Oxygen reduction catalysts and their applications

Abstract

The present invention provides an oxygen reduction catalyst, a cathode for a fuel cell, and a fuel cell, which suppress the interaction between an organic nitrogen compound added to the oxygen reduction reaction catalyst and an ionomer, thereby suppressing the deterioration of ionomer performance at low humidity levels. [Solution] The oxygen reduction catalyst comprises a catalytic metal having oxygen reduction reaction activity and a modified layer containing an organic nitrogen compound having a triazine ring structure represented by the following formula (I) or its cation. TIFF2026169070000006.tif61170

Description

Technical Field

[0001] The present specification relates to an oxygen reduction catalyst and utilization thereof.

Background Art

[0002] It is known that an organic nitrogen compound added to a catalyst metal used in a fuel cell improves the activity of the oxygen reduction reaction in the catalyst, thereby increasing the power generation efficiency of the fuel cell. Examples of such organic nitrogen compounds include melamine-formaldehyde polymers obtained by polymerizing a monomer having an aldehyde group (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, it has been found that in fuel cells, the interaction between the acidic ionomer and the basic group of the organic nitrogen compound reduces the proton conductivity of the ionomer, and under low-humidification conditions, degrades the ionomer performance such as reduced ion conductivity and lowered durability, leading to reduced battery performance.

[0005] The present specification provides a technique that suppresses the interaction between an ionomer and an organic nitrogen compound such as a melamine-formaldehyde polymer added to an oxygen reduction reaction catalyst, and contributes to suppressing the deterioration of ionomer performance under low-humidification conditions.

Means for Solving the Problems

[0006] This disclosure relates to an oxygen reduction catalyst. The oxygen reduction catalyst comprises a catalytic metal having oxygen reduction reaction activity and a modified layer containing an organic nitrogen compound having a triazine ring structure or a cation thereof. The triazine ring structure is represented by the following formula (I).

[0007] [ka]

[0008] In formula (I), R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a group represented by R7OCH2-, or a group represented by -X-*, where R7 represents a hydrogen atom or an alkyl group having 3 to 8 carbon atoms, X represents a methylene group (-CH2-) or a methylene oxy group (-O-CH2-), and * represents a bond. However, at least one of R1, R2, R3, R4, R5, and R6 is a group represented by R7OCH2- (where R7 is an alkyl group having 3 to 8 carbon atoms).

[0009] According to this oxygen reduction catalyst, the modified layer contains an organic nitrogen compound having the triazine ring structure portion. The triazine ring structure portion has a bulky functional group (alkyl group with 3 to 8 carbon atoms), which suppresses the opportunity for physical contact between the organic nitrogen compound and the acidic ionomer, thereby suppressing their interaction. This suppresses the deterioration of ionomer performance at low humidity levels. As a result, the deterioration of battery performance is also suppressed. [Modes for carrying out the invention]

[0010] The oxygen reduction catalysts disclosed herein may further include the following embodiments. Another embodiment of the oxygen reduction catalyst is that the organic nitrogen compound comprises a melamine-formaldehyde polymer. Being such a polymer allows the modification layer to adhere closely to and integrate with the catalyst metal.

[0011] Another embodiment of the oxygen reduction catalyst includes a branched alkyl group for R7. This effectively increases bulk and reduces contact opportunities.

[0012] Another embodiment of the oxygen reduction catalyst includes the fact that R7 is a branched alkyl group, and the ratio of the number of moles of R7 to the total number of moles of triazine rings in the organic nitrogen compound is 0.28 or greater. This more effectively suppresses the opportunity for physical contact between the ionomer and the organic nitrogen compound, thereby inhibiting interaction.

[0013] Another embodiment of the oxygen reduction catalyst includes the fact that R7 is one or more selected from the group consisting of isopropyl group, sec-butyl group, isobutyl group, and tert-butyl group. These functional groups are compatible with both bulkiness and ease of introduction of the functional group.

[0014] Furthermore, the disclosure herein may also be a cathode for a fuel cell, comprising the oxygen reduction catalyst. A fuel cell comprising the cathode may also be provided.

[0015] Furthermore, the disclosures herein may also include an anode for a water electrolysis system, comprising the oxygen reduction catalyst. Or, a water electrolysis system comprising the anode.

[0016] The following describes in detail various embodiments of the oxygen reduction catalyst (hereinafter also simply referred to as the catalyst) disclosed herein.

[0017] The oxygen reduction catalysts disclosed herein may be for fuel cells such as PEFCs, or for water electrolysis systems. The catalysts disclosed herein may be used in the cathode of a fuel cell, in the anode of a fuel cell, in the anode of a water electrolysis system, or in the cathode of a water electrolysis system.

[0018] The oxygen reduction catalyst disclosed in the present specification may take the form of, for example, a catalyst ink, or may take a layered form as a so-called electrode. If necessary, a binder described later can be used.

[0019] (Oxygen Reduction Catalyst) The oxygen reduction catalyst disclosed in the present specification includes a catalyst metal, and a modification layer containing an organic nitrogen compound having a triazine ring structural moiety represented by Formula (I) or a cation thereof. Hereinafter, these elements will be described.

[0020] (Catalyst Metal) The catalyst metal contained in the catalyst is not particularly limited as long as it has oxygen reduction activity. Examples of the catalyst metal include 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. One or a combination of two or more of these metals can be used. Further, the catalyst metal may be an oxide, nitride, sulfide, or phosphide, and one or a combination of two or more of these oxides may be used.

[0021] Among the above, the catalyst metal may preferably be one or two or more selected from platinum, platinum alloys, and composites containing platinum. Examples of metals other than platinum contained in platinum alloys and composites containing platinum include metals such as 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 may be contained. The elemental ratio of metals other than platinum in the platinum alloy is not particularly limited, and may be 0.11 to 50 atm%.

[0022] Such catalytic metals are usually in the form of particles, and the average particle diameter thereof is not particularly limited, and is, for example, 1 nm or more and 100 nm or less.

[0023] The catalytic metal may be supported on a carrier. For the carrier, one type or a combination of two or more types selected from carbon, oxides, and the like can be used. Examples of carbon include conductive carbon, carbon black (acetylene black, ketjen black, channel black, roller black, disc black, oil furnace black, gas furnace black, lamp black, thermal black, VULCAN (registered trademark)-based carbon, and the like), activated carbon, graphite, glassy carbon, graphite, graphene, carbon fiber, carbon nanotube, carbon nitride, carbon sulfide, and carbon phosphide, and two or more of these may be used in combination. As the oxide, for example, titanium oxide, niobium oxide, tin oxide, tungsten oxide, and molybdenum oxide can be used singly or in combination of two or more kinds.

[0024] The carrier may be secondary particles in addition to primary particles. The average particle diameter of the primary particles is not particularly limited, and is, for example, 5 nm or more and 500 nm or less.

[0025] (Modification Layer) The modification layer is provided on the surface of the catalytic metal in order to accelerate the oxygen reduction activity of the catalytic metal. The modification layer contains an organic nitrogen compound or a cation thereof. The term "cation" includes a cation obtained by dissociating a basic functional group contained in the organic nitrogen compound, or a case where the cation forms a salt with an organic acid or an inorganic acid.

[0026] (Organic Nitrogen Compound) The organic nitrogen compound is not particularly limited as long as it has an activity of promoting the oxygen reduction activity by the catalytic metal. The organic nitrogen compound has a triazine ring structural moiety represented by formula (I). As the organic nitrogen compound, for example, any one kind or a combination of two or more of non-polymeric monomers, polymers, or cations (salts) thereof can be used.

[0027] [ka]

[0028] In formula (I), R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a group represented by R7OCH2-, or a group represented by -X-*, where R7 represents a hydrogen atom or an alkyl group having 3 to 8 carbon atoms, X represents a methylene group (-CH2-) or a methylene oxy group (-O-CH2-), and * represents a bond. However, at least one of R1, R2, R3, R4, R5, and R6 is a group represented by R7OCH2- (where R7 is an alkyl group having 3 to 8 carbon atoms).

[0029] The bonding is with the triazine structural portion represented by formula (I).

[0030] The group represented by R7OCH2- (R7: alkyl group with 3 to 8 carbon atoms) in an organic nitrogen compound only needs to be present to the extent that it suppresses opportunities for physical contact with ions, etc., and is not particularly limited. For example, it is preferable that the number of moles of the group represented by R7OCH2- relative to the total number of moles of triazine rings in each organic nitrogen compound is 0.20 or more. This ensures sufficient bulk around the triazine ring, effectively suppressing opportunities for physical contact between the organic nitrogen compound and ions. Other examples include 0.28 or more, 0.30 or more, 0.32 or more, 0.34 or more, 0.35 or more, 0.50 or more, 0.60 or more, 0.70 or more, 0.80 or more, 0.90 or more, 1.0 or more, 1.1 or more, 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.8 or more, 2.0 or more, 2.1 or more, and 2.5 or more. Furthermore, the ranges are 4.0 or less, 3.0 or less, 2.5 or less, 2.2 or less, 2.1 or less, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, 1.6 or less, 1.5 or less, 1.4 or less, 1.3 or less, 1.2 or less, 1.1 or less, 1.0 or less, 0.8 or less, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, and 0.3 or less. The usage range of the base represented by R7OCH2- can be set by combining the above lower and upper limits in any range, for example, 0.28 or more and 2.2 or less, 0.28 or more and 2.1 or less, 0.28 or more and 2.0 or less, 0.30 or more and 2.0 or less.

[0031] The alkyl group having 3 to 8 carbon atoms as R7 can be independently and appropriately selected. This alkyl group may be a linear alkyl group or a branched alkyl group, but a branched alkyl group is sometimes preferred. Because branched alkyl groups are bulky, they can physically reduce the opportunity for contact with the surrounding ionomer. As the branched alkyl group, alkyl groups having 3 to 8 carbon atoms can be used as appropriate. Among these, for example, branched alkyl groups having 3 to 6 carbon atoms, 3 to 5 carbon atoms, or 3 to 4 carbon atoms may be preferred. For example, one or more selected from isopropyl group, sec-butyl group, isobutyl group, and tert-butyl group may be preferred. These functional groups offer a balance between bulkiness and ease of introduction of the functional group. Other examples include isopropyl group and isobutyl group.

[0032] When the organic nitrogen compound is a polymer, it may contain structural units derived from other monomers in addition to the triazine structural portion described above. For example, the polymer may contain structural units derived from thiocyanuric acid. Examples of thiocyanuric acid compounds include thiocyanuric acid and derivatives of thiocyanuric acid. Examples of cyanuric acid compounds include cyanuric acid and derivatives of cyanuric acid.

[0033] Organic nitrogen compounds can be obtained by known methods or commercially. When the organic nitrogen compound is a polymer, polymerization conditions can be similar to those used for the polymerization of melamine and formaldehyde, or for alcohol modification. The degree of polymerization and molecular weight of the polymer are not specifically limited. The presence of the polymer in the modified layer allows for strong adsorption to the catalyst metal, improving adsorption stability. The degree of polymerization of the polymer may be in the range of 1 to 10000. Note that X in the bonding site varies depending on the polymerization conditions, etc.

[0034] Organic nitrogen compounds are not particularly limited, but examples include polymers having the following structural units.

[0035] When the organic nitrogen compound is a polymer, an example is the following melamine-formaldehyde polymer. R' represents at least one alkyl group having 3 to 8 carbon atoms, for example, a branched alkyl group having 3 to 8 carbon atoms, a branched alkyl group having 3 to 5 carbon atoms, or a branched alkyl group having 3 to 4 carbon atoms.

[0036] [ka]

[0037] The content of organic nitrogen compounds in the catalyst is not particularly limited. For example, the total mass of organic nitrogen compounds relative to the mass of the support carrier alone can be 1.0% by mass or more and 15% by mass or less.

[0038] Here, the mass of the organic nitrogen compound is determined by extracting the organic nitrogen compound from the catalyst and measuring it by chromatography such as HPLC or GC. For compound identification, mass spectrometry, NMR, IR, UV, etc., can be used as appropriate. The mass of the support can be measured by the following method: After removing the organic nitrogen compound and other organic compounds such as binders from the catalyst by solvent separation, etc., a support carrying the remaining catalyst metal is obtained. Furthermore, the mass of the support can be determined from the mass of the catalyst metal-supported support, which is obtained by calcining to remove the support.

[0039] (Binder) The catalyst may also contain a binder. The binder is an ionomer having an acidic or basic functional group. An ionomer having an acidic functional group is sometimes preferred. Examples of acidic functional groups include sulfonic acid and phosphoric acid. Examples of basic functional groups include quaternary ammonium cations. The binder may be a perfluorocarbon sulfonic acid polymer, an anion exchange polymer, or a polymer mainly composed of polyether ether ketone and polybenzimidazole.

[0040] The binder content in the catalyst is not particularly limited. For example, the ratio of binder mass to support mass can be between 70% and 115% by mass. The support mass is as previously explained. The binder mass is determined by extracting the binder from the catalyst and measuring its mass. For binder identification, mass spectrometry, NMR, IR, UV, etc., can be used as appropriate.

[0041] As previously described, the catalyst can take the form of a catalyst ink, and after the application of the catalyst ink, it can take the form of a catalyst layer. The formation of the catalyst ink and catalyst layer will be described below.

[0042] (Catalyst ink preparation) First, predetermined amounts of a catalyst metal-supported carrier (catalyst metal-supported carrier), an organic nitrogen compound, a binder, and a solvent are added to a container, and these are stirred using a stirrer to prepare the catalyst ink. The type of solvent is not particularly limited, and any liquid can be used. For example, alcohol or a mixed solution of at least one alcohol and water can be used. Examples of alcohols include methanol, diacetone alcohol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, and propylene glycol.

[0043] The agitator is not particularly limited, and various known agitators can be used as appropriate. Examples include ultrasonic homogenizers, jet mills, ball mills such as bead mills, high-shear mills, film mixers, etc. The stirring conditions, such as stirring speed, stirring time, and rotation speed, are not particularly limited and can be set as appropriate.

[0044] After stirring, perform vacuum degassing and let it stand for one day. There is no limit to the standing time; it can be set as desired. It is also possible to use the product without standing. Furthermore, vacuum degassing may be performed again.

[0045] (Fabrication of the catalyst layer) The prepared catalyst ink is coated onto a predetermined substrate, and the solvent is removed after coating. For example, the catalyst ink is coated onto the substrate, the coated catalyst ink is heated, and the solvent is dried and removed.

[0046] The substrate is not particularly limited, but examples include polytetrafluoroethylene (PTFE), an electrolyte membrane having ion exchange groups, a gas diffusion layer (GDL) composed of carbon fibers or metal fibers, and a gas diffusion layer composed of carbon fibers or metal fibers having a microporous layer (MPL).

[0047] The coating method can be any method that allows for uniform coating of the catalyst ink onto the substrate, and examples include die coating, spin coating, screen printing, doctor blade coating, squeegee coating, spray coating, and applicator coating. The heating rate and heating time can be appropriately set depending on the type of solvent, etc. The removal rate may also be increased by degassing simultaneously with heating. The coating film thickness may be 5 to 30 μm, and the platinum content may be 0.1 to 0.6 mg / cm². -2 The coating may be applied in such a way that it satisfies the following conditions.

[0048] (catalyst layer) This specification provides an oxygen-evolving catalyst layer or an oxygen-reduction reaction catalyst layer containing the catalyst. Since the oxygen reduction reaction of the catalyst metal is reversible, a fuel cell is also provided that has this catalyst layer as a cathode (oxygen-reduction catalyst layer). The fuel cell is, for example, a PEFC. In such a fuel cell, a decrease in the power generation efficiency of the fuel cell due to the deposition of the modification layer is avoided or suppressed. The configuration of the fuel cell other than the cathode is well known to those skilled in the art.

[0049] Furthermore, a water electrolysis system is also provided that includes a catalyst layer containing the catalyst as an anode (oxygen evolution reaction catalyst layer). In such a water electrolysis system, a decrease in the water production efficiency of the water electrolysis device due to the deposition of the modified layer is avoided or suppressed. The components of the water electrolysis system other than the anode are well known to those skilled in the art. [Examples]

[0050] The following describes embodiments that embody the disclosures of this specification. These embodiments are illustrative and not intended to limit the scope of the disclosure. [Examples]

[0051] A catalyst metal particle support, in which platinum particles (particle size: 3-4 nm) as a catalyst metal are supported on a carrier (acetylene black), an organic nitrogen compound (melamine-formaldehyde polymer) having a triazine ring structure as shown in Table 1 below, perfluorocarbon sulfonic acid as a binder, and a water-alcohol mixture as a solvent were added to a container in predetermined amounts. Table 1 shows the type of alkyl group of R7 in formula (I) and the ratio of the number of moles of alkyl group to 1 mole of triazine ring.

[0052] These materials were mixed and stirred using a stirrer (homogenizer, film mixer) to prepare catalyst inks of Examples 1 to 5, which contained an oxygen reduction catalyst.

[0053] Furthermore, catalyst inks for Comparative Examples 1 to 4 were prepared in the same manner as in Example 1, except that no organic nitrogen compounds were used, a triazine (monomer) without an alkoxy group as an organic nitrogen compound was used, a melamine-formaldehyde polymer having a triazine ring structure portion with a methyl group as the alkyl group of the alkoxy group was used, and a melamine-formaldehyde polymer having a triazine ring structure portion with an isobutyl group as the alkyl group of the alkoxy group in a specific amount.

[0054] The melamine-formaldehyde polymers used in the examples and comparative examples were obtained from Tokyo Chemical Industry Co., Ltd. and Sigma-Aldrich.

[0055] A catalyst ink, which had undergone degassing treatment as appropriate, was coated onto a substrate. After coating, the catalyst ions were heated to dry and remove the solvent, thereby creating a catalyst layer.

[0056] Next, each of the fabricated catalyst layers was prepared as a cathode catalyst layer, a perfluorocarbon sulfonic acid polymer (Nafion NR211, EW:1100, manufactured by Chemors) as the electrolyte membrane, and a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo) as the anode catalyst layer. The electrolyte membrane was sandwiched between the cathode catalyst layer and the anode catalyst layer. The cathode catalyst layer, electrolyte membrane, and anode catalyst layer were thermocompressed together under heating (130°C) and pressurizing (3 MPa) conditions. A gas diffusion layer made of carbon fibers (GDL 22BB, manufactured by SGL) was placed outside the cathode catalyst layer and the anode catalyst layer to fabricate a membrane electrode assembly (MEA) (electrode section: 1 cm × 1 cm).

[0057] [Table 1]

[0058] Next, the fabricated MEA was used as a single cell to measure the current-voltage characteristics under low humidity conditions (30%RH) and high humidity conditions (80%RH). The measurement conditions were as follows: sweep rate: 20mA / s (anode sweep), cell temperature: 80℃, pressure: 150kPa_ABS, cathode gas type: air, cathode gas flow rate: 2.0L / min. Current-voltage measurements could be obtained under arbitrary conditions. The results are shown in Table 1.

[0059] As shown in Table 1, it was found that the presence of an alkoxy group in the triazine ring structure suppresses the decrease in cell voltage at low humidity. Furthermore, it was found that the presence of a branched alkyl group is more effective than that of a linear alkyl group in suppressing the decrease in cell voltage at low humidity.

[0060] Furthermore, it was found that the amount of alkoxy groups relative to the triazine ring is preferably above a certain level. For example, it is 0.28 or higher, and also 2 or higher.

Claims

1. An oxygen reduction catalyst, A catalyst metal having oxygen reduction reaction activity, A modified layer comprising an organic nitrogen compound having a triazine ring structure represented by the following formula (I) or its cation, An oxygen reduction catalyst equipped with the following features. 【Chemistry 1】 In formula (I), R １ , R ２ , R ３ , R ４ , R ５ , R ６ each independently represent a hydrogen atom, a group represented by R ７ OCH ２ -, or a group represented by -X-*, wherein R ７ represents a hydrogen atom or an alkyl group having 3 to 8 carbon atoms, X represents a methylene group (-CH ２ -) or a methyleneoxy group (-O-CH ２ -), and * represents a bond; provided that at least one of R １ , R ２ , R ３ , R ４ , R ５ , R ６ is a group represented by R ７ OCH ２ - (wherein R ７ is an alkyl group having 3 to 8 carbon atoms).

2. The oxygen reduction catalyst according to claim 1, wherein the organic nitrogen compound comprises a melamine-formaldehyde polymer.

3. In the above formula (I), R ７ The oxygen reduction catalyst according to claim 1, wherein is a branched alkyl group.

4. In the above formula (I), R ７ is a branched alkyl group, and R is a relative number of moles of the triazine ring of the organic nitrogen compound. ７ The oxygen reduction catalyst according to claim 1, wherein the ratio of the number of moles is 0.28 or greater.

5. In the above formula (I), R ７ The oxygen reduction catalyst according to claim 1, wherein the selected group is one or more selected from the group consisting of isopropyl group, sec-butyl group, isobutyl group, and tert-butyl group.

6. A cathode for a fuel cell comprising the oxygen reduction catalyst according to any one of claims 1 to 5.

7. A fuel cell comprising the cathode described in claim 6.

8. An anode for a water electrolysis system comprising the oxygen reduction catalyst according to any one of claims 1 to 5.

9. A water electrolysis system comprising the anode described in claim 8.

Images