Electrochemical oxygen reduction catalyst

Abstract

To provide an electrochemical oxygen reduction catalyst having high proton conductivity and high durability.SOLUTION: One embodiment of the present invention relates to an electrochemical oxygen reduction catalyst which has a catalytic metal that has oxygen reduction activity and a modifying layer that modifies the catalytic metal, the modifying layer containing a nitrogen-containing cyclic organic compound or a polymer thereof, or a cation of the nitrogen-containing cyclic organic compound or the polymer, and an unsubstituted oxoacid or an anion thereof. Another embodiment of the present invention relates to a fuel cell which includes at least: an electrode catalyst layer of a cathode containing the electrochemical oxygen reduction catalyst of the one embodiment of the present invention; an electrode catalyst layer of an anode; and an electrolyte membrane disposed between the electrode catalyst layer of the cathode and the electrode catalyst layer of the anode. A still another embodiment of the present invention relates to a method for producing the electrochemical oxygen reduction catalyst of the present invention, the method including a modification step of mixing a catalytic metal containing platinum or a platinum alloy with a modifying agent containing a nitrogen-containing cyclic organic compound or a polymer thereof, or a cation of the nitrogen-containing cyclic organic compound or the polymer, and an unsubstituted oxoacid or an anion thereof, and modifying the catalytic metal.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Fuel cells generate electricity through the electrochemical reaction of hydrogen and oxygen. In principle, the only product produced during fuel cell power generation is water. Therefore, fuel cells have attracted attention as a clean power generation system with minimal environmental impact. Fuel cells are constructed using a membrane electrode assembly (MEA) as a basic unit. This assembly consists of an electrolyte membrane, electrode catalyst layers on both sides of which are arranged, and gas diffusion layers on the outer surface of each electrode catalyst layer. Polymer electrolytes (ionomers) with ion-exchange groups are typically used as binders for the electrolyte membrane and electrode catalyst layers. During fuel cell operation, electromotive force is generated by supplying a hydrogen-containing fuel gas to the anode (fuel electrode) electrode catalyst layer and an oxygen-containing oxidizing gas to the cathode (air electrode) electrode catalyst layer. The hydrogen oxidation reaction occurs at the anode, and the oxygen reduction reaction occurs at the cathode, generating electromotive force. Therefore, an oxygen reduction catalyst with oxygen reduction ability is used in the cathode electrode catalyst layer. In order to provide high performance and durable oxygen reduction catalysts that can be used in the electrode catalyst layer of the cathode of a fuel cell, various electrochemical oxygen reduction catalysts have been developed.

[0003] For example, Patent Document 1 describes a catalyst composition comprising a platinum-based catalyst and a salt modified on the platinum-based catalyst, the salt comprising a 1,3,5-triazine derivative cation represented by the following general formula (1) and a perfluoroalkylsulfonylimide anion:

[0004] Patent Document 2 describes a catalyst for electrochemical oxygen reduction containing platinum-containing nanoparticles and at least one selected from the group consisting of a melamine compound, a thiocyanuric acid compound, and a polymer containing the melamine compound or the thiocyanuric acid compound as a monomer.

[0005] Patent Document 3 describes an electrochemical oxygen reduction catalyst containing platinum-containing nanoparticles and at least one compound selected from the group consisting of a polymer having a melamine compound as a monomer and a thiol melamine compound, in which the polymer having a melamine compound as a monomer is a polymer having a repeating unit represented by general formula (1), and the thiol melamine compound is a thiol melamine compound represented by general formula (2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-121010 [Patent Document 2] International Publication No. 2019 / 221156 [Patent Document 3] International Publication No. 2021 / 090746 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, electrochemical oxygen reduction catalysts have been developed that have catalytic metals modified with various nitrogen-containing organic compounds and can be used in the electrode catalyst layer of fuel cell cathodes. Electrochemical oxygen reduction catalysts with modified layers containing nitrogen-containing organic compounds are known to have high performance. However, in the case of such prior art electrochemical oxygen reduction catalysts, due to the water solubility of the nitrogen-containing organic compounds, the modified layer may dissolve in the water generated during fuel cell operation and become detached from the catalytic metal. As a result, prior art electrochemical oxygen reduction catalysts with modified layers containing nitrogen-containing organic compounds have the problem of low durability. Furthermore, in the case of prior art electrochemical oxygen reduction catalysts with modified layers containing nitrogen-containing organic compounds, the nitrogen-containing organic compounds may adsorb to the acid sites of the proton-conducting polymer electrolyte contained in the electrolyte membrane and / or binder, potentially reducing proton conductivity. Therefore, prior art electrochemical oxygen reduction catalysts with modified layers containing nitrogen-containing organic compounds have room for improvement in terms of proton conductivity performance.

[0008] Therefore, an object of the present invention is to provide an electrochemical oxygen reduction catalyst having high proton conductivity and high durability. [Means for solving the problem]

[0009] The present inventors have investigated various means for solving the above-mentioned problems. They have found that by using a modifying layer containing a nitrogen-containing cyclic organic compound or a polymer thereof or a cation thereof, and an unsubstituted oxoacid or an anion thereof as a modifying layer for modifying the catalytic metal of an electrochemical oxygen reduction catalyst, not only the proton conductivity of the electrochemical oxygen reduction catalyst is improved but also the water solubility of the modifying layer is reduced. Based on these findings, the present inventors have completed the present invention.

[0010] That is, the present invention includes the following aspects and embodiments. (Embodiment 1) An electrochemical oxygen reduction catalyst having a catalytic metal having oxygen reduction activity and a modifying layer that modifies the catalytic metal, wherein the modifying layer contains a nitrogen-containing cyclic organic compound or a polymer thereof, or a cation of either, and an unsubstituted oxoacid or an anion thereof. (Embodiment 2) The electrochemical oxygen reduction catalyst according to embodiment 1, wherein the modifying layer comprises an organic salt consisting of a cation of a nitrogen-containing cyclic organic compound or a polymer thereof, and an anion of an unsubstituted oxoacid. (Embodiment 3) The electrochemical oxygen reduction catalyst according to embodiment 1 or 2, wherein the unsubstituted oxoacid or its anion is sulfuric acid, phosphoric acid, polyphosphoric acid, unsubstituted alkylsulfonic acid, unsubstituted alkylphosphoric acid, unsubstituted alkylpolyphosphoric acid, or unsubstituted alkylcarboxylic acid, or an anion thereof. (Embodiment 4) The electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 3, wherein the nitrogen-containing cyclic organic compound or its polymer, or their cation is melamine, ammeline, ammelide, cyanuric acid, or triazine, or their derivatives, or their polymers, or their cations. (Embodiment 5) The electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 4, wherein the content of the modification layer is in the range of 0.001 to 0.1 as a mass ratio to the mass of the catalytic metal in the electrochemical oxygen reduction catalyst. (Embodiment 6) The electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 5, wherein the content of the modification layer is in the range of 0.01 to 0.07 as a mass ratio to the mass of the catalytic metal in the electrochemical oxygen reduction catalyst. (Embodiment 7) The electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 6, wherein the modification layer comprises an organic salt consisting of a melamine cation and a sulfate or polyphosphate anion. (Embodiment 8) The electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 7, wherein the catalytic metal is platinum, a platinum alloy, or a composite containing platinum. (Embodiment 9) A fuel cell comprising at least a cathode electrode catalyst layer containing the electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 8, an anode electrode catalyst layer, and an electrolyte membrane disposed between the cathode electrode catalyst layer and the anode electrode catalyst layer. (Embodiment 10) A modification step in which a catalytic metal containing platinum or a platinum alloy is mixed with a modifying agent containing a nitrogen-containing cyclic organic compound or a polymer thereof or a cation thereof, and an unsubstituted oxoacid or an anion thereof to modify the catalytic metal. 9. A method for producing the electrochemical oxygen reduction catalyst according to any one of embodiments 1 to 8, comprising: [Effects of the Invention]

[0011] The present invention makes it possible to provide an electrochemical oxygen reduction catalyst having high proton conductivity and high durability. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will now be described in detail.

[0013] <1: Electrochemical oxygen reduction catalyst> One aspect of the present invention relates to an electrochemical oxygen reduction catalyst, which comprises a catalytic metal having oxygen reduction activity and a modification layer that modifies the catalytic metal.

[0014] In the electrochemical oxygen reduction catalyst of this embodiment, the catalytic metal may be any metal having oxygen reduction activity (oxygen reduction catalytic ability). Examples of the catalytic metal include 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. The catalytic metal may contain the above-mentioned metals alone or as an alloy of two or more of them. The catalytic metal may also be an oxide, nitride, sulfide, or phosphide of the above-mentioned metals. The catalytic metal is preferably platinum, a platinum alloy, or a platinum-containing composite. In the case of platinum alloys and platinum-containing composites, examples of metals other than platinum include ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. Platinum alloys and platinum-containing composites may contain two or more of the metals exemplified above. By containing the catalytic metals exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high catalytic activity.

[0015] The content of the catalytic metal is typically in the range of 1 to 70 mass%, for example 48 to 50 mass%, and particularly 18 to 50 mass%, relative to the total mass of the electrochemical oxygen reduction catalyst of this embodiment. Furthermore, when the catalytic metal is a platinum alloy or a platinum-containing composite, the content of metals other than platinum is typically in the range of 0.11 to 60 atomic % relative to the total mass of the catalytic metal. By containing the catalytic metal in a content within this range, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high catalytic activity.

[0016] The particle size of the catalyst metal is usually in the range of 1 to 100 nm.

[0017] In the electrochemical oxygen reduction catalyst of this embodiment, the composition and content of the catalytic metal can be determined, for example, by dissolving and extracting the catalytic metal contained in the electrochemical oxygen reduction catalyst of this embodiment, and analyzing the metal elements contained in the extract by thermogravimetric analysis (IG) or inductively coupled plasma optical emission spectroscopy (ICP).

[0018] In the electrochemical oxygen reduction catalyst of this embodiment, the particle size of the catalytic metal can be determined, for example, by measuring the crystallite size by X-ray diffraction and calculating the average crystallite size. Alternatively, the particle size of the catalytic metal may be determined by measuring the particle sizes of 100 to 1,000 catalytic metal particles using an electron microscope and calculating the average value (average particle size) of these.

[0019] In the electrochemical oxygen reduction catalyst of this embodiment, the modifying layer contains a nitrogen-containing cyclic organic compound or its polymer, or a cation thereof, and an unsubstituted oxoacid or its anion. The components are usually in the form of a complex, particularly an organic salt, in the modifying layer. By including the components exemplified above in the modifying layer, the modifying layer exhibits significantly low water solubility. Therefore, the electrochemical oxygen reduction catalyst of this embodiment can suppress detachment of the modifying layer from the catalyst metal even during fuel cell operation, which generates water. Furthermore, when the electrochemical oxygen reduction catalyst of this embodiment is applied to a fuel cell MEA, water can be coordinated around the unsubstituted oxoacid or its anion exemplified above. Therefore, when the electrochemical oxygen reduction catalyst of this embodiment is applied to a fuel cell MEA, adsorption of the nitrogen-containing cyclic organic compound or its polymer, or a cation thereof, to the acid sites of the proton-conducting polymer electrolyte contained in the electrolyte membrane and / or binder can be substantially suppressed. Therefore, by having a catalyst layer containing the components exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0020] In each embodiment of the present invention, the nitrogen-containing cyclic organic compound or its polymer, or their cation means a compound containing nitrogen atoms in the ring members and / or ring-bonding groups of a monocyclic or polycyclic organic compound, or its polymer, or their cation. The nitrogen equivalent of the nitrogen-containing cyclic organic compound is usually in the range of 20 to 270 g / equivalent, particularly in the range of 20 to 70 g / equivalent. The nitrogen equivalent of the nitrogen-containing cyclic organic compound can be calculated by the following formula: Nitrogen equivalent (g / equivalent) = molecular weight of nitrogen-containing organic compound (g / mol) / amount of nitrogen atoms contained in one molecule of nitrogen-containing organic compound (mol N / mol) In the case of a polymer of a nitrogen-containing cyclic organic compound, the nitrogen equivalent of the monomer contained in the polymer may be within the range exemplified above.

[0021] Examples of nitrogen-containing cyclic organic compounds include pyridine, pyrrole, thiazole, isothiazole, oxazole, isoxazole, imidazole, imidazoline, pyrazole, 1,3,5-triazine, pyrimidine, pyritazine, pyrazine, indole, quinoline, isoquinoline, purine, benzimidazole, benzoxazole, benzthiazole, tetrazole, tetrazine, triazole, carbazole, acridine, quinoxaline, and quinazoline. The nitrogen-containing cyclic organic compounds exemplified above may include one or more substituted or unsubstituted amines or aminos (e.g., primary amines, secondary amines, tertiary amines, or quaternary ammonium cations), hydroxyls, halogens (e.g., fluorine, chlorine, bromine, or iodine), nitriles, amides, imides, thiols, sulfonyls, carboxyls, phosphonyls, ketones, aldehydes, esters, substituted or unsubstituted alkyls, substituted or unsubstituted alkenyls, substituted or unsubstituted alkynyls, substituted or unsubstituted cycloalkyls, substituted or unsubstituted cycloalkenyls, substituted or unsubstituted cycloalkynyls, substituted or unsubstituted heterocycloalkyls, substituted or unsubstituted cycloalkyl ... The ring may have an alkylalkyl, a substituted or unsubstituted heterocycloalkylalkyl, a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted heteroaryl, a substituted or unsubstituted heteroarylalkyl, a substituted or unsubstituted alkoxy, a substituted or unsubstituted cycloalkoxy, a substituted or unsubstituted heterocycloalkoxy, a substituted or unsubstituted aryloxy, a substituted or unsubstituted arylalkyloxy, a substituted or unsubstituted arylalkenyloxy, a substituted or unsubstituted heteroaryloxy, a substituted or unsubstituted heteroarylalkyloxy, or a substituted or unsubstituted acyloxy as a ring-linking group. The number of carbon atoms in the above-exemplified groups is usually 1 to 10 in the case of a linear structure, and usually 3 to 16 in the case of a cyclic structure. When the above-exemplified groups are substituted, the substituents are preferably one or more groups selected from the above-exemplified groups.

[0022] The nitrogen-containing cyclic organic compound or its polymer, or its cation is preferably melamine (1,3,5-triazine-2,4,6-triamine), ammeline, ammelide, cyanuric acid, or triazine (1,2,3-triazine, 1,2,4-triazine, or 1,3,5-triazine), or its derivative, or its polymer, or its cation, and examples thereof include melamine or its derivative (nitrogen equivalent: 21 g / equivalent), ammeline, ammelide, 1,3,5-triazine or its derivative (nitrogen equivalent: 27 g / equivalent), thiocyanuric acid or its derivative (nitrogen equivalent: 59 g / equivalent), cyanuric acid or its derivative (nitrogen equivalent: 34 g / equivalent), oleylamine or its derivative (nitrogen equivalent: 267 g / equivalent), tetradecylamine or its derivative (nitrogen equivalent: 213 g / equivalent), g / equivalent), 2,4,6-tris[bis(methoxymethyl)amino]-1,3,5-triazine (nitrogen equivalent 65 g / equivalent), 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol (nitrogen equivalent 68 g / equivalent), 2,4-diamino-6-butylamino-1,3,5-triazine (nitrogen equivalent 30 g / equivalent), or 2,4,6-tris(pentafluoroethyl)-1,3,5-triazine (nitrogen equivalent 145 g / equivalent), or polymers of these monomers, or methylated poly(melamine-co-formaldehyde) (nitrogen equivalent 20 to 40 g / equivalent), or isobutylated poly(melamine-co-formaldehyde) (nitrogen equivalent 20 to 40 g / equivalent). More preferably, the copolymer is a copolymer such as a melamine-formaldehyde copolymer, such as a melamine-formaldehyde copolymer (e.g., 1,3,5-triazine, ammelide, melamine, or a melamine-formaldehyde copolymer, or a cation thereof, and even more preferably, the copolymer is a 1,3,5-triazine, ammelide, melamine, or a melamine-formaldehyde copolymer, or a cation thereof.

[0023] Examples of the polymer of the nitrogen-containing cyclic organic compound include a homopolymer or copolymer containing at least one of the above-mentioned nitrogen-containing cyclic organic compounds as a monomer. In the case of the polymer of the nitrogen-containing cyclic organic compound, the degree of polymerization is preferably in the range of 1 to 10,000, more preferably in the range of 10 to 10,000.

[0024] By having a catalyst layer containing the nitrogen-containing cyclic organic compound or a polymer thereof, or a cation thereof, as exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high durability.

[0025] In each embodiment of the present invention, the term "unsubstituted oxoacid or anion thereof" refers to an inorganic oxygen-containing acid or an anion thereof, or an organic oxygen-containing acid or an anion thereof bonded to a main chain without any substituents. Examples of unsubstituted oxoacids or anions thereof include inorganic acids such as sulfuric acid, phosphoric acid, and polyphosphoric acid, as well as organic acids such as sulfonic acid, phosphoric acid, polyphosphoric acid, and carboxylic acid bonded to an unsubstituted alkyl, unsubstituted alkenyl, unsubstituted alkynyl, unsubstituted cycloalkyl, unsubstituted cycloalkenyl, unsubstituted cycloalkynyl, unsubstituted heterocycloalkyl, unsubstituted cycloalkylalkyl, unsubstituted heterocycloalkylalkyl, unsubstituted aryl, unsubstituted arylalkyl, unsubstituted heteroaryl, or unsubstituted heteroarylalkyl. The unsubstituted oxoacid or anion thereof is preferably sulfuric acid, phosphoric acid, polyphosphoric acid, unsubstituted alkylsulfonic acid, unsubstituted alkylphosphoric acid, unsubstituted alkylpolyphosphoric acid, or unsubstituted alkylcarboxylic acid (e.g., formic acid, acetic acid, or propionic acid), or an anion thereof, and more preferably sulfuric acid or polyphosphoric acid. By having a catalyst layer containing the components exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0026] The modification layer preferably contains an organic salt consisting of a cation of a nitrogen-containing cyclic organic compound or a polymer thereof and an anion of an unsubstituted oxoacid, more preferably an organic salt consisting of a cation of melamine and an anion of sulfate or polyphosphate, and even more preferably melamine sulfate or melamine polyphosphate. By having a catalyst layer containing the components exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0027] The content of the modifying layer, expressed as a mass ratio to the mass of the catalytic metal in the electrochemical oxygen reduction catalyst of this embodiment, is usually in the range of 0.001 to 0.1, particularly in the range of 0.005 to 0.1, and preferably in the range of 0.01 to 0.07. By including the modifying layer in a content within this range, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0028] The composition and content of the nitrogen-containing cyclic organic compound or polymer thereof or cation thereof, and the unsubstituted oxoacid or anion thereof contained in the modified layer can be determined, for example, by dissolving and extracting the modified layer contained in the electrochemical oxygen reduction catalyst of this embodiment, and analyzing the components contained in the extract by elemental analysis, various types of chromatography, ultraviolet-visible spectroscopy (UV-Vis), infrared spectroscopy (IR), or nuclear magnetic resonance (NMR).

[0029] In the electrochemical oxygen reduction catalyst of this embodiment, the modifying layer is usually disposed on the surface of the catalytic metal, for example, coating the surface of the catalytic metal. In this case, it is preferable that at least a portion of the cations of the nitrogen-containing cyclic organic compound or its polymer and the anions of the unsubstituted oxoacid contained in the modifying layer form bonds with the catalytic metal. Furthermore, the modification rate of the catalytic metal surface with the modifying layer is preferably 28 area% or less, more preferably in the range of 1 to 28 area%, relative to the total surface area of ​​the catalytic metal. By disposing the modifying layer in the form described above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high durability.

[0030] In the electrochemical oxygen reduction catalyst of this embodiment, the modification rate of the modifying layer on the surface of the catalytic metal is defined as the percentage of the surface area of ​​the catalytic metal modified with the modifying layer relative to the total surface area of ​​the catalytic metal. The modification rate of the modifying layer on the surface of the catalytic metal can be determined, for example, by the following procedure. The modifying layer contained in the electrochemical oxygen reduction catalyst of this embodiment is dissolved to obtain the catalytic metal from which the modifying layer has been removed. The surface area of ​​the catalytic metal from which the modifying layer has been removed or the surface area of ​​the uncoated catalytic metal having no modifying layer (i.e., the total surface area of ​​the catalytic metal) is measured using an electrochemical metal surface area calculation method such as gas-phase gas adsorption or HUPD. The surface area of ​​the catalytic metal not modified with the modifying layer contained in the electrochemical oxygen reduction catalyst of this embodiment is measured using a similar method. The value obtained by subtracting the surface area of ​​the catalytic metal not modified with the modifying layer from the total surface area of ​​the catalytic metal is the surface area of ​​the catalytic metal modified with the modifying layer. The modification rate of the modifying layer on the surface of the catalytic metal is calculated from the obtained value.

[0031] The electrochemical oxygen reduction catalyst of this embodiment typically has a support that supports the catalytic metal. Examples of the support include electrically conductive carbon and oxides, as well as mixtures of one or more thereof. The carbon is preferably 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, carbon phosphide, channel black, roller black, disc black, oil furnace black, gas furnace black, lamp black, thermal black, or vulcanized carbon, or a mixture of one or more thereof. The oxide is preferably titanium oxide, niobium oxide, tin oxide, tungsten oxide, or molybdenum oxide, or a mixture of one or more thereof. The support is preferably carbon, and more preferably carbon black.

[0032] The carrier may be either primary particles or secondary particles, and the particle size of the primary particles of the carrier is usually in the range of 5 to 5000 nm.

[0033] In the electrochemical oxygen reduction catalyst of this embodiment, the composition, content and particle size of the support can be determined, for example, by the same means as those for determining the composition, content and particle size of the catalytic metal described above.

[0034] The electrochemical oxygen reduction catalyst of this embodiment may contain a binder. The binder is usually a polymer electrolyte (ionomer) having ion exchange groups. Examples of the ion exchange groups contained in the polymer electrolyte include sulfonic acid groups and phosphate groups. Examples of the polymer constituting the polymer electrolyte include polymers containing perfluorocarbon, polyether ether ketone, polybenzimidazole, etc. as main components. The binder is preferably a perfluorocarbon sulfonic acid polymer. By containing the binder exemplified above, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0035] By being provided with the above-described characteristics, the electrochemical oxygen reduction catalyst of this embodiment can exhibit high proton conductivity and high durability.

[0036] The proton conductivity of the electrochemical oxygen reduction catalyst of this embodiment can be evaluated, for example, by preparing an MEA using the electrochemical oxygen reduction catalyst as a cathode and measuring the current-voltage characteristics (e.g., proton transport resistance or cell voltage at a predetermined current density) of the MEA under low humidification conditions (e.g., 30% RH) and high humidification conditions (e.g., 80% RH).

[0037] The durability of the electrochemical oxygen reduction catalyst of this embodiment can be evaluated, for example, by measuring the water solubility of the modification layer contained in the electrochemical oxygen reduction catalyst.

[0038] <2: Applications of electrochemical oxygen reduction catalysts> Another aspect of the present invention relates to a fuel cell, which includes at least a cathode electrode catalyst layer, an anode electrode catalyst layer, and an electrolyte membrane disposed between the cathode electrode catalyst layer and the anode electrode catalyst layer, and the cathode electrode catalyst layer contains the electrochemical oxygen reduction catalyst according to one aspect of the present invention.

[0039] Yet another aspect of the present invention relates to a water electrolysis system, comprising at least a cathode electrode catalyst layer, an anode electrode catalyst layer, an electrolyte membrane disposed between the cathode electrode catalyst layer and the anode electrode catalyst layer, and a water supply unit, wherein the anode electrode catalyst layer contains the electrochemical oxygen reduction catalyst according to one aspect of the present invention.

[0040] Yet another aspect of the present invention relates to a metal-air battery, which includes at least a cathode electrode catalyst layer, an anode metal layer, and an electrolyte membrane disposed between the cathode electrode catalyst layer and the anode metal layer, and the cathode electrode catalyst layer contains the electrochemical oxygen reduction catalyst of one aspect of the present invention.

[0041] The fuel cell, water electrolysis system, and metal-air battery of this embodiment can exhibit high proton conductivity and high durability by including the electrochemical oxygen reduction catalyst of one embodiment of the present invention in the electrode catalyst layer of the cathode or anode. Therefore, the fuel cell of this embodiment can be applied to fuel cells for automobiles, ships, and railway vehicles, for example.

[0042] <3: Method for producing electrochemical oxygen reduction catalyst> Another aspect of the present invention relates to a method for producing the electrochemical oxygen reduction catalyst of an aspect of the present invention. The method of this aspect includes a modification step. The method of this aspect may also include a preparation step and an electrode catalyst layer preparation step, as desired.

[0043] [3-1: Preparation process] The process includes providing a catalytic metal and modifier. The process typically also includes providing a support and binder. The process may also include providing additional materials, such as a solvent and substrate, if desired.

[0044] The catalytic metal, carrier, and binder prepared in this step may be materials having the characteristics described above. For example, the catalytic metal prepared in this step may be supported on the carrier described above.

[0045] The modifying agent prepared in this step contains at least a cation of a nitrogen-containing cyclic organic compound or a polymer thereof, and an anion of an unsubstituted oxoacid. The cation of the nitrogen-containing cyclic organic compound or a polymer thereof, and the anion of the unsubstituted oxoacid contained in the modifying agent may be any material having the characteristics described above.

[0046] In this step, each material may be prepared by self-preparing one having predetermined characteristics, or may be prepared by purchasing a commercially available product.

[0047] [3-2: Modification process] This step involves mixing a catalytic metal with a modifier to modify the catalytic metal. In this step, if desired, a binder may be mixed together in addition to the catalytic metal and the modifier.

[0048] In this step, the catalyst metal, modifier, and optionally a binder are typically mixed together with a solvent. The solvent is not particularly limited, and any liquid can be used. Examples of the solvent include water and alcohol, as well as mixtures of one or more of these. Examples of the alcohol include methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol (tert-butyl alcohol), diacetone alcohol, ethylene glycol, and propylene glycol.

[0049] In this step, the means for mixing the materials is not particularly limited. Examples of mixing means include an ultrasonic homogenizer, a jet mill, a bead mill, a ball mill, a high-shear mill, and a Filmix. The specific conditions for the above-mentioned mixing means (e.g., stirring speed, stirring time, rotation speed, etc.) are not particularly limited and can be set appropriately within any range.

[0050] This step may optionally include a vacuum degassing treatment in which the resulting mixture is degassed under vacuum conditions. In this case, the specific conditions for the vacuum degassing treatment (e.g., pressure, treatment time, etc.) are not particularly limited and can be set appropriately within any range. The vacuum degassing treatment may be performed multiple times.

[0051] In this step, the solvent is removed from the mixture obtained, thereby obtaining the electrochemical oxygen reduction catalyst of one embodiment of the present invention. The method for removing the solvent is not particularly limited, and can be carried out by any means such as heat drying or filtration.

[0052] Alternatively, when the electrode catalyst layer preparation step described below is carried out, the mixture obtained in this step can be used in the electrode catalyst layer preparation step as a catalyst ink containing the electrochemical oxygen reduction catalyst of one embodiment of the present invention. In this case, the mixture obtained in this step may be used as is, or may be used after further adding the solvent exemplified above.

[0053] By carrying out this step, the catalytic metal can be modified with a modifying layer containing a nitrogen-containing cyclic organic compound or a polymer thereof or a cation thereof, and an unsubstituted oxoacid or an anion thereof.

[0054] [3-3: Electrode catalyst layer production process] This step involves applying a catalyst ink containing the electrochemical oxygen reduction catalyst obtained in the modification step to the surface of a substrate.

[0055] The substrate used in this step is not particularly limited, and any material such as polytetrafluoroethylene (PTFE), an electrolyte membrane having an ion exchange group, carbon fiber, and metal fiber can be used.

[0056] In this step, the means for applying the catalyst ink is not particularly limited. Examples of application methods include die coating, spin coating, screen printing, doctor blade, squeegee, spray coating, and applicator. The specific conditions for the application methods exemplified above are not particularly limited and can be set appropriately within any range.

[0057] In this step, the solvent is usually removed from the catalyst ink after application. The method for removing the solvent is not particularly limited and can be carried out by any means, such as heat drying. The specific conditions for removing the solvent (e.g., temperature, pressure, treatment time, etc.) are not particularly limited and can be set appropriately within any range.

[0058] By carrying out this step, an electrochemical oxygen reduction catalyst according to one embodiment of the present invention can be obtained in a form (electrocatalytic layer) arranged on the surface of a substrate. In this case, the thickness of the electrocatalytic layer is usually in the range of 5 to 30 μm. The content of the catalytic metal in the electrocatalytic layer, expressed as a mass relative to the total area of ​​the electrocatalytic layer, is usually 0.1 to 0.6 mg / cm. 2 The range is.

[0059] As described above, by carrying out the method of this embodiment, an electrochemical oxygen reduction catalyst of one embodiment of the present invention can be produced.

[0060] As described in detail herein, the electrochemical oxygen reduction catalyst of one aspect of the present invention has high proton conductivity and high durability. Further, the manufacturing method of one aspect of the present invention can efficiently provide the electrochemical oxygen reduction catalyst of one aspect of the present invention having high proton conductivity and high durability. Therefore, by applying the electrochemical oxygen reduction catalyst of one aspect of the present invention to, for example, the cathode of a fuel cell, a fuel cell having high power generation performance and high durability can be provided. Such a fuel cell can be suitably used, for example, as a fuel cell for an automobile, a ship or a railway vehicle.

Examples

[0061] Hereinafter, the present invention will be described more specifically using examples. However, the technical scope of the present invention is not limited to these examples.

[0062] <I: Manufacture of Electrochemical Oxygen Reduction Catalyst> [I-1: Preparation of Catalyst Ink] A catalyst metal particle-supported carrier (primary particle diameter: 10 to 100 nm) in which platinum particles (particle diameter: 3 to 4 nm) are supported on a carrier (acetylene black), a predetermined nitrogen-containing cyclic organic compound or its polymer, or their cations, and a modifier containing an unsubstituted oxo acid or its anion, a binder (perfluoroalkylsulfonic acid), and a solvent (water) were put into a container in a predetermined amount. These materials were mixed and stirred using a stirrer (homogenizer and film mixer) to prepare a catalyst ink containing an electrochemical oxygen reduction catalyst (modification step). The content of platinum particles in the electrochemical oxygen reduction catalyst was 48 to 50% by mass based on the total mass of the catalyst.

[0063] As Comparative Example 1, a catalyst ink containing an electrochemical oxygen reduction catalyst having a catalyst metal without a modification layer was prepared in the same procedure as above except that no modifier was added.

[0064] The solvent was dried and removed from the prepared catalyst ink to obtain an electrochemical oxygen reduction catalyst. The modifying layer contained in the electrochemical oxygen reduction catalyst was dissolved in any solvent that dissolves the modifier to obtain a catalytic metal from which the modifying layer had been removed. The surface area of ​​the catalytic metal from which the modifying layer had been removed or the surface area of ​​the uncoated catalytic metal without the modifying layer (i.e., the total surface area of ​​the catalytic metal) was measured using an electrochemical metal surface area calculation method such as gas-phase gas adsorption or HUPD. The surface area of ​​the catalytic metal contained in the electrochemical oxygen reduction catalyst that was not modified with a modifying layer was measured using a similar method. The value obtained by subtracting the surface area of ​​the catalytic metal not modified with a modifying layer from the total surface area of ​​the catalytic metal was obtained as the surface area of ​​the catalytic metal modified with a modifying layer. From the obtained value, the modification ratio of the modifying layer on the surface of the catalytic metal was calculated. In the electrochemical oxygen reduction catalysts of Examples 1 to 8 (see Table 1 below), the modification ratio of the modifying layer on the surface of the catalytic metal ranged from 1 to 28 area% relative to the total surface area of ​​the catalytic metal.

[0065] [I-2: Preparation of catalyst layer] The prepared catalyst ink was applied to a substrate (a polytetrafluoroethylene (PTFE) plate) using a homogenizer and a Filmix. The applied catalyst ink was heated to dry and remove the solvent (catalyst ink application process). The resulting catalyst layer had a film thickness of 5 to 30 μm and a thickness of 0.1 to 0.6 mg / cm. 2 The platinum loading was 10 ...

[0066] [I-3: Fabrication of membrane electrode assembly] Using each catalyst layer prepared by the above procedure as the cathode catalyst layer, a perfluorocarbon sulfonic acid polymer (Nafion NR211 manufactured by Chemours, EW: 1100) as the electrolyte membrane, and a platinum-supported carbon catalyst (TEC10E50E manufactured by Tanaka Kikinzoku Kogyo) as the anode catalyst layer, they were each prepared. The electrolyte membrane was sandwiched between the cathode catalyst layer and the anode catalyst layer. Under the conditions of heating (130 °C) and pressurization (3 MPa), the cathode catalyst layer, the electrolyte membrane, and the anode catalyst layer were thermocompression bonded. A gas diffusion layer (GDL 22BB manufactured by SGL) made of carbon fiber was arranged outside the cathode catalyst layer and the anode catalyst layer to fabricate a membrane electrode assembly (MEA) (electrode part: 1×1 cm).

[0067] <II: Performance Evaluation of Electrochemical Oxygen Reduction Catalysts> [II-1: Electrochemical Performance Evaluation of Membrane Electrode Assembly] Using the MEA prepared by the above procedure as a single cell, the current-voltage characteristics under low humidity conditions (30%RH) and high humidity conditions (80%RH) were measured. The measurement conditions are as follows. Sweep rate: 20 mA / s (anode sweep), cell temperature: 80 °C, pressure: 150 kPa_abs, cathode gas species: air, cathode gas flow rate: 2.0 L / min.

[0068] [II-2: Evaluation Results] Table 1 shows the composition of the electrochemical oxygen reduction catalyst prepared by the above procedure and the evaluation results of the MEA having the electrochemical oxygen reduction catalyst.

[0069]

Table 1

[0070] As shown in Table 1, the MEAs having the electrochemical oxygen reduction catalysts of the Examples exhibited high proton conductivity over a wide range of humidity conditions, including low and high humidification conditions. In contrast, the MEAs having the electrochemical oxygen reduction catalyst of Comparative Example 2, which contained only a nitrogen-containing cyclic organic compound (melamine) in the modification layer, exhibited poorer electrochemical performance under low humidification conditions than the MEAs having the electrochemical oxygen reduction catalyst of Comparative Example 1, which did not have a modification layer. These results demonstrate that the electrochemical oxygen reduction catalysts of the present invention, which have a modification layer containing a nitrogen-containing cyclic organic compound and an unsubstituted oxoacid, exhibit high proton conductivity. Furthermore, the water solubility of the modification layer of the Examples, which contained a nitrogen-containing cyclic organic compound and an unsubstituted oxoacid, was more than 85% lower than that of the catalyst layer of Comparative Example 2, which contained only a nitrogen-containing cyclic organic compound (melamine). These results demonstrate that the electrochemical oxygen reduction catalysts of the present invention exhibit high durability by suppressing detachment of the modification layer from the catalyst metal, even during fuel cell operation, which generates water.

[0071] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to add, delete, and / or replace part of the configuration of each embodiment with other configurations.

Claims

1. An electrochemical oxygen reduction catalyst having a catalytic metal having oxygen reduction activity and a modifying layer that modifies the catalytic metal, wherein the modifying layer contains a nitrogen-containing cyclic organic compound or a polymer thereof or a cation thereof, and an unsubstituted oxoacid or an anion thereof.

2. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the modifying layer comprises an organic salt consisting of a cation of a nitrogen-containing cyclic organic compound or a polymer thereof and an anion of an unsubstituted oxoacid.

3. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the unsubstituted oxoacid or its anion is sulfuric acid, phosphoric acid, polyphosphoric acid, unsubstituted alkylsulfonic acid, unsubstituted alkylphosphoric acid, unsubstituted alkylpolyphosphoric acid, or unsubstituted alkylcarboxylic acid, or an anion thereof.

4. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the nitrogen-containing cyclic organic compound, a polymer thereof, or a cation thereof is melamine, ammeline, ammelide, cyanuric acid, triazine, a derivative thereof, a polymer thereof, or a cation thereof.

5. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the content of the modification layer is in the range of 0.001 to 0.1 as a mass ratio to the mass of the catalytic metal in the electrochemical oxygen reduction catalyst.

6. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the content of the modification layer is in the range of 0.01 to 0.07 as a mass ratio to the mass of the catalytic metal in the electrochemical oxygen reduction catalyst.

7. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the modifying layer comprises an organic salt consisting of a melamine cation and a sulfate or polyphosphate anion.

8. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the catalytic metal is platinum, a platinum alloy, or a composite containing platinum.

9. 10. A fuel cell comprising at least a cathode electrode catalyst layer containing the electrochemical oxygen reduction catalyst according to claim 1, an anode electrode catalyst layer, and an electrolyte membrane disposed between the cathode electrode catalyst layer and the anode electrode catalyst layer.

10. a modification step in which a catalytic metal containing platinum or a platinum alloy is mixed with a modifying agent containing a nitrogen-containing cyclic organic compound or a polymer thereof or a cation thereof, and an unsubstituted oxoacid or an anion thereof to modify the catalytic metal; 2. A method for producing the electrochemical oxygen reduction catalyst according to claim 1, comprising:

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