Electrochemical oxygen reduction catalyst

Abstract

To provide an electrochemical oxygen reduction catalyst having improved oxidation resistance.SOLUTION: The present invention relates to an electrochemical oxygen reduction catalyst including metal particles and a modifier that modifies the metal particles, the modifier being an organic nitrogen compound. The organic nitrogen compound includes pyridinic nitrogen and may further include quaternary nitrogen. Also, the organic nitrogen compound may have the total content of the pyridinic nitrogen and the quaternary nitrogen in the organic nitrogen compound is 40 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 cells, etc. Known electrochemical oxygen reduction catalysts include catalysts in which the platinum surface is modified with a melamine compound (Patent Documents 1 and 2).

[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, and the water remaining in the catalyst layer becomes an oxygen source, oxidizing the catalyst support such as carbon and reducing the performance of fuel cells and other devices. [Prior art documents] [Patent documents]

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

[0005] As described above, conventional electrochemical oxygen reduction catalysts have been known to deteriorate the performance of fuel cells, etc. due to oxidation. Therefore, an object of the present invention is to provide an electrochemical oxygen reduction catalyst with improved oxidation resistance. [Means for solving the problem]

[0006] The present inventors have discovered that the oxidation resistance of electrochemical oxygen reduction catalysts can be improved by using, as a modifier, an organic nitrogen compound in which the contents of pyridine-type nitrogen and quaternary nitrogen are controlled within specific ranges, 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, the organic nitrogen compound contains pyridine-type nitrogen and may further contain quaternary nitrogen, and the organic nitrogen compound has a total content of the pyridine-type nitrogen and, if present, the quaternary nitrogen of 40 g / eq or less. (2) The electrochemical oxygen reduction catalyst according to (1) above, wherein the modifying agent is a deammoniation condensate of melamine. (3) The electrochemical oxygen reduction catalyst according to (2) above, wherein the deammoniation condensate of melamine is at least one selected from the group consisting of melem, melam, melon, and g-C3N4. (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. (5) The electrochemical oxygen reduction catalyst according to any one of (1) to (4) above, wherein the metal particles are supported on a carrier, and the carrier is carbon. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an electrochemical oxygen reduction catalyst with improved oxidation resistance. [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 pyridine nitrogen and may further contain quaternary nitrogen. That is, the organic nitrogen compound is a compound containing pyridine nitrogen, or a compound containing pyridine nitrogen and quaternary nitrogen. The organic nitrogen compound may contain a triazine ring containing pyridine nitrogen, or a triazine ring containing pyridine nitrogen and quaternary nitrogen. The organic nitrogen compound used as the modifying agent will be described below.

[0015] The organic nitrogen compound has a total content (nitrogen equivalent) of pyridine-type nitrogen and, if present, quaternary nitrogen of 40 g / eq or less. That is, when the organic nitrogen compound contains pyridine-type nitrogen but not quaternary nitrogen, the content of pyridine-type nitrogen is 40 g / eq or less. Furthermore, when the organic nitrogen compound contains pyridine-type nitrogen and quaternary nitrogen, the total content of pyridine-type nitrogen and quaternary nitrogen is 40 g / eq or less. When the organic nitrogen compound satisfies this nitrogen content, the hydrophobicity is improved compared to conventional melamine, and the oxidation resistance of the catalyst is improved. Furthermore, in the electrochemical oxygen reduction reaction, oxygen transportability is improved and voltage drop in the high current density region is suppressed. From the viewpoint of improving the oxidation resistance of the catalyst, the total content of pyridine-type nitrogen and, if present, quaternary nitrogen in the organic nitrogen compound may be 31.5 g / eq or less. The nitrogen equivalent can be calculated using the following formula. The smaller this value, the greater the amount of nitrogen in the compound. In the case of a polymer, the value of the constituent monomers is regarded as the value of the polymer. Nitrogen equivalent (g / eq) = molecular weight (g / mol) ÷ amount of nitrogen in molecule (molN / mol)

[0016] In the present invention, pyridine-type nitrogen refers to nitrogen in the same bonding state as the nitrogen in pyridine. Pyridine-type nitrogen is bonded to two carbon atoms, one of which is bonded to a single bond and the other to a double bond. On the other hand, quaternary nitrogen refers to a quaternary nitrogen atom bonded to three carbon atoms. Pyridine-type nitrogen and quaternary nitrogen are shown below using the structural formula of melem. In the structural formula of melem, pyridine-type nitrogen is surrounded by a solid line and quaternary nitrogen is surrounded by a dotted line. Note that the exocyclic nitrogen is neither a pyridine-type nitrogen nor a quaternary nitrogen. [ka]

[0017] The organic nitrogen compound may have a water solubility of less than 3 or 1.5 or less at 25°C. The organic nitrogen compound may also have an alcohol (diacetone alcohol or ethanol) solubility of less than 3 or 1.5 or less at 25°C.

[0018] The organic nitrogen compound may have a total nitrogen equivalent of 21.5 g / eq or more. The organic nitrogen compound may have a total nitrogen weight ratio of 65 wt% or less. The organic nitrogen compound may have a ratio of the number of total nitrogen atoms to the total number of atoms in the molecule of 45% or more. Note that "total nitrogen" refers to all nitrogen in the compound, including pyridine-type nitrogen, quaternary nitrogen, and other nitrogen.

[0019] The modifying agent may be a deammoniolated condensation product of melamine. The deammoniolated condensation product of melamine may be at least one selected from the group consisting of melem, melam, melon, and g-CN4 (graphitic carbon nitride). The total content of pyridine nitrogen and quaternary nitrogen is 31.1 g / eq for melem, 39.2 g / eq for melam, 31.1 g / eq for melon, and 23.0 g / eq for g-CN4. Since melam does not contain quaternary nitrogen, the value shown for melam is the content of pyridine nitrogen.

[0020] In one embodiment, the modifying agents are melem and g-C3N4.

[0021] The modifying agent may be a derivative of melem, melam, melon, or g-CN. In one embodiment, the modifying agent is a melem derivative or a polymer containing the derivative as a monomer. In one embodiment, the melem derivative is a compound represented by general formula (1): [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 alkoxy group having 2 to 10 carbon atoms, The functional group is one type of functional group selected from the group consisting of an alkylamino group, 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 have, in the molecular chain, at least one type of 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, provided that R1, R2, and R3 are not all amino groups (—NH2).

[0022] The compound represented by general formula (1) has a total content of pyridine nitrogen and quaternary nitrogen of 40 g / eq or less. Therefore, in general formula (1), R1, R2, and R3 are selected so that the total content of pyridine nitrogen and quaternary nitrogen in the compound represented by general formula (1) is 40 g / eq or less.

[0023] As described above, 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).

[0024] 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.

[0025] In one embodiment, the modifier does not contain fluorine atoms.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The catalyst of the present invention has improved oxidation resistance and therefore suppresses deterioration in the performance of fuel cells, etc. Therefore, the catalyst of the present invention can be suitably used in fuel cells, metal-air batteries, etc. [Example]

[0034] 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.

[0035] <Preparation of catalyst ink> Catalyst inks containing a catalyst and an ionomer in a solvent were prepared. In the catalyst, melem (Example 1), g-CN (Example 2), and melamine (Comparative Example 2) were used as modifiers.

[0036] Example 1 Platinum particles were supported on a carbon support. The carbon support, a Nafion (Chemours)-based perfluorocarbon sulfonic acid polymer (EW:1100) (ionomer), Melem (Tokyo Chemical Industry Co., Ltd.) (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.

[0037] Example 2 A catalyst ink was prepared in the same manner as in Example 1, except that the modifying agent was changed to g-C3N4 (manufactured by Tokyo Chemical Industry Co., Ltd.).

[0038] Comparative Example 1 A catalyst ink was prepared in the same manner as in Example 1, except that no modifier was used.

[0039] 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.).

[0040] <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.

[0041] 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.

[0042] <Evaluation> The membrane-electrode assembly and membrane-electrode gas diffusion layer assembly were 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.

[0043] [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).

[0044] [Actual 1cm 2 Cell 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.

[0045] [Average thickness of catalyst layer] The cross section of each membrane-electrode assembly was cut out using ion milling or a cutter. The thickness of the catalyst layer was measured using a scanning electron microscope (SEM). Measurements were taken at 10 or more points, and the average value was calculated.

[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 total content of pyridine-type nitrogen and quaternary nitrogen of 40 g / eq or less, showed a significantly improved catalyst layer thickness retention rate after the durability test, and improved catalyst oxidation resistance, compared to Comparative Example 1, which used no modifier, and Comparative Example 2, which used melamine as the modifier, which did not satisfy this content. Furthermore, Examples 1 and 2 showed a significantly improved catalyst oxidation resistance at a high current density (3.2 A cm) compared to Comparative Examples 1 and 2. -2 ) The voltage and voltage retention rate after the durability test were also 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 contains pyridine-type nitrogen and may further contain quaternary nitrogen, and the organic nitrogen compound has a total content of the pyridine-type nitrogen and, if present, the quaternary nitrogen of 40 g / eq or less.

2. 2. The electrochemical oxygen reduction catalyst according to claim 1, wherein the modifier is a deammonioated condensate of melamine.

3. The deammoniation condensation product of melamine is melem, melam, melon and g-C ３ N ４ 3. The electrochemical oxygen reduction catalyst according to claim 2, which is at least one selected from the group consisting of:

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.

5. 4. The electrochemical oxygen reduction catalyst according to claim 1, wherein the metal particles are supported on a support, and the support is carbon.

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