Electrochemical oxygen reduction catalyst

Abstract

To provide an electrochemical oxygen reduction catalyst capable of suppressing voltage drop in a high current density region.SOLUTION: An electrochemical oxygen reduction catalyst includes metal particles and a modifier for modifying the metal particles, the modifier being an organic nitrogen compound. The organic nitrogen compound includes a triazine ring and fluorine bonded to the triazine ring through a covalent bond, and the organic nitrogen compound has a fluorine content of 29 g / eq or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electrochemical oxygen reduction catalyst. [Background technology]

[0002] Electrochemical oxygen reduction catalysts are widely used in fuel cells, metal-air batteries, etc. Known electrochemical oxygen reduction catalysts include catalysts in which the platinum surface is modified with a melamine compound (Patent Documents 1 and 2). Patent Document 3 discloses a catalyst composition comprising a platinum catalyst and a salt modified on the platinum catalyst, the salt comprising a specific 1,3,5-triazine derivative cation and a perfluoroalkylsulfonylimide anion.

[0003] However, because melamine compounds are hydrophilic, when a catalyst using a melamine compound as a modifier is used, the generated water remains in the catalyst layer. This inhibits the transport of oxygen, a reactant, and causes a voltage drop in the high current density region. Furthermore, even with the catalyst using salts as disclosed in Patent Document 3, there is still room for improvement in suppressing the voltage drop in the high current density region. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 221156 [Patent Document 2] International Publication No. 2021 / 090746 [Patent Document 3] Japanese Patent Publication No. 2023-121010 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, conventional electrochemical oxygen reduction catalysts have room for improvement in terms of suppressing voltage drop in a high current density region when the catalyst is used. Therefore, an object of the present invention is to provide an electrochemical oxygen reduction catalyst that suppresses voltage drop in a high current density region. [Means for solving the problem]

[0006] The present inventors have discovered that by using a compound having a specific amount of fluorine introduced into the side chain of a triazine ring as a modifying agent, voltage drop in a high current density region can be suppressed when an electrochemical oxygen reduction catalyst is used, and have completed the present invention.

[0007] That is, the gist of the present invention is as follows. (1) An electrochemical oxygen reduction catalyst comprising metal particles and a modifier that modifies the metal particles, wherein the modifier is an organic nitrogen compound that includes a triazine ring and fluorine bonded to the triazine ring via a covalent bond, and the organic nitrogen compound has a fluorine content of 29 g / eq or less. (2) The electrochemical oxygen reduction catalyst according to (1) above, wherein the modifying agent is a compound represented by general formula (1) or a polymer containing the compound as a monomer. [ka] (In the formula, R1, R2, and R3 each represent 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 phosphate 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 aryl group having 2 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkyl group having 2 ... and R1, R2, and R3 are one type of functional group selected from the group consisting of an alkenylsulfonic acid group, a perfluoroalkenyl group having 2 to 10 carbon atoms, and an alkenyl group having 2 to 10 carbon atoms, and each of the functional groups may have in its molecular chain at least one type of functional group selected from the group consisting of an aromatic ring, a heterocycle, an oxygen atom, a sulfur atom, and a nitrogen atom, with the proviso that 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 (2) above, 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 (1) to (3) above, wherein the metal particles are at least one kind selected from the group consisting of platinum particles, platinum alloy particles, and composite particles containing platinum. [Effects of the Invention]

[0008] The present invention makes it possible to provide an electrochemical oxygen reduction catalyst that suppresses voltage drop in a high current density region. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a graph showing cell voltages after a durability test at 3.2 Acm −2 for Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 2]1 is a graph showing voltage retention rates after a durability test at 3.2 Acm −2 for Examples 1 and 2 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] Preferred embodiments of the present invention will now be described in detail.

[0011] The electrochemical oxygen reduction catalyst of the present invention (hereinafter also referred to as the catalyst of the present invention) contains metal particles and a modifier that modifies the metal particles.

[0012] The metal constituting the metal particles may be any metal having oxygen reduction activity (oxygen reduction catalytic activity), 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. Two or more of these metals may be used. The metal may also be an oxide, nitride, sulfide, or phosphide. 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. Examples of metals other than platinum contained in the platinum alloy and platinum-containing composite particles include the metals (excluding platinum) constituting the metal particles, and two or more of these metals may be used. The element ratio of the metal other than platinum in the platinum alloy is not particularly limited and may be 0.11 atm% to 60 atm%. The particle size of the metal particles is not particularly limited and may be 1 nm to 100 nm. In the present invention, the particle size is the average crystallite size measured by X-ray diffraction. The particle size may be determined by measuring the particle sizes of 100 to 1000 particles using an electron microscope and averaging these values ​​to determine the average particle size.

[0013] The metal particles may be supported on a carrier. In this case, the catalyst of the present invention comprises a carrier, metal particles supported on the carrier, and a modifier. The method for supporting the metal particles on the carrier is not particularly limited, and any conventionally known method can be appropriately adopted. The carrier may be primary particles or secondary particles. The particle size of the primary particles of the carrier may be, for example, 5 nm to 5000 nm. The metal loading ratio of the metal particles supported on the carrier is not particularly limited, and may be 1% to 70%, or 18% to 48%. The carrier may be conductive carbon, oxide, etc., or a mixture containing these. The carbon may be carbon black (such as acetylene black, ketjen black, and furnace black), activated carbon, graphite, glassy carbon, graphite, graphene, carbon fiber, carbon nanotubes, carbon nitride, sulfurized carbon, and phosphide carbon, channel black, roller black, disc black, oil furnace black, gas furnace black, lamp black, thermal black, vulcanized carbon, or a mixture containing two or more of these. The oxide may be titanium oxide, niobium oxide, tin oxide, tungsten oxide, and molybdenum oxide, or a mixture containing two or more of these. In one embodiment, the metal particles are supported on a support, and the support is carbon.

[0014] The modifying agent 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 fluorine are bonded to each other via a covalent bond. In the present invention, the phrase "the triazine ring and fluorine are bonded to each other via a covalent bond" includes both cases where the fluorine is directly bonded to the triazine ring via a covalent bond and cases where the fluorine is covalently bonded to the triazine ring via an arbitrary group. Therefore, the modifying agent of the present invention is not in the form of a salt in which the fluorine-containing moiety and the triazine ring-containing moiety are bonded to each other via an ionic bond. The organic nitrogen compound used as the modifying agent will be described below.

[0015] The organic nitrogen compound has a fluorine content (fluorine equivalent) of 29 g / eq or less. When the fluorine content of the organic nitrogen compound is 29 g / eq or less, oxygen transportability is improved in the electrochemical oxygen reduction reaction, and voltage drop in the high current density region is suppressed. Furthermore, the introduction of a specific amount of fluorine suppresses an increase in hydrophilicity of the catalyst (particularly the support surface) in the catalyst's operating environment. This improves the oxidation resistance of the catalyst, and voltage drop in the electrochemical oxygen reduction reaction is suppressed. The fluorine content (fluorine equivalent) in the organic nitrogen compound is preferably 28 g / eq or less, from the viewpoint of suppressing voltage drop in the high current density region. The fluorine equivalent of the organic nitrogen compound can be calculated using the following formula; the smaller this value, the greater the amount of fluorine in the compound. In the case of a polymer, the fluorine equivalent of the constituent monomers is regarded as the fluorine equivalent of the polymer. Fluorine equivalent (g / eq) = molecular weight (g / mol) ÷ amount of fluorine in a molecule (molF / mol)

[0016] From the viewpoint of suppressing voltage drop in a high current density region, the organic nitrogen compound may have a fluorine content (weight ratio) of 65% by weight or more, or 68% by weight or more.

[0017] From the viewpoint of suppressing voltage drop in a high current density region, the organic nitrogen compound may have a ratio of fluorine atoms to the total number of atoms in the molecule of 55% or more, or 58% or more.

[0018] From the viewpoint of suppressing a voltage drop in a high current density region, the organic nitrogen compound may have a nitrogen content (nitrogen equivalent) of 145 g / eq or more, or 195 g / eq or more.

[0019] From the viewpoint of suppressing voltage drop in a high current density region, the organic nitrogen compound may have a ratio of the number of nitrogen atoms to the total number of atoms in the molecule of 11.5% or less, or 8.5% or less.

[0020] In one embodiment, the modifying agent is a compound represented by general formula (1) or a polymer containing the compound as a monomer. [ka] (In the formula, R1, R2, and R3 each represent 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 phosphate 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 aryl group having 2 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an alkyl group having 2 ... and R1, R2, and R3 are one type of functional group selected from the group consisting of an alkenylsulfonic acid group, a perfluoroalkenyl group having 2 to 10 carbon atoms, and an alkenyl group having 2 to 10 carbon atoms, and each of the functional groups may have in its molecular chain at least one type of functional group selected from the group consisting of an aromatic ring, a heterocycle, an oxygen atom, a sulfur atom, and a nitrogen atom, with the proviso that 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.

[0021] The compound represented by general formula (1) has a fluorine content (fluorine equivalent) of 29 g / eq or less. Therefore, in general formula (1), R1, R2, and R3 are selected so that the fluorine content of the compound represented by general formula (1) is 29 g / eq or less.

[0022] In one embodiment, in general formula (1), at least one of R1, R2, and R3 is a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms. R1, R2, and R3 may all be a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms, or 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 may be the same or different. The perfluoroalkyl group or perfluoroalkenyl group refers to an alkyl chain or alkenyl chain in which all hydrogen atoms have been substituted with fluorine atoms.

[0023] 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.

[0024] In one embodiment, the modifying agent may be a polymer containing a compound represented by general formula (1) as a monomer. For example, when R1, R2, and R3 in general formula (1) contain polymerizable functional groups, a polymer can be formed containing a compound represented by general formula (1) as a monomer. Examples of the polymerizable functional group include addition-polymerizable functional groups (e.g., an ethylenic double bond group contained in an alkenyl group) and condensation-polymerizable functional groups (e.g., an amino group and a carboxylic acid group, or a hydroxyl group and a carboxylic acid group).

[0025] The modifying agent may be a mixture of two or more compounds represented by general formula (1), a mixture of two or more polymers having a compound represented by general formula (1) as a monomer, or a mixture of a compound represented by general formula (1) and a polymer having a compound represented by general formula (1) as a monomer.

[0026] In one embodiment, the modifying agent comprises one triazine ring.

[0027] The catalyst of the present invention can be prepared, for example, by preparing metal particles, optionally supported on a carrier, and a modifier, and then subjecting the metal particles to a dissolution-drying method, a vapor-phase method, or the like.

[0028] The catalyst of the present invention can be used, for example, by preparing a catalyst ink containing the catalyst of the present invention and applying the catalyst ink to a substrate. After applying the catalyst ink to the substrate and removing the solvent from the catalyst ink, a catalyst layer is formed on the substrate.

[0029] The catalyst ink may contain the electrochemical oxygen reduction catalyst, an ionomer, and a solvent, i.e., metal particles (which may be supported on a carrier), a modifier, an ionomer, and a solvent.

[0030] An ionomer is a polymer electrolyte having an ion exchange group. The ion exchange group of the ionomer is not particularly limited, and examples thereof include sulfonic acid, phosphoric acid, and quaternary ammonium cations. The ionomer may be a perfluorocarbon sulfonic acid polymer, an anion exchange polymer, or a polymer mainly composed of polyether ether ketone, polybenzimidazole, or the like.

[0031] The solvent for the catalyst ink is not particularly limited and may be, for example, water, alcohol, a mixed solution of water and alcohol, etc. Examples of alcohol include, but are not particularly limited to, methanol, diacetone alcohol, ethanol, 1-propanol, 2-propanol (isopropanol), tert-butyl alcohol, ethylene glycol, and propylene glycol.

[0032] The catalyst ink can be prepared, for example, by placing the above-mentioned components in predetermined amounts in a container and stirring them using a stirrer.

[0033] In forming the catalyst layer, for example, a catalyst ink is applied to a substrate such as an electrolyte membrane having polytetrafluoroethylene (PTFE) or ion exchange groups, a gas diffusion layer (GDL) made of carbon fiber or metal fiber, or a gas diffusion layer made of carbon fiber or metal fiber with a microporous layer (MPL), and the applied catalyst ink is heated to dry and remove the solvent. The coating thickness may be 5 μm to 30 μm. The metal particle content is 0.1 mg cm -2 ~0.6mgcm -2 may be.

[0034] The catalyst of the present invention suppresses voltage drop in the high current density region during electrochemical oxygen reduction reactions, and is therefore suitable for use in fuel cells, metal-air batteries, and the like. [Example]

[0035] The present invention will be described in more detail below using examples, although the technical scope of the present invention is not limited to these examples.

[0036] <Preparation of catalyst ink> Catalyst inks containing a catalyst and an ionomer in a solvent were prepared. In the catalyst, 2,4,6-tris(pentafluoroethyl)-1,3,5-triazine (Compound 1) (Example 1), 2,4,6-tris(heptafluoropropyl)-1,3,5-triazine (Compound 2) (Example 2), and melamine (Comparative Example 2) were used as modifiers.

[0037] Example 1 Platinum particles were supported on a carbon support. The carbon support, a Nafion (Chemours)-based perfluorocarbon sulfonic acid polymer (EW:1100) (ionomer), 2,4,6-tris(pentafluoroethyl)-1,3,5-triazine (Tokyo Chemical Industry Co., Ltd.) (compound 1) (modifier), and a solvent consisting of water and diacetone alcohol were placed in a container and stirred using a stirrer to prepare a catalyst ink.

[0038] Example 2 A catalyst ink was prepared in the same manner as in Example 1, except that the modifying agent was changed to 2,4,6-tris(heptafluoropropyl)-1,3,5-triazine (manufactured by Tokyo Chemical Industry Co., Ltd.) (Compound 2).

[0039] Comparative Example 1 A catalyst ink was prepared in the same manner as in Example 1, except that no modifier was used.

[0040] Comparative Example 2 A catalyst ink was prepared in the same manner as in Example 1, except that the modifying agent was changed to melamine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0041] <Preparation of evaluation samples> Each of the prepared catalyst inks was applied to polytetrafluoroethylene (PTFE), which was then heated to dry and remove the solvent from the catalyst ink, forming a catalyst layer on the PTFE.

[0042] The formed catalyst layer was used as a cathode catalyst layer, an electrolyte membrane (Nafion NR211) was prepared, and an anode catalyst layer containing TEC10E50E (Tanaka Kikinzoku Kogyo Co., Ltd.) was prepared as an anode catalyst. The electrolyte membrane was sandwiched between the cathode catalyst layer and the anode catalyst layer, and these were thermocompression-bonded at 130°C and 3 MPa to produce a membrane-electrode assembly. Two carbon fiber gas diffusion layers (SGL GDL 22BB) were prepared and placed on both sides of the membrane-electrode assembly to produce a membrane-electrode gas diffusion layer assembly.

[0043] <Evaluation> The membrane-electrode gas diffusion layer assembly was evaluated for the following performance before and after a high-potential durability test, and the retention rate of performance after the durability test relative to that before the durability test was evaluated.

[0044] [High voltage durability test] The test was carried out under the conditions of 1.3V, 2h hold, 80°C and excessive humidity (equivalent to 120% RH).

[0045] [Actual 1cm 2Cell Evaluation] The electrode part is 1cm 2 Cell evaluation was carried out using each membrane-electrode gas diffusion layer assembly. Specifically, the current-voltage characteristics were evaluated under highly humidified conditions (80% RH). The current-voltage characteristics were obtained using an anodic sweep at a sweep rate of 20 mA / s. The current-voltage characteristics were also evaluated at a 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.

[0046] The properties and evaluation results of the modifying agents used in Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1. -2 2 shows the cell voltage after the durability test at 3.2 Acm for Examples 1 and 2 and Comparative Examples 1 and 2. -2 The voltage retention rate after the durability test is shown.

[0047] [Table 1]

[0048] As shown in Table 1 and Figures 1 and 2, Examples 1 and 2, which used a modifier with a fluorine content of 29 g / eq or less, had a high current density (3.2 Acm) compared to Comparative Example 1, which did not use a modifier, and Comparative Example 2, which used melamine as the modifier. -2 ) the voltage and voltage retention rate after the durability test were significantly improved.

Claims

1. 1. An electrochemical oxygen reduction catalyst comprising: metal particles; and a modifier that modifies the metal particles, wherein the modifier is an organic nitrogen compound, the organic nitrogen compound comprising a triazine ring and fluorine bonded to the triazine ring via a covalent bond, and the organic nitrogen compound has a fluorine content of 29 g / eq or less.

2. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the modifying agent is a compound represented by general formula (1) or a polymer containing the compound as a monomer: 【Chemical 1】 (In the formula, R １ , R ２ and R ３ are each one 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 phosphate 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 the functional group may each have in its molecular chain at least one functional group selected from the group consisting of the above functional groups, an aromatic ring, a heterocycle, an oxygen atom, a sulfur atom, and a nitrogen atom, with the proviso that R １ , R ２ and R ３ At least one of is a perfluoroalkyl group having 1 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.

3. In the general formula (1), R １ , R ２ and R ３ and each of the groups is a perfluoroalkyl group having 2 to 10 carbon atoms or a perfluoroalkenyl group having 2 to 10 carbon atoms.

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

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