Electrolytic cathode and method for producing electrolytic cathode

Abstract

Provided are an electrolytic cathode that is excellent in reverse current resistance and able to maintain catalytic activity while reducing the amount of a platinum group element used for a catalyst layer, and a method for producing the electrolytic cathode. The electrolytic cathode comprises a catalyst layer provided on a conductive base material made of nickel, the catalyst layer containing ruthenium, praseodymium, neodymium, and iron. The catalyst layer comprises a first catalyst layer provided along the surface of the base material, and a second catalyst layer provided on the first catalyst layer along the surface of the first catalyst layer. The second catalyst layer contains more neodymium and iron than the first catalyst layer.

Description

Electrolytic cathode and method for manufacturing the same

[0001] The present invention relates to a cathode for electrolysis and a method for producing the same.

[0002] On the surface of the electrolytic cathode, hydrogen ions receive electrons from the cathode, becoming atomic hydrogen (H), which then forms hydrogen gas (H ２ ) is produced. In this case, a certain amount of hydrogen overvoltage must be applied for the reaction to proceed. Hydrogen overvoltage varies depending on factors such as the electrode material, the properties of the solution, temperature, and pressure. Electrode materials with low hydrogen overvoltage include platinum group elements such as platinum, palladium, ruthenium, and rhodium.

[0003] Patent Document 1 describes a cathode suitable for generating hydrogen in an industrial electrolysis process, which comprises a metal substrate and a multilayer catalytic coating including at least one inner catalytic layer and one outer catalytic layer provided on the substrate, and both the inner catalytic layer and the outer catalytic layer contain palladium, at least one rare earth element, and at least one noble metal component selected from platinum and ruthenium.

[0004] Patent Document 2 describes that a cathode having excellent reverse current resistance can be obtained by producing a catalyst layer by applying a precursor containing ruthenium nitrate dissolved in a non-chloride aqueous solution containing acetic acid onto a metal substrate.

[0005] Patent Document 3 describes an electrolysis cathode having a low hydrogen overvoltage, which has a conductive substrate and a catalyst layer containing ruthenium.

[0006] Japanese Patent No. 5680655 Japanese Patent No. 5876838 Japanese Patent No. 6389276

[0007] Platinum group elements such as platinum, palladium, ruthenium, and rhodium, which are used in catalytic layers of cathodes suitable for generating hydrogen, generally have the problem of being expensive. Ruthenium is relatively inexpensive among platinum group elements, and therefore catalytic layers of electrodes using only ruthenium as a platinum group element are known. For example, Patent Document 2 discloses a catalytic layer using only ruthenium as a platinum group element and also containing praseodymium. This catalytic layer exhibits excellent performance (e.g., reverse current resistance and catalytic activity) without containing more expensive platinum group elements. However, there is room to reduce the ruthenium content while maintaining equivalent performance. The objective of the present invention is to provide an electrolysis cathode that maintains catalytic activity and has excellent reverse current resistance while reducing the amount of ruthenium used in the catalytic layer, and a method for manufacturing the same.

[0008] A first aspect of the present invention is a cathode for electrolysis comprising: a conductive substrate, at least the surface of which is made of nickel or a nickel-based alloy; and a catalytic layer containing ruthenium, praseodymium, neodymium, and iron, which is provided on the substrate; the catalytic layer comprising: a first catalytic layer provided on the substrate along the surface of the substrate; and a second catalytic layer provided on the first catalytic layer along the surface of the first catalytic layer, the second catalytic layer containing larger amounts of neodymium and iron than the first catalytic layer.

[0009] A second aspect of the present invention is a method for producing an electrolysis cathode, the method comprising: a first coating step of coating a first coating liquid containing a ruthenium compound and a praseodymium compound onto a conductive substrate, at least the surface of which is made of nickel or a nickel-based alloy; a first thermal decomposition step of providing a first catalytic layer by thermally decomposing the ruthenium compound and the praseodymium compound; a second coating step of coating a strongly acidic second coating liquid containing a ruthenium compound, a neodymium compound, and an iron compound onto the first catalytic layer; and a second thermal decomposition step of providing a second catalytic layer by thermally decomposing the ruthenium compound, the neodymium compound, and the iron compound.

[0010] According to the present invention, it is possible to obtain an electrolysis cathode that maintains catalytic activity and has excellent reverse current resistance while reducing the amount of ruthenium used in the catalyst layer.

[0011] FIG. 1 is a cross-sectional view of a catalytic layer formed in Example 6. FIG. 2 is a view showing the distribution of ruthenium in the catalytic layer of Example 6. FIG. 3 is a view showing the distribution of praseodymium in the catalytic layer of Example 6. FIG. 4 is a view showing the distribution of neodymium in the catalytic layer of Example 6. FIG. 5 is a view showing the distribution of iron in the catalytic layer of Example 6. FIG. 6 is a schematic cross-sectional view of an electrolysis cathode according to the present embodiment. FIG. 7 is a cross-sectional view of a catalytic layer formed in Comparative Example 3. FIG. 8 is a view showing the distribution of ruthenium in the catalytic layer of Comparative Example 3. FIG. 9 is a view showing the distribution of praseodymium in the catalytic layer of Comparative Example 3. FIG. 10 is a view showing the distribution of neodymium in the catalytic layer of Comparative Example 3. FIG. 11 is a view showing the distribution of iron in the catalytic layer of Comparative Example 3.

[0012] An embodiment of the present invention will be described in detail below. An electrolytic cathode according to this embodiment comprises: a conductive substrate having at least a surface made of nickel or a nickel-based alloy; and a catalytic layer containing ruthenium, praseodymium, neodymium, and iron provided on the substrate, wherein the catalytic layer comprises: a first catalytic layer provided on the substrate along the surface of the substrate; and a second catalytic layer provided on the first catalytic layer along the surface of the first catalytic layer, wherein the second catalytic layer contains more neodymium and iron than the first catalytic layer. Furthermore, the method for producing an electrolytic cathode according to this embodiment includes: a first coating step of coating a first coating liquid containing a ruthenium compound and a praseodymium compound onto a conductive substrate, at least the surface of which is made of nickel or a nickel-based alloy; a first thermal decomposition step of providing a first catalytic layer by thermally decomposing the ruthenium compound and the praseodymium compound; a second coating step of coating a strongly acidic second coating liquid containing a ruthenium compound, a neodymium compound, and an iron compound onto the first catalytic layer; and a second thermal decomposition step of providing a second catalytic layer by thermally decomposing the ruthenium compound, the neodymium compound, and the iron compound.

[0013] (Nickel substrate) As the conductive substrate, a nickel substrate having at least a surface made of nickel or a nickel-based alloy can be used. Nickel is preferred because its electrical conductivity is 10 times that of stainless steel and it is passivated and therefore resistant to corrosion in alkaline solutions.

[0014] The shape of the nickel substrate is not particularly limited, and an appropriate shape can be selected depending on the purpose. Although not particularly limited, a perforated plate, an expanded shape, or a so-called woven mesh made by knitting nickel wires can be suitably used as the nickel substrate. The shape of the nickel substrate has a suitable specification depending on the distance between the anode and the cathode in the electrolytic cell. Although not particularly limited, a perforated plate or an expanded shape can be used when the anode and the cathode have a finite distance, and a woven mesh made by knitting thin wires can be used in the case of a so-called zero-gap electrolytic cell in which the ion exchange membrane and the electrode are in contact with each other.

[0015] It is preferable to alleviate residual stress during processing by annealing the nickel base material in an oxidizing atmosphere. Also, in order to improve adhesion with the first catalyst layer coated on the surface of the nickel base material, it is preferable to form irregularities on the surface of the nickel base material by, for example, blasting using steel grid, alumina powder, corundum, etc., and then increase the surface area by acid etching.

[0016] (First catalytic layer) The first catalytic layer contains ruthenium (Ru) and praseodymium (Pr). The ruthenium in the first catalytic layer is preferably in the form of an oxide.

[0017] It should be noted that, throughout this specification and claims, ratios of various elements refer to molar ratios of metallic elements unless otherwise specified. The indicated elements may exist as themselves or in the form of oxides or other compounds. For example, when the electrode is fabricated, platinum group elements will exist in metallic or oxide form, and rare earth elements will exist primarily as oxides.

[0018] The ratio of praseodymium contained in the first catalytic layer relative to ruthenium is preferably 95-75% ruthenium to 5-25% praseodymium, and more preferably 90-80% ruthenium to 10-20% praseodymium (total: 100%). Generally, a higher ruthenium ratio increases the catalytic activity of the electrolysis cathode, while a higher praseodymium ratio increases the reverse current resistance of the electrolysis cathode, thus determining the preferred range. The praseodymium in the first catalytic layer is preferably in the form of an oxide. Ruthenium may be present in the first catalytic layer as a ruthenium compound. Examples of the ruthenium compound include ruthenium oxide and ruthenium hydroxide. Praseodymium may be present in the second catalytic layer as a praseodymium compound. Examples of the praseodymium compound include praseodymium oxide and praseodymium hydroxide.

[0019] (Method for forming first catalytic layer) The process for forming the first catalytic layer includes a first coating process of coating a coating liquid (first coating liquid) for forming the first catalytic layer on a nickel base material, a first film formation process of drying the first coating liquid to form a first coating film, as needed, and a first pyrolysis process of heating the first coating film to pyrolyze it.

[0020] (First Coating Step) In the first coating step, the first coating liquid is coated on the surface of the nickel base material.

[0021] (First Coating Liquid) The first coating liquid contains ruthenium and praseodymium. The first coating liquid may contain a ruthenium salt or another soluble ruthenium compound. The first coating liquid may also contain a praseodymium salt or another soluble praseodymium compound.

[0022] Examples of ruthenium salts or other soluble compounds of ruthenium include nitrates, dinitrodiammine complexes, nitrosyl nitrates, chloride salts, and acetate salts. Among these, nitrosyl nitrates are preferred from the viewpoint of ease of thermal decomposition. Examples of ruthenium nitrosyl nitrates include ruthenium(III) nitrosyl nitrate. Ruthenium(III) nitrosyl nitrate has the molecular formula: Ru(NO)(NO ３ ) ｘ Here, the value of x varies depending on the oxidation state of ruthenium, but the average value of x is 3.

[0023] The first coating liquid may contain a salt of a platinum group element other than ruthenium or other soluble compound. Examples of the salt of a platinum group element or other soluble compound that can be used include nitrates, dinitrodiammine complexes, nitrosyl nitrates, chloride salts, and acetates. Among these, nitrates and combinations of nitrates and acetates are preferred from the standpoint of ease of thermal decomposition.

[0024] As the praseodymium salt or other soluble compound of praseodymium, for example, nitrate, dinitrodiammine complex, nitrosyl nitrate, chloride salt, acetate, etc. can be used. Among these, from the viewpoint of ease of thermal decomposition, praseodymium having the molecular formula: Pr(NO ３ ) ２ Praseodymium nitrate represented by the following formula can be suitably used.

[0025] The first coating liquid may contain a salt of a rare earth element other than praseodymium or other soluble compound. Examples of the salt of a rare earth element or other soluble compound that can be used include nitrates, dinitrodiammine complexes, nitrosyl nitrates, chloride salts, and acetate salts. Among these, nitrates are preferably used in view of ease of thermal decomposition.

[0026] Water, acetic acid (glacial acetic acid), and concentrated nitric acid are examples of solvents used in the first coating liquid. The first coating liquid is preferably an acetic acid solution that is substantially chloride-free. An acetic acid solution that is substantially chloride-free can provide a catalyst layer with excellent catalytic activity and reverse current resistance. This is thought to be because ruthenium does not form a coordinate bond with chloride, and a complex species in which ruthenium is coordinated with an acetic acid or a carbonyl group is formed. As will be described later, after the first coating film is formed by applying the first coating liquid, when the first coating film is heated, the complex species in which ruthenium is coordinated with an acetic acid or a carbonyl group is thermally decomposed, thereby forming the first catalyst layer.

[0027] The first coating liquid may contain other components in addition to the platinum group element and the rare earth element.

[0028] (Method for Producing First Coating Liquid) The first coating liquid can be prepared by mixing a ruthenium solution and a praseodymium solution.

[0029] The ruthenium element solution can be prepared, for example, by dissolving ruthenium nitrosyl nitrate in glacial acetic acid while stirring. This allows for the formation of a ruthenium acetate complex. Nitric acid may be added during the process of dissolving ruthenium nitrosyl nitrate in glacial acetic acid while stirring. Alternatively, after dissolving ruthenium nitrosyl nitrate in glacial acetic acid, a 5 to 20 wt % aqueous acetic acid solution may be added.

[0030] The praseodymium elemental solution can be prepared, for example, by dissolving praseodymium nitrate in glacial acetic acid while stirring. Nitric acid may be added during the process of dissolving praseodymium nitrate in glacial acetic acid while stirring. This allows the formation of a praseodymium acetate complex.

[0031] The first coating liquid may be prepared by mixing a ruthenium solution and a praseodymium solution, and then diluting the mixture with a 5 to 20 wt % aqueous solution of acetic acid. The proportion of each solution can be determined appropriately depending on the proportion of ruthenium and praseodymium contained in the first catalyst layer.

[0032] (Method of applying first coating liquid) As a method of applying the first coating liquid onto the nickel base material, a dip method in which the nickel base material is immersed in the first coating liquid, a method in which the first coating liquid is applied to the surface of the nickel base material with a brush, a roll method in which the first coating liquid is impregnated into a sponge-like roll and applied, and an electrostatic application method in which the first coating liquid is charged to an opposite charge to the nickel base material and sprayed using a spray or the like can be suitably used. Among these application methods, the roll method and the electrostatic application method can be suitably used from the viewpoints of productivity and the ability to uniformly apply the coating liquid to the electrode surface.

[0033] (First Film Forming Step) In the first film forming step, the first coating liquid applied to the nickel substrate is dried to form a first coating film. The drying temperature at which the first coating liquid is dried in the first film forming step is not particularly limited, but is preferably, for example, 100°C or lower, and more preferably a temperature of about 30 to 100°C. The drying time is also not particularly limited, but can be, for example, 5 to 60 minutes.

[0034] (First Thermal Decomposition Step) In the first thermal decomposition step, a first coating film obtained by drying the first coating liquid applied to the nickel substrate is subjected to heat treatment to thermally decompose the ruthenium salt or other soluble compound, and the praseodymium salt or other soluble compound. As a result, a first catalytic layer containing ruthenium and praseodymium is formed on the surface of the nickel substrate. When nitrates are used as the ruthenium salt and the praseodymium salt, the ruthenium and praseodymium nitrates are thermally decomposed by heat treatment at a temperature of 350°C to 600°C in the presence of air. As a result, a first catalytic layer is formed on the surface of the nickel substrate. In the first thermal decomposition step, after the first coating liquid is applied to the nickel substrate (after the first coating step), it is preferably dried at a temperature of about 30 to 100°C for 5 to 60 minutes (first film formation step), and then placed in a firing furnace heated to 350°C or higher but lower than 600°C for thermal decomposition (first thermal decomposition step). The term "thermal decomposition" refers to a reaction in which the first coating film (or the second coating film described below) is heated to promote decomposition of the contained metal salt into the metal and a gaseous substance. Although it depends on the atmosphere, in an oxygen atmosphere, many metals tend to bond with oxygen to form oxides.

[0035] In the first pyrolysis step, in order to promote the pyrolysis of the mixture contained in the first coating liquid, the pyrolysis temperature is preferably in the range of 350°C or higher but lower than 600°C. At a temperature of 350°C or higher, the rate of pyrolysis of the mixture can be increased. On the other hand, at a temperature of 600°C or higher, the softening of the nickel substrate proceeds rapidly, making it difficult to maintain the shape of the electrode. Therefore, from the viewpoints of promoting the pyrolysis of the mixture and maintaining the strength of the nickel substrate, a temperature range of 350°C or higher but lower than 550°C is preferred. Furthermore, from the viewpoints of hydrogen overvoltage and ruthenium consumption, a temperature range of 350°C or higher but lower than 500°C is preferred, and 350°C or higher but lower than 450°C is most preferred. To ensure sufficient pyrolysis, a longer pyrolysis time is preferred. However, in order to prevent the pyrolyzate from being completely oxidized and considering the productivity of the electrode, the pyrolysis time per cycle is preferably in the range of 5 to 60 minutes, more preferably 10 to 30 minutes.

[0036] Furthermore, prior to pyrolysis (baking) (after the first film-forming step and before the first pyrolysis step), pre-baking may be carried out at a temperature range of 100°C or higher and lower than 350°C, at which point the water of crystallization of each component contained in the first coating liquid disappears. The pre-baking temperature is preferably 120°C or higher and lower than 350°C, more preferably 150°C to 300°C. The pre-baking time is preferably 1 minute to 60 minutes, more preferably 1 minute to 30 minutes, and even more preferably 1 minute to 15 minutes.

[0037] (Thickness of the First Catalytic Layer) The thickness of the first catalytic layer can be adjusted by repeatedly applying the first coating liquid to the first catalytic layer obtained in the first pyrolysis step, drying, and heat treating the layer. That is, the cycle of the first coating step, the first film-forming step, and the first pyrolysis step may be repeated. From a practical perspective, the number of times the first coating step, the first film-forming step, and the first pyrolysis step are repeated is preferably 20 or less. The thicker the first catalytic layer, the longer the period during which a low overvoltage can be maintained.

[0038] In order to form the first catalytic layer to a predetermined thickness, it is sufficient to increase the amount of coating per application or to increase the metal concentration of the ruthenium compound. However, if the amount of coating per application is large, unevenness may occur during application, and the first catalytic layer may not be formed uniformly. For this reason, in forming the first catalytic layer, it is preferable to carry out coating, drying, and firing by pyrolysis several times. From the above viewpoints, the amount of coating per application of the first coating liquid to the nickel base material in the first coating step is set to 0.1 to 2 g / m2 so that the ruthenium in the formed first catalytic layer is 0.1 to 2 g / m2. ２ It is preferable to set the density to about 0.2 to 1.8 g / m ２ It is more preferable to set the density to 0.5 to 1.6 g / m. ２ is.

[0039] (Second catalytic layer) The second catalytic layer contains ruthenium, neodymium, and iron.

[0040] Ruthenium may be present in the second catalytic layer as a ruthenium compound. Examples of the ruthenium compound that can be used include ruthenium oxide and ruthenium hydroxide. Neodymium may be present in the second catalytic layer as a neodymium compound. Examples of the neodymium compound that can be used include neodymium oxide and neodymium hydroxide. The neodymium compound has the effect of changing the electronic state of ruthenium to reduce hydrogen overvoltage, and is reduced to hydroxide and changes its form to needle-like crystals when hydrogen generating electrolysis is performed in an aqueous sodium hydroxide solution. This is thought to have the effect of preventing the ruthenium compound, which functions as a catalyst, from falling off the substrate and improving durability against reverse current by increasing the surface area.

[0041] From the viewpoint of lowering hydrogen overvoltage and suppressing consumption of ruthenium, the neodymium content in the second catalytic layer is preferably 0.01 mol or more and less than 1 mol, more preferably 0.02 mol or more and less than 1 mol, and even more preferably 0.05 mol or more and 0.5 mol or less, and even more preferably 0.05 mol or more and 0.25 mol or less, per mol of ruthenium element.

[0042] When the ratio of neodymium to 1 mole of ruthenium is 0.01 mole or more, the ruthenium compound can be easily retained, physical detachment is unlikely to occur, the effect of reducing the electrolysis voltage is exhibited, and sufficient durability against reverse current can be imparted. On the other hand, when the ratio of neodymium to 1 mole of ruthenium is less than 1 mole, the oxide having no catalytic activity does not completely cover the surface of the ruthenium compound in the coating layer formed by thermal decomposition, resulting in a cathode with a low electrolysis voltage.

[0043] Iron may be present in the second catalyst layer as an iron compound, such as iron oxide or iron hydroxide.

[0044] From the viewpoint of lowering hydrogen overvoltage and suppressing consumption of ruthenium, the content of iron in the second catalytic layer is preferably 0.01 mol or more and less than 1 mol, more preferably 0.02 mol or more and less than 1 mol, per mol of ruthenium element, and even more preferably 0.05 mol or more and 0.5 mol or less, even more preferably 0.05 mol or more and 0.25 mol or less.

[0045] When the content of iron in the second catalytic layer is 0.01 mol or more per mol of ruthenium, the electronic state of ruthenium is changed to reduce the electrolysis voltage, and sufficient durability against reverse current is obtained by increasing the surface area. On the other hand, when the content is less than 1 mol, iron oxide does not cover the surface of the ruthenium compound, resulting in a cathode with a low electrolysis voltage.

[0046] Neodymium oxide and iron oxide can both form oxides with a valence of less than 4. Therefore, when neodymium and iron are used together with ruthenium in the second catalytic layer, the presence of neodymium and iron oxides around the ruthenium inhibits the ruthenium present at the interface with the neodymium or iron from forming a completely tetravalent oxide, which is thought to partially change the valence of the ruthenium, which is a catalytic component. According to the electrolysis cathode of this embodiment, ruthenium thereby promotes the rate-determining reaction, which may potentially reduce hydrogen overvoltage.

[0047] (Method for forming second catalytic layer) The process for forming the second catalytic layer includes a second coating process for coating a coating liquid for forming the second catalytic layer (second coating liquid) on the first catalytic layer, a second film formation process for drying the second coating liquid to form a second coating film, if necessary, and a second pyrolysis process for heating the second coating film to pyrolyze it.

[0048] (Second Coating Liquid) The second coating liquid contains ruthenium, neodymium, and iron. The ruthenium contained in the second coating liquid may be a ruthenium compound. The ruthenium compound that can be used as a component of the second coating liquid may be in any form, such as a nitrate, a dinitrodiammine complex, or a nitrosyl nitrate, but nitrates are preferred from the standpoint of ease of thermal decomposition. The metal concentration of the ruthenium compound in the second coating liquid is not particularly limited, but is preferably in the range of 10 g / L to 200 g / L, more preferably 50 g / L to 120 g / L, taking into account the coating thickness per coating. The second coating liquid is an aqueous nitric acid solution and is strongly acidic. The pH is preferably 2 or less, more preferably 1 or less.

[0049] The neodymium and iron contained in the second coating liquid are preferably neodymium compounds and iron compounds, and nitrates are preferably used from the viewpoint of ease of thermal decomposition, etc. The second coating liquid may be an aqueous solution of an inorganic acid other than nitric acid, such as hydrochloric acid or sulfuric acid, as long as it is strongly acidic. In this case, the ruthenium, neodymium, and iron contained in the second coating liquid may be their respective inorganic acid salts.

[0050] From the viewpoint of achieving a more satisfactory effect in the second catalytic layer formed by thermal decomposition of the components contained in the second coating liquid, i.e., lowering the hydrogen overvoltage and suppressing consumption of ruthenium, the amount of neodymium in the second coating liquid per mole of ruthenium is preferably 0.01 mol or more and less than 1 mol. More preferably, it is 0.02 mol or more and less than 1 mol. Even more preferably, it is 0.05 mol or more and 0.5 mol or less. Even more preferably, the neodymium content is 0.05 mol or more and 0.25 mol or less per mole of ruthenium.

[0051] From the viewpoint of achieving a more satisfactory effect in the second catalytic layer formed by thermal decomposition of the components contained in the second coating liquid, i.e., lowering the hydrogen overvoltage and suppressing consumption of ruthenium, the content of iron in the second coating liquid is preferably in the range of 0.01 mol or more and less than 1 mol, more preferably in the range of 0.02 mol or more and less than 1 mol, still more preferably in the range of 0.05 mol or more and 0.5 mol or less, and still more preferably in the range of 0.05 mol or more and 0.25 mol or less, relative to 1 mol of ruthenium element.

[0052] (Second Coating Step) In the second coating step, the second coating liquid is applied to the surface of the first catalyst layer. Suitable methods for applying the second coating liquid onto the first catalyst layer include a dip method in which the first catalyst layer is immersed in the second coating liquid, a brush method in which the second coating liquid is applied to the surface of the first catalyst layer, a roll method in which a sponge-like roll is impregnated with the second coating liquid and then coated, and an electrostatic coating method in which the second coating liquid is charged to an opposite charge to that of the first catalyst layer and sprayed using a spray or the like. Among these coating methods, the roll method and the electrostatic coating method are suitable from the viewpoints of productivity and the ability to uniformly apply the coating liquid to the electrode surface.

[0053] (Second Film Forming Step) In the second film forming step, the second coating liquid applied to the surface of the first catalyst layer is dried to form a second coating film. The drying temperature at which the second coating liquid is dried in the second film forming step is not particularly limited, but is preferably, for example, 100°C or lower, and more preferably a temperature of about 30 to 100°C. The drying time is also not particularly limited, but can be, for example, 5 to 60 minutes.

[0054] (Second Pyrolysis Step) In the second pyrolysis step, a heat treatment is performed on the second coating film obtained by drying the second coating liquid applied to the surface of the first catalytic layer, thereby thermally decomposing the ruthenium compound, neodymium compound, and iron compound. As a result, a second catalytic layer containing ruthenium, neodymium, and iron is formed on the surface of the first catalytic layer. When nitrates are used as the ruthenium compound, neodymium compound, and iron compound, the heat treatment is performed in the presence of air at a temperature of 350°C to 600°C, whereby the nitrates of ruthenium, neodymium, and iron are thermally decomposed. As a result, a second catalytic layer is formed on the surface of the first catalytic layer.

[0055] In the second thermal decomposition step, after the second coating liquid is applied to the first catalyst layer (after the second coating step), it is preferably dried at a temperature of about 30 to 100°C for 5 to 60 minutes (second film formation step), and then placed in a firing furnace heated to 350 to less than 600°C for thermal decomposition (second thermal decomposition step). Note that "thermal decomposition" refers to a reaction in which the first or second coating film is heated to promote the decomposition of the contained metal salt into metal and gaseous substance. While it depends on the atmosphere, many metals tend to bond with oxygen to form oxides in an oxygen atmosphere.

[0056] In the second pyrolysis step, in order to promote the pyrolysis of the mixture contained in the second coating liquid, the pyrolysis temperature is preferably in the range of 350°C or higher but less than 600°C. At temperatures of 350°C or higher, the rate of pyrolysis of the mixture can be increased, while at temperatures of 600°C or higher, softening of the nickel substrate proceeds rapidly, making it difficult to maintain the shape of the electrode. Therefore, from the viewpoints of promoting the pyrolysis of the mixture and maintaining the strength of the nickel substrate, a temperature range of 350°C or higher but less than 550°C is preferred. Furthermore, from the viewpoints of hydrogen overvoltage and ruthenium consumption, a temperature range of 350°C or higher but less than 500°C is preferred, and 350°C or higher but less than 450°C is most preferred. To ensure sufficient pyrolysis, a longer pyrolysis time is preferred. However, in order to prevent complete oxidation of the pyrolyzed product and considering the productivity of the electrode, the pyrolysis time per cycle is preferably in the range of 5 to 60 minutes, more preferably 10 to 30 minutes.

[0057] Furthermore, prior to pyrolysis (baking) (after the second film-forming step and before the second pyrolysis step), pre-baking may be carried out at a temperature range of 100°C or higher and lower than 350°C, at which point the crystal water of each component contained in the second coating liquid disappears. The pre-baking temperature is preferably 120°C or higher and lower than 350°C, more preferably 150°C to 300°C. The pre-baking time is preferably 1 minute to 60 minutes, more preferably 1 minute to 30 minutes, and even more preferably 1 minute to 15 minutes.

[0058] (Thickness of the Second Catalytic Layer) The thickness of the second catalytic layer can be adjusted by repeatedly applying a second coating liquid to the second catalytic layer obtained in the second pyrolysis step, drying the layer, and heat-treating the layer. That is, the cycle of the second coating step, the second film-forming step, and the second pyrolysis step may be repeated. From a practical perspective, the number of times the second coating step, the second film-forming step, and the second pyrolysis step are repeated is preferably 20 or less. The thicker the second catalytic layer, the longer the period during which a low overvoltage can be maintained.

[0059] In order to form the second catalytic layer to a predetermined thickness, it is sufficient to increase the amount of coating per time or to increase the metal concentration of the ruthenium compound. However, if the amount of coating per time is large, unevenness may occur during coating, and the second catalytic layer may not be formed uniformly. For this reason, in forming the second catalytic layer, it is preferable to carry out coating, drying, and baking by pyrolysis several times. From the above viewpoints, the amount of coating per time of the second coating liquid in the second coating step is set to 0.1 to 2 g / m of ruthenium in the formed catalytic layer. ２ It is preferable to set the density to about 0.2 to 1.8 g / m ２ It is more preferable to set the density to 0.5 to 1.6 g / m. ２ is.

[0060] (Ruthenium Supported Amount) Generally, the amount of metal element contained in the catalytic layer of the electrode may be appropriately determined depending on the lifespan and various performance requirements of the electrode. The cathode according to this embodiment includes the first catalytic layer and the second catalytic layer as described above, and the total amount of ruthenium contained in both catalytic layers is preferably reduced compared to a conventional cathode composed only of a catalytic layer containing ruthenium and praseodymium. A specific reduction amount is preferably a 30% reduction, more preferably a 40% reduction, and even more preferably a 50% reduction.

[0061] From the viewpoint of economic efficiency, the amount of ruthenium carried in the formed first catalytic layer and second catalytic layer is set to 4 to 20 g / m per unit area of ​​the nickel base material. ２ is preferred, and 4 to 10 g / m ２ More preferably, 4 to 6 g / m ２ is more preferable. Furthermore, the first catalytic layer and the second catalytic layer are preferably provided so that the ratio of the amount of ruthenium supported in each of them is 25% to 75% in the first catalytic layer and 75% to 25% in the second catalytic layer (each range is inclusive, and the sum of the ratios in both catalytic layers is 100%; the same applies below). These ratios are more preferably 30% to 70% in the first catalytic layer and 70% to 30% in the second catalytic layer, even more preferably 35% to 65% in the first catalytic layer and 65% to 35% in the second catalytic layer, and even more preferably 35% to 50% in the first catalytic layer and 65% to 50% in the second catalytic layer. The amount of ruthenium supported in the first catalytic layer can be adjusted by the number of times the first coating step, first film-forming step, and first pyrolysis step are repeated. The amount of ruthenium supported in the second catalytic layer can be adjusted by the number of times the second coating step, the second film-forming step, and the second thermal decomposition step are repeated. The ratio of the amount of ruthenium supported in the first coating step to the amount of ruthenium supported in the second coating step is preferably 30-70:70-30, more preferably 35-65:65-35, and even more preferably 35-50:65-50.

[0062] (Nickel substrate) A nickel 200 mesh having dimensions of 100 mm x 100 mm x 0.89 mm was subjected to a blasting treatment (spraying treatment) with corundum (corundum stone), then etched in 20% boiling HCl for 5 minutes, and further subjected to a thermal annealing process at 500°C for 1 hour to obtain a nickel substrate.

[0063] (Ruthenium elemental solution) A solution of ruthenium nitrosyl nitrate (Ru(NO)(NO)) in an amount equivalent to 100 g of ruthenium was prepared. ３ ) ３ ) was dissolved in 300 ml of glacial acetic acid to which a few ml of concentrated nitric acid had been added. The solution was stirred for 3 hours while maintaining the temperature at 50°C. The solution was then brought to a volume of 500 ml with 10 wt% acetic acid. The ruthenium concentration was 200 g / L.

[0064] (Praseodymium elemental solution) An amount of praseodymium nitrate (Pr(NO)) equivalent to 100 g of praseodymium was ３ ) ２ ) was dissolved in 300 ml of glacial acetic acid to which a few ml of concentrated nitric acid had been added. The solution was stirred for 3 hours while maintaining the temperature at 50°C. The solution was then brought to a volume of 500 ml with 10% by weight acetic acid. The praseodymium concentration was 200 g / L.

[0065] (First Coating Solution) A ruthenium solution and a praseodymium solution were mixed so that the element ratio (molar ratio) of Ru:Pr was approximately 8:2 (80:20 to 90:10), and the mixture was left to stand for 5 minutes under stirring. The solution thus obtained was diluted with 10% by weight of acetic acid to make 1 liter.

[0066] (Second Coating Liquid) A ruthenium nitrate solution (specifically, a ruthenium nitrate nitric acid aqueous solution; ruthenium concentration: 100 g / L), a neodymium nitrate solution (neodymium concentration: 100 g / L), and an iron nitrate solution (iron concentration: 100 g / L) were mixed so that the element ratio (molar ratio) was approximately Ru:Nd:Fe = 8:1:1 to prepare a second coating liquid. The pH was adjusted to 2 or less so that the second coating liquid was strongly acidic. The pH may be adjusted to 1 or less, if necessary.

[0067] [Example 1] (First coating step) The first coating liquid was applied to the nickel substrate using a brush. (First film formation step) After each coating, a drying treatment was carried out at 80 to 90°C for 10 minutes. (First pyrolysis step) Pyrolysis was carried out at 400°C for 10 minutes. The cycle of each of the above steps was repeated until the Ru loading amount reached 2 g / m. ２ The coating amount of ruthenium in the first catalytic layer was calculated from the mass ratio of the added elements, assuming that the oxides were formed while maintaining the ratio of the added elements in the first coating solution.

[0068] (Second coating step) The second coating liquid was applied to the formed first catalyst layer with a brush. (Second film formation step) After each coating, a drying treatment was carried out at 80 to 90°C for 10 minutes. (Second pyrolysis step) Pyrolysis was carried out at 400°C for 10 minutes. The cycle of each of the above steps was repeated until the Ru loading amount reached 3 g / m. ２ The second catalyst layer was provided by repeating the above procedure until the coating amount of ruthenium in the second catalyst layer was calculated in the same manner as in the first catalyst layer.

[0069] [Example 2] A first catalytic layer and a second catalytic layer were provided on a nickel substrate in the same manner as in Example 1. However, in Example 2, pyrolysis was carried out at 500°C for 10 minutes in the first pyrolysis step, and pyrolysis was also carried out at 500°C for 10 minutes in the second pyrolysis step.

[0070] [Example 3] A first catalytic layer and a second catalytic layer were provided on a nickel substrate in the same manner as in Example 1. However, in Example 3, the cycle from the second coating step to the second pyrolysis step was carried out with a Ru loading of 4 g / m ２ This was repeated until the second catalyst layer was formed.

[0071] [Example 4] A first catalytic layer and a second catalytic layer were provided on a nickel substrate in the same manner as in Example 3. However, in Example 4, pyrolysis was carried out at 500°C for 10 minutes in the first pyrolysis step.

[0072] Example 5 A first catalytic layer and a second catalytic layer were provided on a nickel substrate in the same manner as in Example 1. However, in Example 5, the cycle from the first application step to the first pyrolysis step was carried out with a Ru loading of 3 g / m ２ The cycle from the second coating step to the second pyrolysis step was repeated until the Ru loading amount reached 2 g / m ２ This was repeated until the second catalyst layer was formed.

[0073] [Comparative Example 1] The nickel substrate was subjected to a cycle from the first coating step to the first pyrolysis step using the first coating liquid in the same manner as in Example 1, with a Ru loading of 11 g / m ２ This process was repeated until the first coating liquid was applied to the nickel substrate, and an electrolytic cathode was fabricated without applying the second coating liquid. However, in the first coating step, the first coating liquid was applied by electrostatic application. In Comparative Example 1, only the first catalyst layer was provided on the nickel substrate, and the second catalyst layer was not provided.

[0074] [Comparative Example 2] Only the first catalyst layer was provided on the nickel substrate in the same manner as in Comparative Example 1. However, in Comparative Example 2, the first coating liquid was applied by brush coating in the first coating step. In addition, the cycle from the first coating step to the first pyrolysis step was carried out with a Ru loading of 6 g / m ２ The first catalyst layer was provided by repeating this process until the

[0075] Comparative Example 3 A mixed catalyst layer was formed on the above nickel base material using a mixed coating liquid prepared by previously mixing the first coating liquid and the second coating liquid. Specifically, the following steps were carried out.

[0076] (Third coating step) A tray containing the mixed coating liquid was placed at the bottom of the coating roll, and an EPDM (ethylene propylene rubber) coating roll was placed above it so as to be in constant contact with the mixed coating liquid. Furthermore, a PVC (polyvinyl chloride) upper roller was placed above the coating roll. The nickel substrate was passed between the coating roll and the upper roller to coat the mixed coating liquid onto the nickel substrate (roll method).

[0077] (Third film formation step) The mixed coating solution applied to the nickel substrate was then dried for 10 minutes at 50°C to form a mixed coating film. After the mixed coating film was formed, it was pre-baked in the atmosphere at 150°C for 3 minutes using a muffle furnace (product name: KM-600, manufactured by Advantech Co., Ltd.).

[0078] (Third Pyrolysis Step) Subsequently, the mixture coating film was subjected to heating and baking at a baking temperature of 400° C. for 10 minutes to be pyrolyzed.

[0079] The cycle from the third coating step to the third pyrolysis step was carried out with a Ru loading of 4 g / m ２ The amount of ruthenium applied in the mixed catalyst layer was calculated from the mass ratio of the added elements, assuming that the oxides were formed while maintaining the ratio of the added elements in the mixed coating solution.

[0080] Comparative Example 4 A mixed catalyst layer was provided on a nickel substrate in the same manner as in Comparative Example 3. However, in Comparative Example 4, pyrolysis was carried out at 450° C. for 10 minutes in the third pyrolysis step.

[0081] Details of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 1.

[0082] (Cyclic voltammetry) The samples of Examples 1 to 5 and Comparative Examples 1 to 4 were subjected to a performance test under hydrogen generation in an experimental cell supplied with 32% NaOH at a temperature of 90°C. A current density of -6 kA / m was applied until the potential stabilized. ２ The electrode was subjected to cyclic voltammetry at a scan rate of 5 mA / s between −1.070 V and +0.5 V relative to the normal hydrogen electrode (NHE).

[0083] The test results for each sample are shown in Table 1. CV0 is the initial (before test) cathode potential of each sample, and CV25 is the cathode potential of each sample after 25 cycles. ΔCV0 is the difference between the CV0 value of each sample and the CV0 value of Comparative Example 1. ΔCV25 is the difference between the CV25 value of each sample and the CV25 value of Comparative Example 1.

[0084] In general, it can be said that the smaller ΔCV0, the higher the catalytic activity of the electrolysis cathode compared to Comparative Example 1. In addition, it can be said that the smaller ΔCV25, the higher the reverse current resistance of the electrolysis cathode compared to Comparative Example 1.

[0085] Focusing on ΔCV0, Examples 2 to 4 have lower values ​​than Comparative Example 1. Furthermore, Example 1 has a higher value than Comparative Example 1, but the difference is slight. Therefore, it can be said that Examples 1 to 4 have catalytic activity equivalent to or higher than Comparative Example 1, even if the amount of Ru supported is reduced to about half that of Comparative Example 1 as a conventional electrolysis cathode. On the other hand, Comparative Examples 2 to 4 have higher values ​​than Comparative Example 1. Therefore, it can be said that the catalytic activity was lower than that of Comparative Example 1 in Comparative Example 2, in which the amount of Ru supported of Comparative Example 1 was simply reduced to about half, and in Comparative Examples 3 and 4, in which the amount of Ru supported was reduced to about half that of Comparative Example 1 using a mixed coating liquid in which the first coating liquid and the second coating liquid were mixed in advance.

[0086] Focusing on ΔCV25, Examples 3 and 4 have lower values ​​than Comparative Example 1, and Examples 1, 2, and 5 have higher values ​​than Comparative Example 1, but the difference is small. Therefore, it can be said that Examples 1 to 5 maintain the same level of reverse current resistance as Comparative Example 1, even though the amount of Ru supported is about half that of Comparative Example 1 as a conventional electrolysis cathode. On the other hand, Comparative Examples 2 to 4 have significantly higher values ​​than Comparative Example 1. Therefore, it can be said that Comparative Examples 2 to 4 have significantly lower reverse current resistance than Comparative Example 1.

[0087] As described above, according to Examples 1 to 5, it is possible to obtain an electrolysis cathode that maintains catalytic activity and has excellent reverse current resistance while reducing the amount of ruthenium used in the catalyst layer.

[0088] In this example, the application of the coating liquid in the first and second coating steps in the above-described Example 1 was carried out by the electrostatic coating method already described. The other steps were the same.

[0089] FIG. 1A is a cross-sectional view of the catalytic layer formed in Example 6. FIGS. 1B to 1E show the distributions of ruthenium, praseodymium, neodymium, and iron within the catalytic layer of Example 6. As shown in FIGS. 1B and 1C, ruthenium and praseodymium are distributed uniformly throughout the catalytic layer. On the other hand, as shown in FIGS. 1D and 1E, neodymium and iron are distributed unevenly toward the surface. In other words, a second catalytic layer containing neodymium and iron is provided on the first catalytic layer along the surface of the first catalytic layer. In particular, as shown within the circles in FIGS. 1D and 1E, the second catalytic layer is formed near cracks formed in the first catalytic layer, also along the surfaces of the cracks.

[0090] The formation of the first catalytic layer and the second catalytic layer will be specifically described with reference to FIG. 2 . FIG. 2 is a schematic cross-sectional view of an electrolytic cathode according to this embodiment. The electrolytic cathode 1 according to this embodiment includes a conductive nickel base material 3, a first catalytic layer 5 provided along the surface of the nickel base material 3, and a second catalytic layer 7 provided on the first catalytic layer 5 along the surface of the nickel base material 3. When the first catalytic layer 5 is formed by repeating the cycle from the first coating step to the first pyrolysis step described above, a crystalline compound of ruthenium oxide and praseodymium oxide is formed by pyrolysis. During this process, irregularities such as cracks may occur, as shown in FIG. 2 . When the second catalytic layer 7 is formed on the first catalytic layer 5 thus formed by repeating the cycle from the second coating step to the second pyrolysis step described above, the second catalytic layer 7 is formed along the surface of the first catalytic layer 5. As described above, if irregularities such as cracks occur in the first catalytic layer 5, the second catalytic layer 7 is also formed along the surfaces of these cracks. In this way, it can be said that the electrolysis cathode 1 according to this embodiment is characterized in that the second catalytic layer 7 is formed along the irregularities on the surface of the first catalytic layer 5 .

[0091] Here, because the second coating liquid is strongly acidic, it is assumed that when the second coating liquid is applied, some of the oxide crystalline compounds distributed on the surface side (upper part) of the first catalytic layer 5 are dissolved. That is, it is assumed that the second coating liquid penetrates into a portion of the surface side of the first catalytic layer 5 and dissolves some of the praseodymium oxide in the first catalytic layer 5, causing the praseodymium oxide to mix into the second coating liquid (on the other hand, ruthenium oxide generally does not dissolve much even in strong acids). It is assumed that thermal decomposition in this state results in praseodymium oxide also being mixed in the second catalytic layer 7. Note that, since praseodymium oxide in particular is thought to dissolve only in a strongly acidic aqueous solution, it is preferable that the second coating liquid be a strongly acidic inorganic acid aqueous solution containing a strongly acidic nitric acid aqueous solution in this example. When the cycle from the second coating step to the second pyrolysis step is repeated in this manner, some of the praseodymium oxide derived from the first catalytic layer 5 is also distributed in the second catalytic layer 7, so the distribution of praseodymium oxide from the first catalytic layer 5 to the second catalytic layer 7 changes smoothly, making it difficult to see a clear difference. On the other hand, most of the neodymium and iron in the second coating solution remain in the second catalytic layer 7, so the distribution of neodymium oxide and iron oxide in the second catalytic layer 7 is clearly greater than the distribution of neodymium oxide and iron oxide in the first catalytic layer 5. Here, as described in Japanese Patent No. 6615682, for example, if a strong acidic aqueous solution such as nitric acid is used as the coating solution for forming another catalytic layer on an already formed catalytic layer, there is a risk that the already formed catalytic layer will be damaged or peeled off. However, in the Examples of the present application, the second coating liquid is strongly acidic (a nitric acid aqueous solution with a pH of 2 or less), and thus part of the already formed first catalytic layer 5 is dissolved, causing the praseodymium oxide derived from the first catalytic layer 5 to be distributed also within the second catalytic layer 7. On the other hand, most of the neodymium oxide and iron oxide derived from the second coating liquid remain within the second catalytic layer 7. As will be described later, it is believed that controlling the distribution of each oxide in this manner is what achieved the excellent performance of the Examples of the present application.

[0092] Figure 3A is a cross-sectional view of the catalyst layer formed in Comparative Example 3. Figures 3B to 3E show the distributions of ruthenium, praseodymium, neodymium, and iron in the catalyst layer of Comparative Example 3. As shown in each figure, the elements ruthenium, praseodymium, neodymium, and iron are distributed roughly uniformly within the catalyst layer, making it difficult to discern any trends. In other words, in the catalyst layer formed using a mixed coating liquid in which the first and second coating liquids were mixed in advance, a catalyst layer containing a large amount of neodymium and iron was not provided on a catalyst layer containing less neodymium and iron.

[0093] From the above, it can be considered that the reason why Examples 1 to 5 produced better test results than Comparative Examples 3 and 4 in the above-mentioned cyclic voltammetry test is due to the distribution of each metal element within the catalytic layer, particularly the distribution of neodymium and iron within the catalytic layer. That is, in Examples 1 to 5, the second catalytic layer was formed after the first catalytic layer, as in Example 6. Therefore, it is assumed that in the catalytic layers of Examples 1 to 5 as well, the second catalytic layer containing large amounts of neodymium and iron is provided on the first catalytic layer along the surface of the first catalytic layer, as shown in Figures 1A to 1E. It can be considered that these characteristics of the catalytic layer contribute to the excellent catalytic activity and reverse current resistance shown in Examples 1 to 5, i.e., catalytic activity and reverse current resistance comparable to those of Comparative Example 1, which has about twice the amount of ruthenium supported.

Claims

1. An electrolysis cathode comprising: a conductive substrate, at least the surface of which is made of nickel or a nickel-based alloy; and a catalyst layer containing ruthenium, praseodymium, neodymium, and iron provided on the substrate, wherein the catalyst layer comprises: a first catalyst layer provided on the substrate along the surface of the substrate; and a second catalyst layer provided on the first catalyst layer along the surface of the first catalyst layer, wherein the second catalyst layer contains larger amounts of neodymium and iron than the first catalyst layer.

2. A method for manufacturing an electrolysis cathode, comprising: a first coating step of coating a first coating liquid containing a ruthenium compound and a praseodymium compound onto a conductive substrate, at least the surface of which is made of nickel or a nickel-based alloy; a first thermal decomposition step of providing a first catalytic layer by thermally decomposing the ruthenium compound and the praseodymium compound; a second coating step of coating a strongly acidic second coating liquid containing a ruthenium compound, a neodymium compound, and an iron compound onto the first catalytic layer; and a second thermal decomposition step of providing a second catalytic layer by thermally decomposing the ruthenium compound, the neodymium compound, and the iron compound.

3. The method for producing an electrolytic cathode according to claim 2, wherein the ratio of the amount of ruthenium supported in the first coating step to the amount of ruthenium supported in the second coating step is 35 to 50:65 to 50 (the sum is 100).

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