Catalyst powder, catalyst layer, catalyst coating film, and method for producing catalyst powder

Abstract

This invention provides a catalyst powder that combines high initial performance and long-term durability as a catalyst, while also enabling a further reduction in the amount of iridium used. [Solution] A catalyst powder comprising a carrier and a coating covering the surface of the carrier, wherein the coating contains iridium and the coefficient of variation CV of the thickness of the coating is 60% or less.

Description

[Technical Field]

[0001] The present invention relates to catalyst powder, catalyst layer, catalyst coating film, and a method for producing catalyst powder. [Background technology]

[0002] In recent years, with the widespread adoption of renewable energy, the importance of hydrogen as an energy carrier has been increasing. Water electrolysis technology, which produces hydrogen by electrolyzing water, is attracting attention as a core technology for producing green hydrogen. Among these, proton exchange membrane water electrolysis (PEMWE) is being actively developed because of its high energy density and excellent responsiveness to voltage fluctuations.

[0003] The proton exchange membrane (also called a proton conductive membrane) used in water electrolysis is generally used with an anode catalyst coated on one side and a cathode catalyst on the other. A proton exchange membrane coated in this way is called a catalyst-coated membrane (CCM) and is an important factor in determining the performance of a water electrolysis device.

[0004] Here, among the catalysts mentioned above, platinum-supported carbon, which has high activity for the hydrogen evolution reaction, is widely used as the cathode catalyst. On the other hand, iridium-based catalysts such as iridium oxide, which have excellent activity for the oxygen evolution reaction, are generally used as the anode catalyst.

[0005] However, iridium is an especially rare element among the platinum group elements, with an annual production of only about 6-7 tons. As a result, iridium is extremely expensive, which is a factor driving up the cost of water electrolysis equipment and the hydrogen produced by it.

[0006] Given these circumstances, reducing the amount of iridium used as a catalyst has become an extremely important issue.

[0007] Attempts have been made to address this problem by reducing the usage amount of the anode catalyst itself. However, simply reducing the usage amount of the anode catalyst will result in an excessively thin catalyst layer formed on the proton exchange membrane, making it difficult to form a uniform catalyst layer. Furthermore, since iridium and its oxides are substances with high density (low bulk), when the usage amount is low, sufficient conduction paths cannot be secured inside the catalyst layer. For this reason, it has been difficult to significantly reduce the usage amount of iridium.

[0008] Accordingly, in order to secure the bulk (volume) of the catalyst even with a small amount of iridium, it has been proposed to use a supported catalyst in which a material having catalytic activity is adhered (supported) on the surface of a support (core particle) (Patent Documents 1 to 4). By using particles made of a relatively inexpensive material such as titanium oxide as the support, it becomes possible to reduce the usage amount of expensive iridium while securing the catalyst bulk. Note that when the catalyst material supported on the surface of the support forms a layer (shell), it is also called a core-shell catalyst. PRIOR ART DOCUMENTS PATENT DOCUMENTS

[0009] [Patent Document 1] Japanese National Publication of International Patent Application No. 2007-514520 [Patent Document 2] Japanese National Publication of International Patent Application No. 2022-540820 [Patent Document 3] Japanese National Publication of International Patent Application No. 2024-500218 [Patent Document 4] International Publication No. WO 2022 / 210700 SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0010] However, as a result of studies conducted by the present inventors, it has been found that conventional supported catalysts cannot provide sufficient durability performance.

[0011] In other words, catalysts used in electrochemical devices such as water electrolysis equipment require not only excellent initial performance but also high durability to maintain that performance even during long-term operation. No matter how good the initial performance may be, a catalyst that deteriorates significantly over time cannot be considered practical.

[0012] This invention has been made in view of the above circumstances, and aims to provide a catalyst powder that combines high initial performance and long-term durability as a catalyst, and that can further reduce the amount of iridium used. [Means for solving the problem]

[0013] The inventors of this invention conducted studies to achieve the above objectives and obtained the following findings.

[0014] (1) Conventional catalysts, such as those proposed in Patent Documents 1 to 4, were all prepared by adding alkali to a dispersion containing a support and an iridium salt, thereby precipitating iridium oxide on the surface of the support. The catalyst powder obtained by this method had poor coating (shell) uniformity, with some iridium oxide precipitating in clumps.

[0015] (2) Conversely, by preparing a dispersion in which a support is dispersed in a solution of an iridium compound, and then dropping the dispersion into an alkaline solution, a core-shell type catalyst powder with an extremely uniform coating thickness can be produced. The catalyst powder obtained in this way possesses both high initial performance and long-term durability.

[0016] This invention was completed based on the above findings, and its gist is as follows.

[0017] [1] Carrier and It consists of a coating that covers the surface of the carrier, The aforementioned coating contains iridium, A catalyst powder having a coefficient of variation CV of the thickness of the coating of 60% or less.

[0018] [2] The catalyst powder according to [1] above, wherein the support comprises an oxide containing at least one selected from the group consisting of Ti, Ce, Ta, Nb, W, Ru, Sn, Sr, and Ba.

[0019] [3] The catalyst powder according to [1] or [2] above, wherein the carrier is a nonconductive oxide.

[0020] [4] The catalyst powder according to any one of [1] to [3] above, wherein the support is a crystal having a rutile-type structure.

[0021] [5] The catalyst powder according to any one of [1] to [4] above, wherein the coating contains at least one selected from the group consisting of metallic iridium, iridium oxide, iridium hydroxide, and iridium hydroxide oxide.

[0022] [6] The catalyst powder according to any one of [1] to [5] above, wherein the coating is a crystal having a rutile-type structure.

[0023] [7] The catalyst powder according to any one of [1] to [6] above, wherein the coating is epitaxially grown on the surface of the carrier.

[0024] [8] The average thickness t of the coating Ave However, the catalyst powder is 5.0 nm or less, as described in any one of the above [1] to [7].

[0025] [9] A catalyst powder according to any one of the above [1] to [8], wherein the Ir concentration is 50 wt% or less.

[0026]

[10] Of the iridium contained in the coating, trivalent iridium Ir 3+ Tetravalent iridium Ir 4+ Ratio Ir 4+ / Ir 3+is 0.5 to 4.0, the catalyst powder according to any one of the above [1] to [9].

[0027]

[11] The BET specific surface area S is 1 to 50 m 2 / g, the catalyst powder according to any one of the above [1] to

[10] .

[0028]

[12] The BET specific surface area S per 1 g of iridium element Ir is 20 to 500 m 2 / g Ir , the catalyst powder according to any one of the above [1] to

[11] .

[0029]

[13] The catalyst powder according to any one of the above [1] to

[12] , wherein Pt is further supported on the surface of the coating.

[0030]

[14] A catalyst layer comprising the catalyst powder according to any one of the above [1] to

[13] and an ionomer.

[0031]

[15] The catalyst layer according to the above

[14] , wherein a ratio I / C of a mass I of the ionomer to a mass C of the catalyst powder is 0.01 to 0.30.

[0032]

[16] A catalyst-coated membrane comprising an ion exchange membrane and the catalyst layer according to the above

[14] or

[15] coated on at least one surface of the ion exchange membrane.

[0033]

[17] A method for producing catalyst powder, comprising: preparing a dispersion liquid in which a carrier is dispersed in a solution of an iridium compound, dropping the dispersion liquid into an alkaline solution to precipitate a coating containing Ir on the surface of the carrier, separating a solid content from the solution, drying the solid content to obtain catalyst powder, the method for producing catalyst powder.

Effect of the Invention

[0034] The catalyst powder of the present invention has a core-shell structure consisting of a carrier and an iridium-containing coating covering the surface of the carrier, thereby significantly reducing the amount of iridium used. In addition, the catalyst powder of the present invention combines high initial performance with long-term durability. For this reason, the catalyst powder of the present invention can be used very suitably in a variety of applications, including water electrolysis. [Modes for carrying out the invention]

[0035] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the embodiments described below.

[0036] [Catalyst powder] In one embodiment of the present invention, the catalyst powder comprises a carrier and a coating covering the surface of the carrier, wherein the coating contains iridium.

[0037] (carrier) The aforementioned carrier can be any carrier without any particular limitations.

[0038] The material of the support is not particularly limited. In one embodiment of the present invention, a support made of an oxide containing at least one selected from the group consisting of Ti, Ce, Ta, Nb, W, Ru, Sn, Sr, and Ba can be used. The oxide may be an oxide of a single element selected from the group, or a composite oxide of multiple elements. Examples of oxides of a single element include titanium oxide, cerium oxide, tantalum oxide, niobium oxide, tungsten oxide, ruthenium oxide, and tin oxide. Examples of composite oxides include perovskite oxides represented by the compositional formula ABO3, where A and B are elements selected from the group. Examples of perovskite oxides include barium titanate (BaTiO3). Among these, titanium oxide is preferably used as the support.

[0039] The presence or absence of conductivity of the aforementioned carrier is not particularly limited, but it is typically a non-conductive oxide. However, even when using an oxide as described above as a carrier, it may have some degree of conductivity depending on the state of impurities and defects in the oxide. In the present invention, a carrier made of such a conductive oxide can be used without any problems.

[0040] The carrier may be crystalline or amorphous, or it may have a structure in which crystalline and amorphous materials are mixed. From the viewpoint of further improving durability, the carrier is preferably crystalline.

[0041] If the support is crystalline, its crystal structure is not particularly limited. The support may be, for example, a rutile-type or anatase-type crystal. However, from the viewpoint of further improving durability, it is preferable that the support be a rutile-type crystal.

[0042] The carrier is more preferably rutile-type titanium oxide.

[0043] The particle size of the carrier is not particularly limited, and a carrier of any size can be used. However, if the average particle size is excessively small, the specific surface area increases, and as a result, the amount of iridium required for coating increases. Also, if the average particle size is excessively large, coating may become difficult. For this reason, an average particle size of 5 nm to 10 μm is preferable. The average particle size is more preferably 20 nm to 10 μm, and even more preferably 50 nm to 1 μm.

[0044] The average particle size of the carrier can be measured by STEM (scanning transmission electron microscope). Specifically, first, a sample for measurement is taken from the carrier powder and dispersed in a dispersion medium such as alcohol or water to make a dispersion. Next, the dispersion is dropped onto a mesh serving as a sample stage, and the carrier is fixed on the sample stage by evaporating the dispersion medium. The fixed carrier is observed using STEM, and the average equivalent circle diameter of 50 or more particles is taken as the average particle size of the carrier.

[0045] The BET specific surface area of ​​the carrier is not particularly limited and may be any value. However, if the specific surface area is excessively large, the amount of iridium required for coating will increase. On the other hand, if the specific surface area is excessively small, the particle size will be large, which may make coating difficult. Therefore, the BET specific surface area of ​​the carrier should be between 0.1 and 30 m². 2 It is preferable that the value is / g. The BET specific surface area of ​​the carrier can be measured by measuring the amount of adsorption using a gas adsorption method with N2 gas and analyzing it using the BET method.

[0046] (coating) The catalyst powder of the present invention has a core-shell structure in which the surface of the support is covered with a coating. Any coating containing iridium can be used as the coating.

[0047] The state of iridium contained in the coating is not particularly limited and may be metallic iridium or an iridium compound. Examples of the iridium compound include iridium oxide, iridium hydroxide, and iridium hydroxide oxide. The iridium compound is preferably iridium oxide. The coating preferably contains one or both of metallic iridium and an iridium compound, and more preferably consists of one or both of metallic iridium and an iridium compound.

[0048] The coating may be crystalline or amorphous, or it may have a mixed structure of crystalline and amorphous materials. From the viewpoint of further improving durability, the coating is preferably crystalline, and more preferably rutile crystal.

[0049] The coating is more preferably made of rutile-type iridium oxide.

[0050] Preferably, the coating is epitaxially grown on the surface of the support. In other words, preferably, the coating is an epitaxial layer. When the coating is epitaxially grown, even higher durability can be obtained. This is thought to be because the coating has the same crystalline structure as the support, which suppresses the degradation (dissolution and migration) of the coating.

[0051] • Coefficient of variation (CV): 60% or less As described above, the inventors have found that the durability of a core-shell type catalyst can be improved by making the coating thickness uniform. Therefore, in the catalyst powder of the present invention, the coefficient of variation CV of the coating thickness is set to 60% or less, preferably 50% or less. The lower limit of the coefficient of variation CV is not particularly limited, but is typically 10% or more, and may also be 15% or more.

[0052] The coefficient of variation CV is a value defined by the following formula and can be measured by STEM (scanning transmission electron microscope). More specifically, it can be determined by the method described in the examples. CV(%) = σ / t Ave ×100 Here, σ: standard deviation of coating thickness, t Ave This is the average thickness of the coating.

[0053] The average thickness t of the coating Ave While not limited to this, from the perspective of further reducing iridium usage, it is desirable to make it as thin as possible. Therefore, the average thickness t Ave The thickness is preferably 5.0 nm or less, more preferably 3.0 nm or less, even more preferably less than 1.5 nm, and most preferably 1.3 nm or less. On the other hand, the average thickness t Ave The lower limit is also not limited, but for example, it may be 0.5 nm or greater.

[0054] The average thickness t of the coating AveThis can be measured using a STEM (scanning transmission electron microscope). More specifically, it is measured using the method described in the examples.

[0055] The Ir concentration of the catalyst powder is not particularly limited. However, the lower the Ir concentration, the greater the effect of reducing the amount of iridium used. For this reason, the Ir concentration of the catalyst powder is preferably 50 wt% or less, more preferably 25 wt% or less, even more preferably 15 wt% or less, and most preferably less than 5 wt%. On the other hand, the lower limit of the Ir concentration is also not limited, but from the viewpoint of improving the catalytic properties, it is preferably 0.1 wt% or more, more preferably 0.5 wt% or more, even more preferably 1.0 wt% or more, and most preferably 3.0 wt% or more.

[0056] • Iridium valency The valency of the iridium contained in the coating is not particularly limited. However, since the oxidation-reduction reaction of iridium is involved in the catalytic action of the oxygen evolution reaction, trivalent iridium (Ir) is used. 3+ Tetravalent iridium Ir 4+ The atomic ratio of Ir 4+ / Ir 3+ By adjusting the Ir, catalytic performance can be further improved. Specifically, the higher the proportion of trivalent iridium (the lower the proportion of tetravalent iridium), the higher the catalytic activity. Therefore, to further improve catalytic performance (initial performance), Ir 4+ / Ir 3+ It is preferable that the ratio is 4.0 or less, and more preferably 3.5 or less. On the other hand, if there is too much trivalent iridium, the durability performance may decrease. Therefore, in order to further improve the durability performance, Ir 4+ / Ir 3+ It is preferable that the value is 0.5 or higher.

[0057] Ir 4+ / Ir 3+ This can be determined by X-ray photoelectron spectroscopy (XPS). More specifically, it can be determined by the method described in the examples.

[0058] • Specific surface area S of catalyst powder The BET specific surface area S of the catalyst powder is not particularly limited and may be any value, but is between 1 and 50 m 2 It is preferable that the BET specific surface area is 1 to 50 m². The reason is as follows: First, if the specific surface area of ​​the support constituting the catalyst powder is excessively large, the amount of iridium required for coating will increase. On the other hand, if the specific surface area of ​​the support is excessively small, the particle size will be large, which may make coating difficult. Furthermore, the BET specific surface area of ​​the catalyst powder depends on the specific surface area of ​​the support constituting the catalyst. Therefore, the BET specific surface area S of the catalyst powder should be between 1 and 50 m². 2 It is preferable that the concentration is / g. The BET specific surface area S of the catalyst powder can be measured by measuring the amount of adsorption using a gas adsorption method with N2 gas and analyzing it using the BET method.

[0059] BET specific surface area S per gram of iridium Ir This is not particularly limited, but for similar reasons, 20-500m 2 / g Ir It is preferable that this be the case.

[0060] The catalyst powder of the present invention is particularly suitable for use as an anode catalyst in proton exchange membrane water electrolysis, but is not limited to that and can be widely used as an anode catalyst for fuel cells and water electrolysis devices. For example, the catalyst powder of the present invention can also be used as an anode catalyst in anion exchange membrane (AEM) water electrolysis. Furthermore, the catalyst powder of the present invention can be added to the anode catalyst to prevent fuel depletion on the anode side of a fuel cell.

[0061] [Catalyst layer] The catalyst layer in one embodiment of the present invention comprises the catalyst powder and an ionomer.

[0062] The ionomer is not particularly limited, and any ionomer having proton conductivity can be used. The ionomer may be either a fluorine-based ionomer or a non-fluorine-based ionomer. Examples of fluorine-based ionomers include fluororesin-based cation exchange resins having at least one selected from the group consisting of sulfone groups, carboxyl groups, and phosphone groups. Among these, it is preferable to use a perfluorosulfonic acid (PFSA)-based ionomer. A typical example of a perfluorosulfonic acid (PFSA)-based ionomer is Nafion®.

[0063] These ionomers are commercially available as dispersions in which ionomers are dispersed in a solvent, and these can be suitably used. The concentration of the ionomer in the dispersion is generally about 5 to 20% by mass.

[0064] The thickness of the catalyst layer is not particularly limited, but is preferably 1 to 100 μm, and more preferably 1 to 20 μm. If it is 1 μm or more, film formation is possible by general methods, and the durability of the catalyst layer is also improved. On the other hand, if it is 100 μm or less, the amount of catalyst powder used can be reduced. In addition, the decrease in proton conductivity due to the formation of voids in the catalyst layer can be suppressed.

[0065] The ratio I / C of the mass I of the ionomer to the mass C of the catalyst powder in the catalyst layer is not particularly limited. However, from the viewpoint of effectively exhibiting the function of the catalyst powder of the present invention, an I / C of 0.01 to 0.30 is preferable.

[0066] The catalyst layer can be formed by coating an ink composition containing the catalyst powder and ionomer onto a substrate. Examples of the substrate include ion exchange membranes and porous transport layers (PTLs). Ion exchange membranes include proton exchange membranes and anion exchange membranes.

[0067] The solvent (dispersion medium) for the ink composition can be water, 1-propanol, or the like. The application method is not particularly limited. For example, it can be applied by bar coating, die coating, screen printing, dropping method, spray method, etc.

[0068] A catalyst layer can be obtained by applying the ink composition to a substrate and then drying it to evaporate the solvent. Heating is preferable during the drying process. The heating temperature is not particularly limited, but for example, 30 to 100°C is preferred, and 40 to 80°C is more preferred.

[0069] [Catalyst coated membrane] The catalyst coating film (CCM) in one embodiment of the present invention includes an ion exchange membrane and a catalyst layer coated on at least one surface of the ion exchange membrane. The catalyst layer is the catalyst layer described in the above-described embodiment. In other words, the catalyst coating film in one embodiment of the present invention comprises a catalyst layer containing the catalyst powder described above. The ion exchange membrane is not particularly limited and any ion exchange membrane can be used, but it is typically a proton exchange membrane or an anion exchange membrane.

[0070] [Method for producing catalyst powder] As previously mentioned, conventional catalyst powders were prepared by adding alkali to a dispersion containing a carrier and an iridium salt. Catalyst powders obtained by this method suffered from poor coating (shell) uniformity, with some iridium oxide precipitating in clumps. In contrast, the present invention allows for the production of a core-shell type catalyst powder with extremely uniform coating thickness by preparing a dispersion in an iridium chloride solution and then dropping this dispersion into an alkaline solution.

[0071] The reason why the uniformity of the coating differs depending on the manufacturing method is thought to be as follows: The surface of the carrier particles such as titanium dioxide is positively charged in an acidic solution. On the other hand, iridium salts used as coating raw materials dissociate in solution to form iridium complexes ([IrCl6] 2-These form anions such as ). Therefore, in the dispersion, the coating material is attracted to the entire surface of the carrier particles.

[0072] When a dispersion in this state is dropped into a high-temperature alkaline solution, the coating material attracted to the entire surface of the carrier particles immediately precipitates as hydroxides. As a result, a uniform coating can be obtained. In contrast, when alkali is added to the dispersion, the pH of the entire dispersion rises slowly, making it impossible to uniformly precipitate the coating on the surface of the carrier particles.

[0073] For the reasons stated above, in the method for producing catalyst powder in one embodiment of the present invention, A dispersion is prepared by dispersing a support in a solution of an iridium compound. By dropping the dispersion into an alkaline solution, an Ir-containing coating is deposited on the surface of the carrier. The solid component is separated from the aforementioned solution. The catalyst powder is produced by drying the aforementioned solid components.

[0074] The iridium compound is not particularly limited, and any iridium compound can be used. For example, at least one selected from the group consisting of iridium chloride, iridium chloride salt, iridium nitrate, and iridium sulfate can be used as the iridium compound. Among these, it is preferable to use iridium chloride as the iridium compound. In other words, in this embodiment, it is preferable to use a dispersion in which a carrier is dispersed in an iridium chloride solution.

[0075] The iridium compound solution may be an aqueous solution. When an iridium compound as described above is dissolved in water, the solution usually becomes acidic due to hydrolysis. Therefore, a dispersion obtained by dispersing a support in this solution also usually exhibits acidity. Consequently, there is no particular need to adjust the pH. However, a pH adjusting agent such as an acid may be added to the solution.

[0076] Next, the dispersion is dropped into an alkaline solution to precipitate an Ir-containing coating on the surface of the carrier. The alkaline solution is not particularly limited and any can be used. The alkaline solution may be, for example, an aqueous solution of an alkali metal hydroxide. Examples of alkali metal hydroxides include sodium hydroxide and potassium hydroxide.

[0077] The dispersion can be dispensed dropwise by any method. For example, the dispersion can be dispensed dropwise into an alkaline solution at a constant rate from the tip of a tube. A metering pump can be used to dispense the dispersion, for example.

[0078] The temperature of the alkaline solution is not particularly limited. However, if the temperature is too low, the rate of iridium oxide formation decreases, so it is preferable that the temperature of the alkaline solution be 40°C or higher. From the viewpoint of promoting the formation of iridium oxide, it is more preferable that the temperature of the dispersion be 70°C or higher, and even more preferable that it be 80°C or higher. When adjusting the temperature of the alkaline solution, it is preferable to maintain the alkaline solution at the desired temperature while adding the dispersion dropwise into the alkaline solution. On the other hand, since the formation of iridium oxide is promoted at higher temperatures, there is no particular upper limit to the temperature of the alkaline solution. However, since boiling is not necessary, it is preferable that the temperature be less than 100°C, preferably 98°C or lower, and more preferably 95°C or lower.

[0079] Subsequently, the solids are separated from the solution. This separation can be carried out by any method. Typically, it can be done by filtration. After that, it is preferable to wash the separated solids.

[0080] Next, the obtained solid is dried to obtain catalyst powder. The drying can be carried out by any method without particular limitations. The drying temperature is preferably 20 to 150°C, more preferably 40 to 120°C, and even more preferably 40 to 80°C. The drying can be carried out in any atmosphere, but it is usually sufficient to dry it in air.

[0081] Furthermore, it is preferable to subject the catalyst powder to heat treatment after the drying process. Heat treatment can further improve the durability of the catalyst powder. When heat treatment is performed, the heat treatment temperature is preferably 200 to 600°C, more preferably 250 to 500°C, and even more preferably 300 to 450°C.

[0082] The heat treatment time is not particularly limited, but if it is too short, the heat treatment effect will be insufficient. Conversely, if it is too long, the heat treatment effect will saturate. Therefore, the heat treatment time is preferably 30 minutes to 12 hours, more preferably 1 to 8 hours, and even more preferably 2 to 5 hours.

[0083] The heating method used during the heat treatment is not particularly limited. For example, the dried catalyst powder can be placed in a furnace and heated. An electric furnace or muffle furnace can be used as the furnace. The atmosphere during heating is also not particularly limited, but it is usually sufficient to heat it in the atmosphere. [Examples]

[0084] The present invention will be described in more detail below based on the examples. However, the present invention is not limited to these examples.

[0085] (Examples) A catalyst powder having a core-shell structure was produced using the following procedure.

[0086] • Preparation of dispersion Specifically, a support was first added to an aqueous solution of iridium chloride (IrCl4), and the dispersion was prepared by ultrasonic irradiation. The ratio of iridium chloride to the amount of support was varied during this process. As the support, titanium oxide powder having the BET specific surface area and crystal structure shown in Table 1 was used.

[0087] • BET specific surface area (carrier) The BET specific surface area of ​​the carrier was obtained by measuring the amount of adsorption using a gas adsorption method with N2 gas and analyzing it using the BET method.

[0088] Preparation of alkaline solutions On the other hand, an alkaline solution (potassium hydroxide aqueous solution) was prepared by dissolving potassium hydroxide in water. The concentration of potassium hydroxide in the solution was set to twice the amount of Cl contained in the dispersion in molar ratio.

[0089] • Dropping The above alkaline solution was heated to 90°C, and then the above dispersion was added dropwise to the alkaline solution. The temperature of the alkaline solution was maintained at 90°C during the addition.

[0090] The alkaline solution after the above-mentioned dropwise addition was filtered, and the separated solid was washed. The washed powder was then dried at 60°C, and subsequently heat-treated under the conditions (temperature, time) shown in Table 1.

[0091] (Comparative Example 1) For comparison, a catalyst powder was prepared using a conventional method. Specifically, a dispersion prepared in the same manner as in Example 1 was mixed with an aqueous sodium hydroxide solution to adjust the pH to 9.0. As the support, titanium oxide powder having the specific surface area and crystal structure shown in Table 1 was used. The dispersion was then heated to 70°C, and the pH was further adjusted to 9.2. The mixture was then stirred overnight while maintaining the temperature at 70°C.

[0092] Next, the dispersion was filtered, and the separated solids were washed. After that, heat treatment was performed under the conditions (temperature, time) shown in Table 1.

[0093] • Patient Furthermore, in Example 7, Pt fine particles were further supported on the surface of the catalyst powder obtained in Example 6 (i.e., the coating surface) using the following procedure.

[0094] First, a dispersion of the catalyst powder was prepared by adding the catalyst powder to approximately 4 L of pure water. A predetermined amount of a 13% by mass dinitrodiammineplatinum(II) aqueous solution (Pt[(NH3)2(NO2)2] aqueous solution) with a platinum concentration was added to the dispersion to disperse the catalyst powder in a platinum salt solution. Furthermore, 0.5 L of ethanol was added to the dispersion as a reducing agent.

[0095] Subsequently, the dispersion was subjected to a reduction treatment by refluxing it at a temperature near its boiling point to precipitate platinum onto the catalyst powder. After the reflux treatment, the catalyst powder with the supported platinum was filtered, recovered, and washed to obtain the Pt-supported catalyst. (Evaluation of catalyst powder) For each of the catalyst powders obtained in the above examples and comparative examples, the Ir concentration, coating thickness, etc., were measured using the following procedure. The measurement results are shown in Table 1.

[0096] ·Ir concentration The catalyst powder was dissolved, and the iridium concentration in the solution was measured by ICP emission spectroscopy (ICP-OES).

[0097] • Coating thickness The coating thickness was measured using STEM (Scanning Transmission Electron Microscope), and the coefficient of variation CV of the coating thickness and the average thickness t of the coating were determined. Ave The calculation was performed as follows. The specific procedure is as follows:

[0098] First, the obtained catalyst powder was dispersed in a dispersion medium to form a dispersion. A mixed solvent of isopropanol (IPA) and pure water was used as the dispersion medium. Next, the dispersion was dropped onto a mesh serving as a sample stage, and the catalyst powder was fixed onto the sample stage by evaporating the dispersion medium.

[0099] Next, the catalyst powder was observed using a STEM, and STEM images were obtained in three fields of view. A JEM-ARM200F (JEOL Ltd.) STEM was used. The acceleration voltage was set to 200kV. The observation magnification was adjusted in the range of 2 million to 15 million times so that the thickness of the coating could be clearly seen.

[0100] STEM images from the three obtained fields of view were analyzed, and the coating thickness was measured at 50 randomly selected points per field, for a total of 150 points. From the coating thickness values ​​obtained at the 150 points, the average coating thickness t was calculated. Ave , calculate the standard deviation σ, and further, the average thickness t Ave The coefficient of variation CV was calculated from the standard deviation σ. Prior to the above measurement, it was confirmed by EDX measurement that the carrier and the coating could be distinguished by the contrast of the STEM image.

[0101] Furthermore, observations using STEM revealed that in the catalyst powder of the example, a uniform coating was formed on the surface of the support, and no clumps were found. In contrast, in the catalyst powder of the comparative example, areas where iridium oxide and other materials were attached in clumps (particulate form) were observed on the surface of the support.

[0102] • Presence or absence of epitaxial growth Catalyst particles were observed using a scanning transmission electron microscope (STEM) to determine whether the coating was epitaxially growing on the surface of the support. Specifically, epitaxial growth was determined if the lattice fringes in the coating layer were on the extension of or parallel to the lattice fringes on the support.

[0103] ·Ir 4+ / Ir 3+ The catalyst powders of Examples 1 to 3 and Comparative Example 1 were analyzed by X-ray photoelectron spectroscopy (XPS) to determine the presence of trivalent iridium Ir 3+ Tetravalent iridium Ir 4+ The atomic ratio of Ir 4+ / Ir 3+The following measurements were taken. These measurements were performed using the VersaProbe III X-ray photoelectron spectroscopy analyzer manufactured by ULVAC-PHI under the following conditions. • Measurement area: 200 μm φ • X-ray source: Monochromatic Al Kα rays (1486.6 eV), 50 W • Photoelectron extraction angle: 45° (measurement depth: approximately 4 nm) Neutralization conditions: Neither the neutralization electron gun nor the ion gun was used. ·Measurement elements: Ir, C, O, Ti (4 elements)

[0104] For the aforementioned measurement, the catalyst powder sample was fixed to an Au foil and introduced into the apparatus. Charge correction was performed using Au4f, which was measured separately. 7 / 2 This was done by shifting the peak top to 84.0 eV (Au metal).

[0105] The Ir4f spectra obtained were fitted using the following references. Binding energies related to Ir: Chem. Mater. 2016, Vol. 28, Issue 18, pp. 6591-6604 Peaks not explained above (e.g., satellite peaks): J. Phys. Chem. Lett. 2024, Vol. 15, Issue 45, pp. 11217-11223

[0106] • BET specific surface area (catalyst powder) The amount of adsorption was measured using a gas adsorption method with N2 gas, and the BET specific surface area S of the catalyst powder was determined by analysis using the BET method. Table 1 shows the BET specific surface area S and the BET specific surface area S converted to per gram of iridium. Ir The following will be listed together.

[0107] (catalyst layer) Next, catalyst layers were prepared using the catalyst powders obtained in the above examples and comparative examples. A Nafion® film, which is proton conductive, was used as the substrate.

[0108] An ink containing the catalyst powder was spray-coated onto the substrate to form a catalyst layer. The ink was prepared by mixing the catalyst powder, ionomer, water, and 1-propanol, and dispersing them by ultrasonic irradiation. A dispersion of 5% Nafion® was used as the ionomer. After coating, the catalyst layer was dried at 60°C.

[0109] In order to standardize the evaluation conditions for the catalyst properties described later, the iridium loading amount (weight of iridium per unit area) in Examples 1 to 5 was 0.15 mg. Ir / cm 2 The amount of ink applied was adjusted to achieve this result. For similar reasons, in Examples 6 and 7, the iridium loading amount (weight of iridium per unit area) was 0.10 mg. Ir / cm 2 The amount of ink applied was adjusted to achieve this result.

[0110] • Thickness of the catalyst layer For Examples 1 to 5 and Comparative Example 1, the thickness of the catalyst layer formed on the substrate was measured using a scanning electron microscope (SEM). The specific procedure was as follows.

[0111] First, a test specimen for observation was taken from the substrate on which the catalyst layer was formed. Next, the cross-section of the test specimen was processed for SEM observation using an ion milling apparatus. A cross-section polisher IB-09020CP (JEOL Ltd.) was used as the ion milling apparatus.

[0112] Next, the cross-section after processing was observed using a scanning electron microscope (SEM) to measure the thickness of the catalyst layer. A JSM-IT300HR (JEOL Ltd.) SEM was used. The acceleration voltage was set to 10kV. The observation magnification was adjusted within the range of 1000x to 5000x so that the thickness of the catalyst layer could be clearly seen.

[0113] (Evaluation of catalyst performance) Next, the catalytic performance of each of the catalyst powders obtained in the above examples and comparative examples was evaluated. Specifically, the catalyst powder was used as an anode catalyst in proton exchange membrane water electrolysis, and the initial performance and durability were evaluated.

[0114] In the evaluation described above, a CCM was used in which a cathode catalyst was applied to the opposite side of the proton conduction film on which the catalyst layer described above was formed. As the cathode catalyst, platinum-supported carbon (TEC10E50E, Tanaka Kikinzoku Kogyo Co., Ltd.) with a platinum load of 50 wt% was used. The platinum-supported carbon was mixed with an ionomer, water, and 1-propanol, and dispersed by ultrasonic irradiation to prepare an ink for the cathode. As the ionomer, a dispersion of 5% Nafion® was used. After applying the ink to the surface of the proton conduction film, it was dried at 60°C to form the cathode catalyst layer.

[0115] A porous transport layer (PTL) and a separator were laminated on both sides of the aforementioned CCM to form a water electrolysis apparatus. For the anode-side PTL, platinum-plated titanium fibers were used, while carbon was used for the cathode-side PTL. Furthermore, platinum-plated titanium was used as the anode-side separator, and carbon was used as the cathode-side separator.

[0116] ·Initial performance Using the aforementioned water electrolysis apparatus, electrolysis was performed while sweeping the anode-cathode voltage within the range of 1.0 to 2.5 V, and the IV curve representing the relationship between current density and potential was measured. The voltage sweep rate was set to 120 mV / min. The temperature during the measurement was 50°C.

[0117] The voltage measured by the above method includes not only the voltage determined by the electrolytic reaction, but also voltage loss (ohmic loss) due to the resistance of the cell substrate and film. Therefore, an IR-free curve was obtained by subtracting the ohmic loss from the IV curve using the high-frequency resistance measured simultaneously. The current density in the IR-free curve is 2.0 A / cm². 2The voltage values ​​are shown in Table 1. The lower the voltage value in the IR-free curve, the better the initial performance.

[0118] ·Durability Using the aforementioned water electrolysis apparatus, a constant current density (2A / cm²) is obtained. 2 Water electrolysis was performed continuously, and the change in cell voltage during this time was recorded. By subtracting ohmic losses from the measured voltage, an IR-free time-voltage curve was obtained. The rate of voltage increase (μV / h) in the range of 0 to 500 hours in the time-voltage curve was defined as the degradation rate. A smaller degradation rate indicates better durability.

[0119] • Crossover H2 Furthermore, to confirm the effect of Pt support, the crossover H2 concentration of the catalyst powders in Examples 6 and 7 was measured using the following procedure.

[0120] First, the cell temperature is 50°C and the current density is 2A / cm². 2 Water electrolysis was performed under the specified conditions, and the gas generated from the anode was collected by water displacement. The hydrogen concentration (ppm) in the collected gas was measured using a suction-type gas detector (GD-D58, manufactured by Riken Keiki Co., Ltd.).

[0121] [Table 1]

[0122] As shown in Table 1, the catalyst powder of the embodiment of the present invention exhibited excellent initial performance and durability. In contrast, the catalyst powder of Comparative Example 1 had significantly inferior durability, and after 100 hours from the start of the test, degradation had progressed to the point where the voltage could no longer be measured. This is thought to be because the coating thickness was uneven, and with prolonged use, the support material was exposed in areas with thin coatings, making electron conduction between catalyst particles difficult.

[0123] Furthermore, in the examples, those with epitaxially grown coatings exhibited superior durability compared to those without. Observation of the catalyst powder after durability testing using STEM-EDX revealed that in the non-epitaxially grown coatings, some iridium oxide had leached or migrated, exposing the support surface. In contrast, in the epitaxially grown coatings, such leaching and migration of iridium oxide was suppressed, and the uniformity of the coating was relatively well maintained.

[0124] Furthermore, the catalyst powder of Example 7, which had Pt supported, showed a significantly lower concentration of crossover H2 compared to the catalyst powder of Example 6, which did not have Pt supported.

Claims

1. A catalyst powder for the anode of a water electrolysis device or fuel cell, The catalyst powder consists of a carrier and a coating that covers the surface of the carrier. The aforementioned coating contains iridium, The carrier and the coating are both crystalline, The catalyst powder is observed in three fields of view using a scanning transmission electron microscope, and the thickness of the coating is measured at 50 randomly selected points in each field of view, for a total of 150 points. The coefficient of variation CV of the coating thickness, calculated from the obtained coating thickness values ​​at the 150 points, is 50% or less.

2. The catalyst powder according to claim 1, wherein the support comprises an oxide containing at least one selected from the group consisting of Ti, Ce, Ta, Nb, W, Ru, Sn, Sr, and Ba.

3. The catalyst powder according to claim 1 or 2, wherein the carrier is a non-conductive oxide.

4. The catalyst powder according to claim 1 or 2, wherein the support is a crystal having a rutile-type structure.

5. The catalyst powder according to claim 1 or 2, wherein the coating contains at least one selected from the group consisting of metallic iridium, iridium oxide, iridium hydroxide, and iridium hydroxide oxide.

6. The catalyst powder according to claim 1 or 2, wherein the coating is a crystal having a rutile-type structure.

7. The catalyst powder according to claim 1 or 2, wherein the coating is epitaxially grown on the surface of the carrier.

8. The average thickness t of the coating Ａｖｅ The catalyst powder according to claim 1 or 2, wherein the wavelength is 5.0 nm or less.

9. The catalyst powder according to claim 1 or 2, wherein the Ir concentration is 50 wt% or less.

10. Of the iridium contained in the aforementioned coating, trivalent iridium Ir ３＋ Tetravalent iridium Ir ４＋ The atomic ratio of Ir ４＋ / Ir ３＋ The catalyst powder according to claim 1 or 2, wherein the ratio is 0.5 to 4.

0.

11. BET specific surface area S is 1 to 50 m² ２ The catalyst powder according to claim 1 or 2, wherein the amount is / g.

12. BET specific surface area S per 1 g of iridium element Ｉｒ is 20 to 500 m ２ / g Ｉｒ , the catalyst powder according to claim 1 or 2.

13. The catalyst powder according to claim 1 or 2, wherein Pt is further supported on the surface of the coating.

14. A catalyst layer comprising the catalyst powder according to claim 1 or 2 and an ionomer.

15. The catalyst layer according to claim 14, wherein the ratio I / C of the mass I of the ionomer to the mass C of the catalyst powder is 0.01 to 0.

30.

16. A catalyst coating film comprising an ion exchange membrane and a catalyst layer according to claim 14 coated on at least one surface of the ion exchange membrane.

17. A method for producing catalyst powder according to Claim 1, A dispersion is prepared by dispersing a support in a solution of an iridium compound. By dropping the dispersion into an alkaline solution, an Ir-containing coating is deposited on the surface of the carrier. The solid component is separated from the aforementioned solution. The aforementioned solids are dried, A method for producing catalyst powder, further comprising heat treatment at 200 to 600°C to obtain catalyst powder.

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