 mL of ultrapure water and 1.2 mL of 2-propanol, and 20 μL of a 5% by mass Nafion® dispersion solution of polymer electrolyte. The mixture was then stirred with ultrasound for 1 hour to prepare a catalyst ink. In Examples 2-3 and Comparative Example 4, the amounts of Pt / O-BDND particles-A, Pt / O-BDND particles-B, and BDND particles-A used were set to ensure a constant amount of platinum. 10 μL of catalyst ink, 0.196 cm 2 The electrode was fabricated by applying the material evenly to the entire surface of the glassy carbon (GC) electrode and drying it at 60°C for 15 minutes (17.3 μg). Pt / cm 2 geo ))

[0088] [Evaluation of electrodes] (Durability test) A durability test was conducted to accelerate the degradation of conductive diamond particles (carbon support) of an oxygen reduction reaction catalyst (catalyst-supported carbon) according to the potential cycle (start-stop) test method described in the "Cell Evaluation and Analysis Protocol (December 2012)," a publication of the New Energy and Industrial Technology Development Organization (NEDO). The start-stop cycle was set to 5000 cycles. Before and after the durability test, the mass activity was determined in the same manner as described above, and the retention rate of mass activity (%) (mass activity after the durability test / initial mass activity × 100) was calculated to evaluate durability. The results are shown in Table 3.

[0089] As shown in Table 3, Examples 2 and 3 were confirmed to have significantly higher durability compared to Comparative Example 4. Furthermore, it was confirmed that Example 3, in which the heating time for platinum support (heating time for reducing platinum) was shorter than in Example 2, also had a larger platinum surface area than Example 2.

[0090] [Table 3]

Claims

1. It comprises conductive diamond particles and platinum supported on the conductive diamond particles, The conductive diamond particles are a catalyst for an oxygen reduction reaction, with a total acid content of 1.40 mmol / g or more.

2. The conductive diamond particles have a specific surface area of ​​400 m². ２ The catalyst for oxygen reduction reaction according to claim 1, wherein the amount is 1 / g or more.

3. The catalyst for oxygen reduction reaction according to claim 1, wherein the conductive diamond particles have a carboxyl group content of 0.90 mmol / g or more.

4. The catalyst for oxygen reduction reaction according to claim 1, wherein the conductive diamond particles have a phenolic hydroxyl group content of 0.45 mmol / g or less.

5. The catalyst for oxygen reduction reaction according to claim 1, wherein the ratio of the molar amount of the total acid to the molar amount of platinum is 0.38 or more.

6. The catalyst for oxygen reduction reaction according to claim 3, wherein the ratio of the molar amount of the carboxyl group to the molar amount of platinum is 0.26 or more.

7. The catalyst for oxygen reduction reaction according to claim 4, wherein the ratio of the molar amount of the phenolic hydroxyl group to the molar amount of platinum is 0.15 or less.

8. The surface area of ​​the aforementioned platinum is 40 m ２ The catalyst for oxygen reduction reaction according to claim 1, wherein the amount is 1 / g or more.

9. An oxygen reduction electrode comprising an electrode substrate and an oxygen reduction reaction catalyst according to any one of claims 1 to 8.

10. A method for producing a catalyst for an oxygen reduction reaction according to any one of claims 1 to 8, A surface oxidation step is performed by heating conductive diamond particles in the presence of an acid to oxidize the surface of the conductive diamond particles. A supporting step in which a platinum-containing complex is supported on the conductive diamond particles after the surface oxidation step, A method for producing a catalyst for an oxygen reduction reaction, comprising a platinum reduction step of reducing platinum ions in the complex to platinum by heating the conductive diamond particles after the supporting step in a hydrogen-containing atmosphere.

11. The method for producing a catalyst for an oxygen reduction reaction according to claim 10, wherein the heating temperature in the platinum reduction step is 100°C or more and 200°C or less.

12. The method for producing a catalyst for an oxygen reduction reaction according to claim 10, wherein the acid is at least one acid selected from the group consisting of nitric acid, sulfuric acid, and hydrochloric acid.

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