Electrode catalyst

Abstract

[Problem] To provide an electrode catalyst in which crossover is suppressed, particularly an electrode catalyst used for an anode electrode for water electrolysis. [Solution] An electrode catalyst in which platinum particles having an average particle diameter of not more than 10 nm are supported at a support ratio of not less than 1.60 mass% on a noble metal oxide that comprises at least one of iridium and ruthenium.

Description

Electrocatalyst

[0001] The present invention relates to an electrode catalyst in which platinum particles are supported on a precious metal oxide containing at least one of iridium and ruthenium.The present invention also relates to an anode catalyst layer containing the electrode catalyst, a catalyst-coated membrane containing the anode catalyst layer, a membrane electrode assembly containing the catalyst-coated membrane, a water electrolysis cell containing the membrane electrode assembly, and a water electrolysis device containing the water electrolysis cell.The present invention also relates to an electrode catalyst in which a catalyst capable of hydrogen reduction reaction is supported on a catalyst capable of oxygen evolution reaction.

[0002] Iridium oxide has attracted attention as an electrode catalyst for oxygen evolution reactions, oxygen reduction reactions, chlorine evolution reactions, etc. in applications such as proton exchange membrane (hereinafter also referred to as "PEM") water electrolysis, PEM fuel cells, seawater electrolysis, and photocatalytic water splitting. In particular, iridium oxide nanoparticles, which have a particle size reduced to the nano-order, are expected to be used as the electrode catalyst.

[0003] Among electrocatalysts, water electrolysis has been attracting attention in recent years as a method for obtaining hydrogen, which is attracting attention as a clean energy source. Furthermore, producing hydrogen by electrolysis of water using electricity generated from renewable energy sources is also important from the perspective of efficient energy utilization.

[0004] Alkaline water electrolysis systems and PEM water electrolysis systems are being considered as methods for water electrolysis (hereinafter also referred to as "water electrolysis"). PEM water electrolysis systems have the advantages of being able to miniaturize electrolytic cells because they can pass a higher current through the same area than alkaline water electrolysis systems, being able to adapt well to energy sources with rapidly fluctuating voltages such as renewable energy, and producing highly pure hydrogen through electrolysis.

[0005] A PEM water electrolysis cell is constructed by joining multiple membrane electrode assemblies (MEAs) in series via separators, each of which is a structural unit consisting of a catalyst coated membrane (CCM) made of a PEM such as Nafion (registered trademark) sandwiched between an anode catalyst layer and a cathode catalyst layer, and gas diffusion layers on both sides of the membrane electrode assembly (MEA). When water is supplied to the anode catalyst layer, the following chemical reactions occur in the anode catalyst layer and the cathode catalyst layer, resulting in the production of oxygen (O ２ ), and hydrogen (H ２ ) is generated. (Anode side) H ２ O → 1 / 2O ２ (g) + 2H ＋ + 2e － (Cathode side) 2H ＋ + 2e － → H ２ (g) The rate-determining step of the overall reaction is the oxidation of water and the oxygen evolution reaction on the anode side, and the mass activity of the anode catalyst for the oxygen evolution reaction (hereinafter also referred to as "OER") is an important factor that determines the efficiency of the system. Therefore, various studies have been conducted to improve the anode catalyst (for example, Patent Document 1).

[0006] On the other hand, in the MEA, depending on the performance of the catalyst and PEM, H generated on the cathode side ２ However, crossover may occur, where oxygen gas remaining on the anode side mixes with the permeated hydrogen gas, potentially reducing the safety of the PEM water electrolysis cell.

[0007] One possible method for suppressing such crossover is to improve the gas separation performance of the electrolyte membrane and suppress the permeation of hydrogen gas. Another method for suppressing crossover has been proposed, which involves mixing platinum into the anode catalyst (see, for example, Patent Document 2).

[0008] WO2020 / 209195 JP2024-72400

[0009] However, in order to improve electrolysis efficiency, methods for improving gas separation performance and suppressing hydrogen gas permeation also require improving ion permeability at the same time. However, it is difficult to achieve both improved gas separation performance and suppressed hydrogen gas permeation, which could result in a decrease in electrolysis efficiency.

[0010] Furthermore, the method of mixing platinum into the anode-side catalyst may not sufficiently suppress crossover. Therefore, an object of the present invention is to provide an electrode catalyst, particularly an electrode catalyst for use in an anode electrode for water electrolysis, that has PEM water electrolysis efficiency at least equivalent to that of conventional water electrolysis catalysts and that suppresses crossover.

[0011] The present inventors have conducted extensive research to solve the above problems and have completed the present invention. That is, the present invention has the following aspects.

[0012] [1] An electrode catalyst comprising a noble metal oxide containing at least one of iridium and ruthenium, and platinum particles having an average particle size of 10 nm or less as measured by a transmission electron microscope, supported at a loading rate of 1.60 mass % or more. [2] The electrode catalyst according to [1], wherein the noble metal oxide is represented by the following formula (1): Ir ｘ Ru ｙ M ｚ O ｎ (1) In the formula (1), M represents at least one metal selected from Groups 2 to 14 of the periodic table, x, y, and z satisfy 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1.0, and n satisfies 0<n≦3. [3] The electrode catalyst according to the above [2], wherein z=0 in the formula (1). [4] The electrode catalyst according to the above [2], wherein x and y satisfy the following in the formula (1): 0<x<1 0<y<1 [5] The electrode catalyst according to any one of the above [1] to [4], wherein the noble metal oxide has a (1,1,0) crystallite size of 2 nm or more and 8 nm or less, as determined by powder X-ray diffraction (Cu Kα).

[0013] The present invention also has the following aspects. [6] An anode catalyst layer of a water electrolysis device containing the electrode catalyst and an ionomer described in any one of [1] to [5] above. [7] A catalyst-coated membrane comprising the anode catalyst layer and cathode catalyst layer described in [6] above, and a proton exchange membrane. [8] A membrane electrode assembly comprising the catalyst-coated membrane described in [7] above, an anode gas diffusion layer, and a cathode gas diffusion layer. [9] A water electrolysis cell comprising the membrane electrode assembly described in [8] above, an anode separator, a cathode separator, and a current collector.

[10] A water electrolysis device comprising the water electrolysis cell described in [9] above.

[0014] The present invention further includes the following aspects:

[11] An electrode catalyst in which a catalyst capable of hydrogen reduction reaction is supported on a catalyst capable of oxygen evolution reaction.

[0015] According to the present invention, there is provided an electrode catalyst in which the crossover is further suppressed, particularly an electrode catalyst used for an anode electrode for water electrolysis.

[0016] 1 is a schematic diagram of the PEM water electrolysis cell. 2 is a graph showing the change over time in the hydrogen content in oxygen generated on the anode side when the anode catalyst layers of the present example and the comparative example are used. 3 is a TEM image of 11 wt % platinum-supported iridium-ruthenium oxide.

[0017] The present invention will be described in detail below with reference to the following embodiments, but the present invention is not limited to these. Various modifications may be made to the embodiments as long as the effects of the present invention are achieved.

[0018] The electrode catalyst of the present invention (hereinafter also referred to as "the present electrode catalyst") comprises a noble metal oxide containing at least one of iridium and ruthenium, and platinum particles having an average particle size of 10 nm or less as measured by a transmission electron microscope, supported at a loading rate of 1.60 mass% or more.

[0019] The noble metal oxide (hereinafter also referred to as "the noble metal oxide") is an oxide containing at least one of iridium and ruthenium. The noble metal oxide may be iridium oxide or ruthenium oxide, but may also contain, in addition to iridium and ruthenium, a metal from Groups 2 to 14 of the periodic table. The noble metal oxide may contain one or more of these metals from Groups 2 to 14 of the periodic table.

[0020] From the viewpoint of the efficiency of PEM water electrolysis, the element contained in the present noble metal oxide other than iridium and ruthenium is preferably at least one element selected from the group consisting of Groups 2 to 14, and more preferably at least one element selected from the group consisting of Groups 4, 5, 10, and 14.

[0021] As the present noble metal oxide, a ternary oxide represented by the following formula (1) is more preferable from the viewpoint of being able to reduce the amount of iridium used, which is a rare metal, while maintaining the performance of water electrolysis: Ir ｘ Ru ｙ M ｚ O ｎ (1) In the formula (1), M represents at least one metal selected from Groups 2 to 14 of the periodic table, x, y, and z satisfy the following conditions: 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1.0, and n satisfies 0<n≦3.

[0022] In the formula (1), M is more preferably Ti, Ta, Ni or Sn.

[0023] From the viewpoint of water electrolysis performance, it is preferable that the present noble metal oxide has z = 0 in the formula (1), i.e., a noble metal oxide that does not contain M. In the case of the noble metal oxide having z = 0 in the formula (1), i.e., a noble metal oxide that does not contain M, x and y satisfy x + y = 1.0.

[0024] When the present noble metal oxide does not contain M, iridium-ruthenium oxides in which 0<x<1, 0<y<1 in formula (1), i.e., iridium and ruthenium, are contained, are particularly preferred from the viewpoint of water electrolysis performance. In the case of iridium-ruthenium oxides, iridium-ruthenium oxides in which z=0 in formula (1) and 0.2≦x≦0.5, 0.5≦y≦0.8 are preferred, and iridium-ruthenium oxides in which 0.25≦x≦0.45, 0.55≦y≦0.75 are more preferred.

[0025] When the present noble metal oxide is an iridium-ruthenium oxide, from the viewpoint of water electrolysis performance, the present noble metal oxide is preferably a solid solution composite oxide of iridium and ruthenium, and IrO ２ phase and RuO ２ For example, when powder X-ray diffraction (Cu Kα) is measured, the iridium-ruthenium oxide preferably has one diffraction maximum peak in the range of 2θ of 66.10° or more and 67.00° or less.

[0026] From the viewpoint of oxygen generating reaction activity, the present noble metal oxide preferably has a (1,1,0) crystallite size determined by powder X-ray diffraction (Cu Kα) of 2 nm to 8 nm, more preferably 3 nm to 6 nm.

[0027] The BET specific surface area of ​​this precious metal oxide is 20m ２ / g or more 100m ２ / g or less is preferable, and 30m ２ / g or more 80m ２ / g is more preferable. The BET specific surface area is a specific surface area determined by the BET method. The BET method is a method in which the amount of gas physically adsorbed on the particle surface is measured when the particles are kept at a low temperature under certain conditions, and the specific surface area is calculated. Helium gas is usually used as the gas used for physical adsorption.

[0028] In this electrode catalyst, platinum particles having an average particle size of 10 nm or less as measured by TEM are supported on the precious metal oxide at a loading rate of 1.60 mass% or more. Here, the average particle size is the average particle size of primary particles. Normally, multiple primary particles aggregate to form agglomerated particles, but in this electrode catalyst, even if the particle size of the aggregated particles is 10 nm or less, a sufficient crossover suppression effect may not be obtained. Note that a primary particle is a particle that exists alone when observed with a TEM, and an aggregated particle is one in which multiple primary particles are physically adsorbed.

[0029] From the viewpoint of hydrogen oxidation ability, the average particle diameter of platinum particles is preferably 5 nm or less. The average particle diameter can be measured by TEM measurement using a transmission electron microscope (TEM). Among the metal particles captured in a TEM image, 20 or more primary particles are selected, and the longest length of these particles is measured visually as the particle diameter. The average particle diameter is determined by dividing the sum of the particle diameters of each particle by the number of particles whose particle diameters are measured. The number of primary particles measured is not particularly limited as long as it is 20 or more, but is typically 50 or less, approximately 20 to 30. When platinum particles are supported on this precious metal oxide, aggregation of the primary particles of the platinum particles may occur, resulting in an increase in the particle diameter of the platinum particles after support on this precious metal oxide. In this electrode catalyst, the platinum particles maintain the state of primary particles after support on this precious metal oxide, and their average particle diameter is 10 nm or less. The average particle diameter of platinum particles measured by TEM measurement is typically 0.5 nm or more. Platinum is known as a catalyst for the reaction of oxidizing hydrogen and reacting with coexisting oxygen to produce water. Therefore, in the water electrolysis reaction described above, H generated on the cathode side ２ is a proton (H ＋ It is expected that the hydrogen gas that permeates the PEM and enters the anode side will be captured and react with the coexisting oxygen to produce raw water.

[0030] However, simply mixing the anode catalyst with platinum particles does not sufficiently promote the reaction of capturing hydrogen gas and reacting it with coexisting oxygen to produce raw water, resulting in insufficient crossover suppression. As a result of investigations aimed at improving the crossover suppression effect, the present inventors have found that the crossover suppression effect can be enhanced not only by mixing the anode catalyst with platinum particles but also by supporting platinum particles having a specific average particle size or less on the anode catalyst.

[0031] Here, "supported" refers to a state in which a zero-valent metal is attached to a support or in the pores of the support, and typically refers to a state in which a physical force acts between the metal and the support, causing the metal to be attached to the support. In this electrode catalyst, not only is the present noble metal oxide and platinum mixed, but platinum particles having an average particle size of 10 nm or less are supported on the present noble metal oxide. In this electrode catalyst, the present noble metal oxide and platinum particles are not mixed, but the platinum particles are supported on the present noble metal oxide, which is thought to result in the zero-valent platinum particles being attached to the present noble metal oxide or in the pores of the present noble metal oxide. By achieving this state, the present electrode catalyst can efficiently suppress the crossover while maintaining its water electrolysis capability.

[0032] In this electrode catalyst, the loading rate of platinum particles relative to the precious metal oxide is 1.6% by mass or more from the viewpoint of the crossover suppression effect. Furthermore, from the viewpoint that an increase in the loading rate of platinum particles does not significantly change the crossover suppression effect, the loading rate is preferably 20% by mass or less, and more preferably 15% by mass or less. The loading rate is the ratio of the mass of the supported platinum particles to the total mass of the precious metal oxide and the supported platinum particles, which is 100% by mass. The mass of the metal contained in the precious metal oxide and the mass of the supported platinum particles can be determined by ICP measurement as described below. From the viewpoint of the crossover suppression effect, the loading rate of platinum particles is preferably 2% by mass or more, and more preferably 3% by mass or more.

[0033] When the noble metal oxide is iridium oxide or ruthenium oxide, commercially available products may be used as they are, or they may be synthesized by, for example, treating a salt of trivalent or tetravalent iridium such as iridium chloride or a salt of trivalent ruthenium such as ruthenium chloride with oxygen at high temperature. Examples of the salt include halide salts such as chlorides, bromides, and fluorides, nitrates, nitrosyl nitrates, acetates, and acetylacetonates.

[0034] When the present noble metal oxide is the iridium-ruthenium oxide, for example, a mixed solution containing both a salt of trivalent or tetravalent iridium and a salt of trivalent ruthenium, as described above, is prepared, and the resulting mixed solution containing both the iridium salt and the ruthenium salt is subjected to oxygen treatment at high temperature to synthesize the iridium-ruthenium oxide. From the viewpoint of the performance of the resulting noble metal oxide in water electrolysis, it is preferable to react a mixed solution containing both a salt of trivalent or tetravalent iridium and a salt of trivalent ruthenium with an alkaline compound to first generate fine coprecipitated particles of iridium-ruthenium hydroxide. Note that, when preparing a mixed solution containing both a salt of trivalent or tetravalent iridium and a salt of trivalent ruthenium, the desired molar ratio of iridium to ruthenium in the iridium-ruthenium oxide, i.e., the desired values ​​of x and y, can be achieved.

[0035] Examples of the alkaline compound include sodium hydroxide, potassium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, ammonium carbonate, and ammonium hydroxide.

[0036] When the present noble metal oxide contains a metal other than iridium and ruthenium, for example, a metal from Groups 2 to 14 of the periodic table, the present noble metal oxide can be synthesized by adding a salt of these metals to a solution containing both a salt of III- or IV-valent iridium and a salt of III-valent ruthenium, and then subjecting the solution to oxygen treatment at high temperature, or by contacting a mixed solution of the metal salts with supercritical water to oxidize the metal salts. Examples of the salt to be added include halide salts such as chlorides, bromides, and fluorides, nitrates, nitrosyl nitrates, acetates, and acetylacetonates, as described above. From the viewpoint of the dispersibility of platinum particles, it is preferable to mix the present noble metal oxide synthesized as described above with pure water to form a slurry of the present noble metal oxide and use it to support platinum particles, as described below.

[0037] Platinum particles are supported on the present noble metal oxide obtained by the above method. For example, the supporting method involves reducing a solution containing a platinum compound to obtain a platinum-containing precursor, mixing a slurry of the present noble metal oxide with a solution of the platinum-containing precursor, adding a reducing agent, and further heating to obtain a noble metal oxide on which platinum particles are supported, where the platinum-containing precursor has been reduced.

[0038] Platinum compounds include platinum(II) acetylacetonate, chloroplatinic(II), chloroplatinic(IV), tetrachloroplatinic(II) acid, hexachloroplatinic(IV) acid, hexachloroplatinic(IV) acid hexahydrate, ammonium tetrachloroplatinate(II), potassium tetrachloroplatinate(II), sodium tetrachloroplatinate(II), ammonium hexachloroplatinate(IV), potassium hexachloroplatinate(IV), sodium hexachloroplatinate(IV), potassium tetracyanoplatinate(II), potassium trichloroamineplatinate(II), dichloroplatinate(II), dichloroplatinate(IV ...II), dichloroplat Examples include trosulfidic platinum(II) acid, diaminedichloroplatinum(II), tetraammineplatinum(II) hydroxide, tetraamminedichloroplatinum(II) hydrate, tetraammineplatinum(II) nitrate, diaminedinitroplatinum(II), potassium tetranitroplatinum(II), tetrakis(thiourea)platinum(II), hexahydroxoplatinic(IV) acid, ammonium hexahydroxoplatinic(IV) acid, potassium hexahydroxoplatinic(IV) acid, sodium hexahydroxoplatinic(IV) acid, platinum(II) hexafluoroacetylacetonate, etc. Among these, hexahydroxoplatinic(IV) acid is preferred.

[0039] The solution containing the platinum compound is a liquid in which the platinum compound is dissolved in a solvent. The solvent is preferably an organic solvent or water. The organic solvent is preferably a liquid at room temperature and has a boiling point equal to or higher than the reaction temperature at which the platinum compound undergoes a reduction reaction. Examples of organic solvents include 1-octanol, octyl ether, octadecene, triphenylmethane, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, butanol, isobutanol, ethoxyethanol, dimethylformamide, xylene, N-methylpyrrolidinone, dichlorobenzene, toluene, propylene glycol monomethyl ether, ethylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, ethyl lactate, diethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, dipropylene glycol monomethyl ether, diethylene glycol diethyl ether, diethylene glycol monomethyl ether, diethylene glycol butyl methyl ether, tripropylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol monobutyl ether, ethylene glycol monophenyl ether, triethylene glycol monomethyl ether, diethylene glycol dibutyl ether, triethylene glycol butyl methyl ether, polyethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol monomethyl ether, and monoethanolamine. Among the above solvents, monoethanolamine is preferred.

[0040] The amount of the solvent is preferably an amount that can dissolve the platinum compound, and the molar concentration of platinum in the solution of the platinum compound is preferably, for example, 0.001 to 10 mol / L.

[0041] The reaction in which the slurry of this precious metal oxide and a solution of a platinum-containing precursor are mixed, a reducing agent is added, and heating is performed is a reduction reaction. This reduction reaction is a liquid-phase reaction and can be carried out by a liquid-phase reduction method using a reducing agent.

[0042] The reaction temperature of the reduction reaction is the temperature at which the platinum compound is reduced, and is, for example, from room temperature to 400°C, preferably from 20°C to 200°C, more preferably from 50°C to 100°C, and even more preferably from 70°C to 90°C.

[0043] The reaction time of the reduction reaction is the time required for the reduction reaction of the platinum compound to be completed, and is, for example, 1 minute to 24 hours, preferably 10 minutes to 5 hours, and more preferably 30 minutes to 2 hours.

[0044] The reaction to obtain a platinum-containing precursor may be carried out by either a batch system or a flow system. In the reaction to obtain a platinum-containing precursor, an additive may be added as needed. Examples of the additive include a reducing agent.

[0045] The reducing agent is used to reduce the platinum compound to efficiently obtain platinum particles. Examples of the reducing agent include hydrazine, sodium borohydride, sodium hypophosphite, lithium aluminum hydride, sodium sulfite, sodium phosphinate, and formic acid.

[0046] The amount of the reducing agent added relative to platinum is, for example, 1 to 10 times by mole, and preferably 2 to 5 times by mole, per mole of platinum.

[0047] The liquid containing the platinum-containing precursor obtained as described above is mixed with the slurry of the present precious metal oxide. The liquid containing the platinum-containing precursor is in a state in which the precursor is dissolved in at least one of an organic solvent and pure water. The solvent used for the liquid containing the platinum-containing precursor is a solvent that dissolves but does not react with the platinum-containing precursor, such as a mixed solution of a basic solvent and pure water. The basic solvent is preferably an organic solvent that is easily available. For example, monoethanolamine is an example. Furthermore, the solvent used for the solution containing the platinum compound to support platinum particles on the present precious metal oxide may be subsequently used for the liquid containing the platinum-containing precursor, or the present precious metal oxide may be added to the solution from which the precursor was obtained, and the precursor and the present precious metal oxide may be mixed.

[0048] The amount of the solvent is preferably an amount that allows the platinum-containing precursor and the present noble metal oxide to be mixed sufficiently, for example, 10 to 1000 times the volume of the present noble metal oxide.

[0049] If the amount of platinum particles relative to the present precious metal oxide is too small, a sufficient crossover suppression effect cannot be obtained. On the other hand, if the amount of platinum particles relative to the present precious metal oxide is too large, the platinum particles tend to exist in an aggregated state, resulting in many platinum particles that do not contribute to crossover suppression, which is economically disadvantageous. Therefore, the mass ratio of the present precious metal oxide to the mass of platinum is 80:20 to 99.9:0.1, preferably 90:10 to 97:3.

[0050] The solution containing the platinum-containing precursor is mixed with the slurry of the present precious metal oxide. The mixing method involves dropping the solution containing the platinum-containing precursor into the slurry of the present precious metal oxide, mixing, and thoroughly stirring. The resulting slurry dispersion of the platinum-containing precursor and the present precious metal oxide is heated, and a reducing agent is added dropwise to obtain a slurry of the precious metal oxide in which platinum particles have precipitated. The mixture is then subjected to solid-liquid separation using a filter, and thoroughly dried using a dryer. This results in a platinum-supported precious metal oxide in which the platinum-containing precursor is supported on the present precious metal oxide.

[0051] In the platinum-supported noble metal oxide, the loading rate of platinum particles is controlled by adjusting the amount of the platinum-containing precursor relative to the noble metal oxide when mixing the platinum-containing precursor and the noble metal oxide in a liquid.

[0052] The particle size of the platinum particles in the platinum-supported noble metal oxide obtained as described above can be controlled as follows. For example, in the process of obtaining a platinum-containing precursor, the finer the average particle size of the platinum-containing precursor, the easier it is to maintain a fine average particle size of the platinum particles; whereas, the coarser the average particle size of the precursor, the coarser the average particle size of the platinum particles tends to be. In the process of obtaining a platinum-containing precursor, for example, when a platinum-containing precursor is produced by a liquid-phase reduction method, the average particle size can be controlled by adjusting the platinum source concentration and reaction time. The lower the platinum source concentration, the finer the average particle size of the platinum-containing precursor produced, while the higher the platinum source concentration, the coarser the average particle size of the platinum-containing precursor. The formation of a platinum-containing precursor is divided into two stages: nucleation from the platinum source and particle growth to platinum particles. The number of nuclei formed in the solution increases in proportion to the platinum source concentration, increasing the probability that the nuclei will collide with each other, resulting in particle growth and the formation of coarse particles. Furthermore, the shorter the reaction time, the finer the average particle size of the platinum-containing precursor remains, whereas the longer the reaction time, the longer the grain growth time, and therefore the larger the average particle size of the platinum-containing precursor tends to become.

[0053] In the heat treatment of platinum-supported precious metal oxides, increasing the heat treatment temperature tends to coarsen the average particle size of the platinum particles. Furthermore, the longer the heat treatment, the larger the average particle size of the platinum particles tends to become. In the heat treatment of platinum-supported precious metal oxides, the higher the temperature of the platinum-containing precursor, the more vigorously it moves on the surface of the precious metal oxide, and upon contact with other precursors, particles grow and are more likely to form aggregates. Furthermore, the longer the heat treatment time, the higher the probability of particle growth and the more likely aggregates are to form. Therefore, the average particle size of the platinum particles becomes larger. To maintain a fine average particle size of the platinum particles in this electrode catalyst, it is preferable to use a low heat treatment temperature and perform the heat treatment for a short period of time.

[0054] In order to make the average particle size of the platinum particles in this electrode catalyst 10 nm or less, the average particle size of the platinum particles produced can be adjusted by lowering the platinum raw material concentration, shortening the reaction time, and controlling the reaction temperature at a relatively low temperature.

[0055] As described above, it is preferable that the platinum particles in the present electrode catalyst do not become agglomerated particles but remain in the state of primary particles. Therefore, when platinum particles are supported on the present precious metal oxide, it is preferable that the platinum particles are supported so as not to agglomerate. As a method for supporting platinum particles so as not to agglomerate, as described above, a slurry of the present precious metal oxide and a solution of a precursor material containing platinum are mixed, a reducing agent is added, and the mixture is heated. Due to the anchor effect of the present precious metal oxide, the platinum particles do not agglomerate, and a platinum-supported precious metal oxide in which platinum primary particles are highly dispersed on the present precious metal oxide can be synthesized.

[0056] The present electrode catalyst is suitable for use in water electrolysis cells, particularly the PEM water electrolysis cell 1 shown schematically in Fig. 1. Hereinafter, the PEM water electrolysis cell will be described with reference to Fig. 1, but the form of the water electrolysis cell is not limited to that shown in Fig. 1. In Fig. 1, the present electrode catalyst (not shown) constitutes an anode catalyst layer 2 together with an ionomer, an electrolyte material that serves as a binder to bind the present electrode catalyst together. The ionomer also plays a role in supplying ions required for the reaction to the reaction sites.

[0057] As the ionomer, various PEMs such as perfluorosulfonic acid-based, sulfonated polyethylene ether ketone-based, sulfonated polybenzimidazole-based, etc. are used. Among them, perfluorosulfonic acid-based Nafion (registered trademark, manufactured by DuPont), Flemion (registered trademark, manufactured by AGC), Aciplex (registered trademark, manufactured by Asahi Kasei), Fumion (registered trademark, manufactured by Fumatech), Aquivion (registered trademark, manufactured by Solvay), etc. can be preferably used.

[0058] The anode catalyst layer 2 is formed on one side of the PEM 5. Meanwhile, the cathode catalyst layer 6 is formed on the side of the PEM 5 opposite to the side on which the anode catalyst layer 2 is formed. In other words, the anode catalyst layer 2 and the cathode catalyst layer 6 are formed on both sides of the PEM 5, forming a catalyst-coated membrane.

[0059] As the PEM, various PEMs such as perfluorosulfonic acid-based, sulfonated polyethylene ether ketone-based, sulfonated polybenzimidazole-based, etc. Among them, perfluorosulfonic acid-based Nafion (registered trademark, manufactured by DuPont), Flemion (registered trademark, manufactured by AGC), Aciplex (registered trademark, manufactured by Asahi Kasei), Fumion (registered trademark, manufactured by Fumatech), and Aquivion (registered trademark, manufactured by Solvay) can be preferably used.

[0060] The cathode catalyst used in the cathode catalyst layer 6 may be platinum, platinum supported on carbon, or platinum ruthenium supported on carbon. The cathode catalyst layer 6 is also composed of a cathode catalyst and an ionomer. Examples of the ionomer include the same as those mentioned above.

[0061] An anode gas diffusion layer 4 is formed on the anode side of the catalyst-coated membrane, and a cathode gas diffusion layer 7 is formed on the cathode side, thereby forming a membrane electrode assembly 9. An anode electrode 3 including a current collector (not shown) is formed on the anode side of the membrane electrode assembly 9, and a cathode electrode 8 including a current collector (not shown) is formed on the cathode side.Furthermore, an anode separator (not shown) is provided on the anode side, and a cathode separator (not shown) is provided on the cathode side, thereby forming a water electrolysis cell 1.

[0062] Typically, a water electrolysis device uses multiple water electrolysis cells connected in series, and therefore, to prevent electrical short circuits between the cells, an anode separator and a cathode separator are provided as described above. When the water electrolysis device has only one water electrolysis cell, the anode separator and the cathode separator may be omitted.

[0063] In the anode catalyst layer 2, the raw material water is oxidized to protons and oxygen gas. ２ O → 4H ＋ +O ２ +4e － The oxidation reaction produces protons (H ＋ ) and oxygen gas (O ２ ) and electrons (e －) are separated. The protons move through the PEM 5, and the electrons move through an external circuit to the cathode electrode 8 side. Oxygen is discharged to the outside via the anode gas diffusion layer 4. In the cathode catalyst layer 6, 2H ＋ +2e － →H ２ The reduction reaction produces hydrogen (H ２ ) is generated and collected as hydrogen gas via the cathode gas diffusion layer 7, and is used as a raw material for fuel electricity or for other chemical reactions.

[0064] However, there are cases where hydrogen produced in the cathode catalyst layer 6 permeates the PEM 5 and flows back to the anode side, causing crossover and mixing with the oxygen produced in the anode catalyst layer 2. In this case, the supported platinum particles in the anode catalyst layer 2 using this electrode catalyst capture the hydrogen that has flowed back and react with the oxygen to produce water, thereby preventing hydrogen from mixing with the oxygen.

[0065] The anode catalyst layer 2 is preferably formed by stirring and mixing the present electrode catalyst and the ionomer in a solvent to prepare an anode catalyst ink. The ratio of the present electrode catalyst to the ionomer in the anode catalyst ink is, for example, preferably 0.05 to 0.2, more preferably 0.07 to 0.15, per 1 part of the present electrode catalyst. Water or a mixture of water and a lower aliphatic alcohol such as ethanol, propanol, or butanol is preferably used as the solvent.

[0066] The cathode catalyst layer 6 is also preferably formed by stirring and mixing the cathode catalyst and the ionomer in a solvent to prepare a cathode catalyst ink. Water or a mixture of water and a lower aliphatic alcohol such as ethanol, propanol, or butanol is preferably used as the solvent. The anode catalyst ink thus prepared is applied to one side of the PEM 5, and the cathode catalyst ink is applied to the side of the PEM 5 opposite to the side to which the anode catalyst ink is applied, thereby producing a catalyst-coated membrane. Examples of application methods include direct coating methods such as bar coating and spray coating, or forming the anode catalyst layer and cathode catalyst layer separately on Teflon (registered trademark) films in advance and then transferring them to the PEM 5 using a hot press or the like.

[0067] The amount of the anode catalyst layer 2 applied to the PEM 5 is, for example, 0.1 mg / cm ２ 2.0mg / cm or more ２ Preferably, 0.3 mg / cm or less ２ 1.0mg / cm or more ２ The following ranges are used: The amount of the cathode catalyst layer 6 applied to the PEM 5 is the same as above.

[0068] A membrane electrode assembly is obtained by combining the catalyst-coated membrane obtained as described above with an anode-side gas diffusion layer and a cathode-side gas diffusion layer. A water electrolysis cell is obtained by stacking the membrane electrode assembly, an anode separator, and a cathode separator. A PEM water electrolysis device is obtained by stacking multiple water electrolysis cells. The resulting PEM water electrolysis device has the electrode catalyst in the anode catalyst layer, and exhibits water electrolysis efficiency equivalent to that of conventional PEM water electrolysis devices while suppressing crossover.

[0069] The present noble metal oxide is a catalyst capable of generating an oxygen reaction, and the platinum particles are a catalyst capable of reducing a hydrogen reaction. Therefore, the present electrode catalyst comprises a catalyst capable of generating an oxygen reaction supported on a catalyst capable of reducing a hydrogen reaction.

[0070] The above has described the present electrode catalyst, the anode catalyst layer of a PEM water electrolysis device having the present electrode catalyst, the catalyst-coated membrane including the anode catalyst layer, the cathode catalyst layer, and a PEM, the membrane electrode assembly including the catalyst-coated membrane, an anode gas diffusion layer, and a cathode gas diffusion layer, the water electrolysis cell including the membrane electrode assembly, an anode separator, and a cathode separator, and the PEM water electrolysis device including the water electrolysis cell. However, the present invention is not limited to the configurations of the above embodiments. For example, the configurations of the present invention may be supplemented with any other configuration, or may be replaced with any configuration that provides a similar function.

[0071] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0072] Preparation of Iridium-Ruthenium Oxide: In a 5L Teflon beaker, add tetravalent iridium chloride (H ２ IrCl６ ・nH ２ 0) and 30 g of Ir (RuCl 3 ) ３ ・nH ２ 30 g of iridium chloride (Ru) was placed in the flask, and 1.6 L of pure water was added. The liquid temperature was raised to 80° C. while stirring at 200 rpm for 1 hour to prepare a mixed solution of iridium chloride and ruthenium chloride.

[0073] Next, 70 g of NaOH was weighed out and dissolved in 700 mL of pure water to prepare a 10% NaOH solution. The 10% NaOH solution was added dropwise to the mixed solution of iridium chloride and ruthenium chloride at a rate of 12 ml / min. After completion of the dropwise addition, the mixture was stirred for an additional 10 hours while maintaining a liquid temperature of 80°C, thereby preparing an iridium-ruthenium oxide slurry. The resulting slurry was allowed to cool to room temperature, then allowed to stand, and the supernatant was decanted. 1300 ml of pure water was added to the Teflon beaker containing the remaining slurry, and the mixture was stirred for 1 hour while again raising the temperature to 80°C. After cooling to room temperature, the mixture was allowed to stand, and the supernatant was decanted again. This decantation washing was continued until the conductivity of the supernatant no longer changed. The mixture was then filtered, and the filter cake was dried in an electric dryer at 60°C for 20 hours, and then fired in an electric furnace in the air at 350°C for 10 hours to obtain iridium-ruthenium oxide.

[0074] Example 1 Preparation of 11% by mass platinum-supported iridium-ruthenium oxide 9 g of the iridium-ruthenium oxide obtained above was placed in a 500 mL three-neck flask, and 322 g of pure water was added, followed by dispersion for 30 minutes in an ultrasonic cleaner. Next, the liquid temperature was raised to 80°C while stirring with a stirrer, and then stirred for 30 minutes while maintaining the liquid temperature at 80°C, thereby preparing an iridium-ruthenium oxide slurry. After maintaining 80°C for 30 minutes, the liquid was allowed to cool to room temperature.

[0075] 8.0 wt% hexahydroxoplatinic (IV) acid (Furuya Metal H ２ Pt(OH) ６ ) / monoethanolamine solution was placed in a beaker so that the platinum weight was 1 g, and 23 g of pure water was added. ２ Pt(OH) ６Next, the H 2 O 3 solution containing the iridium-ruthenium oxide slurry was added to the 500 mL three-neck flask. ２ Pt(OH) ６ An aqueous solution of platinum / monoethanolamine was added dropwise at a rate of 2.5 mL / min while stirring with a stirrer. After the addition, the temperature was raised to 80°C, and a formic acid solution containing 0.77 g of formic acid and 38 g of pure water was added dropwise at a rate of 2.5 mL / min while maintaining the temperature at 80°C. After the addition was completed, the mixture was stirred for an additional 2 hours while maintaining the liquid temperature at 80°C, thereby producing a platinum / iridium-ruthenium oxide slurry. The resulting platinum / iridium-ruthenium oxide slurry was filtered and washed until the conductivity of the filtrate remained unchanged. The filter cake was then dried at 60°C for 20 hours or more, and the cake was pulverized in an agate mortar to obtain an iridium-ruthenium oxide powder supporting 11.0 wt% platinum. The obtained iridium-ruthenium oxide powder carrying 11.0 wt% platinum was examined using a transmission electron microscope (hereinafter also referred to as TEM) to measure the average particle size of the supported platinum particles using the method described below, and the result was 1.52 nm. The obtained iridium-ruthenium oxide powder carrying 11.0 wt% platinum was used as an anode electrode catalyst.

[0076] Example 2 Preparation of 4.84 mass% platinum-supported iridium-ruthenium oxide 9.5 g of the iridium-ruthenium oxide obtained above was placed in a 500 mL three-neck flask, and 322 g of pure water was added, followed by dispersion for 30 minutes in an ultrasonic cleaner. Next, the liquid temperature was raised to 80°C while stirring with a stirrer, and then stirred for 30 minutes while maintaining the liquid temperature at 80°C, thereby preparing an iridium-ruthenium oxide slurry. After maintaining 80°C for 30 minutes, the liquid was allowed to cool to room temperature.

[0077] 8.0 wt% hexahydroxoplatinic (IV) acid (Furuya Metal H ２ Pt(OH) ６ ) / monoethanolamine solution was placed in a beaker so that the platinum weight was 0.5 g, and 23 g of pure water was added. ２ Pt(OH) ６An aqueous solution of 0.37 g of formic acid and 38 g of pure water was prepared. In the subsequent operations, an iridium-ruthenium oxide powder carrying 4.84 wt % of platinum was obtained in the same manner as in Example 1, except that a formic acid solution containing 0.37 g of formic acid and 38 g of pure water was prepared. The obtained iridium-ruthenium oxide powder carrying 4.84 wt % of platinum was used as the anode electrode catalyst.

[0078] Example 3 Preparation of 2.82 mass% platinum-supported iridium-ruthenium oxide 9.7 g of the iridium-ruthenium oxide obtained above was placed in a 500 mL three-neck flask, and 347 g of pure water was added, followed by dispersion for 30 minutes in an ultrasonic cleaner. Next, the liquid temperature was raised to 80°C while stirring with a stirrer, and then stirred for 30 minutes while maintaining the liquid temperature at 80°C, thereby preparing an iridium-ruthenium oxide slurry. After maintaining 80°C for 30 minutes, the liquid was allowed to cool to room temperature.

[0079] 8.0 wt% hexahydroxoplatinic (IV) acid (Furuya Metal H ２ Pt(OH) ６ ) / monoethanolamine solution was placed in a beaker so that the platinum weight was 0.3 g, 24 g of pure water was added, and H ２ Pt(OH) ６ An aqueous solution of 0.23 g of formic acid and 43 g of pure water was prepared. In the subsequent operations, an iridium-ruthenium oxide powder carrying 2.82 wt % of platinum was obtained in the same manner as in Example 1, except that a formic acid solution containing 0.23 g of formic acid and 43 g of pure water was prepared. The obtained iridium-ruthenium oxide powder carrying 2.82 wt % of platinum was used as the anode electrode catalyst.

[0080] Comparative Example 1: Preparation of a mixed sample of iridium-ruthenium oxide containing 8.23 ​​wt% platinum black 0.9 g of iridium-ruthenium oxide and 0.1 g of platinum black (manufactured by Sigma-Aldrich) were weighed and mixed in an agate mortar to obtain a mixed sample of iridium-ruthenium oxide containing 8.23 ​​wt% platinum black. The obtained mixed sample of iridium-ruthenium oxide containing 8.23 ​​wt% platinum black was used as the anode electrode catalyst.

[0081] Comparative Example 2 In Comparative Example 2, only the iridium-ruthenium oxide obtained above was used as the anode electrode catalyst.

[0082] (Comparative Example 3) Preparation of 1.58 mass% platinum-supported iridium-ruthenium oxide 9.9 g of the iridium-ruthenium oxide obtained above was placed in a 500 mL three-neck flask, and 322 g of pure water and 0.1 g of acetic acid were added, followed by dispersion for 30 minutes in an ultrasonic cleaner. Next, the liquid temperature was raised to 80°C while stirring with a stirrer, and the liquid temperature was maintained at 80°C while stirring for 30 minutes to prepare an iridium-ruthenium oxide slurry. After maintaining 80°C for 30 minutes, the liquid was allowed to cool to room temperature.

[0083] 8.0 wt% hexahydroxoplatinic (IV) acid (Furuya Metal H ２ Pt(OH) ６ ) / monoethanolamine solution was placed in a beaker so that the platinum weight was 0.1 g, and 23 g of pure water was added. ２ Pt(OH) ６ An aqueous solution of 0.23 g of formic acid and 43 g of pure water was prepared. In the subsequent operations, an iridium-ruthenium oxide powder supporting 1.58 wt % of platinum was obtained in the same manner as in Example 1, except that a formic acid solution containing 0.23 g of formic acid and 43 g of pure water was prepared. The obtained iridium-ruthenium oxide powder supporting 1.58 wt % of platinum was used as the anode electrode catalyst.

[0084] (Comparative Example 4) Preparation of iridium-ruthenium oxide with an average platinum particle size of 10 nm or more. ２ PtCl ６ ) into a 1 L beaker so that the platinum weight is 1 g, add 300 g of pure water, and ２ PtCl ６ Then, H ２ PtCl ６Platinum nanoparticle slurry was obtained by adding 2.64 g of ascorbic acid to the aqueous solution and heating at 80°C for 5 hours. The resulting platinum nanoparticle slurry was filtered and washed with pure water until the filtrate's conductivity was 100 μS / cm or less. The filter cake was then dried at 80°C for 20 hours or more and crushed in an agate mortar to obtain platinum nanoparticles. The average particle size of the platinum nanoparticles was measured by gas adsorption and found to be 17 nm. Platinum-loaded iridium-ruthenium oxide was synthesized by impregnation using an evaporator. 0.1 g of platinum nanoparticles and 0.9 g of iridium-ruthenium oxide were placed in a 100 mL eggplant flask, 10 g of pure water was added, and the mixture was dispersed in an ultrasonic cleaner for 1 minute. Drying to dryness at 60°C yielded an iridium-ruthenium powder loaded with 11.71 wt% platinum. The obtained iridium-ruthenium oxide powder carrying 11.71 wt% platinum was observed under TEM to measure the average particle size of the supported platinum nanoparticles using the method described below. However, the particles aggregated, making it difficult to distinguish between primary particles, and the average particle size could not be measured. As described above, the average particle size of the platinum nanoparticles before loading was 17 nm, and it is unlikely that the average particle size of platinum would decrease after loading. Therefore, the platinum particle size of the iridium-ruthenium oxide carrying 11.71 wt% platinum is thought to be approximately 17 nm. The obtained iridium-ruthenium oxide powder carrying 11.71 wt% platinum was used as an anode electrode catalyst.

[0085] (Comparative Example 5) Preparation of Iridium-Ruthenium Oxide with an Average Platinum Particle Size of 10 nm or More An iridium-ruthenium oxide powder supporting 9.51 wt% platinum was obtained using the same production method as Comparative Example 4, except that 10.4 g of 10 wt% sodium formate was used instead of the 2.64 g of the ascorbic acid aqueous solution used in Comparative Example 4. As in Comparative Example 4, due to particle aggregation, the average particle size could not be confirmed by TEM. The average particle size before support was measured by gas adsorption and was found to be 67 nm. Since it is unlikely that the average particle size would decrease after support, the platinum particle size of the iridium-ruthenium oxide supporting 9.51 wt% is thought to be approximately 67 nm. The obtained iridium-ruthenium oxide powder supporting 9.51 wt% platinum was used as an anode electrode catalyst.

[0086] The platinum-supported iridium-ruthenium oxide powders of Examples 1 to 3, the mixed sample of platinum black and iridium-ruthenium oxide of Comparative Example 1, the iridium-ruthenium oxide of Comparative Example 2, and the platinum-supported iridium-ruthenium oxides of Comparative Examples 3 to 5 were subjected to the following measurements and evaluations.

[0087] [BET Specific Surface Area Measurement] The specific surface areas of the platinum-supported iridium-ruthenium oxide powders of Examples 1 to 3, the mixed sample of platinum black and iridium-ruthenium oxide of Comparative Example 1, the iridium-ruthenium oxide of Comparative Example 2, and the platinum-supported iridium-ruthenium oxides of Comparative Examples 3 to 5 were measured by the nitrogen adsorption method using a specific surface area / pore distribution measuring device BELSORP-mini II (Microtrac BEL Corporation). In addition, the obtained adsorption isotherm data was analyzed by the BET method using the analysis software BEL Master built into the specific surface area / pore distribution measuring device to determine the BET specific surface area. The results are summarized in Table 1.

[0088] [Metal Content Analysis] Measurement solutions were prepared by the alkali fusion method for the platinum-supported iridium-ruthenium oxide powders of Examples 1 to 3, the mixed sample of platinum black and iridium-ruthenium oxide of Comparative Example 1, the iridium-ruthenium oxide of Comparative Example 2, and the platinum-supported iridium-ruthenium oxides of Comparative Examples 3 to 5, and the metal contents in the oxides were measured using an inductively coupled plasma optical emission spectroscopy (hereinafter referred to as ICP-OES) apparatus (Agilent Technologies, Inc., Agilent 5800). The results are summarized in Table 2. A current was swept at a constant current density, and the voltage during the current sweep and the hydrogen concentration in oxygen were measured using an suction-type hydrogen gas detector (GP-5001 manufactured by Riken Keiki) installed near the outlet of the anode piping. The results are summarized in Table 2.

[0089] [Measurement of Average Particle Diameter of Platinum] The platinum particle diameter was confirmed using a TEM. Twenty or more primary particles of the supported platinum metal particles were selected from an image of platinum-supported iridium-ruthenium oxide (iridium-ruthenium oxide) taken using a TEM, and the longest length of these particles was visually measured as the particle diameter. The average particle diameter was determined by dividing the sum of the particle diameters of each particle by the number of particles whose particle diameters were measured. There is no particular restriction on the number of primary particles to be measured, as long as it was 20 or more, but it is usually 50 or less, approximately 20 to 30. The values ​​in Table 1 were obtained by measuring the lengths of 29 platinum metal particles. However, in Comparative Example 1, the particle diameter of the primary particles of unagglomerated platinum particles before mixing with iridium-ruthenium oxide was measured using the above method. In Comparative Examples 4 and 5, the primary particles agglomerated, making it difficult to identify the primary particles using a TEM. The results are shown in Table 1.

[0090] [Measurement of Average Particle Size of Iridium-Ruthenium Oxide] The average particle size of iridium-ruthenium oxide was measured using an X-ray diffractometer (hereinafter referred to as XRD). XRD was performed using CuKα radiation at a tube voltage of 40 kV and a tube current of 40 mA. First, the diffraction angle was adjusted so that the diffraction angle 2θ of Si (220) was 48.28° using angle standard Si powder. The iridium-ruthenium oxide powder was filled into a sample holder, and the scan was performed from 2θ = 10° to 90°, with a sampling interval of 0.05° in 2θ and a scan rate of 10° / min in 2θ. A main diffraction peak of (101) was found near 28°. The crystallite size obtained from the main diffraction peak was taken as the average particle size, and the results are summarized in Table 1.

[0091]

[0092] Using the obtained platinum-supported iridium-ruthenium oxide powders of Examples 1 to 3, the mixed sample of platinum black and iridium-ruthenium oxide of Comparative Example 1, the iridium-ruthenium oxide of Comparative Example 2, and the platinum-supported iridium-ruthenium oxides of Comparative Examples 3 to 5, CCMs and single cells were produced by the following method, and the amount of hydrogen permeated to the anode side was measured.

[0093] <Preparation of anode catalyst sheet> - Preparation of platinum-supported iridium-ruthenium oxide anode catalyst sheet 0.2 g of the platinum-supported iridium-ruthenium oxide of Examples 1 to 3 and Comparative Examples 3 to 5 was weighed out, and 0.1 g of ultrapure water, 0.3 g of 2-ethoxyethanol, and 0.075 g of 20% Nafion dispersion (manufactured by DuPont) were added. The mixture was stirred with a magnetic stirrer for 5 minutes, and then stirred and dispersed with an ultrasonic homogenizer for 10 minutes while stirring in a thermostatic bath at 5°C, to obtain an anode catalyst paste. A 50 μm-thick Teflon sheet was attached to the glass surface of a wire bar coater with a doctor blade (PM-9050MC, manufactured by SMT), and the anode catalyst paste was applied to the Teflon sheet surface by sweeping the blade at a sweep speed of 0.80 m / min. This was air-dried for 1 hour and then dried in a vacuum dryer at 120°C for 1 hour to obtain an anode catalyst sheet. The catalyst coating amount per unit area of ​​the catalyst sheet was about 1 mg / cm. ２The dried anode catalyst sheet was cut into 9 cm 2 for the effective electrode area using a Thompson blade. ２ The anode catalyst sheet was cut into a circular shape to obtain an anode catalyst sheet for evaluating the durability of a PEM single water electrolysis cell.

[0094] Preparation of an anode catalyst sheet of a mixed sample of platinum and iridium-ruthenium oxide containing 8 wt % platinum black An anode catalyst sheet was obtained in the same manner as in the preparation of the anode catalyst sheet of platinum-supported iridium-ruthenium oxide, except that the mixed sample of iridium-ruthenium oxide containing 8 wt % platinum black obtained in Comparative Example 1 was used instead of the platinum-supported iridium-ruthenium oxide of Examples 1 to 3.

[0095] Preparation of Iridium-Ruthenium Oxide Anode Catalyst Sheet An anode catalyst sheet was obtained in the same manner as in the preparation of the platinum-supported iridium-ruthenium oxide anode catalyst sheet described above, except that the iridium-ruthenium oxide of Comparative Example 2 was used instead of the platinum-supported iridium-ruthenium oxide of Examples 1 to 3.

[0096] <Preparation of cathode catalyst sheet> Ketjen Black EC300J (manufactured by AKZO NOBEL) was ultrasonically dispersed in deionized water, and high specific surface area platinum black (FHPB manufactured by Furuya Metal, BET specific surface area 85 m) was added to the dispersion. ２ A slurry of 50 wt% platinum supported carbon powder was prepared by ultrasonically dispersing 50 wt% platinum (Pt / g) in deionized water, and the resulting mixture was added to prepare a cathode catalyst. The 50 wt% platinum supported carbon powder was weighed, and ultrapure water, 2-ethoxyethanol, IPA, and a 5 wt% Nafion dispersion were added. The mixture was stirred and mixed using a magnetic stirrer and a powerful ultrasonic disperser to obtain a cathode catalyst paste.

[0097] A 50 μm thick Teflon® sheet was attached to the glass surface of a wire bar coater equipped with a doctor blade, and the cathode catalyst paste was applied to the surface of the Teflon® sheet. The sheet was then swept with the blade to coat the cathode catalyst paste on the surface of the sheet. This sheet was air-dried for 15 hours and then dried at 100°C for 1 hour in a vacuum dryer to obtain a cathode catalyst sheet. The catalyst coating amount per unit area of ​​the catalyst sheet was approximately 1 mg / cm. ２ The dried cathode catalyst sheet was cut with a Thomson blade to obtain a 9 cm ２ The cathode catalyst sheet CS-1 for evaluating the durability of a PEM single water electrolysis cell was obtained.

[0098] [Production of CCM for water electrolysis cell] Nafion 115 (manufactured by DuPont) was cut to a diameter of 70 mm as the PEM, and the anode catalyst sheet and cathode catalyst sheet cut to the above-mentioned electrode effective area were sandwiched together with their centers aligned with the catalyst-coated surfaces facing inward. The CCM was then pressed in a high-precision hot press (manufactured by Tester Sangyo) at 160°C and 2 kN / cm. ２ After pressing, the Teflon (registered trademark) sheets attached to the anode and cathode were peeled off to obtain a CCM.

[0099] [Evaluation of constant current durability of PEM single cell for water electrolysis] Effective electrode area: 9 cm ２ A water electrolysis cell unit (manufactured by FC Development Co., Ltd.) was prepared. A platinum-plated titanium sintered body was used as the gas diffusion layer on the anode side, and carbon paper was used as the gas diffusion layer on the cathode side. These and the CCM of the catalyst of the example prepared above were assembled into a single cell and fastened with fastening bolts. The anode and cathode sides of this single cell were connected to the gas line and pure water line of a water electrolysis evaluation device (Toyo Corporation, AutoPEM-Dual / APMD-12), respectively. The constant current durability evaluation of the PEM water electrolysis single cell was performed by setting the cell temperature to 80°C, supplying hot pure water with a conductivity of 0.1 mS / m or less to the anode at a flow rate of 30 ml / min, and measuring the initial IV characteristics. Next, when the current density was increased to 2 A / cm ２A current was swept at a constant current density of 0.05 V, and the voltage during the current sweep and the change over time in the hydrogen concentration in oxygen were measured using an aspirating hydrogen gas detector (GP-5001 manufactured by Riken Keiki) installed near the outlet of the anode pipe. The results are shown in Figure 2. Finally, the IV characteristics were measured again. The evaluation results of the PEM water electrolysis single cell are summarized in Table 2. In Table 2, IV characteristics (before) and IV characteristics (after) are the initial IV characteristics and the IV characteristics measured at the end, respectively, and represent the voltage characteristics of the water electrolysis catalyst. The smaller the difference between the IV characteristics (before) and IV characteristics (after), the longer the life of the water electrolysis catalyst.

[0100]

[0101] The results in Table 2 show that the water electrolysis cells using the platinum-supported iridium-ruthenium oxides of Examples 1 to 3 as anode catalysts exhibited water electrolysis efficiencies equal to or greater than those using the conventional iridium-ruthenium oxide (Comparative Example 2) and the mixed sample of iridium-ruthenium oxide containing 8 wt % platinum black (Comparative Example 1) as anode catalysts.

[0102] 2, when the platinum black and iridium-ruthenium oxide mixed sample of Comparative Example 1 was used as the anode catalyst, the hydrogen concentration in oxygen increased with the time of water electrolysis. In contrast, when the platinum-supported iridium-ruthenium oxides of Examples 1 to 3 were used as the anode catalyst, the hydrogen concentration in oxygen was lower than in Comparative Examples 1 and 2, and in platinum-supported iridium-ruthenium oxides with a platinum loading of 3 wt % or more, the hydrogen concentration in oxygen was almost zero. Furthermore, the results of Table 2 show that the platinum-supported iridium-ruthenium oxides of Comparative Examples 3 and 4 suppressed crossover compared to Comparative Examples 1 and 2, but the crossover suppression effect was smaller than that of the platinum-supported iridium-ruthenium oxides of Examples 1 to 3. Therefore, it can be seen that hydrogen crossover was sufficiently suppressed when the platinum-supported iridium-ruthenium oxide of this electrode catalyst was used as the anode catalyst. It is also found that hydrogen crossover can be sufficiently suppressed by using an electrode catalyst in which a catalyst capable of hydrogen reduction reaction is supported on a catalyst capable of oxygen evolution reaction.

[0103] 1: PEM water electrolysis cell 2: Anode catalyst layer 3: Anode separator 4: Anode gas diffusion layer 5: PEM 6: Cathode catalyst layer 7: Cathode gas diffusion layer 8: Cathode separator 9: Membrane electrode assembly

Claims

1. An electrode catalyst comprising a noble metal oxide containing at least one of iridium and ruthenium, on which platinum particles with an average particle size of 10 nm or less, as measured by transmission electron microscopy, are supported at a loading rate of 1.60% by mass or more.

2. The electrode catalyst according to claim 1, wherein the noble metal oxide is represented by the following formula (1). Ir ｘ Ru ｙ M ｚ O ｎ (1) However, in equation (1) above, M represents at least one metal selected from groups 2 to 14 of the periodic table, x, y, and z satisfy 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, x + y + z = 1.0, and n satisfies 0 < n ≤ 3.

3. The electrode catalyst according to claim 2, wherein z = 0 in formula (1).

4. The electrode catalyst according to claim 2, wherein in formula (1), x and y satisfy the following conditions. 0 < x < 1 0 < y < 1

5. The electrode catalyst according to claim 1, wherein the noble metal oxide has a (1,1,0) crystallite size determined by powder X-ray diffraction (Cu Kα) of 2 nm or more and 8 nm or less.

6. An anode catalyst layer for a water electrolysis apparatus containing the electrode catalyst and ionomer described in claim 1.

7. A catalyst coating film comprising an anode catalyst layer, a cathode catalyst layer, and a proton exchange membrane as described in claim 6.

8. A membrane electrode assembly comprising a catalyst coating film, an anode gas diffusion layer, and a cathode gas diffusion layer as described in claim 7.

9. A water electrolytic cell comprising a membrane electrode assembly, an anode separator, a cathode separator, and a current collector plate as described in claim 8.

10. A water electrolysis apparatus including the water electrolysis cell described in claim 9.