Water electrolysis anode catalyst, water electrolysis anode catalyst layer, and water electrolysis membrane electrode assembly

Abstract

[Problem] To provide a water electrolysis anode catalyst capable of reducing an increase in hydrogen concentration caused by cross leakage in the anode of a membrane electrode assembly of a water electrolysis apparatus. [Solution] The present invention pertains to a water electrolysis anode catalyst in which platinum fine particles are supported on a base catalyst having an oxygen evolution activity. A base catalyst of the present invention contains a metal or a metal compound that has a higher oxygen evolution activity than platinum. The platinum fine particles supported on the base catalyst have an average particle diameter of 1-55 nm. In the particle size distribution of the platinum fine particles supported on the base catalyst, the number-based cumulative 90% particle diameter is preferably 90 nm or less.

Description

Anode catalyst for water electrolysis, anode catalyst layer for water electrolysis, and membrane electrode assembly for water electrolysis

[0001] The present invention relates to an anode catalyst for water electrolysis that generates hydrogen and oxygen by electrolyzing water, an anode catalyst layer for water electrolysis containing the catalyst, and a membrane electrode assembly for water electrolysis. More specifically, the present invention relates to a water electrolysis catalyst applied to the anode electrode of a water electrolysis apparatus that can reduce the hydrogen concentration entering the anode due to cross-leakage.

[0002] Hydrogen is expected to play a significant role as one of the renewable energy sources that are gaining importance against the backdrop of environmental and energy problems. Examples of hydrogen-based energy applications include hydrogen engines as power plants and fuel cells as power generators, and their practical application is progressing. However, for hydrogen to be effectively utilized as an energy source, its stable and safe production and supply are essential.

[0003] Furthermore, in recent years, with the shift towards a carbon-neutral society being advocated, the hydrogen production process using water electrolysis is gaining traction due to its good compatibility with other renewable energy sources such as solar power. The progress in the development of hydrogen storage and supply systems is also contributing to the advancement of the hydrogen production process using water electrolysis.

[0004] While there are several processes for hydrogen production using water electrolysis, the polymer electrolyte membrane (polymer) water electrolysis method is considered promising due to its high efficiency in hydrogen production. Water electrolysis devices to which the polymer electrolyte membrane method is applied include proton exchange type (PEM) and anion exchange type (AEM). These types of water electrolysis devices are formed by stacking multiple electrolytic cells, each consisting of an ion exchange membrane (solid electrolyte membrane) and anode and cathode catalyst layers on both sides of the membrane. The anode and cathode catalyst layers are bonded to the surface of the ion exchange membrane, and the resulting membrane electrode assembly (CCM: Catalytic Coated Membrane) is a major component of the water electrolysis device.

[0005] The ion exchange membrane constituting the CCM of the water electrolysis device is an ion exchange membrane having proton exchange ability in the PEM, and an ion exchange membrane having hydroxide ion (OH － ) exchange ability is used in the AEM. The anode catalyst layer and the cathode catalyst layer are formed by mixing a water electrolysis catalyst having catalytic activity corresponding to the reactions at both electrodes with an appropriate ionomer.

[0006] At the anode of the water electrolysis device, the electrolysis reaction of water (2H ２ O → O ２ + 4H ＋ + 4e － ) is assumed in the PEM, and the production reaction of water and oxygen from hydroxide ions (4OH － → O ２ + 2H ２ O + 4e － ) is assumed in the AEM. Therefore, a metal or metal compound having oxygen evolution activity (OER activity: Oxygen Evolution Reaction) is applied as the anode catalyst for water electrolysis constituting the anode catalyst layer. Examples of this metal or metal compound having oxygen evolution activity include noble metals such as iridium, ruthenium, and rhodium, and their compounds such as iridium oxide, ruthenium oxide, and rhodium oxide. In addition, base metals that do not belong to noble metals such as manganese, nickel, lead, cobalt, and iron also exhibit oxygen evolution activity when they become compounds (oxides). In the present specification, hereinafter, the anode catalyst for water electrolysis may sometimes be simply referred to as a water electrolysis catalyst.

[0007] There are also several aspects regarding the mode of the anode catalyst for water electrolysis composed of the above-mentioned metal or metal compound having oxygen evolution activity. That is, a water electrolysis catalyst composed only of the metal or metal compound as the active component, with the active component in particulate or powder form can be mentioned. For example, the applicant of the present application discloses in Patent Document 1 a water electrolysis anode catalyst composed only of powder of iridium oxide as the active component and having excellent water electrolysis characteristics. As another aspect of the water electrolysis anode catalyst, a water electrolysis catalyst in which fine particles of the metal or metal compound as the active component are supported on an inorganic oxide carrier as the support is also known. For example, in Patent Document 2 and Patent Document 3, SnO ２ , TiO２ Al ２ O ３ , ZrO ２ Anode catalysts for water electrolysis are disclosed, in which iridium oxide or ruthenium oxide is supported on an inorganic oxide such as the above. These anode catalysts for water electrolysis have advantages depending on their form, and are used in a manner that appropriately meets the requirements of the water electrolysis apparatus to which they are applied, such as specifications, characteristics, and cost.

[0008] Patent No. 7297962 Publication Patent No. 5199575 Publication Special Publication No. 2020-500692

[0009] Incidentally, one issue that is expected to come into focus in the future regarding water electrolysis as a hydrogen generation process is cross-leakage between the anode and cathode electrodes. Cross-leakage is a phenomenon in which hydrogen generated at the cathode moves to the anode through the ion exchange membrane, while oxygen generated at the anode moves to the cathode through the ion exchange membrane. Cross-leakage is caused by pressure differences, hydrogen concentration differences, and oxygen concentration differences between the anode and cathode. Problems caused by cross-leakage include potential changes at both electrodes and the reaction of the cross-leaked hydrogen and oxygen with the membrane components of the ion exchange membrane, causing the membrane components to leach out.

[0010] Furthermore, a significant problem arising from cross-leakage is the concern regarding safety during the operation of water electrolysis equipment. The minimum ignition energy of hydrogen gas in oxygen gas (oxygen gas in hydrogen gas) is 0.013 mJ, which is low compared to the ignition energy of hydrogen gas in air. Therefore, there is a risk of explosion when hydrogen enters the anode side where oxygen is being produced and reaches the explosive limit concentration range (lower limit 4 vol%). While this cross-leakage problem can occur in both the anode and cathode, the problem of hydrogen contamination of the anode is more serious in the operation of water electrolysis cells, as the pressure on the cathode side is often high.

[0011] A simple and effective measure to reduce cross-leakage in water electrolysis is to increase the thickness of the ion exchange membrane. This is because increasing the thickness of the ion exchange membrane reduces the permeability of hydrogen and oxygen gases. However, increasing the thickness of the ion exchange membrane reduces the permeability of ions (H ＋ or OH － This leads to an increase in the movement resistance of the ion exchange membrane, causing a decrease in electrolysis efficiency due to the rise in voltage. In recent water electrolysis devices that aim to improve electrolysis efficiency, it is rather necessary to make the ion exchange membrane thinner, and thickening the membrane is not a feasible solution. In this respect, it can be said that improvements should be made to reduce only the gas permeability of the ion exchange membrane, but it is not easy to improve only the gas permeability without affecting the original function of ion conductivity.

[0012] Furthermore, there is currently very little concrete evidence, including practical demonstrations, regarding cross-leakage countermeasures from the perspective of water electrolytic catalysts. The direction of water electrolytic catalyst development to date has mainly focused on high activity, low oxygen overpotential, and durability (maintenance of activity) in the environment of water electrolysis equipment. This trend is necessary for the development and widespread adoption of hydrogen production processes using water electrolysis. However, it would be desirable to develop water electrolytic catalysts that can address the new problem of cross-leakage.

[0013] Therefore, the present invention provides a water electrolysis anode catalyst that constitutes the anode electrode of a membrane electrode assembly of a water electrolysis apparatus, while maintaining suitable activity and other properties, and addressing the problem of cross-leakage. Specifically, it provides a water electrolysis anode catalyst that can suppress the increase in hydrogen gas concentration at the anode, which is a concern due to cross-leakage.

[0014] The inventors conducted thorough research and concluded that a suitable means of reducing the hydrogen concentration at the anode is to react the cross-leaked hydrogen with oxygen to convert it into water. This is because oxygen produced by the anode reaction is present in the anode of a water electrolysis device, and it is thought that this can be used to carry out the hydrogen conversion reaction relatively easily. However, most conventional anode catalysts for water electrolysis have high oxygen evolution activity (OER activity) but poor activity for the hydrogen conversion reaction. Therefore, the inventors considered it preferable to use the conventional water electrolysis catalysts of the various embodiments described above as the basis for improvement (base catalyst) and to add an additional catalytic component that is active for the conversion of hydrogen into water to the base catalyst.

[0015] Furthermore, when adding additional catalytic components to the base catalyst acting as an anode water electrolytic catalyst, it is important to suppress the degradation of the base catalyst's properties caused by the addition. In other words, the addition of additional catalytic components leads to a decrease in the proportion of water electrolytic catalyst to the total catalyst. This raises concerns about a decrease in the efficiency of the water electrolysis reaction that is originally required. In addition, depending on the type of catalytic component selected, its very presence may adversely affect the water electrolysis properties.

[0016] Therefore, the inventors conducted further studies and concluded that it is preferable to apply platinum (metallic platinum) as an additional catalyst that has little effect on the activity of the water electrolytic catalyst, which is the base catalyst, and that can convert hydrogen to water by utilizing the oxygen in the anode. Platinum is a precious metal that has low oxygen evolution activity (OER activity) but excellent oxygen reduction activity (ORR activity). It is thought that by utilizing the ORR activity of platinum, hydrogen can be converted to water through the reduction of oxygen in the anode.

[0017] When it is preferable to add platinum to a water electrolytic catalyst to reduce the hydrogen concentration in the anode, the next thing to consider is the manner of addition. Platinum is well known as a general catalyst that has activity for various chemical reactions. Platinum is known to be used as a catalyst in powder form and in fine particle form. When using the former, powdered platinum, a water electrolytic anode catalyst can be formed by simply mixing it with a water electrolytic catalyst. The latter, platinum fine particles, can be made usable by using a water electrolytic catalyst as a base catalyst support (carrier). The inventors of the present invention came to the conclusion that in order to optimize both the OER activity that the water electrolytic catalyst should inherently exhibit and the ORR activity that the added platinum can exhibit, it is preferable to support finely particleized platinum on the base catalyst to a suitable state.

[0018] In other words, the present invention relates to an anode catalyst for water electrolysis comprising platinum nanoparticles supported on a base catalyst having oxygen-evolving activity, wherein the base catalyst contains a metal or metal compound with higher oxygen-evolving activity than platinum, and the platinum nanoparticles have an average particle diameter of 1 nm or more and 55 nm or less. The structure and manufacturing method of the anode catalyst for water electrolysis according to the present invention, and an anode catalyst layer and membrane electrode assembly (CCM) for water electrolysis that suitably utilizes this anode catalyst for water electrolysis will be described below.

[0019] (A) Anode catalyst for water electrolysis and method for producing the same according to the present invention As described above, the anode catalyst for water electrolysis according to the present invention is composed of a base catalyst having oxygen evolution activity and platinum nanoparticles. The base catalyst is a catalyst having oxygen evolution activity that is used as an anode catalyst for water electrolysis in the description of the prior art above. In other words, the anode catalyst for water electrolysis according to the present invention has a configuration in which a base catalyst for water electrolysis is used as a carrier, and platinum nanoparticles, which are additional catalytic components, are supported thereon. The base catalyst and platinum nanoparticles constituting the anode catalyst for water electrolysis according to the present invention will be described below.

[0020] (A-1) Base Catalyst As described above, the base catalyst is a catalyst containing a metal or metal compound that has oxygen-evolving activity, and is a catalyst that has been conventionally known as a water electrolysis catalyst. The base catalyst promotes the anodic reaction in a water electrolysis apparatus based on the oxygen-evolving activity that the anode catalyst for water electrolysis should inherently exhibit. Furthermore, the base catalyst also acts as a support (carrier) for supporting platinum nanoparticles, which are catalysts for hydrogen-water conversion.

[0021] The metals or metal compounds that make up the base catalyst and have higher oxygen evolution activity than platinum are specifically the noble metals iridium (Ir), ruthenium (Ru), rhodium (Rh), palladium (Pd), and compounds of these noble metals iridium oxide (IrO2). ｘ ), ruthenium oxide (RuO ｘ ), rhodium oxide (RhO ｘ ), palladium oxide (PdO ｘ Examples include the following. Compounds of precious metals also include composite oxides of different precious metals, for example, the composite oxide of iridium and ruthenium (IrRuO2). ｘ Other metal compounds are also applicable. Furthermore, as metal compounds with higher oxygen evolution activity than platinum, oxides (MO) of base metals (M) such as manganese, nickel, lead, cobalt, and iron are also applicable. ｘ (MnO ｘ NiO ｘ , PbO ｘ CoO ｘ FeO ｘ The following can be applied: (etc.) Furthermore, the above-mentioned composite oxides of noble and base metals (IrMO) ｘ RuMO ｘ IrRuMO ｘ ) can also be applied.

[0022] As mentioned above, the base catalyst is a catalyst that has been conventionally known as a water electrolytic catalyst. Therefore, as for the composition of the base catalyst, firstly, a catalyst consisting only of the metal or metal compound specifically exemplified above can be mentioned.

[0023] Furthermore, conventional water electrolytic catalysts include those in which particles made of a metal or metal compound with higher oxygen evolution activity than platinum are supported on a carrier made of an inorganic oxide or the like. Such water electrolytic catalysts can also serve as the base catalyst of the present invention. The carrier of the base catalyst in this embodiment only needs to have durability (acid resistance, etc.) in the environment to which the anode catalyst layer is exposed, and does not need to have catalytic activity such as oxygen evolution activity. Examples of such carriers include inorganic oxides such as tin oxide, silicon oxide, zirconium oxide, and titanium oxide. The base catalyst is constructed by supporting particles made of a metal or metal compound with higher oxygen evolution activity than platinum on a carrier made of such an inorganic oxide or the like. In this case, the particles made of a metal or metal compound with higher oxygen evolution activity than platinum that are supported on the carrier consist of the metals and their compounds described above.

[0024] The shape and form of the base catalyst are not particularly limited, but a powder form is preferred. A base catalyst consisting only of a metal or metal compound with higher oxygen evolution activity than platinum will be a powder of the said metal or metal compound (e.g., iridium oxide powder, ruthenium oxide powder). In addition, a base catalyst in which a metal or metal compound with higher oxygen evolution activity than platinum is supported on an inorganic oxide support will be a catalyst powder in which particles of the active component, the metal or metal compound, are supported on the inorganic oxide support powder. In these cases, the particle size of the base catalyst is preferably 0.01 μm or more and 100 μm or less in average particle size. The average particle size of the base catalyst can be measured using various commercially available particle size and particle size distribution analyzers. For example, laser diffraction / scattering particle size distribution analyzers can measure the particle size of various powders in a dry manner.

[0025] Furthermore, there are no particular restrictions on which of the two embodiments of the base catalyst described above should be applied to the present invention. This is because the reduction of hydrogen concentration due to cross-leakage, which is the problem of the invention, is not solved by the composition of the base catalyst, but by the platinum nanoparticles added to the base catalyst, and by the addition form and state (particle size, etc.) described later. Based on this technical significance, there is no need to restrict the selection of the embodiment of the base catalyst. The two embodiments of water electrolytic catalysts described above each have their own advantages. Water electrolytic catalysts consisting only of metals or metal compounds with higher oxygen evolution activity than platinum, such as iridium oxide powder, have excellent properties in terms of water electrolytic activity. In addition, many catalysts of this embodiment are conductive themselves, so they are useful in anode catalyst layers that inherently require conductivity. On the other hand, water electrolytic catalysts in which metals or metal compounds with higher oxygen evolution activity than platinum are supported on a carrier such as an inorganic oxide suppress the amount of active component used and exhibit maximum catalytic activity under those conditions, so they are superior in terms of catalyst cost. In addition, the requirement for conductivity can be addressed by adjusting the carrier composition, etc. Therefore, the selection of the base catalyst form should, as in the past, be based on the specifications, characteristics, and cost requirements of the water electrolysis device.

[0026] Furthermore, the present invention is applicable to both PEM water electrolysis apparatuses and AEM water electrolysis apparatuses. The anode water electrolysis catalyst of the present invention can use an anode water electrolysis catalyst suitably used in each type of water electrolysis apparatus as its base catalyst.

[0027] (A-2) Platinum nanoparticles The anode catalyst for water electrolysis according to the present invention is constructed by supporting the above-described base catalyst as a support and carrying particulate metallic platinum thereon. When the base catalyst consists only of a metal or metal compound with higher oxygen evolution activity than platinum, the platinum nanoparticles are carried on the metal or metal compound. Furthermore, when the base catalyst is an inorganic oxide support on which a metal or metal compound with higher oxygen evolution activity than platinum is carried, the platinum nanoparticles are carried on the inorganic oxide support and / or on the metal or metal compound.

[0028] When platinum (platinum nanoparticles) is added (supported) to a water electrolytic catalyst, if the amount added (supported) is excessive, there is a concern that the proportion of the active component in the entire water electrolytic catalyst will decrease, leading to a decline in oxygen evolution activity. In this invention, in order to effectively convert cross-leaked hydrogen while suppressing the decrease in oxygen evolution activity by limiting the amount of platinum added (platinum supported), the platinum is micronized to increase the active area of ​​the water conversion reaction.

[0029] In this invention, the particle size of the platinum nanoparticles supported on the base catalyst is set to an average particle size of 1 nm or more and 55 nm or less. Platinum nanoparticles with an average particle size exceeding 55 nm will have insufficient activity when converting hydrogen to water. Also, platinum nanoparticles with a particle size of less than 1 nm may dissolve under the operating conditions of the water electrolysis device. The average particle size of the platinum nanoparticles is preferably 1 nm or more and 30 nm or less, more preferably 1 nm or more and 20 nm or less. In this invention, the platinum nanoparticles include not only primary platinum particles but also aggregates of primary particles. The particle size of the platinum nanoparticles is measured for the primary particles and aggregates of primary particles. It is preferable to measure the particle size of the platinum nanoparticles based on observation images obtained by an electron microscope (SEM, TEM, STEM). In this case, for spherical platinum nanoparticles, the diameter is defined as the particle size, and for platinum nanoparticles with shapes other than spherical (elliptical, irregular, etc.), the major axis is defined as the particle size. Furthermore, the average particle size is the arithmetic mean of the particle sizes of a plurality of platinum nanoparticles (preferably 50 or more) arbitrarily selected within the observed image.

[0030] Furthermore, even if the average particle size of platinum nanoparticles falls within the range described above, there is a possibility that some coarse platinum nanoparticles may be present. In order to optimize the hydrogen conversion efficiency of platinum nanoparticles, such coarse particles should be eliminated as much as possible. Therefore, in this invention, the cumulative 90% particle size (D) in the particle size distribution based on the number of platinum nanoparticles is determined. ９０ It is preferable that the cumulative 90% particle size (D) of the platinum nanoparticles be 90 nm or less. ９０ The particle size distribution of these platinum nanoparticles is more preferably 50 nm or less, and even more preferably 30 nm or less. The lower limit is preferably 1 nm or more, the same as the average particle size. The particle size distribution of these platinum nanoparticles can be measured during the particle size measurement of the platinum nanoparticles described above.

[0031] As described above, the present invention provides a catalyst in which platinum nanoparticles of a predetermined average particle size are supported on a base catalyst, which is a water electrolytic catalyst. Since these platinum nanoparticles partially coat the base catalyst, water electrolytic activity may be suppressed in the coated areas. The present invention provides a catalyst that imparts the hydrogen-to-water conversion function to a water electrolytic catalyst, but a reduction in the original function of water electrolytic properties should be avoided.

[0032] From this perspective, the present invention defines a suitable range for the ratio of the total surface area of ​​platinum nanoparticles to the specific surface area of ​​the entire water electrolysis anode catalyst as a platinum coating state that ensures water electrolysis characteristics. Specifically, the specific surface area of ​​the entire water electrolysis anode catalyst is defined as S ｃ Based on the average crystallite size of the platinum nanoparticles, the total surface area of ​​platinum is determined as S. Ｐｔ When this is the case, S ｃ S for Ｐｔ The ratio of platinum specific surface area ratio S is the ratio of platinum specific surface area ratio S. Ｐｔ / S ｃ It is preferable to keep it below 80%.

[0033] The total specific surface area S of the anode catalyst for water electrolysis. ｃ This corresponds to the effective specific surface area for both water electrolysis and hydrogen conversion reactions. The specific surface area S of the entire anode catalyst for water electrolysis. ｃ This can be measured by the gas adsorption method. In the gas adsorption method, an adsorption-desorption isotherm is measured using an inert gas such as nitrogen as the adsorbing gas, and the specific surface area obtained from this is then applied by methods such as the BET multipoint method.

[0034] On the other hand, the total surface area of ​​platinum S Ｐｔ This is the total area of ​​the platinum nanoparticles that cover the base catalyst. Total platinum surface area S Ｐｔ This is calculated based on the average crystallite diameter of the platinum nanoparticles. In this invention, the platinum nanoparticles are primary particles and aggregates of primary particles, and it is preferable to apply the average crystallite diameter of their smallest unit. Also, the total platinum surface area S Ｐｔ Since this represents the area where platinum nanoparticles (crystallites) coat the base catalyst, the calculation assumes hemispherical crystallites.

[0035] Platinum total surface area S Ｐｔ In the calculation, the volume of hemispherical platinum crystallites (in units: m) is obtained from the average crystallite diameter (in units: nm) of platinum. ３ The average crystallite size of platinum can be calculated by measuring the full width at half maximum of the diffraction peak of platinum from the diffraction profile obtained by XRD analysis of the anode catalyst for water electrolysis, and then calculating it based on the Scherrer equation. In addition, the total mass of the anode catalyst for water electrolysis (unit: g), the amount of platinum loaded (unit: mass%), and the theoretical density of platinum (21.45 g / cm³) are used. ３ ) and the volume of the platinum crystallites are used to calculate the number of hemispherical platinum crystallites. Then, the surface area of ​​the hemispherical platinum crystallites (unit: m) is calculated. ３ The total surface area S of platinum is calculated by multiplying this by the number of platinum crystallites mentioned above. Ｐｔ The following is calculated. Based on the above explanation, the total surface area of ​​platinum S Ｐｔ The formula for calculating this is shown below.

[0036]

[0037] S measured and calculated as described above Ｐｔ and S ｃ The ratio of platinum specific surface area S Ｐｔ / S ｃ The following can be calculated: the platinum specific surface area ratio S of the anode catalyst for water electrolysis of the present invention. Ｐｔ / S ｃ It is preferable that the platinum specific surface area ratio S be 80% or less. Ｐｔ / S ｃ An increase in the platinum specific surface area ratio tends to increase the amount of hydrogen converted while decreasing the amount of water electrolysis. To optimize both, the platinum specific surface area ratio S Ｐｔ / S ｃ It is preferable to keep it below 80%. Platinum specific surface area ratio S Ｐｔ / S ｃ It is more preferable to keep it below 60%. Also, the platinum specific surface area ratio S Ｐｔ / S ｃ The lower limit is preferably 7% or more, more preferably 10% or more, and even more preferably 14% or more.

[0038] Furthermore, the total surface area S of the platinum nanoparticles described above. ＰｔThe calculation method involves the amount of platinum supported on the anode catalyst for water electrolysis. The amount of platinum supported is the ratio of the mass of platinum to the total mass of the anode catalyst for water electrolysis. In this invention, the amount of platinum supported is preferably 5% by mass or more and 70% by mass or less. If the amount of platinum supported is less than 5% by mass, even if the platinum nanoparticles are sufficiently refined, there will be insufficient catalyst volume, making it difficult to sufficiently reduce the hydrogen concentration. Furthermore, in anode catalysts for water electrolysis with a platinum support exceeding 70% by mass, the proportion of the base catalyst, the water electrolysis catalyst, becomes too low, making it difficult to ensure water electrolysis activity. The amount of platinum supported is more preferably 5% by mass or more and 40% by mass or less, and even more preferably 10% by mass or more and 35% by mass or less.

[0039] Furthermore, the specific surface area S of the entire anode catalyst for water electrolysis ｃ No particular limitations are necessary regarding this. The specific surface area S of the entire anode catalyst for water electrolysis. ｃ The value of is basically greatly influenced by the specific surface area of ​​the water electrolysis catalyst that serves as the base catalyst. As mentioned above, several types of metals or metal compounds can be used as base catalysts, and they have a wide range of specific surface areas. Furthermore, the water electrolysis characteristics of the base catalyst cannot be determined solely by its specific surface area. Therefore, the specific surface area S of the entire anode catalyst for water electrolysis is also important. ｃ A wide range is set for this. However, considering specific examples of the various base catalysts mentioned above, the specific surface area S of the entire anode catalyst for water electrolysis is ｃ is 5m ２ / g or more 200m ２ A range of less than / g is expected.

[0040] The anode catalyst for water electrolysis according to the present invention is formed by supporting platinum nanoparticles of a predetermined average particle size on the base catalyst described above. Supporting refers to a state in which the platinum nanoparticles are bonded to the base catalyst by physical or chemical adsorption forces such as intermolecular forces.

[0041] (B) Method for Producing the Anode Catalyst for Water Electrolysis According to the Present Invention Next, the method for producing the anode catalyst for water electrolysis according to the present invention will be described. The anode catalyst for water electrolysis according to the present invention is produced by supporting platinum nanoparticles of a predetermined particle size on a base catalyst consisting of various water electrolysis catalysts. In this regard, if it is not necessary to consider the average particle size of the platinum nanoparticles after support, there are no restrictions on the method of supporting the platinum particles on the base catalyst. For example, even by a method of mixing the base catalyst and platinum particles wet or dry (simple mixing), it is possible to support the platinum particles in the sense described above. However, with such simple mixing, there is a concern that the platinum nanoparticles will coarseen due to aggregation during the mixing process, making it difficult to support the platinum nanoparticles in a suitable state. The present inventors consider that a supporting method using platinum particles that have already been atomized in the stage prior to addition (mixing) to the base catalyst cannot form the supported state of platinum nanoparticles that is essential in the present invention. This consideration applies when it is assumed that the platinum nanoparticles to be added are within the range of the average particle size specified in this application.

[0042] Furthermore, in order to form a state in which platinum nanoparticles of a predetermined particle size are supported on the base catalyst after the support process, it is necessary to bond (support) the platinum nanoparticles to the base catalyst at the same time as they are generated. That is, in the method for producing an anode catalyst for water electrolysis according to the present invention, it is preferable to apply a wet reduction method as the method for supporting platinum nanoparticles on the base catalyst. The wet reduction method is a method in which the base catalyst is dispersed in a solution of platinum salt, and a reduction treatment is performed to deposit metallic platinum on the surface of the base catalyst. According to the wet reduction method, the platinum initially added to the base catalyst is platinum ions, and the generation (deposition) of platinum nanoparticles by reduction occurs simultaneously with their support on the base catalyst. This makes it possible to produce an anode catalyst for water electrolysis in which platinum nanoparticles of a specified average particle size are highly dispersed and supported.

[0043] Furthermore, anode catalysts for water electrolysis on which platinum particles have been coarseened by aggregation through simple mixing or the like cause an uneven distribution of platinum in the anode catalyst layer, which affects the activity of hydrogen-water conversion. The wet reduction method applied in this invention makes it possible to obtain anode catalysts for water electrolysis on which platinum nanoparticles are supported in a relatively uniform dispersion state, thus reducing the likelihood of uneven distribution of platinum in the anode catalyst layer. The method for producing an anode catalyst for water electrolysis according to the present invention, including the step of supporting platinum nanoparticles by the wet reduction method, will be described below.

[0044] (B-1) Preparation of the base catalyst First, a base catalyst that will serve as the basis for the anode catalyst according to the present invention is prepared. The embodiment of the water electrolytic catalyst containing a metal or metal compound with higher oxygen evolution activity than platinum, which will serve as the base catalyst, is as described above. The water electrolytic catalyst that will serve as the base catalyst may be procured from commercially available products on the market, or it may be manufactured by a known manufacturing method.

[0045] For example, when iridium oxide powder is used as a base catalyst, the ADAMS method (dry method) has long been known as a method for producing iridium oxide powder. In the ADAMS method, nitrate (NaNO) is added to an iridium chloride solution. ３ A solution containing (etc.) can be evaporated to dryness and then calcined at high temperature to obtain crystalline iridium oxide powder. Another known preferred method for producing iridium oxide powder is the precipitation method. In the precipitation method, an iridium compound solution such as iridium chloride is neutralized with an alkali to obtain hydroxide (Ir(OH) ４ Iridium oxide powder is produced by precipitating and recovering the iridium hydroxide, and then dehydrating and calcining the recovered iridium hydroxide. In the precipitation method, crystalline iridium oxide powder, amorphous iridium oxide powder, or mixtures thereof can be produced by adjusting the drying and calcination conditions. Furthermore, the iridium oxide powder produced in this way has an iridium content of 70% to 90% by mass, which is the metallic component, making it a suitable base catalyst.

[0046] The same or similar wet and dry synthesis methods described above can be applied to the production of base catalysts consisting of metal compounds other than iridium oxide. Furthermore, the same dry methods described above are also known for complex oxides containing noble metals such as iridium. In addition, methods for producing base catalysts (water electrolytic catalysts) in which a metal or metal compound with higher oxygen evolution activity than platinum is supported on a carrier such as an inorganic oxide are also known (Patent Documents 2 and 3), and these can be applied. The metal or metal compound contained in the base catalyst may be entirely crystalline, or partially or entirely amorphous.

[0047] (B-2) Supporting Platinum Nanoparticles on the Base Catalyst A preferred method for supporting platinum nanoparticles on the base catalyst is to reduce a platinum salt solution in which the base catalyst powder is dispersed. Through reduction, suitable platinum nanoparticles precipitate from the platinum salt solution and are supported on the base catalyst. Suitable platinum salts for the platinum salt solution at this time include dinitrodiammineplatinum(II), chloroplatinic acid(II), diamminedinitroplatinum(II) nitric acid solution, hexachloroplatinum(IV) acid hexahydrate, hexaammineplatinum(IV) chloride solution, tetraammineplatinum(II) chloride, and tetraammineplatinum(II) hydroxyl solution. Water is preferred as the solvent. When dispersing the base catalyst in the platinum salt solution, it is also possible to prepare the platinum salt solution by dissolving the platinum salt in the solvent beforehand and then dispersing the base catalyst, or to dissolve the platinum salt after dispersing the base catalyst in the solvent.

[0048] Furthermore, in the reduction treatment for precipitating platinum nanoparticles, it is preferable to add a reducing agent such as ethanol or other alcohol, formic acid, hydrazine, or sodium borohydride to the solution. In this case, it is preferable to heat the solution to a temperature near its boiling point under reflux. Through this reduction treatment, metallic platinum is precipitated from the platinum salt, forming platinum nanoparticles and supporting them on the base catalyst, thereby producing the anode catalyst for water electrolysis according to the present invention. After this, the catalyst may be recovered and washed as appropriate.

[0049] Furthermore, in the anode catalyst for water electrolysis according to the present invention, the average particle size of the platinum nanoparticles supported on the base catalyst is defined, and a preferred configuration of the particle size distribution is clarified. In the method for producing the anode catalyst for water electrolysis according to the present invention based on a wet reduction method, the properties of these platinum nanoparticles can be optimized by adjusting the volume of the platinum salt solution, the concentration of the platinum salt in the platinum salt solution, the type of platinum salt, the type of reducing agent (reducing power), the amount and rate of reducing agent input, the reduction temperature and reduction time, etc. For example, when the platinum salt concentration is adjusted and other conditions are kept the same, the amount of platinum deposited increases with increasing platinum salt concentration, and the amount of platinum supported in the anode catalyst for water electrolysis and the average particle size of the platinum nanoparticles increase.

[0050] (C) Anode catalyst layer and membrane electrode assembly for water electrolysis according to the present invention The anode catalyst for water electrolysis according to the present invention described above forms the anode catalyst layer of a water electrolysis apparatus. The anode catalyst layer, together with the ion exchange membrane, forms a membrane electrode assembly (CCM) for water electrolysis.

[0051] (C-1) Anode catalyst layer The anode catalyst layer can be formed by mixing an anode catalyst for water electrolysis with an ionomer. The ionomer is formed by ions (H) generated by water electrolysis. ＋ , OH － It is an ion conductor that rapidly transfers ions to the ion exchange membrane.

[0052] In a PEM water electrolysis apparatus, known ionomers can be used for mixing with the water electrolytic catalyst of the present invention. Examples of ionomers include fluororesin-based cation exchange resins having sulfone groups, carboxyl groups, and phosphone groups. For example, Nafion® is widely known, and among them, the dispersion solution type can be suitably used. The concentration of the dispersion solution is generally 5% by mass or more and 20% by mass or less, and it is available from the market. In addition, known ionomers can be used in AEM water electrolysis apparatuses as well. Examples include resin-based anion exchange ionomers having cationic groups such as ammonium groups and imidazolium groups.

[0053] The mixing ratio of the anode catalyst and the ionomer is expressed as the mixing ratio per unit area, based on the total mass of the anode catalyst for water electrolysis (mg / cm²). ２ ) and the mass of the ionomer (mg / cm³) ２ The ratio of catalyst component to ionomer is preferably 10:1 or more and 1:1 or less. The anode catalyst layer requires a good balance of conductivity from the anode catalyst and ionic conductivity from the ionomer, so the mixing ratio takes this into consideration.

[0054] Furthermore, the platinum content in the anode catalyst layer is 0.05 mg / cm³. ２ 3.5mg / cm or more ２ The following is preferable: 0.05 mg / cm ２ Below this level, the effect of reducing hydrogen concentration diminishes. 3.5 mg / cm³ ２ Beyond this point, a significant decrease in the electrolytic properties of the water electrolytic catalyst occurs. The platinum content in the anode catalyst layer is 0.15 mg / cm³. ２ 3.0mg / cm or more ２ The following is more preferable. Note that the anode catalyst layer according to the present invention only needs to contain at least a portion of the anode catalyst according to the present invention; it is not necessary to use the anode catalyst according to the present invention for the entire layer. The anode catalyst supported by the platinum nanoparticles of the present invention may be combined with a conventional water electrolytic catalyst (such as iridium oxide powder) to form the anode catalyst layer. Furthermore, multiple water electrolytic catalysts with different platinum supporting amounts may be combined to form an anode catalyst layer in which the amount of platinum changes in a gradient or stepwise manner.

[0055] The thickness of the anode catalyst layer is preferably between 0.5 μm and 50 μm. Manufacturing a catalyst layer thinner than 0.5 μm is difficult, and even if possible, its durability will be poor. On the other hand, increasing the thickness of the catalyst layer increases the number of active sites due to factors such as an increase in the water electrolytic catalyst content, but at the same time, ionic conductivity decreases. Considering the balance between active sites and ionic conductivity, it is preferable to keep the catalyst layer thickness within the aforementioned range.

[0056] (C-2) Membrane electrode assembly for water electrolysis (CCM) A membrane electrode assembly for water electrolysis is formed by joining the anode catalyst layer and the ion exchange membrane described above, and then joining the cathode catalyst layer. The anode catalyst layer of the membrane electrode assembly can be formed, for example, by coating the surface of the ion exchange membrane with a mixed solution containing the anode water electrolysis catalyst.

[0057] The material of the ion exchange membrane is the same as in the conventional technology and there are no particular restrictions. For ion exchange membranes (proton exchange membranes) in PEM water electrolysis devices, fluororesin-based cation exchange membranes having sulfone groups, carboxyl groups, and phosphone groups, which have the same composition as the ionomer of the catalyst layer, are known. For example, Nafion® is widely known, and trade names such as 112, 115, 117, and 450 can be suitably used. Similarly, for ion exchange membranes (anion exchange membranes) in AEM water electrolysis devices, fluororesin-based anion exchange membranes into which cationic groups such as ammonium groups have been introduced are known. For example, Neosepta®, Selemion®, and Sustenion are widely known.

[0058] Regarding the thickness of the ion exchange membrane, conventional technology required increasing the film thickness to suppress cross-leakage. However, increasing the thickness of the ion exchange membrane increases the overpotential generated by the membrane's resistance. Therefore, conventionally, there was a trade-off between addressing cross-leakage and reducing the membrane's overpotential. In contrast, the present invention provides a means for reducing cross-leakage to the anode catalyst, thereby solving the cross-leakage problem while reducing the thickness of the ion exchange membrane. Specifically, in the present invention, the thickness of the ion exchange membrane is preferably 1 μm to 150 μm, more preferably 1 μm to 100 μm, and particularly preferably 1 μm to 50 μm.

[0059] Furthermore, a cathode catalyst layer is formed in the membrane electrode assembly for water electrolysis along with the anode catalyst layer. The cathode catalyst layer is a mixture of a hydrogen generation catalyst and an ionomer. Platinum-supported carbon catalysts (Pt / C catalysts) and the like are used as hydrogen generation catalysts. However, in this invention, the composition and thickness of the cathode catalyst layer are not particularly limited. The cathode catalyst layer can be manufactured simultaneously with the anode catalyst layer during the manufacturing stage of the membrane electrode assembly for water electrolysis. The cathode catalyst layer is also manufactured by coating and pressing a mixture of hydrogen generation catalysts. The membrane electrode assembly for water electrolysis is manufactured by coating the mixture that will become the anode catalyst layer and the cathode catalyst layer on each surface of an ion exchange membrane as a substrate and simultaneously hot-pressing it.

[0060] A membrane electrode assembly for water electrolysis, together with components such as a power supply, constitutes a single electrolytic cell. The power supply uses a structure that is conductive while allowing the flow of gas and liquid. Examples include fiber sintered bodies of metals such as Ti, Ta, and stainless steel, powder sintered bodies, expanded metal, metal mesh, and porous bodies (sponge metal), and these are generally plated with highly corrosion-resistant metals (such as Pt or Au).

[0061] As described above, the anode catalyst for water electrolysis according to the present invention is composed of supporting fine particles of metallic platinum on a water electrolysis catalyst such as iridium oxide powder, and exhibits both water electrolysis activity and hydrogen-to-water conversion activity. According to the present invention, hydrogen gas that has entered the anode due to cross-leakage can be effectively converted to water, and the hydrogen concentration in the anode can be reduced. The anode catalyst for water electrolysis according to the present invention ensures safety in the operation of a water electrolysis device by solving the problem of cross-leaked hydrogen gas while maintaining water electrolysis activity at the same level as the conventional technology.

[0062] STEM image of the water electrolysis anode catalyst of Example 2. Graph showing the measurement results of the particle size distribution of platinum nanoparticles supported on the water electrolysis anode catalysts of Examples 1, 2, and 5. XRD diffraction patterns of the water electrolysis anode catalysts and base catalysts of Examples 1, 2, and 5. STEM image of the water electrolysis anode catalyst manufactured as Comparative Example 2. Figure showing the relationship between the platinum content in the anode catalyst layer, the cell potential, and the hydrogen concentration in the anode, obtained from the water electrolysis test results in Examples 1 to 7, Reference Example 1, and Comparative Example 1.

[0063] Next, embodiments of the present invention will be described. In this embodiment, iridium oxide powder (IrO2) is used as the water electrolytic catalyst that serves as the base catalyst. ｘ We applied a powder to produce an anode catalyst for water electrolysis in which platinum nanoparticles were supported on a base catalyst. Then, we constructed a PEM water electrolysis apparatus using the produced anode catalyst as the anode electrode, and confirmed the water electrolysis performance and the effect of reducing hydrogen concentration due to cross-leakage.

[0064] As the base catalyst, iridium oxide powder was prepared from commercially available products (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., product name: TEC77100, iridium concentration 74.10% by mass, average particle size 3.52 μm).

[0065] In the process of supporting platinum nanoparticles onto iridium oxide powder (base catalyst), first, 100 g of iridium oxide powder was added to approximately 4 L of pure water to prepare a dispersion of iridium oxide powder. To this dispersion of iridium oxide powder, a dinitrodiammineplatinum(II) aqueous solution with a platinum concentration of 13% by mass (Pt[(NH ３ ) ２ (NO ２ ) ２ A predetermined amount of [aqueous solution] was added to the platinum salt solution to disperse the iridium oxide powder. Then, 0.5 L of ethanol was added as a reducing agent, and the solution was refluxed near its boiling point to perform a reduction treatment. Platinum was deposited onto the iridium oxide powder by the reduction treatment, thereby supporting platinum nanoparticles. After the reflux reaction, the platinum-supported iridium oxide powder was filtered, recovered, and washed, and this was used as an anode catalyst for water electrolysis.

[0066] In this embodiment, eight types of anode catalysts for water electrolysis were produced by adjusting the amount of dinitrodiammineplatinum(II) aqueous solution added to the dispersion of iridium oxide powder, resulting in different amounts of platinum supported on the iridium oxide powder and different average particle sizes of platinum nanoparticles. Specifically, the amounts of dinitrodiammineplatinum(II) aqueous solution added were 60 mL (Example 1), 150 mL (Example 2), 250 mL (Example 3), 350 mL (Example 4), 550 mL (Example 5), 1300 mL (Example 6), 1650 mL (Example 7), and 2750 mL (Reference Example 1).

[0067] [Analysis of the composition and particle size measurement of the anode catalyst for water electrolysis] For the anode catalysts for water electrolysis manufactured in Examples 1 to 7 and Reference Example 1, the amount of platinum supported and the specific surface area of ​​the catalyst were measured. For the analysis of the amount of platinum supported in the anode catalyst for water electrolysis, the catalyst was completely dissolved and the platinum concentration was measured using ICP-OES (analytical instrument: iCAP PRO manufactured by Thermo Scientific). For the measurement of the specific surface area, N ２ The BET specific surface area was determined by the BET multipoint method after analysis using the gas adsorption method (analytical instrument: MICROTRAC BEL BELSORP MINIx).

[0068] Furthermore, STEM observation was performed on arbitrary locations on the catalyst surface of the anode catalysts for water electrolysis in each example. In the STEM image, platinum nanoparticles and iridium oxide can be distinguished by the intensity of the color. As an example, Figure 1 shows the STEM image of the anode catalyst for water electrolysis in Example 2. In Figure 1, the black particles are platinum, and the gray particles are iridium oxide. In the anode catalyst for water electrolysis in Example 2, a large number of minute platinum nanoparticles with a particle size of several nanometers or less, which appear to be composed of primary particles, are observed. Aggregates consisting of multiple primary particles in contact at least at one location are also observed. These platinum nanoparticles made up of aggregates have a particle size of several nanometers to tens of nanometers, but their number is small. The anode catalyst for water electrolysis is formed by dispersing and supporting these platinum nanoparticles on the surface of iridium oxide powder. The anode catalysts for water electrolysis in Examples 1, 3 to 7, and Reference Example 1 were also observed in the same way as in Example 2, and similar morphologies were found.

[0069] In this embodiment, 50 platinum fine particles (primary particles and aggregates of primary particles) in the STEM image of the anode catalyst for water electrolysis of each example were arbitrarily selected and their particle diameters were measured. In the measurement of the particle diameter, referring to the scale of the STEM image, for spherical or substantially spherical platinum fine particles mainly seen in primary particles, the diameter was measured and taken as the particle diameter. Also, for elliptical and irregular platinum fine particles mainly seen in aggregates of primary particles, the major axis (maximum length) was measured and taken as the particle diameter. Then, the particle diameters of the 50 selected platinum fine particles were measured, the average particle diameter, which is the arithmetic mean thereof, was calculated, and a particle size distribution based on the number was created. FIG. 2 shows the particle size distribution based on the number of platinum fine particles in the anode catalysts for water electrolysis of Example 1, Example 2, and Example 5. And Table 1 shows the analysis results and measurement results for the anode catalysts for water electrolysis of Examples 1 to 7 and Reference Example 1.

[0070]

[0071] Referring to Table 1, when the reaction conditions for platinum loading are the same, as the amount of the aqueous solution of dinitrodiammineplatinum(II) added to the base catalyst (iridium oxide powder), that is, as the platinum loading amount increases, the average particle diameter and the cumulative 90% particle diameter of the supported platinum fine particles tend to increase. Also, the platinum specific surface area ratio S Ｐｔ / S ｃ also tends to increase. In the anode catalysts for water electrolysis of Examples 1 to 7, the average particle diameter of the platinum fine particles was 55 nm or less, and the cumulative 90% particle diameter of the platinum fine particles was 90 nm or less. Also, the value of the platinum specific surface area ratio S Ｐｔ / S ｃ was also within the range of 80% or less. In Reference Example 1 with the maximum platinum loading amount, the particle diameter of the supported platinum fine particles exceeded 100 nm, and the cumulative 90% particle diameter was also quite large.

[0072] Figure 3 also shows the XRD diffraction patterns of the anode catalysts for water electrolysis in Examples 1, 2, and 5, and the iridium oxide powder without platinum support, which is the base catalyst. In the diffraction patterns of the water electrolysis catalysts in Examples 1, 2, and 5, a peak is observed near 2θ = 40° that is not seen in the base catalyst. This is the peak of platinum nanoparticles supported on the iridium oxide powder. This peak of platinum nanoparticles is observed in the water electrolysis catalysts in Examples 1, 2, and 5, and becomes more clearly visible with increasing platinum support. The XRD diffraction patterns of the anode catalysts for water electrolysis in Examples 3, 4, 6, 7, and Reference Example 1 showed a similar trend.

[0073] In this embodiment, the full width at half maximum (FWHM) of the Pt diffraction peak near 2θ = 40° in the XRD diffraction patterns of the water electrolysis anode catalysts for Examples 1 to 7 and Reference Example 1 was measured. At this time, the diffraction peaks were separated by waveform as needed to measure the FWHM. The average crystallite diameter of Pt was then calculated from the measured FWHM using the Scherrer equation. Furthermore, the platinum specific surface area ratio S of each water electrolysis anode catalyst was calculated using the average crystallite diameter of Pt and the above-mentioned equation 1. Ｐｔ / S ｃ They sought it.

[0074] [Manufacturing of Water Electrolysis Membrane Electrode Assembly (CCM)] Next, water electrolysis membrane electrode assembly (CCM) was manufactured using the water electrolysis anode catalysts of Examples 1 to 7 and Reference Example 1. For each example, the water electrolysis catalyst and ionomer (Aldrich Chemicals' Nafion dispersion (DE520)) were mixed in a mass ratio of 1:0.25 to produce a catalyst ink. During mixing, the catalyst powder and ionomer were mixed while being crushed, pulverized, and stirred using a bead mill (pulverizing medium: 2 mm).

[0075] Then, the catalyst ink from each example was applied onto a 50 μm thick proton exchange film (NR212, dimensions: 70 mm x 70 mm). The ink was applied using a spray coater, with an Ir coating amount of 1.0 mg / cm². ２ It was applied in this manner.

[0076] As described above, an anode catalyst layer was formed on one surface of the proton exchange membrane, and an ink obtained by mixing a commercially available platinum catalyst and an ionomer was applied by a spray coater to the other surface of the proton exchange membrane for forming a cathode catalyst layer (Pt coating amount: 1.0 mg / cm ２ ).

[0077] After applying the catalyst ink to both surfaces of the proton exchange membrane as described above, pressure bonding was performed using a hot press device. The conditions for pressure bonding at this time were a temperature of 120°C, a pressing force of 50 kgf / cm ２ , and pressure application and heating were performed for 30 minutes. Thereby, a CCM with an anode catalyst layer thickness of about 10 μm and a cathode catalyst layer thickness of about 20 μm was manufactured.

[0078] Comparative Example 1: In this embodiment, in addition to the CCMs applying the anode catalysts for water electrolysis of the above Examples 1 to 7 and Reference Example 1, for comparison, iridium oxide powder as a base catalyst was used as Comparative Example 1, and a CCM applying this anode catalyst for water electrolysis was manufactured. The manufacturing conditions of the CCM according to Comparative Example 1 were the same as those of each example.

[0079] Comparative Example 2: Further, in this embodiment, an anode catalyst for water electrolysis applying simple mixing of platinum powder was manufactured as a method for adding platinum to the base catalyst, and a CCM applying this was manufactured. This is taken as Comparative Example 2.

[0080] The anode catalyst for water electrolysis of Comparative Example 2 used the same iridium oxide powder as that of Examples 1 to 7 and Reference Example 1 as the base catalyst, and commercially available platinum powder (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., primary particle diameter: 8.0 nm) was mixed as the platinum powder. At this time, the amounts of the respective powders were adjusted so that the platinum mass with respect to the entire catalyst after mixing was about 15% by mass, the same as that of Example 2. Further, at the time of mixing, iridium oxide powder and platinum powder were put into a dispersion liquid containing the same ionomer as described above, and pulverized and stirred with a bead mill (pulverizing medium: 2 mm).

[0081] Figure 4 shows a STEM image of the anode catalyst for water electrolysis in Comparative Example 2, observed under the same conditions as in each example. From Figure 4, it can be observed that in the anode catalyst for water electrolysis produced by simple mixing of the base catalyst and platinum powder, only platinum particles that have been coarsely enlarged due to excessive aggregation are present on the base catalyst. The average particle size of the platinum particles in the anode catalyst for water electrolysis in Comparative Example 2 was measured using the same method as in the examples and was found to be 56.1 nm. Furthermore, the cumulative 90% particle size of the platinum nanoparticles in the anode catalyst for water electrolysis in Comparative Example 2 was 100 nm. From the observation results of Comparative Example 2, it can be seen that when platinum powder is simply mixed (wet mixing in Comparative Example 2), there is a tendency for the platinum particles to become coarser due to excessive aggregation. In addition, for the anode catalyst for water electrolysis in Comparative Example 2, the full width at half maximum of the Pt diffraction peak (near 2θ = 40°) in the XRD diffraction pattern was measured and the average crystallite diameter of Pt was calculated from the Scherrer equation. Furthermore, the platinum specific surface area ratio S was calculated from the average crystallite diameter of Pt. Ｐｔ / S ｃ The value of was calculated.

[0082] For the water electrolysis anode catalyst in Comparative Example 2, the dispersion obtained by the wet mixing process described above was used as an ink for manufacturing the anode catalyst layer, and CCM was manufactured under the same conditions as in each example.

[0083] [Evaluation of Hydrogen Concentration Reduction Effect and Water Electrolysis Characteristics] A water electrolysis cell for evaluation was fabricated by incorporating Pt-plated Ti fiber sintered bodies as power supply elements on both sides of the CCM manufactured above. Water electrolysis treatment was performed in this water electrolysis cell, and the electrolysis characteristics and hydrogen concentration at the anode were measured. The conditions for water electrolysis treatment were a current density of 2.0 A / cm². ２ The electrolysis temperature was set to 50°C. Measurements were taken at the stage when the hydrogen concentration stabilized after the start of electrolysis and the cell potential and the hydrogen concentration in oxygen at the anode were measured. The hydrogen concentration was measured using a semiconductor-type hydrogen detector (GD-D58-AC). The evaluation results of the water electrolysis catalysts for Examples 1 to 7, Reference Example 1, and Comparative Examples 1 and 2 are shown in Table 2.

[0084]

[0085] Referring to Table 2, we will examine the evaluation results of each water electrolysis anode catalyst. First, looking at the results for Comparative Example 1, a water electrolysis catalyst consisting of iridium oxide powder (base catalyst) without platinum addition, the hydrogen concentration measured at the anode after water electrolysis treatment was 2000 ppm or higher, confirming that hydrogen intrusion occurred due to cross-leakage. In Examples 1 to 7, Reference Example 1, and Comparative Example 2, the hydrogen concentration at the anode was reduced in all cases, confirming that the addition of platinum to the water electrolysis catalyst contributes to the reduction of hydrogen concentration in the anode due to cross-leakage.

[0086] However, when comparing Example 2 and Comparative Example 2, which have almost the same amount of supported platinum but different addition methods, the hydrogen concentration reduction rate of the water electrolysis anode catalyst in Example 2 is clearly higher. Therefore, it was found that Example 2 is optimal as a water electrolysis anode catalyst for reducing hydrogen concentration by cross-leakage. In this regard, Comparative Example 2 is a water electrolysis anode catalyst in which a simple mixture of platinum powder is applied as the form of platinum addition. As explained in the STEM observation results above, in a simple mixture of platinum powder and base catalyst, the aggregation of platinum particles is large and coarse platinum particles are formed. It is thought that the hydrogen concentration reduction rate was smaller than in Example 2 due to this coarsening of the particle size of the platinum particles.

[0087] In Examples 1 to 7 and Reference Example 1, which involve supporting platinum onto iridium oxide powder, it can be seen that increasing the amount of supported platinum, i.e., increasing the average particle size and the cumulative 90% particle size, increases the hydrogen concentration reduction effect. On the other hand, increasing the average particle size and the cumulative 90% particle size of the platinum nanoparticles increases the cell potential. Considering both the hydrogen concentration reduction rate and the cell potential increase rate, the CCMs of Examples 1 to 7 can reduce the cross-leak hydrogen concentration while keeping the cell potential increase rate to less than 10% compared to the CCM of Comparative Example 1 without platinum addition. In particular, the water electrolysis anode catalysts of Examples 1 to 4 showed a cell potential increase rate of less than 1%. These water electrolysis anode catalysts can be said to completely achieve the prevention of a decrease in activity as a water electrolysis catalyst. In Reference Example 1, the cell potential increase rate was 18.4%. Therefore, if cell potential is considered important, it is presumed that it is preferable to adjust the amount of platinum supported to bring the average particle size and cumulative 90% particle size of the platinum nanoparticles into a suitable range.

[0088] [Investigation of Platinum Content in the Anode Catalyst Layer] Finally, we will investigate the relationship between the platinum content in the anode catalyst layer and the water electrolysis characteristics and hydrogen conversion characteristics of the anode catalyst layer. Figure 5 is a graph created based on the results of water electrolysis tests using the anode catalyst for water electrolysis manufactured in this embodiment. The graph in Figure 5 shows the unit area (cm²) of the anode catalyst layer, calculated from the amount of catalyst used, platinum content, and area of ​​the anode catalyst layer of the anode catalyst for water electrolysis in Examples 1 to 7, Reference Example 1, and Comparative Example 1. ２ Platinum content per unit (mg / cm³) ２ The horizontal axis represents the platinum content. The vertical axis shows the cell potential and hydrogen concentration in the anode, measured in water electrolysis tests using anode catalyst layers with different platinum content.

[0089] In Figure 5, considering the balance between the cell potential and hydrogen concentration of the anode catalyst layer, the platinum content of the anode catalyst layer that is considered to exhibit suitable characteristics is 3.5 mg / cm³. ２ The following can be assumed (the region enclosed by the thick solid line in Figure 5, with a lower limit of 0.05 mg / cm² being the preferred range for platinum content). ２(This is set to [value]). Regarding the applicability of the water electrolytic catalyst of the present invention in the anode catalyst layer, we believe that suitable results can be obtained both when applied to the entire catalyst layer and when applied to a part of the catalyst layer, by considering the platinum content of the anode catalyst layer.

[0090] The present invention provides a water electrolytic catalyst comprising a base catalyst containing a metal or metal compound applicable as a water electrolytic catalyst, on which fine platinum nanoparticles are supported. In this invention, hydrogen that moves and enters the anode through the ion exchange membrane via cross-leak is converted into water by oxygen on the anode side, thereby reducing the hydrogen concentration. According to this invention, changes in electrolytic properties due to cross-leak can be suppressed, as can the risk of ignition and explosion due to an increase in hydrogen concentration in the anode side atmosphere.

[0091] The water electrolysis hydrogen production process is an important technology that can promote the transition to a carbon-neutral society in conjunction with other renewable energy sources. The present invention is a technology that enhances the safety of such water electrolysis processes while ensuring their efficiency, and will greatly contribute to the widespread adoption of water electrolysis processes. The anode water electrolytic catalyst according to the present invention and the CCM to which it is applied are effective in solid electrolyte type water electrolysis systems and are applicable to both proton exchange water electrolysis devices (PEMs) and anion exchange water electrolysis devices (AEMs).

Claims

1. An anode catalyst for water electrolysis comprising a base catalyst having oxygen-evolving activity on which platinum nanoparticles are supported, wherein the base catalyst contains a metal or metal compound having higher oxygen-evolving activity than platinum, and the platinum nanoparticles have an average particle diameter of 1 nm or more and 55 nm or less.

2. The anode catalyst for water electrolysis according to claim 1, wherein the cumulative 90% particle diameter in the particle size distribution based on the number of platinum nanoparticles is 90 nm or less.

3. The specific surface area of ​​the entire anode catalyst for water electrolysis is S. ｃ Based on the average crystallite size of the platinum nanoparticles, the total surface area of ​​platinum is determined as S. Ｐｔ When this is the case, S ｃ S for Ｐｔ The ratio of platinum specific surface area ratio S is the ratio of platinum specific surface area ratio S. Ｐｔ / S ｃ A water electrolysis anode catalyst according to claim 1 or claim 2, wherein the content is 7% or more and 80% or less.

4. The anode catalyst for water electrolysis according to claim 1 or claim 2, wherein the amount of platinum supported relative to the total mass of the anode catalyst for water electrolysis is 5% by mass or more and 70% by mass or less.

5. The anode catalyst for water electrolysis according to claim 1 or claim 2, wherein the base catalyst is a powdered catalyst consisting only of a metal or metal compound that has higher oxygen evolution activity than platinum.

6. The anode catalyst for water electrolysis according to claim 1 or claim 2, wherein the base catalyst comprises iridium and / or ruthenium.

7. A water electrolysis anode catalyst layer comprising the water electrolysis anode catalyst described in either claim 1 or claim 2, wherein the water electrolysis anode catalyst and an ionomer are mixed.

8. Platinum content is 0.05 mg / cm³ ２ 3.5mg / cm or more ２ The anode catalyst layer for water electrolysis according to claim 7, which is as follows:

9. A membrane electrode assembly for water electrolysis comprising an anode catalyst layer for water electrolysis according to claim 7 or claim 8, and an ion exchange membrane in contact with the anode catalyst layer for water electrolysis.

10. The membrane electrode assembly for water electrolysis according to claim 9, wherein the thickness of the ion exchange membrane is 1 μm or more and 150 μm or less.

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