Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
  • C25B11/03—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form perforated or foraminous
  • C25B11/031—Porous electrodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/052—Electrodes comprising one or more electrocatalytic coatings on a substrate
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
  • C25B11/057—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of a single element or compound
  • C25B11/061—Metal or alloy
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
  • C25B11/077—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the compound being a non-noble metal oxide
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Definitions

• the present invention relates to a metal nonwoven fabric for use in water electrolysis electrodes.
• Patent Document 1 proposes an electrode catalyst that contains copper oxide and a composite oxide of two types of metals on an electrode substrate that contains copper. The document states that this electrode catalyst is unlikely to cause an increase in overvoltage when used as an electrode for producing hydrogen, and has long-term stability.
• Patent Document 2 also describes a method for producing an anode for use in water electrolysis, in which a nanosheet material containing NiFe hydroxide and Ce oxide is formed on a sponge-like substrate (nickel foam) made of nickel metal wire.
• Non-Patent Document 1 describes the use of electrodes made of porous felt using nickel microfibers in alkaline water electrolysis. According to this document, due to the good balance between the surface area of the electrode and the ability to remove bubbles, oxygen generation in alkaline water electrolysis occurs efficiently, improving the rate at which hydrogen is produced at the counter electrode, and it is possible to increase the amount of hydrogen produced per unit area, which ultimately leads to reduced equipment costs for the water electrolysis system.
• JP 2021-70864 A Chinese Patent Publication No. 108447703
• an object of the present invention is to provide a metal nonwoven fabric having sufficient catalytic activity and sufficient corrosion resistance in a water electrolysis reaction environment.
• the catalytic activity and corrosion resistance of the metal nonwoven fabric of the present invention can be improved by configuring the fibers constituting the metal nonwoven fabric to be fibers composed of a core and a coating, using a combination of specific metals and/or compounds as the metals and/or compounds constituting each part, and setting the mass of the metal element contained in the coating within a specific range.
• the present invention has been made based on the above findings, and provides a nonwoven metal fabric containing metal fibers,
• the metal fiber has a core containing a first metal and a coating disposed on a surface of the core and containing a second metal and/or a compound of the second metal;
• the first metal is composed of one or more metal elements
• the second metal and/or the compound of the second metal is composed of one or more metal elements and/or a compound containing one or more metal elements, the second metal and/or a compound of the second metal has catalytic activity and corrosion resistance in a water electrolysis reaction environment;
• the geometric area of the surface of the core per unit mass of the metal nonwoven fabric is S, When the sum of the mass of the second metal present in the coating portion and the mass of the metal element present as a compound of the second metal per unit mass of the metal nonwoven fabric is M2,
• M2/S is 0.01 g/m2 or more and 20.0
• the present invention also provides a nonwoven metal fabric comprising metal fibers,
• the metal fiber has a core containing a first metal and a coating disposed on a surface of the core and containing a second metal and/or a compound of the second metal;
• the first metal is composed of one or more metal elements
• the second metal and/or the compound of the second metal is composed of one or more metal elements and/or a compound containing one or more metal elements, the second metal and/or a compound of the second metal has catalytic activity and corrosion resistance in a water electrolysis reaction environment;
• the present invention provides a nonwoven metal fabric for a water electrolysis electrode, wherein the thickness T of the covering portion is 5 nm or more and 1,500 nm or less.
• FIG. 1 is a schematic diagram of a scanning electron microscope image of a cross section of metal fibers constituting the nonwoven metal fabric of the present invention, cut along a direction perpendicular to the longitudinal direction of the metal fibers.
• FIG. 2 is a graph showing the results of an electrochemical durability test of the nonwoven metal fabric for water electrolysis electrodes of Example 1.
• FIG. 3 is a graph showing the results of an electrochemical durability test of the nonwoven metal fabric for water electrolysis electrodes of Example 4.
• FIG. 4 is a graph showing the results of an electrochemical durability test of the nonwoven metal fabric for water electrolysis electrodes of Example 6.
• FIG. 5 shows the first and second cycles of the cyclic voltammogram of the copper nonwoven fabric electrode not including a coating layer of Comparative Example 1.
• the present invention relates to a metal nonwoven fabric for a water electrolysis electrode.
• the metal nonwoven fabric of the present invention contains metal fibers made of metal or metal compounds.
• the term "metal fiber” may refer to an individual fiber or an aggregate of multiple fibers, depending on the context.
• the nonwoven metal fabric of the present invention is suitably used as an electrode catalyst for water electrolysis.
• a metal nonwoven fabric is a sheet-like fiber assembly mainly made of metal fibers.
• "mainly made of metal fibers” refers to a state in which the content of metal fibers in the metal nonwoven fabric is 50 mass% or more.
• the metal nonwoven fabric may contain other constituent materials such as organic fibers, carbon fibers, and oxide fibers. It may also contain substances in shapes other than fibers. Therefore, the metal nonwoven fabric may be a sheet-like fiber assembly containing multiple constituent materials including metal fibers.
• "made essentially of metal fibers only” means to exclude intentional addition of components other than metal fibers to the nonwoven fabric, and to allow trace components that are inevitably mixed in during the manufacturing process of the metal nonwoven fabric.
• the metal fibers constituting the nonwoven metal fabric have a core and a coating disposed on the surface of the core.
• the core of the metal fiber contains a first metal, and preferably consists of the first metal and unavoidable impurities.
• the first metal is composed of one or more metal elements. Examples of the first metal include copper, silver, gold, aluminum, magnesium, nickel, cobalt, tin, iron, zinc, and alloys containing these metals. Among these, a core part made of copper or a copper alloy is particularly preferable. Note that "made of copper or a copper alloy" means that the proportion of copper in the metal fiber is 60 mass% or more.
• the coating of the metal fiber contains a second metal and/or a compound of the second metal, and when it contains a substance other than the second metal and the compound of the second metal, it preferably contains only inevitable impurities. From the viewpoint of improving catalytic activity and corrosion resistance, it is preferable that the second metal is different from the above-mentioned first metal.
• the second metal and/or the compound of the second metal is a metal consisting of one or more metal elements and/or a compound containing one or more metal elements, that is, the metal elements are classified into those present as the second metal in the coating portion (those contained in the second metal) and those present as a compound of the second metal in the coating portion (those contained in the compound of the second metal).
• the metal nonwoven fabric of the present invention is preferably used as a catalyst for water electrolysis reaction. Since the second metal and/or the compound of the metal are arranged on the surface of the metal fiber, those having catalytic activity and corrosion resistance in a water electrolysis reaction environment are preferably used. This imparts catalytic activity to the metal nonwoven fabric and can increase the corrosion resistance of the metal nonwoven fabric in the reaction environment.
• the water electrolysis reaction environment can be, for example, a solution with a pH of 5 or more.
• the second metal and/or the compound of the metal preferably contains one or more metal elements selected from nickel, cobalt, iron, molybdenum, tungsten, vanadium, zirconium, titanium, aluminum, platinum, iridium, palladium, rhodium, gold, and silver, and more preferably contains one or more metal elements selected from nickel, cobalt, iron, molybdenum, tungsten, platinum, and iridium.
• the metal compound contained in the coating portion include oxides, hydroxides, and oxyhydroxides of the second metal, which are generated by air oxidation of the second metal, as well as phosphorus compounds and sulfur compounds, etc.
• the metal compound contained in the coating portion is preferably a hydroxide or oxyhydroxide.
• the type of the second metal and/or the compound of the metal is preferably selected appropriately according to the type of water electrolysis cell to be applied and the type of reaction.
• the second metal and/or the compound of the metal preferably contains one or more metal elements selected from iridium, copper, cobalt, iron, nickel, ruthenium, manganese, cerium, palladium, aluminum, lead, titanium, strontium, barium, praseodymium, tungsten, vanadium, and zirconium, more preferably one or more metal elements selected from iridium, copper, cobalt, iron, nickel, ruthenium, manganese, cerium, palladium, aluminum, lead, titanium, strontium, barium, praseodymium, tungsten, vanadium, and zirconium, more preferably one or more metal elements selected from iridium, copper, cobalt, iron, nickel, ruthenium, manganese, cerium, palladium, aluminum, lead, titanium, strontium, barium,
• the second metal and/or a compound of the second metal preferably contains one or more metal elements selected from platinum, palladium, ruthenium, iron, nickel, cobalt, copper, tungsten, molybdenum, cerium, lanthanum, titanium, aluminum, gold, and silver, more preferably contains one or more metal elements selected from platinum, palladium, ruthenium, iron, nickel, cobalt, copper, tungsten, molybdenum, cerium, lanthanum, titanium, and aluminum, and even more preferably contains one or more metal elements selected from platinum, palladium, ruthenium, iron, nickel, cobalt, copper, tungsten, and molybdenum.
• the first metal preferably contains one or more metal elements selected from the group consisting of copper, silver,
• the second metal and/or a compound of the second metal preferably contains one or more metal elements selected from the group consisting of ruthenium, iridium, platinum, palladium, rhodium, tin, tantalum, molybdenum, cobalt, manganese, niobium, titanium, tungsten, antimony, silver, copper, nickel, iron, strontium, lithium, thulium, erbium, yttrium, and praseodymium, more preferably one or more metal elements selected from the group consisting of ruthenium, iridium, platinum, palladium, rhodium, tin, tantalum, molybdenum, cobalt, manganese, niobium, titanium, tungsten, and antimony, and even more preferably one or more metal elements selected from the group consisting of ruthenium, iridium, platinum, palladium, rhodium, tin, tantalum, molybdenum, co
• the second metal and/or a compound of the second metal preferably contains one or more metal elements selected from platinum, iridium, palladium, ruthenium, rhodium, molybdenum, tungsten, titanium, vanadium, selenium, tantalum, cobalt, nickel, copper, iron, and silicon, more preferably contains one or more metal elements selected from platinum, iridium, palladium, ruthenium, rhodium, molybdenum, tungsten, titanium, vanadium, selenium, tantalum, cobalt, nickel, and copper, and even more preferably contains one or more metal elements selected from platinum, iridium, palladium, ruthenium, rhodium, molybdenum, and tungsten.
• the first metal preferably contains one or more metal elements selected from platinum, iridium, palladium, ruthenium, rhodium, molybdenum, and tungsten.
• the first metal preferably contains one or more metal
• M2/S is set within a specific range, where S is the geometric area of the surface of the core per unit mass of the metal nonwoven fabric, and M2 is the sum of the mass of the second metal present in the coating portion and the mass of the metal element present as a compound of that metal per unit mass of the metal nonwoven fabric.
• the metal element contained in the coating portion can sufficiently coat the surface of the metal fiber made of the metal constituting the core portion, thereby improving the corrosion resistance.
• the inventors have found that the larger the M2/S, i.e., the thicker the coating, the higher the proportion of the second metal element present in the metallic state in the vicinity of the outermost surface of the coating.
• the electrical conductivity of a metal element is highest in the metallic state, and decreases in the state of a compound such as an oxide or hydroxide.
• transition metal elements spontaneously oxidize the metal surface in air or in an aqueous solution, or at the interface with a proton or hydroxide ion conductor, forming a compound such as an oxide, hydroxide, or oxyhydroxide, and a layer with low electrical conductivity is formed on the electrode surface. Therefore, by increasing the thickness of the coating, the proportion of metal elements in the metallic state with high electrical conductivity near the outermost surface of the coating can be increased, and the catalytic activity of the metal nonwoven fabric of the present invention can be improved.
• the corrosion resistance of the metal nonwoven fabric can be made sufficient while suppressing the amount of the second metal used. Furthermore, the decrease in gas diffusivity of the metal nonwoven fabric caused by the coating being too thick can be suppressed.
• the electrical conductivity of the metal nonwoven fabric of the present invention can be increased, particularly when a metal with high electrical conductivity is used as the first metal.
• M2/S is preferably 0.01 g/ m2 or more and 20.0 g/ m2 or less, more preferably 0.05 g/ m2 or more and 18.0 g/m2 or less , even more preferably 0.10 g/ m2 or more and 15.0 g/ m2 or less, and even more preferably 0.20 g/ m2 or more and 8.0 g/ m2 or less.
• M2 and S used in the calculation of M2/S are measured as follows.
• the measurement sample used is a cross-sectional sample obtained by cutting the metal nonwoven fabric along a plane perpendicular to its length.
• the geometric area S of the surface of the core of the measurement sample, the metal nonwoven fabric, per unit mass can be calculated by assuming that the metal fiber is a perfect cylinder and considering only the outer circumferential surface as the surface, since the aspect ratio of the metal fiber is high.
• the mass M1 of the first metal per unit mass of the metal nonwoven fabric, and the sum M2 of the mass of the second metal and the mass of the metal element present as a compound of the second metal per unit mass of the metal nonwoven fabric, can be measured, for example, by analyzing the concentration of the metal element contained in an aqueous solution obtained by dissolving a unit mass of the metal nonwoven fabric in a strong acid using inductively coupled plasma atomic emission spectrometry. They can also be evaluated by X-ray fluorescence analysis (hereinafter also referred to as "XRF").
• XRF X-ray fluorescence analysis
• the coating portion contains a second metal and/or a compound of the second metal, and from the viewpoint of improving the electrical conductivity of the coating portion, it is preferable that the coating portion contains at least the second metal. Specifically, it is preferable that the coating portion is composed of the second metal and inevitable impurities, the second metal and a compound of the second metal, or the second metal, a compound of the second metal, and inevitable impurities. It is believed that the inclusion of the second metal in the coating portion increases the electrical conductivity of the coating portion and improves the catalytic activity.
• the ratio of the metal element present as the second metal to the total of the metal element present as the second metal in the coating portion and the metal element present as a compound of the metal is preferably 2.0 atm% or more, more preferably 2.5 atm% or more, and even more preferably 3.0 atm% or more.
• the ratio can be calculated by the following calculation formula using the peak area of the peak observed by X-ray photoelectron spectroscopy (hereinafter also referred to as "XPS").
• XPS X-ray photoelectron spectroscopy
• the coating covers the surface of the core evenly, since this can improve the catalytic activity and corrosion resistance of the metal nonwoven fabric.
• the degree of coating of the core by the coating in the metal fiber can be evaluated by analyzing the chemical composition of the metal fiber surface using XPS.
• the ratio (element composition percentage) of the metal element present as the first metal to the total of the metal element present as the first metal on the outermost surface of the metal fiber, the metal element present as the second metal, and the metal element present as a compound of the metal can be calculated from the photoelectron spectrum from the inner shell electrons obtained by XPS, so that the ratio of the metal element present as the first metal exposed on the outermost surface can be evaluated.
• the ratio of the first metal measured by XPS is preferably 5.0 atm% or less, more preferably 4.0 atm% or less, even more preferably 3.0 atm% or less, even more preferably 2.5 atm% or less, and ideally zero atm%. Details of the measurement conditions in the above XPS measurements will be described in the Examples section below.
• the first metal has a higher electrical conductivity than the second metal and/or the compound of said metal.
• the value of ⁇ 1/ ⁇ 2 is preferably 1.05 or more, more preferably 1.50 or more, and even more preferably 2.00 or more.
• the metal nonwoven fabric may be composed of a single type of metal fiber, or may contain multiple types of metal fibers with different types of metal elements contained in the core or coating.
• the metal fibers constituting the metal nonwoven fabric are randomly stacked, and the metal fibers are bonded to each other by entanglement and/or fusion, etc., thereby maintaining the form of a fabric.
• a metal nonwoven fabric due to the random stacking of the metal fibers constituting the metal nonwoven fabric, a plurality of voids formed between the fibers by the plurality of fibers are continuous in the thickness direction of the metal nonwoven fabric. This allows the metal nonwoven fabric to have gas diffusibility in the thickness direction.
• the metal nonwoven fabric of the present invention When used as an electrode catalyst for water electrolysis, the metal nonwoven fabric of the present invention having gas diffusibility can have a catalytic effect on the water electrolysis reaction not only on the surface of the metal nonwoven fabric but also inside the metal nonwoven fabric, improving the catalytic activity.
• the metal nonwoven fabric may have a single layer structure or a laminate structure in which a plurality of layers are laminated together. When the metal nonwoven fabric has a laminate structure, the layers may be the same or different from each other.
• the term "same" as used here means that the metal fibers constituting the metal nonwoven fabric are the same.
• Such a metal nonwoven fabric can be suitably manufactured, for example, by the manufacturing method described below.
• the average diameter D of the core of the metal fiber is preferably 20 nm or more and 10.0 ⁇ m or less, more preferably 20 nm or more and 6.00 ⁇ m or less, even more preferably 30 nm or more and 3.00 ⁇ m or less, even more preferably 120 nm or more and 2.50 ⁇ m or less, and particularly preferably 0.50 ⁇ m or more and 2.00 ⁇ m or less.
• the average diameter D of the core is preferably 20 nm or more and 10.0 ⁇ m or less, more preferably 20 nm or more and 6.00 ⁇ m or less, even more preferably 30 nm or more and 3.00 ⁇ m or less, even more preferably 120 nm or more and 2.50 ⁇ m or less, and particularly preferably 0.50 ⁇ m or more and 2.00 ⁇ m or less.
• the thickness T of the coating of the metal fibers is preferably 5 nm or more and 1500 nm or less, more preferably 5 nm or more and 1000 nm or less, even more preferably 10 nm or more and 800 nm or less, even more preferably 40 nm or more and 500 nm or less, even more preferably 70 nm or more and 500 nm or less, and particularly preferably 100 nm or more and 500 nm or less.
• the thickness T of the coating is preferably 5 nm or more and 1500 nm or less, even more preferably 5 nm or more and 1000 nm or less, even more preferably 10 nm or more and 800 nm or less, even more preferably 40 nm or more and 500 nm or less, even more preferably 70 nm or more and 500 nm or less, and particularly preferably 100 nm or more and 500 nm or less.
• the value of T/D which is the ratio of the thickness T of the coating to the average diameter D of the core, is preferably 0.01 to 1.00, more preferably 0.01 to 0.70, and even more preferably 0.02 to 0.40.
• T/D the ratio of the thickness T of the coating to the average diameter D of the core.
• the first metal (core) and the second metal or compound (coating) are different, by increasing the volume ratio of the first metal with high electrical conductivity to the whole, the average electrical conductivity ⁇ av of the entire metal nonwoven fabric including the coating is increased, and water electrolysis can be performed with high efficiency, thereby improving the catalytic activity of the nonwoven fabric of the present invention.
• the average length of the metal fibers is preferably 0.5 ⁇ m or more and 5000 ⁇ m or less, more preferably 0.5 ⁇ m or more and 2000 ⁇ m or less, even more preferably 1 ⁇ m or more and 500 ⁇ m or less, even more preferably 3 ⁇ m or more and 200 ⁇ m or less, even more preferably 3 ⁇ m or more and 150 ⁇ m or less, even more preferably 3 ⁇ m or more and 90 ⁇ m or less, and particularly preferably 25 ⁇ m or more and 90 ⁇ m or less.
• the average diameter of the core of the metal fibers in the metal nonwoven fabric and the thickness and average length of the covering portion may be the same on one side of the metal nonwoven fabric or may be different. Alternatively, when the metal nonwoven fabric is viewed along the thickness direction, the average diameter of the core of the metal fibers and the thickness and average length of the covering portion may each be changed stepwise, continuously, or a combination thereof.
• Such a metal nonwoven fabric is formed, for example, by using multiple types of metal fibers.
• the metal fibers in the metal nonwoven fabric are preferably very thin and long, with the average diameter of the core and the thickness and average length of the covering falling within the above-mentioned ranges. This enhances the gas diffusivity of the metal nonwoven fabric, and increases the electrolytic current. In addition, it becomes easier to control these characteristic values of the metal fibers, and also makes them easier to handle.
• the aspect ratio of the metal fibers (average length of the metal fibers/average diameter D of the core of the metal fibers) is preferably 5 or more and 5,000 or less, more preferably 20 or more and 3,000 or less, and even more preferably 20 or more and 1,500 or less.
• the average diameter D of the core and the thickness T of the coating can be measured by the following method.
• the cross section observation sample can be cut after embedding the nonwoven fabric in resin, such as polyepoxy resin.
• the metal nonwoven fabric is observed at a magnification at which the average diameter D of the core and the thickness T of the coating are easily observed, specifically, at any magnification between 1000 and 10000 times.
• FIG. 1 shows a schematic diagram of an SEM image of a metal fiber 1 constituting a metal nonwoven fabric.
• the 1 is composed of a core 2 and a coating 3 covering its surface.
• 20 to 100 cores 2 are randomly selected, and for each core 2, an imaginary straight line passing through two points on the surface (outer circumference) of the core 2 that are the longest distance between the two points is considered.
• a line perpendicular to the imaginary straight line and having the longest length is considered.
• the lengths of the line segments are measured, and the arithmetic mean value is taken as the average diameter D of the core portion.
• 20 to 100 measurement points are randomly selected on the surface of the core 2, and the distance between each measurement point and the outer surface of the coating 3 is measured.
• the arithmetic mean value of the measured distances is then taken as the thickness T of the coating 3.
• the average diameter D and thickness T may be measured using image analysis software such as image analysis type particle size distribution measurement software Mac-View Version. 4 (manufactured by Mountec Co., Ltd.).
• image analysis software such as image analysis type particle size distribution measurement software Mac-View Version. 4 (manufactured by Mountec Co., Ltd.).
• a region where the surface of the core portion 2 is convex toward the inside region 4 in FIG. 1).
• a region where multiple covering portions 3 are in contact with each other and the boundaries between them are unclear (region 5 in FIG. 1).
• An area where part of the covering portion 3 is missing and the entire core portion 2 is not covered.
• the average length of the metal fibers in the metal nonwoven fabric can be measured by the following method.
• the metal nonwoven fabric is observed by SEM.
• the metal nonwoven fabric is observed at a magnification at which the length of the metal fibers constituting the metal nonwoven fabric can be easily observed, specifically at any magnification between 200x and 2000x.
• 20 metal fibers are taken out, excluding those that are entangled with other metal fibers and cannot be measured individually, and the length of each metal fiber is measured. If the metal fibers extend beyond one screen of the SEM image, the length of each metal fiber may be calculated by connecting multiple consecutive SEM images in a panoramic manner.
• the arithmetic mean value of these is calculated to obtain the average length of the metal fibers.
• image analysis software such as image analysis type particle size distribution measurement software Mac-View Version. 4 (manufactured by Mountec Co., Ltd.) may be used.
• the metal fibers in the metal nonwoven fabric may have a form in which the average diameter of the core and the thickness of the coating are almost uniform over the entire length, or the average diameter and thickness are not uniform but are bead-shaped. It is preferable that at least one end of the metal fibers is tapered.
• a “tapered shape” refers to a shape in which the thickness gradually decreases toward the tip when observing the end region of the metal fiber.
• metal fibers with at least one end tapered reduce unevenness of the contact surface compared to metal fibers with a uniform fiber diameter, so that the variation in the size of the voids in the metal nonwoven fabric can be suppressed and stable catalytic activity can be obtained.
• the angle of the tip of the tapered shape is 90 degrees or less, more preferably 80 degrees or less, even more preferably 70 degrees or less, even more preferably 60 degrees or less, even more preferably 50 degrees or less, and particularly preferably 45 degrees or less.
• the angle of the tip of the tapered shape is measured by the following procedure.
• the fiber diameter of the metal fiber calculated as the sum of the diameter of the core and the thickness of the coating, is measured by the above-mentioned method using a SEM at a magnification between 1000x and 10000x.
• an arc with a diameter equal to the fiber diameter of the metal fiber is drawn with the end tip of the metal fiber as the center, and two intersections of the arc and the metal fiber are obtained.
• the angle between the two intersections and the end tip of the metal fiber is measured as the tip angle. Note that if the cross section of the end of the metal fiber is linear or approximately linear, the center of the end tip is taken as the end tip.
• the metal fiber is not measured. This measurement is performed for 10 or more metal fibers, and the arithmetic average value is taken as the angle of the tip of the tapered shape.
• the shape of the metal fibers in metal nonwoven fabrics is typically that of fibers extending in one direction, and these metal fibers may or may not have a main chain portion extending in one direction and a branched structure in which the main chain portion branches off halfway. From the viewpoint of controlling the size of the voids in the metal nonwoven fabric and improving gas diffusion properties, it is preferable that the metal fibers have a non-branched structure having only a main chain portion. On the other hand, from the viewpoint of making the metal nonwoven fabric have a bulky structure and further improving the contact area with liquid, it is preferable that the metal fibers have one or more branched portions. Furthermore, the metal nonwoven fabric may be formed using only metal fibers that have a non-branched structure having only a main chain portion, or may be formed using only metal fibers that have one or more branched portions, or may be formed using a combination of these.
• the number of metal fibers having curved parts with a radius of curvature of 5 times or less the length of the metal fiber accounts for 20% or more of the total number of metal fibers in the metal nonwoven fabric.
• the metal fibers constituting the metal nonwoven fabric have curved parts from the beginning, because this can improve the resistance of the metal nonwoven fabric when the metal nonwoven fabric is compressed, pulled, or subjected to bending stress.
• contact across multiple metal fibers along the lateral (width) direction of the metal fibers is easily achieved, which has the advantage of reducing electrical resistance.
• the radius of curvature is calculated as follows. First, the metal nonwoven fabric is observed using an SEM.
• Both ends of the metal fibers in the SEM image are connected by a straight line, and the length (chord length) of the line is measured. Furthermore, an auxiliary line perpendicular to the line is drawn from the midpoint of the line toward the metal fiber, and the distance (arrow height) between the midpoint and the point where the line intersects with the metal fiber is measured.
• the above-mentioned radius of curvature is preferably 0.5 ⁇ m or more and 1000 ⁇ m or less.
• the radius of curvature is calculated by approximating the shape of the metal fibers as having a curved portion from the above formula.
• the radius of curvature is measured as if the straight line were a different metal fiber at the crossing point.
• metal nonwoven fabric it is not prohibited to contain particles having a shape other than that of metal fibers.
• the metal nonwoven fabric is produced based on the idea of controlling the gaps in the metal nonwoven fabric by randomly stacking the fibers, from the viewpoint of improving the gas diffusion properties of the metal nonwoven fabric even when it is subjected to deformation such as bending and stretching, it is preferable that particles having a shape other than that of fibers are not present in the metal nonwoven fabric as much as possible.
• the peak top diameter in the pore size distribution of voids measured by mercury porosimetry in the metal nonwoven fabric is preferably 0.01 ⁇ m or more and 30 ⁇ m or less, more preferably 0.02 ⁇ m or more and 25 ⁇ m or less, even more preferably 0.05 ⁇ m or more and 20 ⁇ m or less, even more preferably 0.10 ⁇ m or more and 20 ⁇ m or less, and even more preferably 0.15 ⁇ m or more and 15 ⁇ m or less.
• the distribution peak top diameter of the peak with the highest height falls within the above-mentioned range, since this makes the above-mentioned advantages even more pronounced.
• the void distribution peak top diameter is obtained by the mercury porosimetry method.
• the size of the voids is obtained from the average value of the sphere-equivalent diameters of the pores divided by the image analysis of the X-ray CT measurement.
• Mercury porosimetry is measured using an Autopore IV9510 (Micromeritics).
• the mercury pressure input is set to about 0.5 to 60,000 psi (about 3 kPa to 400 MPa)
• the measurement mode is set to the pressure increase (pressure intrusion) process
• the mercury contact angle is set to 141.3°
• the mercury surface tension is set to 484 dyn/cm.
• X-ray CT measurement is performed using a SKYSCAN AN2214 (manufactured by Bruker), and the average value of the sphere-equivalent diameter of the pores dividing the voids is determined using image analysis software Avizo 3D (manufactured by Thermo Fisher Scientific).
• the distribution peak top diameter of the voids in the above-mentioned metal nonwoven fabric measured by mercury porosimetry, and the sphere equivalent diameter (average) of the pores dividing the voids determined by X-ray CT can be achieved, for example, by adjusting the average diameter of the cores of the metal fibers, the thickness of the coating, and the average length of the metal fibers. It can also be achieved by intentionally mixing multiple types of metal fibers with different average core diameters, coating thicknesses, and average lengths of the metal fibers. In addition to this, or instead, it can also be achieved by appropriately adjusting the conditions in the manufacturing method of the metal nonwoven fabric described below.
• the nonwoven metal fabric of the present invention has appropriate voids as described above and has high gas diffusivity, so that the decrease in reaction efficiency caused by the decrease in effective surface area is unlikely to occur and the fabric has high catalytic activity.
• an electrode catalyst for water electrolysis containing a metal nonwoven fabric having pores wider than the appropriate pore size has excellent gas diffusivity, but the catalytic efficiency is inferior to that of the metal nonwoven fabric of the present invention because the surface area of the metal nonwoven fabric is small.
• the average thickness of the metal nonwoven fabric is preferably 3 ⁇ m or more and 50 mm or less, more preferably 50 ⁇ m or more and 30 mm or less, and even more preferably 90 ⁇ m or more and 10 mm or less. If the average thickness of the metal nonwoven fabric is 3 ⁇ m or more, the metal nonwoven fabric can maintain a self-supporting state and has excellent practicality. Furthermore, if the average thickness of the metal nonwoven fabric is 50 mm or less, the gas diffusion properties of the metal nonwoven fabric can be increased, improving the catalytic activity.
• the average thickness of the metal nonwoven fabric can be measured by the following method. First, cut the metal nonwoven fabric into a length of 2 cm x width of 2 cm to prepare a cut piece of the metal nonwoven fabric. However, if it is not possible to prepare a cut piece of this size as the cut piece to be measured, prepare a cut piece as large as possible. Next, place the cut piece on a flat plate, and place a flat plate weighing 1 g and having dimensions larger than the cut piece on top of it. Under this condition, measure the distance from the lower surface of the flat plate below the cut piece to the upper surface of the flat plate above the cut piece with a vernier caliper.
• Subtract the thickness of the two flat plates measured in advance from the obtained measurement value is the thickness of the metal nonwoven fabric.
• the thickness of the nonwoven metal fabric exceeds 10 ⁇ m, the thickness is measured by a vernier caliper as described above.
• the average thickness is measured by a cross-sectional SEM image.
• the specific surface area of the metal nonwoven fabric is preferably 0.02 m 2 /g or more and 22 m 2 /g or less, more preferably 0.1 m 2 /g or more and 10 m 2 /g or less, even more preferably 0.2 m 2 /g or more and 3 m 2 /g or less, and even more preferably 0.25 m 2 /g or more and 2 m 2 /g or less.
• the specific surface area of the metal nonwoven fabric can be measured by the following method.
• the specific surface area is measured by the krypton gas adsorption-BET multipoint method using, for example, a specific surface area/pore distribution measuring device BELSORP-max (manufactured by BEL Japan).
• X-ray CT measurement is performed using, for example, a nanofocus X-ray CT scanner SKYSCAN AN2214 (manufactured by Bruker), and the specific surface area is determined using image analysis software Avizo 3D (manufactured by Thermo Fisher Scientific), which digitizes the structural features of the three-dimensional object being measured, such as the area and volume of the object, based on the obtained three-dimensional information.
• image analysis software Avizo 3D manufactured by Thermo Fisher Scientific
• the specific surface area of a metal nonwoven fabric can be adjusted, for example, by forming the metal nonwoven fabric using metal fibers with different average core diameters or different thicknesses of coating, or by stacking multiple types of metal nonwoven fabric.
• the apparent density of the metal nonwoven fabric is preferably 0.01 g/cm3 or more and 2.0 g/cm3 or less , more preferably 0.05 g/cm3 or more and 1.7 g/cm3 or less , and even more preferably 0.1 g/cm3 or more and 1.4 g/ cm3 or less.
• the apparent density can be measured as follows. First, the volume V1 of the metal nonwoven fabric is calculated from the length and width of the metal nonwoven fabric and the thickness of the metal nonwoven fabric measured by the above-mentioned method. Next, the mass of the metal nonwoven fabric is measured and the apparent density is calculated by dividing the mass by the volume V1.
• the apparent density of metal nonwoven fabrics can be adjusted, for example, by increasing the average diameter of the core of the metal fibers or the thickness of the coating, by increasing the average thickness of the metal fibers, or by stacking multiple types of metal nonwoven fabrics.
• the metal nonwoven fabric may be used as a three-layer laminate, for example, with one or more layers of mesh or perforated foil (hereinafter also referred to as "mesh, etc.") disposed between two layers of metal nonwoven fabric.
• mesh perforated foil
• it may be used as a three-layer or more laminate, with one layer of metal nonwoven fabric disposed between two layers of mesh, etc. This can improve the strength of the metal nonwoven fabric.
• the average opening size of the mesh (JIS Z8801) or the average opening size of the perforated foil is preferably 900 ⁇ m or less.
• the mesh may be made of metal or nonmetal. When the mesh is made of metal, the type of metal constituting the mesh may be the same as or different from the type of metal constituting the metal nonwoven fabric.
• the metal fibers that mainly constitute the nonwoven metal fabric are preferably produced by first producing fibers (corresponding to the core of the nonwoven metal fabric) made of a first metal, piling up the fibers to form a nonwoven fabric (hereinafter also referred to as a "nonwoven fabric intermediate"), and then forming a coating containing a second metal and/or a compound of the second metal on the surface of the fibers.
• the fibers made of the first metal can be produced by an electroless method or an electrolytic method. When producing fibers by an electroless method, for example, the method described in ACS Appl. Mater. Interfaces, 2017, 9, 34715-34721 can be adopted.
• the electrolytic method When fibers are produced by the electrolytic method, an electrolyte containing a first metal source is used, and fibers made of the first metal are precipitated on a cathode by electrolytic reduction.
• fibers with a low degree of aggregation can be easily obtained.
• the electrolytic method has the advantage that the fibers can be easily controlled to a desired shape.
• the electrolyte can be reused, and less liquid is required to produce the fibers, so that the amount of waste liquid to be treated can be reduced at the same time. From the above points, the electrolytic method is preferably used in the present invention.
• fibers made of the first metal are produced by electrolysis, for example, an anode and a cathode are immersed in a sulfuric acid electrolyte containing a source of the first metal, and a direct current is passed through the electrolyte to perform electrolytic reduction.
• a preferred current density is 50 A/m2 or more and 600 A/m2 or less.
• the oily substance to be attached to the surface of the cathode when manufacturing the fibers made of the first metal includes various organic compounds that are poorly soluble or insoluble in water and have a viscosity that allows them to be retained on the surface after being attached to the surface of the cathode.
• Such organic compounds include aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic alcohols, aromatic alcohols, aliphatic aldehydes, aromatic aldehydes, aliphatic ethers, aromatic ethers, aliphatic ketones, aromatic ketones, aliphatic carboxylic acids and their salts, aromatic carboxylic acids and their salts, amides of aliphatic carboxylic acids, amides of aromatic carboxylic acids, esters of aliphatic carboxylic acids, esters of aromatic carboxylic acids, and the like, provided that they are poorly soluble or insoluble in water.
• higher fatty acids or their salts, esters or amides, or organic solvents in which they are mixed are particularly preferred.
• the higher fatty acids may be monobasic acids or polybasic acids.
• Examples of higher fatty acids include saturated or unsaturated aliphatic carboxylic acids having preferably 10 to 25 carbon atoms, more preferably 10 to 22 carbon atoms, and even more preferably 11 to 20 carbon atoms.
• Methods for attaching an oily substance to the surface of a cathode when manufacturing fibers made of a first metal include, for example, a method of directly applying the oily substance to the surface of the cathode, a method of immersing the cathode in a container containing the oily substance to attach it, and a method of floating the oily substance on the electrolyte and immersing the cathode from above to attach it.
• Another method is to suspend the oily substance in the electrolyte and stir the suspended electrolyte to collide the suspended oily substance with the surface of the cathode and attach it directly to the surface of the cathode.
• the oily substance has a property of dissolving in a small amount in the electrolyte, even if the oil droplets mainly composed of the oily substance in the suspension containing the oily substance and the electrolyte do not directly touch the electrode, the oily substance once dissolved in the electrolyte is continuously adsorbed onto the electrode surface, and the same effect as when the oil droplets are in direct contact with the electrode can be obtained.
• the amount of oily substance adhered to the surface of the cathode when producing fibers made of the first metal is preferably 1 g/m2 or more and 500 g/m2 or less per unit surface area of the cathode, more preferably 3 g/m2 or more and 200 g/m2 or less , and even more preferably 5 g/m2 or more and 100 g/m2 or less .
• the precipitated fibers of the first metal are left to settle, forming a nonwoven fabric intermediate.
• a slurry of the fibers of the first metal is poured into a water-impermeable mold, the mold is left to stand to deposit the fibers, and then the supernatant of the slurry is removed from the mold, and the mold is further dried to obtain a nonwoven fabric intermediate.
• a method for removing the supernatant of the slurry for example, a method using a centrifuge, a method of decanting the supernatant, or a method of volatilizing the supernatant can be used.
• the dimensions of the mold may be approximately the same as the dimensions of the target metal nonwoven fabric, or may be larger than the dimensions.
• a metal nonwoven fabric having a desired size of voids in the metal nonwoven fabric and a desired porosity can be obtained.
• a slurry of fibers made of the first metal can be filtered using a filter to deposit the fibers on the filter, and then dried to obtain a nonwoven intermediate.
• the nonwoven fabric intermediate can be dried in an air atmosphere or an inert gas atmosphere. Since the metal fibers in the nonwoven fabric intermediate are very thin and long, voids are generated in the nonwoven fabric intermediate.
• the nonwoven fabric intermediate can be pressed in the thickness direction using a press.
• This process allows the thickness of the metal nonwoven fabric to be adjusted as desired. Furthermore, by adjusting the pressure, time, temperature, etc., when pressing the nonwoven fabric intermediate in the thickness direction, the size of the voids in the produced metal nonwoven fabric and the porosity can be adjusted to the desired value.
• the nonwoven fabric intermediate can be removed from the mold, surrounded on all four sides by metal plates (spacers) having the desired thickness, and pressed from above the metal plates.
• the size of the voids and the porosity in the produced metal nonwoven fabric can be adjusted to the desired value.
• the pressure applied to the nonwoven fabric intermediate is preferably 0.01 MPa or more and 50 MPa or less, more preferably 1 MPa or more and 30 MPa or less, and even more preferably 1 MPa or more and 20 MPa or less, from the viewpoint of obtaining a metal nonwoven fabric having the desired size of voids and porosity in the metal nonwoven fabric.
• the time for pressing the nonwoven fabric intermediate is preferably from 3 minutes to 5 hours, more preferably from 10 minutes to 3 hours, and even more preferably from 30 minutes to 2 hours.
• the nonwoven intermediate may be heated.
• the nonwoven intermediate may be pressed without heating.
• the temperature is preferably 500° C. or less, more preferably 350° C. or less, from the viewpoint of preventing excessive fusion between the metal fibers.
• the nonwoven fabric intermediate After drying the nonwoven fabric intermediate or pressing the nonwoven fabric intermediate in the thickness direction, the nonwoven fabric intermediate can be calcined without pressure in an argon, nitrogen, or hydrogen-containing atmosphere as necessary to fuse the intersections of the fibers made of the first metal in the nonwoven fabric intermediate.
• the nonwoven fabric intermediate is calcined without pressure in a hydrogen-containing atmosphere. Since the fibers made of the first metal are very long and thin, the surfaces of the metal fibers in the nonwoven fabric intermediate are easily oxidized. Therefore, it is advantageous to calcinate the nonwoven fabric intermediate to reduce the surfaces of the metal fibers.
• the fibers made of the first metal are obtained by the above-mentioned electrolysis method, there is also the advantage that at least a part of the metal fibers in the nonwoven fabric intermediate is easily fused by reducing and calcining the nonwoven fabric intermediate.
• the time for calcining the nonwoven fabric intermediate is preferably from 3 minutes to 24 hours, and more preferably from 30 minutes to 5 hours, from the viewpoint of sufficiently and efficiently reducing the surface oxide and fusing the fibers made of the first metal together.
• the temperature at which the nonwoven fabric intermediate is fired is preferably 800°C or lower, and more preferably 150°C or higher and 500°C or lower, from the viewpoint of efficiently reducing the metal and achieving adequate fusion between the fibers made of the first metal.
• a coating containing a second metal and/or a compound of the second metal is formed on the surface of the fibers made of the first metal that constitute the nonwoven fabric intermediate.
• the method of forming the coating on the surface of the fibers made of the first metal is not particularly limited.
• electrolytic plating in an electrolytic solution containing a source of the second metal, cationic electrodeposition coating, anionic electrodeposition coating, surface oxidation treatment (a method of forming a film by reacting a chemical substance in a solution by electrically or chemically oxidizing the surface), displacement plating and electroless plating, a method of plating a target substance after attaching a catalyst for these plating methods to the nonwoven fabric intermediate, liquid phase deposition, electrophoresis, a method using a surface potential difference, a sol-gel method, a gel-sol method, a polyol method, an evaporation method, a sputtering method, a CVD method, a pyrolysis method, a plasma film formation method, and a method of physically or chemically coating fine particles, such as a dipping method, a spray method, a cold spray method, a spray drying method, an atomic layer deposition method, a coating method, an ink method, and a dry method.
• surface oxidation treatment
• electrolytic plating is preferred for the production of the metal nonwoven fabric of the present invention because it allows uniform coating in a short time. Furthermore, by using electrolytic plating, it is expected that the electrochemically highly active surface of the second metal will be preferentially exposed for coating.
• an electrolytic solution containing a second metal source is used, and a coating containing a second metal and/or a compound of the second metal is precipitated on the surface of the fibers constituting the nonwoven fabric intermediate by electrolytic reduction to form the coating.
• the electrolytic reduction can be carried out, for example, by using the nonwoven fabric intermediate as a cathode, immersing the nonwoven fabric intermediate and the anode in an electrolytic solution containing the second metal source, and passing a direct current therethrough.
• the preferred current density in this case is 50 A/ m2 or more and 1500 A/ m2 or less.
• the second metal and/or the compound of the second metal is a metal consisting of one or more metal elements and/or a compound containing one or more metal elements.
• the ion concentration of the second metal source in the electrolytic solution is preferably 10 g/L or more and 140 g/L or less, more preferably 20 g/L or more and 100 g/L or less, and even more preferably 40 g/L or more and 70 g/L or less.
• the metal nonwoven fabric obtained in this manner may be cut into a desired shape.
• multiple pieces of the obtained metal nonwoven fabric may be laminated to form a metal nonwoven fabric having a laminated structure.
• the metal nonwoven fabric has sufficient catalytic activity and is also able to have sufficient corrosion resistance in a water electrolysis reaction environment.
• the catalyst containing the metal nonwoven fabric of the present invention obtained by the above method can be applied to water electrolysis reactions.
• the water electrolysis electrode catalyst containing the metal nonwoven fabric of the present invention has excellent catalytic activity and corrosion resistance, and is therefore suitable for use.
• the metal nonwoven fabric of the present invention is used as an electrode catalyst for water electrolysis, there is no limitation on the type of water electrolysis.
• the metal nonwoven fabric of the present invention can be applied to water electrolysis methods such as alkaline water electrolysis, PEM water electrolysis, and AEM water electrolysis.
• the water electrolysis electrode catalyst containing the metal nonwoven fabric of the present invention can be incorporated into a water electrolysis electrode or processed into a water electrolysis electrode, and can be used as a water electrolysis device containing the electrode.
• the metal nonwoven fabric of the present invention is incorporated into a water electrolysis electrode or processed into a water electrolysis electrode, it is preferable that the pores in the metal nonwoven fabric are maintained in order to exhibit excellent catalytic activity.
• a metal nonwoven fabric for a water electrolysis electrode comprising metal fibers,
• the metal fiber has a core containing a first metal and a coating disposed on a surface of the core and containing a second metal and/or a compound of the second metal;
• the first metal is composed of one or more metal elements
• the second metal and/or the compound of the second metal is composed of one or more metal elements and/or a compound containing one or more metal elements, the second metal and/or a compound of the second metal has catalytic activity and corrosion resistance in a water electrolysis reaction environment;
• the geometric area of the surface of the core per unit mass of the metal nonwoven fabric is S, When the sum of the mass of the second metal present in the coating portion and the mass of the metal element present as a compound of the second metal per unit mass of the metal nonwoven fabric is M2,
• a metal nonwoven fabric for a water electrolysis electrode having an M2/S of 0.01 g/m 2 or more and 20.0
• a metal nonwoven fabric containing metal fibers The metal fiber has a core containing a first metal and a coating disposed on a surface of the core and containing a second metal and/or a compound of the second metal;
• the first metal is composed of one or more metal elements
• the second metal and/or the compound of the second metal is composed of one or more metal elements and/or a compound containing one or more metal elements, the second metal and/or a compound of the second metal has catalytic activity and corrosion resistance in a water electrolysis reaction environment;
• An electrode catalyst for water electrolysis comprising the metal nonwoven fabric according to any one of [1] to [11].
• a water electrolysis device comprising the electrode catalyst for water electrolysis according to [12].
• Example 1 the metal nonwoven fabric was manufactured using copper as the first metal and nickel as the second metal.
• An electrolyte solution was prepared from copper sulfate and sulfuric acid so that the copper ion concentration was 40 g/L and the free sulfuric acid concentration was 19.6 g/L, and 800 mL of the solution was placed in an electrolytic cell measuring 10 cm x 8 cm x 12 cm (capacity: approximately 1000 mL) and stirred.
• the electrolyte temperature was 40°C.
• a copper plate of 8 cm x 8 cm was used as the cathode. Oleic acid was evenly applied to the surface of the cathode. The amount of application was 7 g/ m2 .
• a copper plate of 8 cm x 8 cm was used as the anode. Both electrodes were suspended in an electrolytic cell so that the distance between the cathode and anode was 8 cm. The current density was adjusted to 313 A/ m2 , and electrolysis was carried out for 30 minutes. In this manner, fibrous copper was electrodeposited on the surface of the cathode. The fibrous copper thus deposited was deposited on the electrode in a state of low degree of aggregation, and was peeled off and dispersed in 2-propanol to form a slurry.
• the formation of the coating was carried out by electrolytic plating.
• a Watts bath was used as the plating solution.
• the plating solution was prepared so that the nickel sulfate concentration was 0.84 mol/L, the nickel chloride concentration was 0.13 mol/L, and the boric acid concentration was 0.49 mol/L, and 800 mL of the plating solution was placed in a plating tank measuring 10 cm x 8 cm x 12 cm (capacity approximately 1000 mL) and stirred.
• the temperature of the plating solution was 40°C.
• a nonwoven fabric intermediate (50 mm x 12 mm) sandwiched between two copper meshes (20 mesh) of 70 mm x 20 mm was used as the cathode, and the nonwoven fabric intermediate was nickel-plated together with the mesh.
• a nickel plate of 70 mm x 20 mm was used as the anode. Both electrodes were suspended in a plating tank so that the distance between the cathode and the anode was 3 cm. The current density was adjusted to 538 A/ m2 with respect to the area of the nonwoven fabric intermediate, and electrolysis was performed for 416 seconds. In this way, a metal nonwoven fabric for water electrolysis electrodes was obtained, which included metal fibers with a nickel-containing coating formed on the surface of the core. The thickness of the metal nonwoven fabric was 1 mm.
• the coating portion of the obtained metal nonwoven fabric for water electrolysis electrode contained metallic nickel (Ni) as the second metal, as well as NiO and Ni(OH) 2 as compounds of the second metal.
• Examples 2 to 6 A nonwoven metal fabric for water electrolysis electrodes having a core and a nickel-containing coating formed on the surface thereof was obtained in the same manner as in Example 1, except that the electrolytic plating time in the electrolytic plating step was set to the time shown in Table 1. In all of the Examples, the thickness of the nonwoven fabric was 1 mm.
• the coating portion of the obtained metal nonwoven fabric for water electrolysis electrodes contained metallic nickel (Ni) as the second metal, as well as NiO and Ni(OH) 2 as compounds of the second metal.
• Comparative Example 1 A nonwoven metal fabric for water electrolysis electrodes having no covering portion was obtained in the same manner as in Example 1, except that the electrolytic plating step was not performed. Comparative Example 2 To confirm that it is preferable that the core contains a first metal and the coating contains a second metal element, a porous metal body made of nickel metal (Ni-Celmet (registered trademark), manufactured by Sumitomo Electric Industries, Ltd.) was used instead of a metal nonwoven fabric. To clean the surface of the Ni-Celmet, it was immersed in a 1 mol/L hydrochloric acid aqueous solution, ultrasonically cleaned for 1 hour, rinsed with water, and then air-dried in the air.
• Ni-Celmet registered trademark
• Ni-Celmet was cut to 50 mm x 25 mm to obtain an electrode catalyst for water electrolysis having no coating.
• values evaluated by XRF based on the above-mentioned method were used for M1 and M2.
• the average diameter D, the thickness of the metal nonwoven fabric, and the average length of the metal fibers were measured. Based on these measured values, the aspect ratio of the metal fibers was calculated.
• the average diameter D of the washed Ni-ceramic metal was measured by obtaining the arithmetic mean value of measurements taken at 10 points using an optical microscope DIGITAL MICROSCOPE VHX-900 (manufactured by KEYENCE CORPORATION) and image processing software ImageJ. The other items were measured by the methods described above.
• Example 2 the washed Ni-ceramic metal of Comparative Example 2 was a single continuous fiber, and since it was difficult to measure the average length, the corresponding portion of Table 2 is labeled "continuum.”
• the ratio (A) of the metal element present as the first metal to the total of the metal element present as the first metal, the metal element present as the second metal, and the metal element present as a compound of the first metal was measured by XPS measurement.
• the ratio (B) of the metal element present as the second metal and the ratio (C) of the metal element present as a compound of the second metal to the total of the metal element present as the second metal (metallic nickel) in the coating portion and the metal element present as a compound of the second metal (nickel oxide and nickel hydroxide) were measured by XPS measurement. These results are shown in Table 3.
• the ratios (A) to (C) were calculated using the following formulas.
• Ratio (A) (peak area of metal element present as first metal)/((peak area of metal element present as first metal)+(peak area of metal element present as second metal)+(peak area of metal element present as compound of second metal))
• Ratio (B) (peak area of the metal element present as the second metal)/((peak area of the metal element present as the second metal)+(peak area of the metal element present as a compound of the second metal))
• Ratio (C) (peak area of Ni element present as NiO or Ni(OH) 2 ) / ((peak area of metal element present as second metal) + (peak area of metal element present as compound of second metal))
• Measurement device PHI Quantes (manufactured by ULVAC-PHI, Inc.)
• Excitation X-ray Monochromatic Al-K ⁇ ray (1486.7 eV)
• Output 25W Acceleration voltage: 15 kV ⁇ X-ray irradiation diameter: 100 ⁇ m ⁇ ⁇ Measurement area: 1000 x 300 ⁇ m2
• Detection angle 45°
• Pass energy 26.0 eV Energy step: 0.1 eV/step
• Hg/HgO and Pt foil were used as the reference electrode and counter electrode, respectively, and the OER overpotential in the alkaline solution was evaluated by LSV.
• the OER current was recorded at room temperature under nitrogen bubbling in a 1 mol/L potassium hydroxide solution at a scan rate of 10 mV/s.
• LSV measurement was repeated four times by sweeping from 0.9 V to 1.6 V vs. reversible hydrogen electrode (RHE) in the anode direction. Then, LSV measurements were performed four times by sweeping from 1.6 V to 0.9 V vs. RHE in the cathodic direction.
• the OER current was recorded from the fourth LSV measurement by sweeping in the cathodic direction.
• LSV measurement was repeated four times by sweeping from 0.8 V to 2.5 V in the anode direction at a scanning speed of 10 mV/s. Then, LSV measurement was repeated four times by sweeping from 2.5 V to 0.8 V in the cathode direction.
• the water electrolysis current of the fourth LSV measurement by sweeping in the cathode direction was recorded and used as the evaluation result before durability fluctuation.
• the potential fluctuation range was set to 0.1 V to 2.5 V, and 1000 potential fluctuations were performed at a scanning speed of 100 mV/s. Then, the water electrolysis current was recorded under the same conditions as the evaluation before durability fluctuation and used as the evaluation result after durability fluctuation.
• the metal nonwoven fabrics for water electrolysis electrodes of each Example had lower overvoltages in the oxygen generation reaction and superior catalytic activity than the metal nonwoven fabrics for water electrolysis electrodes of each Comparative Example and the Ni-Celmet after washing. Furthermore, as shown in Figures 2 to 4, the metal nonwoven fabrics for water electrolysis electrodes of Examples 1, 4, and 6 did not show an increase in overvoltage before and after 1,000 potential fluctuations, confirming that the nonwoven fabrics of the present invention have sufficient corrosion resistance in a water electrolysis reaction environment. On the other hand, as shown in Table 2, the metal nonwoven fabrics for water electrolysis electrodes and the washed Ni-Celmet of Comparative Examples 1 and 2 had significantly high overvoltages in the oxygen generation reaction. Furthermore, as shown in Fig.
• the metal nonwoven fabric for water electrolysis electrodes of Comparative Example 1 did not have a coating layer, and therefore when the potential was increased to 1.23 V or more at which the oxygen generation reaction occurs in the electrochemical measurement, a large oxidation current was generated, and even when the potential was increased again, no current flowed, and no oxygen generation current was observed. Furthermore, the metal nonwoven fabric for water electrolysis electrodes of Comparative Example 1 turned black after the electrochemical measurement. This is thought to be due to the surface being oxidized and inactivated.
• the proportion of nickel present in a metallic state in the coating measured by XPS increases as M2/S increases, and therefore it is believed that the electrical conductivity in the coating also increases as M2/S increases.
• the average electrical conductivity of the entire electrode decreases as M2/S increases, and therefore it is clear that by setting M2/S within an appropriate range as in the present invention, the catalytic activity of the metal nonwoven fabric for water electrolysis electrodes can be maximized.
• the present invention provides a metal nonwoven fabric for water electrolysis electrodes that has sufficient catalytic activity and sufficient corrosion resistance in a water electrolysis reaction environment.

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