Electrode and manufacturing method thereof

Abstract

To provide an electrode having a sufficiently low hydrogen overvoltage, a fast hydrogen generation rate, and no deactivation of an active material on a hydrogen generating electrode.SOLUTION: A surface of a substrate containing 50 mass% or more iron atoms shall have a layer of mackinawite with a thickness of 10 micrometers or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention uses an alkaline aqueous solution such as a KOH aqueous solution to generate hydroxide ions (OH - The present invention relates to an electrode, such as a hydrogen generating electrode for generating hydrogen or a catalytic electrode, which is used in a water electrolysis device that exchanges hydrogen ions, and a method for manufacturing such an electrode. [Background technology]

[0002] In response to the global trend toward decarbonization, the use of hydrogen gas is being explored. This is because the use of hydrogen gas has a low environmental impact. For example, in the case of fuel cells, the only waste product after power generation is water, which has a low environmental impact. Hydrogen gas can also be used to reuse CO2. A typical example is methanation, which synthesizes methane from CO2 and hydrogen gas. As such, hydrogen gas is considered to be an extremely useful substance for realizing a decarbonized society. However, when we look at the production of hydrogen gas, it cannot be said that the production process is sufficiently decarbonized.

[0003] Currently, there are four methods for producing hydrogen gas: reforming fossil fuels, by-product gases from steel mills and chemical plants, biomass, and water electrolysis. When it comes to reforming fossil fuels and by-product gases from steel mills and chemical plants, the original fuel is an underground resource such as petroleum, and naturally CO2 emissions are unavoidable during the hydrogen gas production process. Biomass is not derived from underground resources, but it will be difficult to obtain sufficient amounts of hydrogen gas from biomass alone to support the decarbonized society of the future. In the first place, the main primary product gases from biomass are methanol and methane, and in many cases it is more efficient to use them as is without reforming them into hydrogen.

[0004] Here, water electrolysis (also known as water electrolysis) is a method of splitting water into oxygen gas and hydrogen gas using electricity. When electricity using fuel derived from underground resources is used, it cannot be said that the hydrogen gas obtained is a decarbonized production method, but when electricity derived from so-called natural energy is used, it can be said that the hydrogen gas obtained is a decarbonized production method.

[0005] As described above, when comparing methods for producing hydrogen gas, water electrolysis using natural energy is the most promising method for producing hydrogen as it does not produce CO2 during the production process.

[0006] One type of water electrolysis method involves electrolyzing an alkaline aqueous solution containing KOH or other substances, such as alkaline or anion exchange membrane (AEM). Compared to proton exchange membrane (PEM)-type water electrolysis, these electrolysis methods do not require expensive alloys for the hydrogen- or oxygen-generating electrodes, making them relatively inexpensive. However, these water electrolysis methods have their own problems due to the use of alkaline aqueous solutions. Specifically, when the power supply drops or is interrupted, the catalyst used in the hydrogen-generating electrode changes or dissolves in the electrolyte, resulting in a loss of catalytic activity, a phenomenon known as deactivation. This has made it difficult to produce hydrogen gas using renewable energy sources such as wind and solar power, which have large fluctuations in power output and are subject to power interruptions at night.

[0007] To solve the above problems, it has been considered to continue water electrolysis by using stored hydrogen to generate electricity when the power supply is reduced or cut off, but it goes without saying that this is an inefficient method of obtaining hydrogen that can be used in industry.In addition, since it is necessary to construct huge hydrogen storage tanks and fuel cells near the water electrolysis equipment to produce hydrogen, the ancillary facilities become large-scale, and it is difficult to say that this method reduces the total production cost.

[0008] To solve the problem of catalyst deactivation in the hydrogen generating electrode, for example, Patent Document 1 proposes a hydrogen generating electrode in which nickel oxide is formed on a nickel substrate and a manufacturing method thereof. That is, it discloses a method in which nickel oxide, which is resistant to deterioration due to oxidation, is applied to a nickel substrate and sintered at a temperature of 600°C or less, and a hydrogen generating electrode obtained in this manner. However, even at a temperature of 600°C or less, it is difficult to completely prevent deterioration of the strength of the nickel substrate due to sintering.

[0009] Furthermore, Patent Document 2 proposes an electrode that combines a metal that easily adsorbs hydrogen and a metal that easily desorbs hydrogen. That is, the electrode discloses an electrode that has high hydrogen catalytic activity (referred to as having a low hydrogen overvoltage) and excellent stability by combining two types of metals that have the above-mentioned properties, each of which is highly stable as a substance. However, because the hydrogen adsorption site and hydrogen desorption site on the electrode are different, the rate of hydrogen generation is not sufficiently fast, and the efficiency of hydrogen generation is not necessarily high.

[0010] Furthermore, Patent Document 3 proposes an electrode for hydrogen generation that exhibits low hydrogen overvoltage and excellent durability, in which nickel fine particles are supported on a carrier made of one or more compounds selected from carbides, oxides, and sulfides of molybdenum or tungsten. However, because the manufacturing method described in Patent Document 3 includes a sintering step, it is difficult to completely prevent deterioration in the strength of the electrode substrate.

[0011] Furthermore, Patent Document 4 proposes an electrode for hydrogen generation that has low hydrogen overvoltage and durability, in which an alloy of nickel and molybdenum with a lattice constant of 3.566 Å or less is laminated on an electrode substrate. However, because the manufacturing method described in Patent Document 4 includes a sintering step, it is difficult to completely prevent deterioration in the strength of the electrode substrate.

[0012] Furthermore, Patent Document 5 proposes an electrode in which an Fe-Co-Ni-C alloy is coated on an electrode substrate by a wet method (electroplating method). However, as described in detail in Patent Document 5, the stability of the electrode disclosed in Patent Document 5 is achieved by suppressing the elution of Fe from the electrode into the solution when electrolysis is interrupted to the extent that it can be avoided by adjusting the conditions for using the electrode, and therefore the effect is limited, and a more fundamental solution is desired.

[0013] Furthermore, Patent Document 6 proposes an electrode for alkaline water electrolysis that can be used as both a cathode and an anode, in which an Fe-Ni-W alloy film is formed on a substrate. However, looking at the examples disclosed in Patent Document 6, it is clear that LSV measurement to investigate the characteristics as an anode electrode was performed only once, and it is unclear whether the alloy will be oxidized and deactivated even when sweeping from the cathode polarization side to the anode polarization side is repeated many times, and a more fundamental solution is desired. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-234379 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-234380 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-317289 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-013271 [Patent Document 5] Japanese Patent Application Laid-Open No. 2015-178666 [Patent Document 6] Japanese Patent Application Publication No. 2018-127664 Summary of the Invention [Problem to be solved by the invention]

[0015] An object of the present invention is to provide an electrode that overcomes the above-mentioned problems of the prior art, i.e., an electrode that has a sufficiently low hydrogen overvoltage, a high rate of hydrogen evolution, and is free from deactivation of the active material on the electrode. Another object of the present invention is to provide a method for producing such an electrode. [Means for solving the problem]

[0016] The inventors conducted a wide-ranging search for a material that could solve the above-mentioned problems and discovered an electrode in which an iron sulfide called mackinawite is formed to a thickness of 10 micrometers or more on the surface of an electrode substrate containing 50 mass% or more of iron atoms. First, the inventors discovered that the surface of the iron sulfide called mackinawite has a low hydrogen overvoltage. That is, mackinawite satisfies the low hydrogen overvoltage required for a hydrogen generation electrode in a water electrolysis device. Incidentally, mackinawite is an n-type semiconductor. Furthermore, because mackinawite is a homogeneous compound, it is not necessary to combine a metal that easily adsorbs hydrogen and a metal that easily desorbs hydrogen, as described in Patent Document 2. That is, because hydrogen adsorption and desorption occur on the same compound in mackinawite, hydrogen does not need to migrate from the adsorbed site to the desorbed site by diffusion or other means, and hydrogen generation is sufficiently fast.

[0017] When mackinawite is used for the hydrogen generating electrodes of a water electrolysis device that uses an alkaline aqueous solution as the solution and exchanges hydroxide ions between electrodes, such as alkaline water electrolysis or AEM water electrolysis, the behavior of mackinawite when the supply of power fluctuates or is interrupted is as follows. First, when hydrogen is generated, that is, when the hydrogen generating electrode is polarized to the cathode side, the reaction of water being decomposed by electricity to generate hydrogen is proceeding on the Mackinawite electrode. Meanwhile, a small amount of sulfur atoms are ionized and sulfur ions (S 2- ) is also in progress. In other words, some of the sulfur that made up the mackinawite is ionized and dissolved into the electrolyte. The sulfur ions (S 2-) is a hydrogen oxide ion (OH - ), the ion size is larger than that of the sulfur ions (S 2- ) remains on the cathode side.

[0018] On the other hand, in the case of conventional hydrogen generating electrodes (described in the above-mentioned patent documents), which in the case of AEM-type electrodes are sometimes called catalytic electrodes, distinguished from the main electrode to which a conductor is generally connected, a state occurs in which the electrodes are polarized toward the anode side, i.e., the side where electrons are removed from hydroxide ions and a reaction occurs to generate water and oxygen, and from the electrode's perspective, the side where the oxidation reaction is promoted. This is the cause of oxidation of the hydrogen generating electrode when the power supply fluctuates or is interrupted. In this way, when the hydrogen generating electrode is polarized toward the anode side, oxygen in the electrolyte releases electrons and bonds with the material that constitutes the hydrogen generating electrode, causing the material to oxidize. This is the mechanism of deactivation.

[0019] The reason why the hydrogen generating electrode can become polarized toward the anode side when the power supply fluctuates or is interrupted is that if the power supply fluctuates or is interrupted suddenly, the hydrogen generating electrode is temporarily polarized toward the anode side, not toward the oxygen generating electrode at the opposite electrode, but toward the ion exchange membrane separating the electrodes, or toward the material separating the electrolysis cell. For this reason, even if the hydrogen generating electrode should be polarized toward the cathode side when viewed as a whole, the hydrogen generating electrode becomes oxidized when the power supply fluctuates or is interrupted suddenly, and if this occurs repeatedly, it will eventually become deactivated. Incidentally, in PEM water electrolysis devices, protons (H + In a system in which hydroxide ions are exchanged between electrodes, even if the hydrogen evolution electrode is polarized toward the anode side, deactivation does not occur because there are no hydroxide ions.

[0020] On the other hand, if the hydrogen generation electrode is made of mackinawite, even if the hydrogen generation electrode is polarized to the anode side, the oxidation described above does not occur, and instead sulfur ions (S 2-) loses electrons and is oxidized, and mackinawite is regenerated on the electrode. In other words, the mackinawite that was partially deactivated during hydrogen generation is regenerated. In this way, when mackinawite is used as a hydrogen generation electrode, the electrode does not oxidize due to fluctuations or interruptions in the power supply, and there is no deactivation due to these causes. Furthermore, both iron and sulfur are very common elements, and are advantageously very inexpensive compared to materials that have been considered so far as hydrogen generating electrodes for alkaline water electrolysis devices. The inventors have obtained the above findings and have completed the present invention.

[0021] The present invention has been made based on the above findings, and the gist of the present invention is as follows. 1. An electrode having a layer of mackinawite with a thickness of 10 micrometers or more on the surface of a substrate containing iron atoms at 50 mass% or more.

[0022] 2. The method for producing an electrode according to the above item 1, wherein the substrate is coated with a liquid having a specific liquid amount relative to the surface area of ​​the substrate of 50 mL / cm. 2 and maintaining the temperature above 60°C, the substrate is immersed in an aqueous solution of ammonium thiocyanate having a concentration of 1.0 mass% or more for 15 hours or more, to form mackinawite on the surface of the substrate. [Effects of the Invention]

[0023] The electrode of the present invention has a hydrogen overvoltage equal to or lower than that of the prior art, yet exhibits the remarkable effects of high hydrogen gas generation rate because hydrogen adsorption and desorption occur on the same compound, and the hydrogen generation electrode does not oxidize or deactivate even when the power supply fluctuates or is interrupted. Furthermore, since the electrode of the present invention is made of inexpensive compounds, it is also possible to reduce manufacturing costs. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 10 is a diagram illustrating a method for determining an angle at which a peak exists from an X-ray spectrum. [Figure 2] FIG. 1 shows a coupon-embedded test specimen. [Figure 3] FIG. 2 is a diagram illustrating a cathodic polarization curve. [Figure 4] FIG. 1 is a diagram illustrating a polarization cycle. DETAILED DESCRIPTION OF THE INVENTION

[0025] The electrode for hydrogen generation of the present invention will be described below. That is, the electrode for hydrogen generation of the present invention is characterized by having a layer of mackinawite with a thickness of 10 micrometers or more on the surface of a substrate containing 50 mass% or more of iron atoms. There are several types of iron sulfide (FeS) with a crystalline structure in addition to mackinawite. Specifically, troilite, puroite, etc. exist, but mackinawite is optimal for the present invention because it has the characteristic of low hydrogen overvoltage. Each of the above configurations will be described in detail below.

[0026] [Base material] Substrate containing iron atoms at 50 mass% or more The hydrogen generation electrode in an alkaline water electrolysis device generally comprises a metal or alloy mesh, expanded metal, or porous sheet on which a catalyst that promotes hydrogen generation is formed. The substrate is required to have the following characteristics: chemical stability in the electrolyte and sufficient strength to maintain its shape at the operating temperature of the device. Furthermore, if the electrode manufacturing process includes a heat treatment step such as calcination, deformation during the heat treatment step must be small enough to not be detrimental to the device design.

[0027] Specifically, an iron-based alloy containing 50 mass% or more of iron atoms is chemically stable in alkaline conditions (aqueous solutions with a pH of 7 or higher). Furthermore, such an alloy does not change in hardness (i.e., strength as a substrate) at temperatures of 90°C or lower used in alkaline water electrolysis devices or AEM water electrolysis devices. This hardness is preferably 150 or higher in Vickers hardness. Furthermore, if a heat treatment is performed in the electrode manufacturing process, and the Vickers hardness value does not change by ±20 or more before and after the heat treatment, it can be said that deformation of the substrate is small enough not to be detrimental to the design.

[0028] In addition to the above, in the case of the electrode substrate of the present invention, it is preferable that iron atoms or iron ions are supplied from the electrode substrate during the mackinawite regeneration process when the power supply fluctuates or is interrupted. This is because, as sulfur ions are eluted during hydrogen generation, some of the iron atoms that originally formed mackinawite together with sulfur are lost, and this iron atom replacement is necessary to supply the iron atoms that were lost. Because power supply fluctuations and interruptions are frequently repeated, even if the amount of eluted iron atoms is extremely small, the amount of eluted iron atoms cannot be ignored over a long cumulative device usage time. Therefore, it is preferable that iron is supplied from the electrode substrate during mackinawite regeneration. To achieve this, the substrate itself must contain 50 mass% or more of iron atoms. More preferably, it should contain 60 mass% or more, and even more preferably, it should contain 70 mass% or more. Since such an electrode substrate is essentially an iron alloy, the substrate's strength is also excellent.

[0029] [McKennawight] Mackinawite is the core compound of the present invention. Specifically, the surface layer of mackinawite is prone to the adsorption of hydrogen atoms from the electrolyte (generally referred to as the Volmer process) and the conversion of the adsorbed hydrogen atoms into hydrogen gas molecules (generally referred to as the Tafel process). This Tafel process is particularly favorable when the hydrogen overvoltage is low. Furthermore, mackinawite is stable in alkaline solutions, making it chemically stable in the electrolytes used in alkaline water electrolysis and AEM water electrolysis. Mackinawite is also an iron sulfide. Therefore, it is less expensive than the Ni-S alloys, Raney nickel, precious metals such as platinum and ruthenium, nickel oxide, and Ni-Sn alloys that have been investigated as catalysts for hydrogen generation electrodes, as well as the materials, metals, alloys, and compounds disclosed in the above-mentioned patent documents. Furthermore, as described below, mackinawite is formed as a result of a corrosion reaction when immersed in a solution at or below 100°C, eliminating the need for high-temperature firing at several hundred degrees. This means that there is no need to worry about a decrease in the strength of the substrate.

[0030] Next, a method for identifying mackinawite will be described in detail. Mackinawite can be identified by X-ray diffraction. X-ray diffraction (XRD) is preferably performed using thin-film XRD. Specifically, the object to be measured is placed in a thin-film XRD apparatus, and the X-ray incident angle is set to 5°. The X-ray source is CuKα radiation. The detector angle of the diffracted X-rays is then set to 90° from the incident angle. The diffraction peaks in the X-ray intensity spectrum measured under these conditions are compared with the XRD peaks of mackinawite provided by the International Centre for Diffraction Data (ICDD). The presence of three or more diffraction peaks that match the ICDD peaks indicates the presence of a mackinawite layer on the surface of the object to be measured.

[0031] Here, the criterion for determining whether the peak positions match is that if the measured peak exists within ±1° of the peak position provided by ICDD, the peak positions are considered to match. The method for determining the angle at which a peak exists from the X-ray spectrum of the measurement results will be explained with reference to Figure 1. First, the minimum X-ray intensity (referred to as "X-ray intensity Y") present within ±5° (A°±5°) of a certain angle (referred to as "A°") in the spectrum of X-ray intensity versus detector angle is obtained. By collecting A° where the X-ray intensity at A° is more than twice the X-ray intensity Y, a certain width of A° is obtained for each peak. The median angle of this width is determined to be the angle at which the peak exists.

[0032] Mackinawite layer thickness: 10 micrometers or more The thickness of the mackinawite layer formed on the electrode substrate must be 10 micrometers or more. If the mackinawite layer thickness is less than 10 micrometers, there is a concern that the mackinawite layer will completely disappear while the water electrolysis device is operating to generate hydrogen. For example, when natural energy is used, fluctuations and interruptions in the power supply from natural energy basically occur on a 24-hour cycle. In this environment, a mackinawite layer thickness of 10 micrometers is the minimum thickness at which the mackinawite layer is lost and portions of the electrode substrate surface (or the surface of the layer of iron atoms concentrated on the electrode substrate surface after sulfur ion desorption) become exposed to the solution when the water electrolysis device is operated continuously under constant hydrogen generation conditions for two weeks. A thickness greater than this is even more preferable, more preferably 15 micrometers or more, and even more preferably 20 micrometers or more.

[0033] On the other hand, the thickness of the mackinawite layer is preferably 100 micrometers or less. If the thickness of the mackinawite layer exceeds 100 micrometers, the mackinawite layer may easily peel off from the substrate. In other words, if the thickness of the mackinawite layer is too thick, the mismatch caused by the difference in crystal structure and crystal lattice constant between the mackinawite and the substrate becomes large, causing a large stress to concentrate between the layer and the substrate, which may make the layer easily peel off from the substrate. For this reason, the layer thickness is preferably 100 micrometers or less. More preferably, it is 90 micrometers or less.

[0034] The method for producing an electrode for hydrogen generation according to the present invention comprises: disposing the substrate in a solution containing 50 mL of water at a specific liquid volume relative to the surface area of ​​the substrate; 2 The substrate is immersed for 15 hours or more in an aqueous solution of ammonium thiocyanate having a concentration of 1.0 mass % or more and maintained at a temperature above 60° C., thereby forming mackinawite on the surface of the substrate. Each production condition is described in detail below.

[0035] [Ammonium thiocyanate aqueous solution] Specific liquid volume to substrate surface area: 50mL / cm 2 End The substrate can be in various forms such as a plate or mesh, but the amount of solution relative to the total area of ​​the area where you want to form the mackinawite, i.e., the specific liquid volume, is 50 mL / cm. 2 If the specific liquid volume is less than 50 mL / cm, it will be impossible to form a uniform layer of mackinawite with sufficient thickness. 2 If the concentration is less than 55 mL / cm, the decrease in ammonium thiocyanate in the solution during the formation treatment is significant, and a layer of the desired thickness cannot be obtained even if the solution is immersed for 15 hours or more. 2 That's all.

[0036] On the other hand, there is no need to set an upper limit for the specific solution volume, since there are no technical problems caused by an excessively large specific solution volume. In other words, as long as the immersion treatment is performed in an aqueous solution with a specific solution volume equal to or greater than the lower limit of the specific solution volume described above, the specific solution volume itself will not affect the type of iron sulfide formed on the substrate, its thickness, the properties of the mackinawite, or the like. Therefore, the specific solution volume can be determined appropriately by the practitioner, taking into account the economic costs of film formation.

[0037] Ammonium thiocyanate aqueous solution temperature: above 60°C If the temperature of the aqueous solution in which the substrate is immersed is 60°C or lower, mackinawite will not form on the substrate. That is, the temperature at which the iron sulfide formed on the substrate surface during immersion takes on a crystalline structure rather than an amorphous structure and becomes single-phase mackinawite is above 60°C. Therefore, the temperature of the aqueous solution must be above 60°C. It is preferably above 65°C, and more preferably above 70°C.

[0038] There is no specific upper limit to the temperature of the aqueous solution. This is because, even if the water in the aqueous solution evaporates, it can be reduced to a solution as water droplets by cooling the gas phase, eliminating the need to worry about changes in the specific liquid volume or the concentration of the aqueous solution due to evaporation. However, if the aqueous solution boils, the stirring of the solution is significantly accelerated, which may inhibit the formation of a mackinawite coating, which forms on the surface of the substrate as a corrosion film during immersion. Furthermore, even if the solution is not boiling, bubbles tend to form on the inner wall of the aqueous solution container and on the surface of the test piece, which can also inhibit the formation of mackinawite. While this can be resolved by extending the immersion time, it is of course preferable to avoid this situation. To achieve this, it is recommended that the aqueous solution temperature be kept below 90°C.

[0039] Ammonium thiocyanate aqueous solution concentration: 1.0 mass% or more When a metal (substrate) containing 50 mass% or more of iron atoms is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 1.0 mass% or more, thiocyanic acid (SCN - ) decomposes to carbon dioxide, ammonium and S 2-ions and hydrogen atoms are produced. To obtain mackinawite through this corrosion reaction, an aqueous solution containing 1.0 mass% or more of thiocyanic acid is required. This is because, at concentrations below 1.0 mass%, the thiocyanic acid concentration is too low, slowing the decomposition rate of thiocyanic acid, and as a result, it takes a long time, for example, one week or more, to obtain mackinawite of a sufficient thickness. A more preferred concentration is 2 mass% or more, and even more preferably 5 mass% or more.

[0040] On the other hand, there is no need to limit the upper limit of the concentration of the ammonium thiocyanate aqueous solution, since the concentration of the aqueous solution does not affect the sulfide species obtained by immersion or the properties of the mackinawite layer. Therefore, the concentration of the aqueous solution can be appropriately determined by the practitioner, taking into account the economic cost of film formation.

[0041] Soaking time: 15 hours or more The immersion time in the above solution must be at least 15 hours. Therefore, if the immersion time is less than 15 hours, an FeS layer thicker than 10 μm cannot be obtained. This is because the iron sulfide does not form a crystalline structure and becomes amorphous. The iron sulfide formed on the substrate is initially amorphous because it is significantly influenced by the crystalline structure of the substrate. A certain thickness is required for the iron sulfide to form a crystalline structure. This minimum thickness is not entirely clear, but is estimated to be approximately 3–8 μm. While the iron sulfide is amorphous, its chemical stability is low. Therefore, the rate of decomposition of the previously formed iron sulfide is high relative to the generation of new iron sulfide, and the thickness of the iron sulfide does not increase significantly. However, as mentioned above, if the thickness exceeds approximately 3–8 μm and the iron sulfide forms a crystalline structure, chemical stability is increased, decomposition is suppressed, and the growth of the iron sulfide layer as mackinawite is promoted. The minimum time required for iron sulfide to take on a crystalline structure and become mackinawite is 15 hours or more, and more preferably 20 hours or more.

[0042] On the other hand, there is no need to restrict the upper limit of the immersion time. As long as the immersion conditions are within the range specified in the present invention, the mackinawite obtained in the early stages of immersion will not deteriorate in properties or be transformed into a compound with a different crystal structure due to an excessively long immersion time. In other words, the immersion time is merely one experimental variable for controlling the thickness of the mackinawite layer. Therefore, the immersion time can be set appropriately to a preferred value in accordance with the other immersion conditions, taking into account the process and economic costs involved in film formation. [Example]

[0043] The present invention will be described in more detail below based on examples, but the present invention is not limited to the scope described in the examples. Preparation of electrode substrates and test specimens Five kg of material consisting of the chemical composition shown in Table 1 was melted and hot rolled at temperatures between 1000 and 1200°C to produce a rolled plate with a thickness of approximately 10 mm. A rolled plate with an area of ​​approximately 1.5 cm2 was cut from this 10 mm thick rolled plate. 2 A square plate-shaped test piece was cut out so that it had a thickness of 2 mm. This will be referred to as a coupon hereafter. Both sides of the cut coupon were polished to #1000 grit, and a conductor wire was soldered to one side. Thereafter, as shown in Figure 2, coupon 1 was embedded in one side of a cubic resin block 3 made of non-conductive epoxy resin so that the side opposite to the side soldered with conductor wire 2 was exposed and the side soldered with conductor wire 2 was embedded. Note that conductor wire 2 penetrates resin block 3 and is pulled out to the outside of resin block 3. After embedding coupon 1, the exposed side of coupon 1 in resin block 3 was polished again to #1000 grit together with the resin. Next, the boundary between coupon 1 and the epoxy resin was polished so that the area of ​​the part exposed to the outside of coupon 1 was approximately 1.0 cm2. 2 The specimen was then coated with a silicone-type non-conductive sealant to obtain a test piece.

[0044] [Table 1]

[0045] Mackinawite formation on the specimen A treatment to form a Mackinawite layer on the surface of the test piece prepared as described above was carried out as follows. First, a specific liquid volume of 50 mL / cm 2 , that is, the approximately 1.0 cm prepared above 2 For each of the two test pieces, 100 mL of an aqueous solution of ammonium thiocyanate was prepared. The concentration of the aqueous solution of ammonium thiocyanate is shown in Table 2. Next, the prepared aqueous solution was poured into a thermostatic bath heated to the immersion temperature shown in Table 2, and left to stand for at least one hour, maintaining the immersion temperature.

[0046] Two test pieces were immersed in the aqueous solutions in the thermostatic bath prepared in this way for the immersion times shown in Table 2 to form coatings on the surfaces of the test pieces. Thereafter, XRD and coating thickness measurements were carried out on one of the two test pieces as follows. That is, the X-ray diffraction peaks of the obtained coating were obtained by thin-film XRD, and the number of peaks that matched the peaks of mackinawite was determined. The results are shown as the number of peaks in Table 2. Furthermore, a cross section of the test piece on which the thin-film XRD measurement was performed was cut out, and the thickness of the coating formed on the surface layer of the test piece was measured using an optical microscope. The results are shown as coating thickness in Table 2.

[0047] The remaining test piece from the test pieces prepared under the conditions in Table 2 was used to measure the polarization curve for hydrogen evolution. The solution used for polarization was a 30 mass% KOH aqueous solution at 90°C. The sweep rate for polarization measurement was 5 mV / min, and the counter electrode was a platinum foil (Ag / AgCl electrode) with a platinum concentration of 99.9 mass% or more. The polarization curve for hydrogen evolution measured in this way is shown in Figure 3. Figure 3 shows the polarization curve for hydrogen evolution measured at a current of 0.1250 A / cm 2 The relationship between the log exchange current density and the hydrogen evolution overvoltage at the current density (absolute value) is shown in a Tafel plot. The log exchange current density and the hydrogen evolution overvoltage were calculated as follows according to the diagram in Figure 3. Table 2 shows the log exchange current density and the overvoltage, respectively.

[0048] The reference potential V is used to calculate the log exchange current density and hydrogen evolution overpotential. t (V t Ag / AgCl (vs. KCl) is calculated from the solution pH using the following formula: V t =-0.0592pH-0.199

[0049] First, the log exchange current density was calculated by Tafel extrapolation. That is, as shown in Figure 3, the log exchange current density was calculated as 1250 A / m 2 Find the tangent to the polarization curve at the current density of V t The current density at the intersection is calculated by extrapolating up to the current density. The unit of this current density is A / cm 2 The logarithm to the base 10 of the converted value is the value shown in Table 2 as log exchange current density.

[0050] Here, the log exchange current density is an index of the amount of hydrogen generated. The amount of exchange current is the number of electrons, which corresponds to the amount of hydrogen generated. In other words, the larger the log exchange current density, the greater the amount of hydrogen generated per unit area. Therefore, a material with a high exchange current density is an excellent hydrogen-activating catalyst. The log exchange current density is also an index of the hydrogen generation rate. As mentioned above, the elementary process of hydrogen gas generation is the adsorption of hydrogen atoms onto the surface of the material (catalyst) (Volmer process) and their desorption after hydrogen gasification (Tafel process). The adsorption of hydrogen atoms is limited to one per atom exposed on the surface of the material (catalyst). In other words, the upper limit of the number of hydrogen atoms that can be adsorbed per unit area is determined by the material (catalyst). Nevertheless, a large amount of hydrogen gas generated (desorption after hydrogen gasification) (high exchange current density) indicates a fast reaction rate for the two elementary processes (Volmer process and Tafel process) mentioned above, i.e., a fast hydrogen generation rate. Since the log exchange current density of platinum is −1.7 to −3.4, it can be said that a catalyst having a log exchange current density in a similar range is excellent as a hydrogen generation catalyst.

[0051] The hydrogen generation overvoltage, like the log exchange current density, is another index that indicates the likelihood of hydrogen generation. If this hydrogen generation overvoltage value is less than 0.150 V, it can be said that the substance has a hydrogen overvoltage suitable for use in a hydrogen generation electrode in an alkaline water electrolysis device.

[0052] Inventive Examples 1 to 19 shown in Table 2 are examples according to the present invention. That is, a mackinawite layer having a thickness of 10 micrometers or more was obtained on an electrode substrate containing 50 mass% or more of iron atoms, so the log exchange current density was equivalent to that of platinum, and the hydrogen overvoltage was also less than 0.150 V.

[0053] On the other hand, in Comparative Examples 1, 5, 6, and 10 shown in Table 2, the substrate was an electrode substrate containing 50 mass% or more of iron atoms, but the concentration of the ammonium thiocyanate aqueous solution used to form the mackinawite layer was less than 1 mass%, so the mackinawite layer was not obtained and the desirable characteristics of both the log exchange current density and overvoltage were not obtained in the polarization curve for hydrogen evolution.

[0054] In Comparative Examples 2 and 7, the substrate is an electrode substrate containing 50 mass% or more of iron atoms, but the specific liquid volume when forming the mackinawite layer is 50 mL / cm 2 Since the thickness was less than 100 nm, the mackinawite layer was not obtained, and the desired characteristics of both the exchange current density and the overpotential were not obtained in the polarization curve for hydrogen evolution.

[0055] In Comparative Examples 3 and 8, the substrate was an electrode substrate containing 50 mass% or more of iron atoms, but the temperature of the solution during film formation was 60°C or lower, so a mackinawite layer was not obtained, and desirable characteristics of both the exchange current density and overpotential were not obtained in the polarization curve for hydrogen evolution.

[0056] In Comparative Examples 4 and 9, the substrate was an electrode substrate containing 50 mass% or more of iron atoms, but the immersion time during film formation was less than 15 hours, so a mackinawite film was obtained, but its thickness was less than 10 micrometers. Because a mackinawite film was obtained, the desired characteristics were obtained for both the log exchange current density and overpotential in the polarization curve for hydrogen evolution.

[0057] Furthermore, for the test pieces of Comparative Examples 4 and 9, in the polarization test for hydrogen generation, the current density was 1250 A / m 2 After reaching the σ (absolute value), the specimen was maintained at this current density for two weeks. After two weeks of maintenance, the specimen was removed from the solution, washed, and then a cross section of the specimen was cut out. The thickness of the mackinawite layer on the specimen surface was measured using an optical microscope. Ten fields of view were measured at random positions. Of these ten fields of view, the number of fields in which a mackinawite layer of 1 micrometer or more was observed was three in Comparative Example 4 and five in Comparative Example 9. No mackinawite layer was observed in the remaining fields. In other words, when the mackinawite layer thickness was less than 10 micrometers, the mackinawite layer dissolved into the solution during two weeks of maintenance, exposing some of the substrate surface. When such exposed portions of the substrate surface were formed, the electrode no longer exhibited the characteristics expected of a hydrogen generation electrode.

[0058] Similar tests were also carried out on invention examples 1 and 10, and the thickness of the surface mackinawite layer was measured, and it was confirmed that there were no defects in these invention examples.

[0059] [Table 2]

[0060] Next, the case where Comparative Materials 1 and 2 in Table 1 were used as the substrate will be described below. First, two test pieces were prepared from each of Comparative Materials 1 and 2 according to the method described above. These two test pieces were placed in a liquid volume of 50 mL / cm. 2The samples were then immersed in a 1.0 mass% aqueous solution of ammonium thiocyanate at 61°C for 15 hours. As a result, the number of mackinawite peaks obtained by thin-film XRD was 5 for both Comparative Materials 1 and 2, and the mackinawite layer thickness was 25 micrometers for Comparative Material 1 and 29 micrometers for Comparative Material 2.

[0061] Another test piece immersed in the same batch was polarized on the cathode side in a 30 mass% KOH aqueous solution at 90°C, with a current density of 1250 A / m 2 The test piece was maintained at this temperature (absolute value) for one week. The specific liquid volume of the KOH aqueous solution at this time was 1000 mL / cm 2 After one week of storage, the electrode was polarized to the anode side under the same conditions, with the current density increased to 1250 A / m 2 (absolute value) for 6 hours.

[0062] The polarization curve for hydrogen evolution of a test piece having a substrate of Comparative Material 1 that had undergone the above process was measured in a newly prepared 30 mass% KOH aqueous solution at 90°C. The sweep rate was 5 mV / min. The log exchange current density was -4.92 and the overvoltage was 1.38 V, which did not satisfy the conditions expected in the present invention. This is because, after maintaining the cathode polarized state for one week, the supply of iron atoms from the substrate was insufficient when polarized to the anode side, and therefore the mackinawite film did not regenerate during anodic polarization. Furthermore, the polarization curve for Comparative Material 2 was measured in the same manner as for Comparative Material 1. The log exchange current density was -4.10 and the overvoltage was 1.15 V, which did not satisfy the conditions expected in the present invention. The cause of this is the same as for Comparative Material 1.

[0063] The above tests were performed using Examples 2 and 11 of the present invention in Table 2. As a result, it was confirmed that in the Examples of the present invention, the supply of iron atoms from the substrate was sufficient when polarized to the anode side, and therefore the mackinawite film was sufficiently regenerated during anodic polarization, the log exchange current density was equivalent to that of platinum, and the overvoltage was also less than 0.150 V.

[0064] Finally, the following additional tests were conducted on a test piece (referred to as Comparative Example 11) in which an Fe-Ni-W layer was formed on the surface layer of Substrate Example 1 in Table 1 according to the method of Experiment 1 described in the examples of Patent Document 6, and on Invention Example 5 to confirm whether or not the surface catalyst layer (the Fe-Ni-W layer in Comparative Example 11 and the mackinawite layer in Invention Example 5) had been deactivated.

[0065] The additional test was conducted in a 90°C KOH solution with an initial concentration of 30 mass%, in which the polarization cycle shown in Figure 4 was repeated 30 times in the same solution. The immersion potential is the potential at zero current density. This immersion potential was not set to a fixed value because it varies depending on the solution concentration, temperature, and the combination of the immersed specimen. In other words, the immersion potential changes from moment to moment during the polarization test. However, since the immersion potential must be determined at the start of a given cycle, it was determined from the measurement results of the potential sweep from the anode side to the cathode side at the end of the cycle immediately preceding that cycle.

[0066] The two types of test pieces that had undergone such cycle testing were subjected to polarization measurements according to the method for determining the log exchange current density described above. As a result, in Comparative Example 11, the catalyst layer was deactivated, resulting in a log exchange current density of -6.23 and an overvoltage of 2.22 V, which did not satisfy the conditions expected by the present invention. On the other hand, in Inventive Example 5, the Mackinawite membrane was sufficiently regenerated during the cycles, so that the log exchange current density was equivalent to that of platinum and the overvoltage was also confirmed to be less than 0.150 V. [Explanation of symbols]

[0067] 1 coupon 2 conductors 3 Resin lump

Claims

[Claim 1] A method for manufacturing an electrode for use in alkaline water electrolysis, having a layer of mackinawite with a thickness of 10 micrometers or more on the surface of a substrate containing 50 mass% or more of iron atoms, comprising immersing the substrate for 15 hours or more in an aqueous solution of ammonium thiocyanate with a concentration of 1.0 mass% or more, the aqueous solution being maintained at a temperature above 60°C and with a liquid volume relative to the surface area of ​​the substrate being 50 mL / cm2 or more, to form mackinawite on the surface of the substrate.

Images