Electrochemical oxygen reduction catalyst

Abstract

The present invention relates to an electrochemical oxygen reduction catalyst comprising metal particles and a modifier for modifying the metal particles. The modifier is an organic nitrogen compound, the organic nitrogen compound contains a triazine ring and fluorine bonded to the triazine ring via a covalent bond, and the fluorine content of the organic nitrogen compound is 29 g / eq or less.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrochemical oxygen reduction catalyst. BACKGROUND

[0002] Electrochemical oxygen reduction catalysts are widely used in fuel cells, metal-air batteries, and the like. As an electrochemical oxygen reduction catalyst, a catalyst obtained by modifying the surface of platinum with a melamine compound is known (International Publication No. 2019 / 221156, International Publication No. 2021 / 090746). In addition, a catalyst composition provided with a platinum-based catalyst and a salt that modifies the platinum-based catalyst is disclosed in Japanese Patent Application Publication No. 2023-121010. In the catalyst composition, the above-mentioned salt is composed of a specific 1,3,5-triazine derivative cation and a perfluoroalkylsulfonylimide anion.

[0003] However, since melamine compounds have hydrophilicity, when a catalyst that uses a melamine compound as a modifier is used, water generated can remain in the catalyst layer. Therefore, the transport of oxygen as a reactant can be hindered, leading to a decrease in voltage in a high current density region. In addition, in a catalyst that uses a salt as disclosed in Japanese Patent Application Publication No. 2023-121010, there is still room for improvement in the suppression of a decrease in voltage in a high current density region. SUMMARY

[0004] As described above, in the conventional electrochemical oxygen reduction catalyst, there is still room for improvement in the suppression of a decrease in voltage in a high current density region when the catalyst is used. Therefore, an object of the present application is to provide an electrochemical oxygen reduction catalyst that suppresses a decrease in voltage in a high current density region.

[0005] The present inventors found that by using a compound in which a specific amount of fluorine is introduced to the side chain of a triazine ring as a modifier, a decrease in voltage in a high current density region when an electrochemical oxygen reduction catalyst is used is suppressed, and thus completed the present application.

[0006] That is, the gist of the present application is as follows.

[0007] Mode 1

[0008] An electrochemical oxygen reduction catalyst comprising metal particles and a modifier that modifies the above-mentioned metal particles,

[0009] The above-mentioned modifier is an organic nitrogen compound, the above-mentioned organic nitrogen compound comprises a triazine ring and fluorine bonded to the above-mentioned triazine ring via a covalent bond, and the fluorine content of the above-mentioned organic nitrogen compound is 29 g / eq or less.

[0010] Mode 2

[0011] The electrochemical oxygen reduction catalyst according to Mode 1, wherein the modifier is a compound represented by General Formula (1) or a polymer having the compound as a monomer.

[0012]

[0013] (In the formula, R1, R2, and R3 are each a functional group selected from the group of functional groups consisting of a hydrogen atom, a halogen atom, a nitrile group, an amide group, an imine group, an amino group, a thiol group, a hydroxyl group, a sulfo group, a carboxylic acid group, a phosphoric acid group, a ketone group, an aldehyde group, an ester group, an alkoxy group, a phenol group, a cyclopentyl group, a cyclohexyl group, an alkylamino group having 1 to 10 carbon atoms, an alkylsulfo group having 1 to 10 carbon atoms, a perfluoroalkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkenylamino group having 2 to 10 carbon atoms, an alkenylsulfo group having 2 to 10 carbon atoms, a perfluoroalkenyl group having 2 to 10 carbon atoms, and an alkenyl group having 2 to 10 carbon atoms, each of which can have at least one selected from the group consisting of at least one functional group selected from the above group of functional groups, an aromatic ring, a heterocyclic ring, an oxygen atom, a sulfur atom, and a nitrogen atom in a molecular chain, wherein at least one of R1, R2, and R3 is a perfluoroalkyl group having 1 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.)

[0014] Mode 3

[0015] The electrochemical oxygen reduction catalyst according to Mode 2, wherein R1, R2, and R3 in General Formula (1) are each a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.

[0016] Mode 4

[0017] The electrochemical oxygen reduction catalyst according to any one of Modes 1 to 3, wherein the metal particles are at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles.

[0018] According to the present application, it is possible to provide an electrochemical oxygen reduction catalyst that suppresses a decrease in voltage in a high current density region. BRIEF DESCRIPTION OF DRAWINGS

[0019] Hereinafter, features, advantages, and technical and industrial significance of illustrative embodiments of the present application will be described with reference to the accompanying drawings, wherein like numerals indicate like elements.

[0020] Figure 1 is a graph showing the single cell voltage after the durability test at 3.2 Acm －2 for Examples 1 and 2 and Comparative Examples 1 and 2.

[0021] Figure 2 is a graph showing the single cell voltage after the durability test at 3.2 Acm－2 a graph of voltage retention rate after the durability test of Example 1. DETAILED DESCRIPTION

[0022] Hereinafter, a preferred embodiment of the present application will be described in detail.

[0023] The electrochemical oxygen reduction catalyst of the present application (hereinafter, also referred to as the catalyst of the present application) contains metal particles and a modifier that modifies the metal particles.

[0024] The metal that constitutes the metal particles is only required to be a metal having oxygen reduction activity (oxygen reduction catalytic ability). As examples of the metal that constitutes the metal particles, there are metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium, and two or more of these metals can be used. In addition, the metal can be an oxide, a nitride, a sulfide, a phosphide, or the like. In one embodiment, the metal particles are at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles. As the metal other than platinum included in the platinum alloy and the platinum-containing composite particles, for example, there are the above-mentioned metals (excluding platinum) as the metal that constitutes the metal particles, and two or more of these metals can be included. The elemental ratio of the metal other than platinum in the platinum alloy is not particularly limited, and can be 0.11 atm% to 60 atm%. The particle diameter of the metal particles is not particularly limited, and can be 1 nm to 100 nm. In the present application, the particle diameter of the particles is the average crystallite diameter measured by X-ray diffraction method. For the particle diameter of the particles, the particle diameters of 100 to 1000 particles can be measured using an electron microscope, and the average value thereof can be used as the average particle diameter of the particles.

[0025] The metal particles can be supported on a carrier. In this case, the catalyst of the present application comprises a carrier, metal particles supported on the carrier, and a modifier. The method of supporting the metal particles on the carrier is not particularly limited, and a method known in the past can be appropriately employed. The carrier can be a primary particle, or a secondary particle. The particle diameter of the primary particle of the carrier can be, for example, 5 nm to 5000 nm. The metal support ratio of the metal particles supported on the carrier is not particularly limited, and can be 1% to 70%, or 18% to 48%. The carrier can be carbon, oxides, or the like having an electric conductivity, or a mixture containing them. The carbon can be carbon black (acetylene black, ketjen black, and furnace black, or the like), activated carbon, graphite, glassy carbon, graphite, graphene, carbon fiber, carbon nanotube, carbon nitride, carbon sulfide, and carbon phosphide, channel black, roll tube black, disc black, oil furnace black, gas furnace black, lamp black, thermal cracking black, VULCAN black, or a mixture containing two or more of them. The oxides can be titanium oxide, niobium oxide, tin oxide, tungsten oxide, and molybdenum oxide, or a mixture containing two or more of them, or the like. In one embodiment, the metal particles are supported on a carrier, and the carrier is carbon.

[0026] The modifier is an organic nitrogen compound. The organic nitrogen compound contains a triazine ring and fluorine bonded to the triazine ring via a covalent bond. That is, in the organic nitrogen compound, the triazine ring and the fluorine are bonded via a covalent bond. In the present application, "the triazine ring and the fluorine are bonded via a covalent bond" includes both cases where the fluorine is directly bonded to the triazine ring with a covalent bond, and where the fluorine is bonded to the triazine ring with a covalent bond via an arbitrary group. Therefore, the modifier of the present application is not in the form of a salt in which a portion containing fluorine and a portion containing a triazine ring are bonded with an ionic bond. Hereinafter, the organic nitrogen compound used as the modifier will be described.

[0027] The fluorine content (fluorine equivalent) of the organic nitrogen compound is 29 g / eq or less. If the fluorine content of the organic nitrogen compound is 29 g / eq or less, in the electrochemical oxygen reduction reaction, the oxygen transportability is improved, and the decrease in the voltage in the high current density region is suppressed. In addition, by introducing a specific amount of fluorine, the increase in the hydrophilicity of the catalyst (particularly, the surface of the carrier) is suppressed in the environment in which the catalyst is used. Therefore, the oxidation resistance of the catalyst is improved, and the decrease in the voltage in the electrochemical oxygen reduction reaction is suppressed. From the viewpoint of suppressing the decrease in the voltage in the high current density region, the fluorine content (fluorine equivalent) in the organic nitrogen compound is preferably 28 g / eq or less. The fluorine equivalent of the organic nitrogen compound can be calculated from the following formula, and the smaller the value, the more the amount of fluorine in the compound. Note that, in the case of a polymer, the fluorine equivalent of the monomer constituting the polymer is regarded as the fluorine equivalent of the polymer.

[0028] Fluorine equivalent (g / eq) = molecular weight (g / mol) ÷ amount of fluorine in the molecule (mol F / mol)

[0029] From the viewpoint of suppressing a decrease in voltage in a high current density region, the fluorine content (weight ratio) of the organic nitrogen compound can be 65% by weight or more, or 68% by weight or more.

[0030] From the viewpoint of suppressing a decrease in voltage in a high current density region, the proportion of the number of fluorine atoms in the total number of atoms in the molecule of the organic nitrogen compound can be 55% or more, or 58% or more.

[0031] From the viewpoint of suppressing a decrease in voltage in a high current density region, the nitrogen content (nitrogen equivalent) of the organic nitrogen compound can be 145 g / eq or more, or 195 g / eq or more.

[0032] From the viewpoint of suppressing a decrease in voltage in a high current density region, the proportion of the number of nitrogen atoms in the total number of atoms in the molecule of the organic nitrogen compound can be 11.5% or less, or 8.5% or less.

[0033] In one embodiment, the modifier is a compound represented by General Formula (1), or a polymer in which the compound is a monomer.

[0034]

[0035] (In the formula, R1, R2, and R3 are each a functional group selected from the group of functional groups consisting of a hydrogen atom, a halogen atom, a nitrile group, an amide group, an imine group, an amino group, a thiol group, a hydroxyl group, a sulfo group, a carboxylic acid group, a phosphoric acid group, a ketone group, an aldehyde group, an ester group, an alkoxy group, a phenol group, a cyclopentyl group, a cyclohexyl group, an alkylamino group having 1 to 10 carbon atoms, an alkylsulfonic acid group having 1 to 10 carbon atoms, a perfluoroalkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkenylamino group having 2 to 10 carbon atoms, an alkenylsulfonic acid group having 2 to 10 carbon atoms, a perfluoroalkenyl group having 2 to 10 carbon atoms, and an alkenyl group having 2 to 10 carbon atoms, each of which can have at least one selected from the group consisting of at least one functional group selected from the above-mentioned group of functional groups, an aromatic ring, a heterocyclic ring, an oxygen atom, a sulfur atom, and a nitrogen atom in the molecular chain, wherein at least one of R1, R2, and R3 is a perfluoroalkyl group having 1 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.)

[0036] The fluorine content (fluorine equivalent) of the compound represented by General Formula (1) is 29 g / eq or less. Therefore, in General Formula (1), R1, R2, and R3 are selected in such a way that the fluorine content of the compound represented by General Formula (1) is 29 g / eq or less.

[0037] In one embodiment, at least one of R1, R2, and R3 in General Formula (1) is a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms. All of R1, R2, and R3 can be a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms. All of R1, R2, and R3 can be a perfluoroalkyl group having 2 or 3 carbon atoms or a perfluoroalkenyl group having 2 or 3 carbon atoms. In this embodiment, R1, R2, and R3 can be the same or different. Note that the perfluoroalkyl group or the perfluoroalkenyl group refers to a group in which all of the hydrogen atoms of the alkyl chain or the alkenyl chain are replaced with fluorine atoms.

[0038] In one embodiment, the compound represented by General Formula (1) is 2,4,6-tris(pentafluoroethyl)-1,3,5-triazine or 2,4,6-tris(heptafluoropropyl)-1,3,5-triazine.

[0039] In one embodiment, the modifier can be a polymer in which the compound represented by General Formula (1) is a monomer. For example, in a case where R1, R2, and R3 in General Formula (1) include a polymerizable functional group, a polymer in which the compound represented by General Formula (1) is a monomer can be formed. As the polymerizable functional group, an addition polymerizable functional group (for example, an ethylenic double bond group included in an alkenyl group), a condensation polymerizable functional group (for example, an amino group and a carboxylic acid group, a hydroxyl group and a carboxylic acid group) can be cited.

[0040] The modifier can be a mixture of two or more kinds of the compound represented by General Formula (1). The modifier can also be a mixture of two or more kinds of the polymer in which the compound represented by General Formula (1) is a monomer. The modifier can be a mixture of the compound represented by General Formula (1) and the polymer in which the compound represented by General Formula (1) is a monomer.

[0041] In one embodiment, the modifier contains one triazine ring.

[0042] The catalyst of the present application can be prepared, for example, by preparing metal particles to be supported on a support and a modifier as needed and using a method such as a sol-gel method or a vapor phase method.

[0043] The catalyst of the present application can be used, for example, by preparing a catalyst ink containing the catalyst of the present application and applying the catalyst ink to a substrate. When the catalyst ink is applied to a substrate and the solvent of the catalyst ink is removed, a catalyst layer is formed on the substrate.

[0044] The catalyst ink can contain the above-described electrochemical oxygen reduction catalyst, the ionomer, and the solvent. That is, the catalyst ink can contain metal particles (which can be supported on a support), a modifier, an ionomer, and a solvent.

[0045] The ionomer is a high-molecular electrolyte having ion exchange groups. The ion exchange groups of the ionomer are not particularly limited, and examples thereof include sulfonic acid, phosphoric acid, and quaternary ammonium cation. The ionomer can be a perfluorocarbon sulfonic acid polymer, an anion exchange polymer, or a polymer having polyether ether ketone and polybenzimidazole as main components.

[0046] The solvent of the catalyst ink is not particularly limited, and examples thereof include water, alcohol, or a mixed solution of water and alcohol. The alcohol is not particularly limited, and examples thereof include methanol, diacetone alcohol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), tert-butyl alcohol, ethylene glycol, and propylene glycol.

[0047] The catalyst ink can be prepared, for example, by putting the above-described components in a container in a prescribed amount and stirring them using a stirrer.

[0048] In the formation of the catalyst layer, for example, the catalyst ink is applied to a substrate such as an electrolyte membrane, a gas diffusion layer (GDL), a gas diffusion layer, and the like, and the applied catalyst ink is heated and dried to remove the solvent. The electrolyte membrane can have polytetrafluoroethylene (PTFE), ion exchange groups. The gas diffusion layer can be composed of carbon fibers, metal fibers. The gas diffusion layer can be composed of carbon fibers, metal fibers having a microporous layer (MPL). The applied film thickness can be 5 μm to 30 μm. The metal particle content can be 0.1 mg cm －2 ~ 0.6 mg cm －2 .

[0049] The catalyst of the present application suppresses the decrease in voltage in a high current density region in an electrochemical oxygen reduction reaction. Therefore, the catalyst of the present application can be appropriately used in fuel cells, metal-air batteries, and the like.

[0050] The present application will be described more specifically using examples below. However, the technical scope of the present application is not limited to these examples.

[0051] Preparation of catalyst ink

[0052] A catalyst ink containing a catalyst and an ionomer in a solvent was prepared. As a modifier, 2,4,6-tris(pentafluoroethyl)-l,3,5-triazine (compound 1) (Example 1), 2,4,6-tris(heptafluoropropyl)-l,3,5-triazine (compound 2) (Example 2), and melamine (Comparative Example 2) were used, respectively, in the catalyst.

[0053] Example 1

[0054] Platinum particles were supported on a carbon carrier. The carbon carrier supporting platinum particles, a Nafion (manufactured by Chemours) matrix of a perfluorocarbon sulfonic acid polymer (EW: 1100) (ionomer), 2,4,6-tris(pentafluoroethyl)-1,3,5-triazine (manufactured by Tokyo Chemical Industry Co., Ltd.) (modifier), and a solvent composed of water and diacetone alcohol were put into a container. They were stirred using a stirrer to prepare a catalyst ink.

[0055] Example 2

[0056] The modifier was changed to 2,4,6-tris(heptafluoropropyl)-1,3,5-triazine (manufactured by Tokyo Chemical Industry Co., Ltd.) (compound 2), and otherwise, a catalyst ink was prepared in the same manner as in Example 1.

[0057] Comparative Example 1

[0058] A catalyst ink was prepared in the same manner as in Example 1, except that no modifier was used.

[0059] Comparative Example 2

[0060] The modifier was changed to melamine (manufactured by FUJIFILM and Otsuka Pharmaceutical Co., Ltd.), and otherwise, a catalyst ink was prepared in the same manner as in Example 1.

[0061] Preparation of evaluation samples

[0062] Each of the prepared catalyst inks was applied to polytetrafluoroethylene (PTFE), heated, and dried to remove the solvent of the catalyst ink, thereby forming a catalyst layer on the PTFE.

[0063] The formed catalyst layer was prepared as a cathode catalyst layer, an electrolyte membrane (Nafion NR211) was prepared, and an anode catalyst layer containing TEC10E50E (manufactured by Tanaka Kikinzoku Kogyo K.K.) as an anode catalyst was prepared. The electrolyte membrane was sandwiched with the cathode catalyst layer and the anode catalyst layer, and they were hot-pressed at 130°C and 3 MPa, thereby preparing a membrane-electrode assembly. Two gas diffusion layers (GDL22BB manufactured by SGL) composed of carbon fibers were prepared, and they were disposed on both sides of the membrane-electrode assembly, thereby preparing a membrane-electrode gas diffusion layer assembly.

[0064] Evaluation

[0065] The membrane-electrode gas diffusion layer assembly was evaluated for the following properties before and after the high potential durability test, and the retention rate of the properties after the durability test with respect to the properties before the durability test was evaluated.

[0066] High potential durability test

[0067] The test was carried out under the conditions of 1.3 V, 2 h, 80° C., and excessive humidity (equivalent to 120% RH).

[0068] Actual 1cm 2 Single cell evaluation

[0069] The electrode area used is 1 cm 2 Single cell evaluation was performed on each membrane-electrode-gas diffusion layer assembly. Specifically, current / voltage characteristics were evaluated under high humidity conditions (80% RH). Current / voltage characteristics were acquired using an anodic sweep (Anodicsweep) at a sweep rate of 20 mA / s. The current / voltage characteristics were evaluated at a single cell temperature of 80°C, a pressure of 150 kPa_ABS, air as the cathode gas, and a cathode gas flow rate of 2.0 L / min.

[0070] The properties and evaluation results of the modifiers used in Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1. Figure 1 3.2Acm is shown for Examples 1, 2 and Comparative Examples 1, 2. －2 The single cell voltage after the endurance test. Figure 2 3.2Acm is shown for Examples 1, 2 and Comparative Examples 1, 2. －2 Voltage holding rate after endurance test under .

[0071] [Table 1]

[0072]

[0073] As shown in Table 1 and Figure 1 、 2 As shown, Examples 1 and 2 use a modifier with a fluorine content of 29 g / eq or less. Comparative Example 1 does not use a modifier. The modifier of Comparative Example 2 is melamine. Compared with Comparative Examples 1 and 2, Examples 1 and 2 have a high current density (3.2 Acm －2 ) after the endurance test under , the voltage and voltage holding rate are significantly improved.

Claims

1. An electrochemical oxygen reduction catalyst comprising metal particles and a modifier for modifying the metal particles, The modifying agent is an organic nitrogen compound including a triazine ring and fluorine bonded to the triazine ring via a covalent bond, and the fluorine content of the organic nitrogen compound is 29 g / eq or less.

2. The electrochemical oxygen reduction catalyst according to claim 1, wherein The modifier is a compound represented by the general formula (1), or a polymer containing the compound as a monomer. In formula (1), R1, R2 and R3 are each a functional group selected from the group consisting of a hydrogen atom, a halogen atom, a nitrile group, an amide group, an imine group, an amino group, a thiol group, a hydroxyl group, a sulfo group, a carboxylic acid group, a phosphoric acid group, a ketone group, an aldehyde group, an ester group, an alkoxy group, a phenol group, a cyclopentyl group, a cyclohexyl group, an alkylamino group having 1 to 10 carbon atoms, an alkylsulfonic acid group having 1 to 10 carbon atoms, a perfluoroalkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkenylamino group having 2 to 10 carbon atoms, an alkenylsulfonic acid group having 2 to 10 carbon atoms, a perfluoroalkenyl group having 2 to 10 carbon atoms and an alkenyl group having 2 to 10 carbon atoms, and each of these functional groups may have at least one selected from the following groups on its molecular chain: The group is: at least one functional group selected from the functional group, an aromatic ring, a heterocyclic ring, an oxygen atom, a sulfur atom and a nitrogen atom, At least one of R1, R2 and R3 is a perfluoroalkyl group having 1 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.

3. The electrochemical oxygen reduction catalyst according to claim 2, wherein In the general formula (1), R1, R2 and R3 are all perfluoroalkyl groups having 2 to 10 carbon atoms or perfluoroalkenyl groups having 2 to 10 carbon atoms.

4. The electrochemical oxygen reduction catalyst according to any one of claims 1 to 3, wherein The metal particles are at least one selected from the group consisting of platinum particles, platinum alloy particles, and composite particles containing platinum.

Images