Electrode and water electrolysis apparatus

Abstract

To provide an electrode for a water electrolysis apparatus that can demonstrate excellent electrode performance.SOLUTION: An electrode 1 disclosed herein comprises a conductive base material 10 and a catalyst layer 20 containing at least Ni-Fe oxide and metallic Ni. The catalyst layer 20 comprises a first layer 21 formed on the conductive base material 10 and a second layer 22 formed on the first layer 21 with Fe element content larger than that for the first layer. In the elemental analysis from the conductive base material 10 toward a surface 20a of the catalyst layer 20, the increase ratio of the Fe ratio in the first layer 21 is 0.17% / nm or more and the increase ratio of the Fe ratio in the second layer 22 is less than 0.17% / nm. The ratio of thickness T2 of the second layer 22 to thickness T1 of the first layer 21 is 0.9 or less. The electrode 1 in such a configuration can demonstrate excellent performance as an electrode for a water electrolysis apparatus.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The technology disclosed herein relates to an electrode and a water electrolysis device. [Background technology]

[0002] From the perspective of energy and environmental issues, various technologies (such as solar cells and wind power generation) have been proposed for converting renewable energy into electricity. However, storing renewable energy in the form of electricity requires large-scale power storage facilities, which causes equipment costs to soar. In recent years, water electrolysis devices have been proposed that use electricity to decompose aqueous electrolytes (such as alkaline aqueous solutions) into oxygen and hydrogen. This allows renewable energy to be converted into useful gases, reducing the equipment costs required for storing renewable energy.

[0003] An example of this type of water electrolysis device is an AEM type water electrolysis device. In the AEM type water electrolysis device, an oxygen generating electrode (anode) and a hydrogen generating electrode (cathode) face each other through an anion exchange membrane. Patent Document 1 (International Publication No. 2022 / 250119) discloses an example of an oxygen generating electrode of an AEM type water electrolysis device. Specifically, the electrode (catalyst) described in Patent Document 1 has a nickel oxide layer containing NiOOH and a layer containing NiFe on a nickel base material in this order. Since the nickel oxide layer is interposed between the nickel base material and the NiFe layer, the electrode having the above configuration is said to be able to exhibit excellent durability and catalytic activity even in an alkaline environment. Patent Document 1 discloses that the electrode having the above configuration is manufactured by performing an oxidation process and an Fe electrodeposition process on the nickel base material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 250119 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a water electrolysis device is used in an actual industry, high-performance electrodes capable of generating a sufficient amount of useful gas even at a low voltage are required. However, conventional electrodes do not have sufficiently high electrode performance, and there is still room for improvement. The technology disclosed herein has been made to solve such problems. [Means for solving the problem]

[0006] As a result of various experiments, the present inventors discovered that, when the manufacturing method including the above-mentioned oxidation step and Fe electrodeposition step is carried out, an electrode having remarkably excellent electrode performance is very rarely manufactured. Then, as a result of further investigation into this discovery, it was found that an electrode with higher performance can be stably manufactured by adjusting the electrodeposition conditions (voltage, processing time) in the Fe electrodeposition step and the Fe concentration in the electrolyte for Fe electrodeposition. Then, when the structure of this high-performance electrode was analyzed, it was surprisingly found to have a structure completely different from that of the electrode described in Patent Document 1. The electrode disclosed herein was made based on this knowledge.

[0007] The electrode disclosed herein includes a conductive substrate and a catalytic layer formed on the surface of the conductive substrate and containing at least Ni-Fe oxide and metal Ni. The catalytic layer includes a first layer formed on the conductive substrate and including a high Ni region in which the Fe element content is lower than the Ni element content, and a second layer formed on the first layer and having a higher Fe element content than the first layer. The first layer has an increase rate of Fe ratio along a TEM-EDX line scan of 0.17% / nm or more. The second layer has an increase rate of Fe ratio along a TEM-EDX line scan of less than 0.17% / nm. In the electrode disclosed herein, the thickness ratio of the second layer to the first layer is 0.9 or less. The "Fe ratio along a TEM-EDX line scan" here refers to the ratio of the count number of Fe element to the total count number of Ni element and Fe element in a TEM-EDX line scan from the conductive substrate toward the surface of the catalytic layer.

[0008] As described above, the inventors have confirmed through experiments that the electrode having the above-mentioned configuration has remarkably excellent performance. Although it is not intended to limit the technology disclosed herein, it is speculated that this significant improvement in electrode performance is caused by the following action.

[0009] First, in the electrode disclosed herein, a first layer is formed on a conductive substrate. This first layer contains a high Ni region where the Fe element is less than the Ni element. For this reason, it is expected that the first layer contains a large amount of metallic Ni, which has excellent electronic conductivity. Meanwhile, a second layer is formed on the first layer. This second layer contains more Fe element than the first layer. For this reason, it is expected that the second layer contains a large amount of Ni-Fe oxide, which has excellent catalytic activity. In such a multi-layered catalyst layer, hydroxide ions (OH - ) is easily in contact with the second layer. This allows the excellent catalytic activity to be exerted efficiently. In addition, because the first layer, which has excellent electronic conductivity, is in contact with the conductive base material, the exchange of electrons between the conductive base material and the catalyst layer can be promoted.

[0010] In addition, in the first layer, a concentration gradient occurs in which the amount of Fe increases and the amount of Ni decreases toward the surface (upper layer side) of the catalyst layer. That is, the first layer has a characteristic that the catalytic activity increases toward the upper layer where hydroxide ions are easily supplied. On the other hand, in this first layer, the amount of Fe decreases and the amount of Ni increases toward the conductive substrate (lower layer side). For this reason, the first layer has high electronic conductivity in the lower layer where electrons are directly exchanged with the conductive substrate. In this way, the balance between catalytic activity and electronic conductivity of this first layer changes favorably at each point in the thickness direction.

[0011] Furthermore, as described above, the second layer exhibits excellent catalytic activity because it contains a large amount of Ni-Fe oxide. However, the second layer tends to have low electronic conductivity because the content of metallic Ni is relatively low. Therefore, if the second layer becomes too thick, the transfer of electrons from the surface of the catalyst layer to the conductive base material is hindered, and the electrode performance may be reduced. In contrast, in the electrode having the above configuration, the thickness of the second layer is thinner than the thickness of the first layer. This allows the transfer of electrons on the surface of the catalyst layer to occur appropriately.

[0012] As described above, the catalyst layer of the electrode disclosed herein has a favorable balance between catalytic activity and electronic conductivity at each point in the thickness direction, which is expected to result in excellent electrode performance.

[0013] In one embodiment of the electrode disclosed herein, the first layer has an Fe fraction increase rate of 3.17% / nm or more along a TEM-EDX line scan. The increased concentration gradient in the first layer can favorably change the catalytic performance and electronic conductivity in the first layer.

[0014] In one embodiment of the electrode disclosed herein, the average Fe ratio in the first layer is 5% or more and 45% or less, which increases the amount of metallic Ni present in the first layer and allows the electrode to exhibit more suitable electronic conductivity.

[0015] In one embodiment of the electrode disclosed herein, the average Fe ratio in the second layer is 40% or more and 85% or less, which increases the amount of Ni-Fe oxide present in the second layer and allows the electrode to exhibit more suitable catalytic activity.

[0016] In one embodiment of the electrode disclosed herein, the density of the second layer based on electron microscope observation of a cross section of the catalyst layer is 50% or less, which improves the contact efficiency with hydroxide ions in the second layer, thereby further improving the electrode performance.

[0017] In one embodiment of the electrode disclosed herein, the density of the first layer based on electron microscope observation of a cross section of the catalyst layer is 50% or more, which makes it easier for an electronic conduction path to be formed in the first layer, thereby further improving the electrode performance.

[0018] In one embodiment of the electrode disclosed herein, the catalyst layer is substantially free of metallic Fe having a BCC structure, which facilitates cooperative catalytic action by multiple Fe ions, thereby enabling more suitable catalytic activity to be exhibited.

[0019] In one embodiment of the electrode disclosed herein, the catalytic layer is substantially free of NiO, which increases the amount of Ni-Fe oxide in the catalytic layer and thus allows the electrode to exhibit more favorable catalytic activity.

[0020] In one embodiment of the electrode disclosed herein, the conductive base material is a Ni base material, which can improve the adhesive strength between the conductive base material and the catalyst layer.

[0021] As another aspect of the technology disclosed herein, a water electrolysis device is provided. The water electrolysis device disclosed herein includes an oxygen generating electrode having a first catalytic layer on the surface of a first substrate, a hydrogen generating electrode having a second catalytic layer on the surface of a second substrate, an anion exchange membrane interposed between the oxygen generating electrode and the hydrogen generating electrode, a water supply pipe attached near the surface of the first catalytic layer, and a conductive line electrically connecting the first substrate and the second substrate. The oxygen generating electrode of this water electrolysis device is an electrode according to any one of the above-mentioned aspects. Since this water electrolysis device has an electrode with excellent performance, it can efficiently convert electricity from a power generation facility into useful gas. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a cross-sectional view that illustrates a water electrolysis device according to one embodiment. [Diagram 2] FIG. 2 is a cross-sectional view illustrating an electrode according to an embodiment. [Diagram 3] FIG. 3 is a graph illustrating a method for calculating the increase rate of the Fe ratio in each layer. [Figure 4] FIG. 4 is a flowchart illustrating an example of a method for producing an electrode. [Diagram 5] FIG. 5(a) is a cross-sectional TEM photograph (650,000x magnification) of Example 1, and FIGS. 5(b) to (d) are elemental maps of Fe, O, and Ni, respectively, based on EDX analysis. [Figure 6] FIG. 6(a) is a cross-sectional TEM photograph (magnification: 2,500,000) of Example 2, and FIGS. 6(b) to 6(d) are elemental maps of Fe, O, and Ni based on EDX analysis, respectively. [Figure 7] FIG. 7(a) is a cross-sectional TEM photograph (magnification: 2,500,000) of Example 3, and FIGS. 7(b) to (d) are elemental maps of Fe, O, and Ni based on EDX analysis, respectively. [Figure 8] FIG. 8(a) is a cross-sectional TEM photograph (1250,000x magnification) of Example 4, and FIGS. 8(b) to 8(d) are elemental maps of Fe, O, and Ni, respectively, based on EDX analysis. [Figure 9]FIG. 9 is a graph showing the gradient of the Fe fraction of Example 1 based on a TEM-EDX line scan. [Figure 10] FIG. 10 is a graph showing the gradient of the Fe fraction for Example 2 based on a TEM-EDX line scan. [Figure 11] FIG. 11 is a graph showing the gradient of the Fe fraction for Example 3 based on a TEM-EDX line scan. [Figure 12] FIG. 12 shows the measurement results showing the oxidation state of Ni element in Example 3. [Figure 13] FIG. 13 shows the measurement results showing the oxidation state of the Fe element in Example 3. [Figure 14] FIG. 14 shows the measurement results showing the oxidation state of Ni element in Example 5. [Figure 15] FIG. 15 shows the measurement results showing the oxidation state of the Fe element in Example 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] An embodiment of the technology disclosed herein will be described below. Matters other than those specifically mentioned in this specification and necessary for implementing the technology disclosed herein can be understood as design matters of a person skilled in the art based on the conventional technology in the field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the field. In this specification, the expression "A to B" indicating a numerical range means "A or more and B or less" unless otherwise specified. The drawings are drawn diagrammatically, and the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships.

[0024] 1.Water electrolysis device First, a water electrolysis device using the electrodes disclosed herein will be described. Fig. 1 is a cross-sectional view that illustrates a water electrolysis device according to the present embodiment. Note that the following describes an example of the use of the electrodes, and is not intended to limit the use of the technology disclosed herein.

[0025] The water electrolysis device 100 shown in FIG. 1 is an AEM type water electrolysis device. The water electrolysis device 100 includes an oxygen generating electrode (anode) 110 and a hydrogen generating electrode (cathode) 120. The oxygen generating electrode 110 is an electrode having a first catalyst layer 112 on the surface of a first substrate 111. As will be described in detail later, the electrode 1 according to the present embodiment is used for the oxygen generating electrode 110. On the other hand, the hydrogen generating electrode 120 is an electrode having a second catalyst layer 122 on the surface of a second substrate 121. An anion exchange membrane 130 is interposed between the oxygen generating electrode 110 and the hydrogen generating electrode 120. Note that, for the parts other than the oxygen generating electrode 110, parts that can be used in this type of water electrolysis device can be used without any particular restrictions, and therefore detailed description thereof will be omitted.

[0026] A water supply channel 162 is attached to the hydrogen generating electrode 120. An aqueous electrolyte is supplied to the hydrogen generating electrode 120 through the water supply channel 162. An alkaline aqueous solution such as an NaOH aqueous solution is preferably used as the aqueous electrolyte. An oxygen recovery pipe 164 is attached to the oxygen generating electrode 110. The oxygen recovery pipe 164 passes through the first substrate 111 and is connected to the first catalyst layer 112. On the other hand, a hydrogen recovery pipe 166 is attached to the hydrogen generating electrode 120. The hydrogen recovery pipe 166 passes through the second substrate 121 and is connected to the second catalyst layer 122.

[0027] Next, the oxygen evolving electrode 110 and the hydrogen evolving electrode 120 are electrically connected by a conductive line 140. Specifically, the conductive line 140 connects the first substrate 111 of the oxygen evolving electrode 110 and the second substrate 121 of the hydrogen evolving electrode 120. In addition, a power source 150 is installed on this conductive line 140. For example, a generator (such as a solar cell or a wind power generator) that converts renewable energy into electricity is used as the power source 150.

[0028] Next, the operation of the water electrolysis device 100 will be described. First, in the water electrolysis device 100, an aqueous electrolyte is supplied from a water supply channel 162 to the hydrogen generating electrode 120. In addition, electrons (e -At this time, in the hydrogen generating electrode 120, the aqueous electrolyte (H2O) is converted into hydrogen gas (H2) and hydroxide ions (OH - ) (see formula (1) below). The hydrogen gas produced in the hydrogen generating electrode 120 is collected from the hydrogen recovery pipe 166. Meanwhile, the hydroxide ions produced in the hydrogen generating electrode 120 pass through the anion exchange membrane 130 and move to the oxygen generating electrode 110. As a result, oxygen gas (O2) and water (H2O) are produced in the oxygen generating electrode 110, as shown in formula (2) below. The oxygen gas is then collected from the oxygen recovery pipe 164. The water is discharged to the outside of the device through a drain pipe (not shown). As described above, with this water electrolysis device 100, the power generated by the power source 150 can be converted into oxygen gas and hydrogen gas. 4H2O+4e - →2H2+4OH - (1) 4OH - →O2+2H2O+4e - (2)

[0029] 2.Electrode Next, one embodiment of the electrode disclosed herein will be described. Fig. 2 is a cross-sectional view that shows a schematic diagram of the electrode according to this embodiment. In Fig. 2, the symbol Z indicates the "thickness direction (of the electrode)". The symbol U indicates the "upper side" and the symbol D indicates the "lower side". However, these directions are merely defined for the convenience of explanation, and do not limit the installation mode of the electrode disclosed herein.

[0030] As shown in Fig. 2, the electrode 1 according to this embodiment includes a conductive substrate 10 and a catalyst layer 20. As described above, the electrode 1 is used as the oxygen evolving electrode 110 of the water electrolysis device 100 shown in Fig. 1. The structure of the electrode 1 will be specifically described below.

[0031] (1) Conductive base material The conductive substrate 10 is a metal member having electrical conductivity. When constructing the water electrolysis device 100 shown in FIG. 1, the conductive substrate 10 is connected to the conductive line 140 as the first substrate 111. The conductive substrate 10 can be any conventionally known substrate that can be used for an electrode for a water electrolysis device, without any particular limitation. Examples of the material of the conductive substrate 10 include Ni, Ti, NiCr alloy, and SUS. Among these, the conductive substrate 10 made of Ni (Ni substrate) is particularly suitable because of its excellent bonding with the catalyst layer 20 containing Ni. In addition, the Ni substrate also has the advantage that the catalyst layer 20 can be easily formed, as will be described in detail later. As shown in FIG. 2, the conductive substrate 10 in this embodiment is a plate-shaped member. However, the shape of the conductive substrate does not limit the technology disclosed herein. The shape of the conductive substrate can be appropriately changed depending on the structure of the water electrolysis device to which the conductive substrate is applied.

[0032] The conductive substrate 10 is preferably a porous body having a plurality of pores. This allows fluids such as water and oxygen gas to easily pass through the conductive substrate 10. As a result, this contributes to improving the operation efficiency of the water electrolysis device 100. For example, the average pore diameter in the conductive substrate 10 is preferably 0.05 mm or more, more preferably 0.1 mm or more, even more preferably 0.15 mm or more, and particularly preferably 0.2 mm or more. This allows the fluid permeability of the conductive substrate 10 to be sufficiently ensured. On the other hand, the average pore diameter in the conductive substrate 10 is preferably 5.5 mm or less, more preferably 5.0 mm or less, even more preferably 4.5 mm or less, and particularly preferably 4.0 mm or less. This allows the strength of the conductive substrate 10 to be sufficiently ensured.

[0033] The porosity of the conductive substrate 10 is preferably 70% or more, more preferably 75% or more, even more preferably 80% or more, and particularly preferably 85% or more. This allows the fluid permeability of the conductive substrate 10 to be more suitably improved. On the other hand, taking into consideration the strength of the conductive substrate 10, the porosity of the conductive substrate 10 is preferably 98% or less, more preferably 97% or less, and particularly preferably 96% or less. In this specification, the "porosity of the conductive substrate" is measured according to the following procedure. First, the conductive substrate is cut into pieces of 1 cm3 The actual specific gravity is calculated by cutting out a piece and measuring its weight. Next, the apparent specific gravity is calculated based on the specific gravity of the conductive base material (Ni, Ti, etc.). The result of the calculation of the apparent specific gravity / actual specific gravity is then taken as the "porosity."

[0034] In addition, the thickness T of the conductive substrate 10 S is preferably 50 μm or more, more preferably 100 μm or more, and particularly preferably 150 μm or more. This allows the conductive substrate 10 to have a sufficient strength. S The upper limit of the thickness is preferably 500 μm or less, more preferably 450 μm or less, and particularly preferably 400 μm or less, whereby the fluid permeability in the conductive substrate 10 can be sufficiently ensured.

[0035] The thickness of each layer in this specification is measured according to the following procedure. First, a TEM-EDX line scan is performed from the conductive substrate 10 toward the surface 20a of the catalyst layer 20. In this line scan, the measurement interval is set to every 1 second, and the scanning speed is set so that the surface 20a of the catalyst layer 20 is reached from the conductive substrate 10 in 75 seconds. Next, the Fe ratio of each of the 75 measurement points is calculated. The "Fe ratio" here is the ratio (%) of the count number of the Fe element when the total count number of the Ni element and the Fe element is set to 100%. Then, from among the 75 measurement points, an arbitrary measurement point where the Fe ratio is about 1% (for example, 0.9% to 1.1%) is selected, and this measurement point is set as the "high Ni reference point". Next, the total count number of the Ni element and the Fe element at this high Ni reference point is set as the reference count number. Then, the region where the count is 0.1% or more of this reference count number, which is set to 100%, is regarded as the "region where the electrode exists in the thickness direction," and the region where the count number is less than 0.1% is regarded as the "region where the electrode (measurement object) does not exist." In this way, the "thickness T of electrode 1" is determined based on the TEM-EDX line scan. A " can be measured.

[0036] Next, the gradient of the Fe ratio at each of the 75 measurement points is measured. In this specification, the "gradient of the Fe ratio at the n-th measurement point" can be measured by drawing a line segment connecting the n-th measurement point and the n+2-th measurement point, and calculating the gradient of the line segment. Next, the high Ni reference point is defined as the "starting point of the conductive substrate 10". In addition, in the measurement of the gradient of the Fe ratio from the high Ni reference point toward the surface of the electrode 1, if three or more consecutive measurement points are confirmed to have a gradient of the Fe ratio of more than 0.5, the measurement point closest to the starting point of the conductive substrate 10 among the consecutive measurement points is defined as the "end point of the conductive substrate 10". Then, the distance from the starting point to the end point of the conductive substrate 10 is defined as the "thickness T of the conductive substrate 10". S ". Next, the end point of the conductive base material 10 is defined as the "start point of the first layer 21". In addition, when three or more consecutive measurement points are confirmed in the measurement of the gradient of the Fe ratio from the start point of the first layer 21 toward the surface of the electrode 1, the measurement point closest to the start point of the first layer 21 among the consecutive measurement points is defined as the "end point of the first layer 21". Then, the distance from the start point to the end point of the first layer 21 is defined as the "thickness T1 of the first layer 21". In other words, the "thickness T1 of the first layer 21" in this specification can be measured by taking the measurement point where the gradient of the Fe ratio begins to exceed 0.5 in the measurement of the gradient of the Fe ratio in succession as the start point, and the measurement point where the gradient of the Fe ratio falls again to 0.5 or less as the end point. Next, the end point of the first layer 21 is defined as the "start point of the second layer 22". The end point of the region where the electrode 1 exists is defined as the "end point of the second layer 22." The distance from the start point to the end point of the second layer 22 is defined as the "thickness T2 of the second layer 22." That is, the "thickness T2 of the second layer 22" in this specification is a region starting from the measurement point where the slope of the Fe fraction drops to 0.5 or less in continuous measurements of the slope of the Fe fraction, and ending at the surface of the electrode 1.

[0037] (2)Catalyst layer The catalytic layer 20 is formed on the surface of the conductive substrate 10. As shown in FIG. 1, when constructing the water electrolysis device 100, the catalytic layer 20 (first catalytic layer 112) is disposed so as to face the anion exchange membrane 130. A water supply channel 162 is connected to the vicinity of the surface 20a of the catalytic layer 20 (for example, the second layer 22 described later). The catalytic layer 20 contains at least Ni-Fe oxide and metallic Ni. The Ni-Fe oxide is a component with excellent catalytic activity for promoting the oxygen generation reaction. On the other hand, metallic Ni is a component with excellent electronic conductivity. The catalytic layer 20 may contain other Ni-based components and Fe-based components. An example of the Ni-based component that may be contained in the catalytic layer 20 is Ni oxide. An example of the Fe-based component is Fe oxide, metallic Fe, etc.

[0038] Moreover, the main metal elements of the catalyst layer 20 are Ni and Fe. Here, "the main metal elements are Ni and Fe" means that metal elements other than Ni and Fe are not intentionally added to the catalyst layer. In other words, the catalyst layer 20 may contain metal elements other than Ni and Fe as inevitable impurities derived from raw materials, manufacturing processes, etc. Examples of impurities that may be contained in the catalyst layer 20 include potassium (K), aluminum (Al), copper (Cu), zinc (Zn), calcium (Ca), magnesium (Mg), manganese (Mn), chromium (Cr), and molybdenum (Mo). More specifically, in this specification, "the main metal elements are Ni and Fe" means that when the total number of metal elements in the catalyst layer is 100 atm%, the total number of atoms of Ni and Fe is 70 atm% or more (preferably 75 atm% or more, more preferably 80 atm% or more, and particularly preferably 85 atm% or more). The upper limit of the total number of Ni and Fe atoms in the catalyst layer 20 is not particularly limited, and may be 100 atm%, 95 atm% or less, or 90 atm% or less. Note that the "atomic number" here is a value based on elemental analysis obtained by carrying out energy dispersive X-ray spectroscopy (EDX) on a cross-sectional TEM image of the catalyst layer 20.

[0039] In addition, it is preferable that the catalyst layer 20 does not substantially contain metallic Fe having a body-centered cubic (BCC) structure. Specifically, when a cooperative action of a plurality of Fe ions occurs in the catalyst layer 20, the generation of oxygen gas is favorably promoted. However, in the case of metallic Fe having a BCC structure, the distance between the Fe elements is long, so that the cooperative action of the Fe ions is difficult to occur. For this reason, it is understood that the catalytic activity improves as the amount of metallic Fe having a BCC structure in the catalyst layer 20 decreases. The presence or absence of metallic Fe having a BCC structure can be evaluated by performing X-ray diffraction analysis (XRD: X-ray diffraction) on the electrode. Specifically, when an XRD analysis is performed by irradiating the surface of the catalyst layer 20 with X-rays at an incident angle of 1.0°, a "peak indicating the (200) plane of metallic Fe having a BCC structure" is confirmed near 2θ=65°. In addition, in this XRD analysis, a "peak indicating the (200) plane of metallic Ni having an FCC structure" is confirmed near 2θ=52°. and the intensity I of the peak indicating metallic Ni with FCC structure Ni and the intensity of the peaks showing metallic Fe with a BCC structure I Fe Ratio to (I Fe / I Ni The amount of metallic Fe in the BCC structure can be evaluated by calculating the peak intensity ratio I Fe / I Ni is 20 or less (preferably 15 or less, more preferably 10 or less, and particularly preferably 5 or less).

[0040] Furthermore, it is preferable that the catalyst layer 20 does not substantially contain NiO. Specifically, NiO is a very stable oxide into which Fe is not easily introduced. For this reason, if NiO is produced in the process of producing the catalyst layer 20, the amount of Ni-Fe oxide produced in the Fe electrodeposition step S20 described below will decrease. In other words, a catalyst layer 20 containing a large amount of NiO may have a reduced catalytic activity due to the small amount of Ni-Fe oxide produced. Note that, in this specification, "substantially does not contain NiO" refers to the NiO content measured by X-ray photoelectron spectroscopy (XPS) of the catalyst layer 20.2p In the analysis, the integral value of the NiO peak is 45% or less (preferably 40% or less, more preferably 35% or less, and particularly preferably 30% or less). Note that the integral value (%) of the NiO peak is determined by the above-mentioned XPS spectrum. 2p In the analysis of the above, the integral value of the peaks other than the satellites is taken as 100%. The Ni peak of NiO is confirmed at around 854 eV in the XPS measurement.

[0041] In addition, the total thickness T of the catalyst layer 20 C is preferably 5 nm or more, more preferably 10 nm or more, further preferably 15 nm or more, and particularly preferably 20 nm or more. This allows the electrode 1 to have a suitable catalytic activity. On the other hand, the total thickness T C The upper limit of the total thickness T of the catalyst layer 20 is preferably 100 nm or less, more preferably 90 nm or less, further preferably 80 nm or less, and particularly preferably 70 nm or less. This shortens the distance between the surface 20a of the catalyst layer 20 and the conductive substrate 10, so that electrons can be more suitably transferred to and from the surface 20a of the catalyst layer 20. C " is the total thickness T of the electrode 1 described above. A and the thickness T of the conductive substrate 10 S It is obtained by taking the difference between

[0042] Here, the electrode 1 according to this embodiment includes a catalyst layer 20 having a multi-layer structure including a first layer 21 and a second layer 22. Each layer included in the catalyst layer 20 will be described below.

[0043] (2-a) First layer The first layer 21 is a layer formed on the conductive substrate 10. The first layer 21 includes a high Ni region in which the Fe element content is lower than the Ni element content. Therefore, metallic Ni consisting only of Ni element is present in a part of the first layer 21. This metallic Ni has excellent electrical conductivity, and can promote the transfer of electrons between the conductive substrate 10 and the catalyst layer 20. From the viewpoint of further promoting the transfer of electrons with the conductive substrate 10, it is preferable that the high Ni region is adjacent to the conductive substrate 10.

[0044] In addition, in the electrode 1 according to this embodiment, a TEM-EDX line scan from the conductive substrate 10 toward the surface 20a of the catalyst layer 20 (straight line L in FIG. 2) S ) is performed, the increase rate of the Fe rate in the first layer 21 is 0.17% / nm or more. In other words, in the first layer 21, a concentration gradient is generated in which the Fe element increases toward the surface 20a side (upward U) of the catalyst layer 20. As a result, Ni-Fe oxide with excellent catalytic activity increases in the upper layer of the first layer 21, which is likely to come into contact with hydroxide ions from the counter electrode (hydrogen generating electrode 120). On the other hand, in the first layer 21, Ni element increases toward the conductive substrate 10 side (downward D). Therefore, in the lower layer of the first layer 21, where electrons are directly exchanged with the conductive substrate 10, there is a lot of metal Ni with excellent electronic conductivity. In this way, in the first layer 21 in this embodiment, the balance between the catalytic activity and the electronic conductivity changes appropriately at each point in the thickness direction Z.

[0045] The increase rate of the Fe percentage in the first layer 21 is preferably 1.17% / nm or more, more preferably 2.17% / nm or more, and even more preferably 3.17% / nm or more. This allows the balance between the catalytic activity and the electronic conductivity in the first layer 21 to be changed more suitably. On the other hand, the upper limit of the increase rate of the Fe percentage in the first layer 21 is preferably 15% / nm or less, more preferably 14% / nm or less, even more preferably 13% / nm or less, and particularly preferably 12% / nm or less. This prevents the concentration gradient of the Fe element (Ni element) in the first layer 21 from becoming unnecessarily steep. As a result, the balance between the catalytic activity and the electronic conductivity in the thickness direction can be changed more suitably.

[0046] In addition, the "increase in Fe ratio" in this specification is measured according to the following procedure. First, a TEM-EDX line scan is performed, and regions representing each layer (conductive substrate 10, first layer 21, second layer 22) are identified according to the procedure for measuring the thickness of each layer described above. Then, a straight line L1 connecting the start point and end point of the first layer, and a straight line L2 connecting the start point and end point of the second layer are set. The slope of the straight line L1 is defined as the "increase in Fe ratio in the first layer (% / nm)," and the slope of the straight line L2 is defined as the "increase in Fe ratio in the second layer (% / nm)."

[0047] The average Fe ratio in the first layer is preferably 5% or more, more preferably 10% or more, even more preferably 15% or more, and particularly preferably 20% or more. This allows sufficient Ni-Fe oxide to be generated in the first layer, thereby improving the catalytic activity of the first layer. Meanwhile, the average Fe ratio in the first layer is preferably 45% or less, more preferably 40% or less, even more preferably 35% or less, and particularly preferably 30% or less. This allows sufficient metallic Ni to be generated in the first layer, thereby improving the electronic conductivity of the first layer.

[0048] Moreover, the thickness T1 of the first layer 21 is preferably 5 nm or more, more preferably 10 nm or more, and particularly preferably 15 nm or more. By forming the first layer 21 with a certain thickness or more, the electronic conductivity of the entire catalyst layer 20 can be further improved. On the other hand, the upper limit of the thickness T1 of the first layer 21 is preferably 35 nm or less, more preferably 30 nm or less, and particularly preferably 25 nm or less. This allows the thickness T2 of the second layer 22 described later to be sufficiently secured, so that the catalyst layer 20 having better catalytic activity can be constructed. Note that the "thickness T1 of the first layer 21" is the thickness of a region in which the Fe rate increase rate is 0.17% / nm or more (a region in which a substantial concentration gradient of Fe element occurs) among the regions regarded as the catalyst layer in the above TEM-EDX line scan is regarded as the "first layer", and the thickness of the region is measured.

[0049] In addition, the first layer 21 is preferably a denser layer than the second layer 22. This makes it easier to form an electron conduction path in the first layer 21, and thus the exchange of electrons between the catalyst layer 20 and the conductive substrate 10 can be further promoted. Specifically, the density of the first layer 21 is preferably 50% or more, more preferably 55% or more, even more preferably 60% or more, and particularly preferably 65% ​​or more. On the other hand, the density of the first layer 21 is preferably 95% or less, more preferably 94% or less, even more preferably 93% or less, and particularly preferably 92% or less. This increases the surface area of ​​the first layer 21, and therefore the catalytic performance in the generation of oxygen gas can be improved. Note that the "density of the first layer" and the "density of the second layer" in this specification are calculated according to the following procedure. First, a TEM-EDX line scan is performed, and the sum of the net counts of Ni, Fe, and O elements is calculated for each of the conductive substrate, the first layer, and the second layer. Next, when the sum of the net counts of the conductive substrate is 100%, the ratio (%) of the sum of the net counts of the first layer is defined as the "density of the first layer." Similarly, when the sum of the net counts of the conductive substrate is 100%, the ratio (%) of the sum of the net counts of the second layer is defined as the "density of the second layer."

[0050] In addition, the average value of the total count number of Ni elements and Fe elements in the first layer 21 is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more. This allows the first layer 21 to be formed with a certain degree of density, so that the electronic conductivity of the first layer 21 can be sufficiently ensured. On the other hand, the average value of the total count number of Ni elements and Fe elements in the first layer 21 is preferably 95% or less, more preferably 90% or less, even more preferably 85% or less, and particularly preferably 80% or less. This increases the surface area of ​​the first layer 21, so that the catalytic performance in generating oxygen gas can be improved. The "average value of the total count number of Ni elements and Fe elements in the first layer 21" can be obtained by taking the reference count number at the high Ni reference point as 100% and calculating the average value of the total count number of a plurality of measurement points present in the region regarded as the first layer 21.

[0051] (2-b) Second layer The second layer 22 is formed on the first layer 21. In the electrode 1 according to this embodiment, the second layer 22 is disposed on the surface 20a of the catalyst layer 20. As described above, hydroxide ions are supplied to the surface 20a of the catalyst layer 20 from the hydrogen generating electrode 120 via the anion exchange membrane 130. In contrast, the second layer 22 in this embodiment is characterized in that it contains more Fe elements than the first layer 21. Therefore, the second layer 22 contains a large amount of Ni-Fe oxide, which has excellent catalytic activity. This allows oxygen gas to be generated particularly efficiently on the surface 20a of the catalyst layer 20.

[0052] Moreover, in the second layer 22, unlike the first layer 21, there is substantially no concentration gradient of the Fe element in the thickness direction Z. Specifically, in the second layer 22, the increase rate of the Fe ratio along the TEM-EDX line scan is less than 0.17% / nm. Since a large amount of Ni-Fe oxide is uniformly present in the second layer 22 in the thickness direction Z, the second layer 22 can exhibit better catalytic activity. The increase rate of the Fe ratio in the second layer 22 is preferably 0.15% / nm or less, more preferably 0.13% / nm or less, and particularly preferably 0.1% / nm or less. This can further improve the catalytic activity of the second layer 22. On the other hand, the lower limit of the increase rate of the Fe ratio in the second layer 22 is not particularly limited, and may be -0.17% / nm or more, -0.1% / nm or more, 0% / nm or more, or 0.01% / nm or more.

[0053] The average Fe ratio in the second layer 22 is preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, and particularly preferably 55% or more. Meanwhile, the average Fe ratio in the second layer 22 is preferably 85% or less, more preferably 80% or less, even more preferably 75% or less, and particularly preferably 70% or less. As a result, both the Fe element and the Ni element constituting the Ni-Fe oxide are sufficiently present in the second layer 22, and therefore the catalytic activity in the second layer 22 can be more suitably improved.

[0054] Furthermore, in the electrode 1 according to this embodiment, the ratio (T2 / T1) of the thickness T2 of the second layer 22 to the thickness T1 of the first layer 21 is 0.9 or less. This makes it possible to realize an electrode 1 with even better performance. Specifically, since the second layer 22 has a low content of metal Ni, the electronic conductivity tends to be low. Therefore, if the second layer 22 becomes too thick, the transfer of electrons from the surface 20a of the catalyst layer 20 to the conductive substrate 10 becomes difficult. In contrast, in the electrode 1 according to this embodiment, since the thickness T2 of the second layer 22 is thin, the transfer of electrons to the conductive substrate 10 can be prevented from being hindered by the second layer 22. From the viewpoint of further improving the electronic conductivity of the catalyst layer 20, the thickness ratio (T2 / T1) is preferably 0.8 or less, more preferably 0.7 or less, even more preferably 0.6 or less, and particularly preferably 0.5 or less. From the viewpoint of ensuring that the second layer 22 has excellent catalytic activity, the lower limit of the thickness ratio (T2 / T1) is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.15 or more, and particularly preferably 0.2 or more.

[0055] The specific value of the thickness T2 of the second layer 22 is preferably 30 nm or less, more preferably 25 nm or less, even more preferably 20 nm or less, and particularly preferably 15 nm or less. This can more suitably improve the electronic conductivity of the catalyst layer 20. On the other hand, in consideration of the catalytic activity of the entire catalyst layer 20, the lower limit of the thickness T2 of the second layer 22 is preferably 0.5 nm or more, more preferably 1 nm or more, preferably 2 nm or more, and particularly preferably 3 nm or more. The "thickness T2 of the second layer 22" is the thickness of a region that is regarded as the catalyst layer in the above TEM-EDX line scan, in which the increase rate of the Fe ratio is less than 0.17% / nm, is regarded as the "second layer", and the thickness of the region is measured.

[0056] In addition, it is preferable that the second layer 22 is a sparse layer compared to the first layer 21. This increases the surface area of ​​the second layer 22, making it possible to generate oxygen gas more efficiently. Specifically, the density of the second layer 22 is preferably 50% or less, more preferably 48% or less, even more preferably 46% or less, and particularly preferably 44% or less. On the other hand, the density of the second layer 22 is preferably 2% or more, more preferably 4% or more, even more preferably 6% or more, and particularly preferably 8% or more. This makes it possible to ensure sufficient strength of the second layer 22.

[0057] Moreover, the average value of the total count number of Ni elements and Fe elements in the second layer 22 is preferably 40% or less, more preferably 35% or less, even more preferably 30% or less, and particularly preferably 25% or less. This increases the surface area of ​​the second layer 22, thereby enabling more suitable catalytic performance to be exhibited. Meanwhile, the average value of the total count number of Ni elements and Fe elements in the second layer 22 is preferably 0.1% or more, more preferably 0.5% or more, even more preferably 1% or more, and particularly preferably 1.5% or more. This ensures a certain degree of electronic conductivity in the second layer 22. The "average value of the total count number of Ni elements and Fe elements in the second layer 22" can be measured in the same manner as the "average value of the total count number of Ni elements and Fe elements in the first layer 21" described above.

[0058] (3) Effects of this embodiment It has been experimentally confirmed that the electrode 1 having the above-described configuration can exhibit significantly superior electrode performance. Although it is not intended to limit the technology disclosed herein, it is presumed that this improvement in electrode performance occurs due to the following mechanism.

[0059] First, the catalytic layer 20 of the electrode 1 has a multilayer structure including a first layer 21 and a second layer 22. The first layer 21 in the lower portion D includes a high Ni region in which Fe element is less than Ni element. From this, it is expected that a large amount of metallic Ni is present in the first layer 21. As a result, high electronic conductivity can be exhibited in the first layer 21 in contact with the conductive base material 10. On the other hand, the second layer 22 in the upper portion U has a larger amount of Fe element than the first layer 21. From this, it is expected that a large amount of Ni-Fe oxide is present in the second layer 22. In the catalytic layer 20 having such a configuration, excellent catalytic activity can be exhibited in the second layer 22 to which hydroxide ions are directly supplied.

[0060] In addition, the first layer 21 has a concentration gradient in which the amount of Fe increases toward the surface 20a of the catalytic layer 20. That is, the catalytic activity of the first layer 21 increases toward the upper side U where hydroxide ions are easily supplied. Meanwhile, the first layer 21 has an increased amount of Ni and a decreased amount of Fe toward the conductive substrate 10. Therefore, the electronic conductivity increases toward the lower side D where the conductive substrate 10 is located. In this way, the balance between the catalytic activity and the electronic conductivity of the first layer 21 changes appropriately at each point in the thickness direction Z.

[0061] Furthermore, the second layer 22 exhibits excellent catalytic activity because it contains a large amount of Ni-Fe oxide. On the other hand, the second layer 22 has a low content of metal Ni, so that the electronic conductivity tends to be low. Therefore, if the second layer 22 becomes too thick, the transfer of electrons from the surface 20a of the catalyst layer 20 to the conductive substrate 10 is hindered, and the electrode performance may be deteriorated. In contrast, in the electrode 1 according to this embodiment, the thickness T2 of the second layer 22 is thinner than the thickness T1 of the first layer 21. This allows the transfer of electrons in the catalyst layer 20 to occur appropriately.

[0062] As described above, in the catalyst layer 20 of this embodiment, the balance between catalytic activity and electronic conductivity changes appropriately at each point in the thickness direction Z. This is expected to result in extremely excellent electrode performance.

[0063] 3. Electrode manufacturing method Fig. 4 is a flow chart illustrating an example of a method for producing an electrode. As shown in Fig. 4, the method for producing an electrode includes an oxidation step S10 and an Fe electrodeposition step S20.

[0064] (1) Oxidation process In the oxidation step S10, a nickel oxide layer is formed on the surface of the conductive substrate 10. For example, when the conductive substrate 10 is made of Ni, the surface of the conductive substrate 10 may be oxidized. This allows a uniform nickel oxide layer to be easily formed. An example of the oxidation treatment of the conductive substrate 10 is electrolytic oxidation treatment. In this electrolytic oxidation treatment, a voltage is applied between the conductive substrate 10 and the counter electrode while the conductive substrate 10 and the counter electrode are immersed in an electrolytic solution (such as an alkaline solution). This allows a uniform nickel oxide layer to be formed on the surface of the Ni conductive substrate 10. This step is not limited to electrolytic oxidation treatment as long as a nickel oxide layer can be formed on the surface of the conductive substrate 10. For example, a nickel oxide layer can be formed on the surface of the Ni conductive substrate 10 by chemical oxidation treatment using an acid or an oxidizing agent. The surface of the Ni conductive substrate 10 may be oxidized by heat treatment. Nickel oxide may be attached to the surface of the conductive substrate 10 by chemical vapor deposition or physical vapor deposition. A Ni thin film may be formed on the surface of the conductive substrate 10, and the Ni thin film may be oxidized. When these treatments are used, a nickel oxide layer can be formed on the surface of the conductive substrate 10 other than the Ni substrate.

[0065] In the oxidation step, it is preferable to form a nickel oxide layer containing nickel oxide with a valence of +3. This makes it easier for the Fe element to be introduced into the nickel oxide layer in the Fe electrodeposition step described below. As a result, a large amount of Ni-Fe oxide can be present in the catalyst layer 20 after production. On the other hand, as described above, it is difficult for the Fe element to be introduced into NiO. For this reason, it is preferable to form a nickel oxide layer in the oxidation step so that NiO is not substantially contained. Among the above-mentioned oxidation treatments, electrolytic oxidation treatment and chemical oxidation treatment are particularly suitable because they produce a small amount of NiO and a large amount of nickel oxide with a valence of +3.

[0066] (2) Fe electrodeposition process Next, in the Fe electrodeposition step S20, Fe elements are electrodeposited onto the nickel oxide layer on the surface of the conductive substrate 10. As a result, Fe elements are introduced into the nickel oxide layer, and a catalyst layer 20 containing Ni-Fe oxide is generated. In this Fe electrodeposition step S20, as in the above-mentioned electrolytic oxidation treatment, a voltage may be applied between the conductive substrate 10 and the counter electrode while the conductive substrate 10 and the counter electrode are immersed in an electrolyte. At this time, the Fe element can be introduced into the nickel oxide layer by using an electrolyte containing Fe elements.

[0067] Here, in this manufacturing method, the Fe concentration in the electrolytic solution is reduced, the voltage during Fe electrodeposition is lowered, and the processing time is shortened. As a result, a first layer 21 having a concentration gradient of Fe element and a second layer 22 containing a large amount of Ni-Fe oxide uniformly can be formed. Although it is not intended to limit the technology disclosed herein, it is presumed that the catalyst layer 20 having such a configuration is formed by the following action. First, by reducing the Fe concentration in the electrolytic solution, it is possible to suppress the introduction of a large amount of Fe to the surface of the nickel oxide layer immediately after the start of electrodeposition. This makes it possible to prevent the nickel oxide layer from remaining inside (the lower layer side) of the catalyst layer 20. Next, when the voltage in the Fe electrodeposition step S20 is lowered, the Fe element is gradually introduced into the inside of the nickel oxide layer. As a result, the first layer 21 having a concentration gradient of Fe element is formed on the lower layer side of the catalyst layer 20. Meanwhile, a second layer 22 containing a large amount of Ni-Fe oxide uniformly is formed on the surface side of the catalyst layer 20 where the Fe element is directly introduced. Here, by shortening the processing time and lowering the voltage, it is possible to prevent the second layer 22 from being formed with an excessive thickness.

[0068] The Fe concentration of the electrolyte, the voltage at the time of Fe introduction, and the treatment time are not limited to specific conditions (numerical values). This is because the conditions under which the catalyst layer 20 having the above-mentioned configuration is appropriately formed vary depending on the area and surface roughness of the conductive substrate 10. However, according to an experiment conducted by the present inventors, it has been confirmed that the catalyst layer described in Patent Document 1 (catalyst layer having a nickel oxide layer and a NiFe layer) is formed when the Fe concentration of the electrolyte is increased, the voltage at the time of Fe introduction is increased, and the treatment time is extended. For this reason, in order to mass-produce the electrode disclosed herein, it is preferable to conduct a preliminary experiment in which the Fe concentration of the electrolyte, the voltage at the time of Fe introduction, and the treatment time are each gradually decreased and the structure of the catalyst layer after production is analyzed every time the type of conductive substrate is changed. This allows mass production of the electrode to be started under conditions under which a catalyst layer with an appropriate structure can be formed.

[0069] An example of a method for manufacturing the electrode 1 according to the present embodiment has been described above. Note that the above description is not intended to limit the electrodes disclosed herein to those manufactured by a specific manufacturing method. For example, even if the above-mentioned multi-layered catalyst layer is formed on the surface of the conductive substrate by other means such as chemical vapor deposition, an electrode with extremely high performance can be realized.

[0070] 4. Other embodiments An embodiment of the electrode disclosed herein has been described above. However, the electrode disclosed herein is not limited to the above embodiment. For example, as shown in FIG. 2, the electrode 1 according to the above embodiment is composed of two members, a conductive substrate 10 and a catalyst layer 20. However, the electrode disclosed herein may include a layer member other than the conductive substrate and the catalyst layer. For example, in another embodiment of the technology disclosed herein, an adhesive layer may be formed on the catalyst layer (second layer). This adhesive layer is a layer mainly composed of an ion-conductive resin, a catalyst material, a conductive material, etc. An electrode having this adhesive layer can further contribute to improving cell performance because it has improved adhesion to the ion exchange membrane of a water electrolysis device.

[0071] [Test example] Next, test examples related to the technology disclosed herein will be described. Note that the technology disclosed herein is not limited to the following test examples.

[0072] 1. Preparation of Test Electrodes In this test, five types of test electrodes (Examples 1 to 5) with different catalyst layer configurations were prepared. Each example will be described in detail below.

[0073] (1) Example 1 (a) Preparation of conductive substrate In this test, a conductive substrate made of Ni was first prepared. Specifically, a porous Ni material (nickel Celmet #8) manufactured by Sumitomo Electric Industries, Ltd. was rolled to a thickness of 200 μm. Next, a piece with an area of ​​25 cm2 was cut from the rolled Ni plate. 2The conductive substrate was cut out. Next, the surface of the substrate was washed successively with ethanol, hexane, and water. After that, the surface of the conductive substrate was washed with 1 mol / L hydrochloric acid to remove the oxide film, and then the hydrochloric acid was washed away with water and ethanol.

[0074] (b) Oxidation treatment Next, a nickel oxide layer was formed on the surface of the conductive substrate by electrolytic oxidation. Specifically, a nickel counter electrode (with an area of ​​30 cm) was prepared on the conductive substrate by the same procedure as in (a) Preparation of the conductive substrate. 2 A nickel plate (Ni plate) was prepared. Next, the conductive substrate and the counter electrode were placed facing each other and immersed in a 1 mol / L KOH solution. A voltage was applied between the conductive substrate and the counter electrode to form a nickel oxide layer on the substrate surface. In this electrolytic oxidation treatment, the voltage was 1.7 V and the treatment time was 3 minutes.

[0075] (c) Fe electrodeposition treatment Next, Fe was introduced into the nickel oxide layer on the substrate surface by Fe electrodeposition. 2+ A mixed solution was prepared by mixing 25 mmol / L of (NH4)2SO4, 1 mmol / L of H2SO4, and water. The pH of this mixed solution was adjusted to 3 or less, and then degassed with nitrogen to prepare an electrolyte for Fe electrodeposition. Next, a Ni counter electrode (with an area of ​​30 cm2) was prepared by the same procedure as in the preparation of the conductive substrate (a) above. 2 A Ni plate (a Ni plate made of Ni alloy) was prepared. The conductive substrate and the counter electrode were placed opposite each other and immersed in an electrolyte for Fe electrodeposition. A voltage was applied between the conductive substrate and the counter electrode to introduce Fe elements into the nickel oxide layer on the substrate surface. In this way, the electrode of Example 1 was prepared. In the Fe electrodeposition treatment in Example 1, the voltage was set to -1.3 V and the treatment time was set to 3 minutes.

[0076] (2) Example 2 The electrode of Example 2 was prepared in the same manner as in Example 1, except that the treatment time for the Fe electrodeposition treatment was set to 1 minute.

[0077] (3) Example 3 The electrode of Example 3 was prepared in the same manner as in Example 1, except that the voltage for the Fe electrodeposition treatment was −1.0 V and the treatment time was 5 minutes.

[0078] (4) Example 4 For the electrode of Example 4, the treatment time for Fe electrodeposition was 120 minutes, and the Fe electrodeposition electrolyte was Fe 2+ It was prepared in the same manner as in Example 1, except that the concentration was 125 mmol / L.

[0079] (5) Example 5 In Example 5, only Fe electrodeposition was performed without performing oxidation treatment. The voltage of the Fe electrodeposition treatment was -1.5 V, and the treatment time was 1 minute. Other procedures were the same as those in Example 1.

[0080] 2.Performance evaluation test In this test, the overpotential in the oxygen evolution reaction (OER) was measured to evaluate the electrode performance of each example. In this evaluation, a three-electrode electrochemical cell was constructed in which a working electrode, a counter electrode, and a reference electrode were immersed in an aqueous electrolyte. Specifically, the electrode of each example was placed 1 cm 2 The counter electrode was cut into pieces with an area of ​​18 cm. 2 The Au substrate was used. A silver-silver chloride (Ag / AgCl) reference electrode was used. 1 mol / L KOH was used as the aqueous electrolyte.

[0081] Next, the electrochemical cell with the above configuration was subjected to linear sweep voltammetry (LSV) analysis to measure the overpotential in the OER activity. Specifically, in this test, the decomposition of the aqueous electrolyte was carried out while changing the potential of the working electrode at a sweep rate of 10 mV / sec. Then, when the current density was 10 mA / cm 2 The potential (V) at which the decomposition of the aqueous electrolyte solution started (in other words, the potential at which the reactions of the above-mentioned formulas (1) and (2) started) was measured as the "first potential E" in this test.2 The potential when the potential reached the equilibrium electrode potential E was measured as the "second potential E2". The first potential E1 and the second potential E2 are potential values ​​converted to the reversible hydrogen electrode (RHE). The theoretical potential of the decomposition reaction calculated based on thermodynamics (equilibrium electrode potential E e ) was set to 1.23 V, and the first overvoltage V1 and second overvoltage V2 were calculated based on the following formulas (3) and (4). When there are two electrodes with the same first overvoltage V1, it can be interpreted that the electrode with the smaller second overvoltage V2 has a larger effective catalyst loading per unit area. The calculation results of these overvoltages are shown in Table 1. First overvoltage V1 = E1-E e (3) Second overvoltage V2 = E2 - E e (4)

[0082] [Table 1]

[0083] As shown in Table 1, the first overvoltage V1 was lower in Examples 1 to 3 compared to Examples 4 to 5. That is, the electrodes in Examples 1 to 3 exhibited excellent electrode performance in that the decomposition of the aqueous electrolyte solution started at a low voltage. Furthermore, the second overvoltage V2 was also lowered in Examples 1 to 3. From this, it can be inferred that the electrodes in Examples 1 to 3 also have a large amount of effective catalyst support per unit area. Furthermore, among the electrodes in Examples 1 to 3, the first overvoltage V1 and the second overvoltage V2 were particularly significantly lowered in Example 3. From these points, it was found that when an electrode is manufactured by performing the oxidation treatment and the Fe electrodeposition treatment, an electrode having significantly excellent electrode performance may be manufactured. Then, as a result of comparing and examining the manufacturing conditions of each example, it was found that the electrode performance tends to improve when the voltage and treatment time of the Fe electrodeposition treatment and the Fe concentration in the electrolyte for Fe electrodeposition are reduced.

[0084] 3.Structural analysis test Next, in this test, the structure of the electrode of each example was analyzed. Here, in the analysis of the electrode structure, TEM-EDX analysis, TEM-EDX line scan, XRD analysis, XPS measurement, and ICP emission analysis were performed. Each test will be described below.

[0085] (1)TEM-EDX analysis In this test, TEM-EDX analysis was performed on the electrodes of Examples 1 to 4, and element maps of Ni, Fe, and O were obtained. Specifically, the electrodes of Examples 1 to 4 were subjected to a focused ion beam (FIB) treatment to obtain thin sliced ​​samples. Next, cross-sectional TEM observation was performed on the analysis samples, and element maps were created based on EDX analysis. Detailed measurement conditions are as follows. Cross-sectional TEM photographs and element maps of each example are shown in Figs. 5 to 8. Fig. 5 shows the analysis results of Example 1. Fig. 6 shows the analysis results of Example 2. Fig. 7 shows the analysis results of Example 3. Fig. 8 shows the analysis results of Example 4.

[0086] [Measurement conditions] Equipment used: JEOL JEM-F200 Acceleration voltage: 200kV Probe diameter: 0.3 nm

[0087] As shown in FIG. 8, in Example 4, a layer (nickel oxide layer) containing a large amount of Ni element and O element was formed on the conductive base material (Ni base material). Then, a layer (NiFe layer) containing Ni element and zero-valent Fe was formed on this nickel oxide layer. That is, the electrode of Example 4 had a configuration equivalent to the electrode (catalyst) described in Patent Document 1. On the other hand, as shown in FIGS. 5 to 7, in Examples 1 to 3, the nickel oxide layer as shown in Example 4 was not formed, and each of Ni element, Fe element, and O element was present throughout the entire catalyst layer. This is considered to be one of the reasons why Examples 1 to 3 can exhibit better electrode performance than Example 4.

[0088] (2) TEM-EDX line scan Next, in order to investigate the reason why Example 3 exhibited particularly remarkable electrode performance, TEM-EDX line scanning was carried out on Examples 1 to 3. Specifically, a measurement line was drawn from the conductive substrate toward the surface of the catalyst layer (in other words, along the thickness direction) on the element map obtained in the above TEM-EDX analysis. In this line scanning, the measurement interval was set to every second, and the scanning speed was set so that the conductive substrate 10 reached the surface 20a of the catalyst layer 20 in 75 seconds. Measurement was carried out under these conditions, and a graph of the TEM-EDX line scanning was obtained. Then, the graph was converted into data using WebPlotDigitizer. The analysis conditions at that time were as follows.

[0089] [Analysis conditions] Analysis software: WebPlotDigitizer Algorithm:ΔXstep w / Interpolation ΔXstep: 1 nm Smoothing: 0% of ΔX

[0090] Next, the ratio of the count number of Fe element (Fe ratio) was calculated when the total count number of Ni element and Fe element was set to 100% at each measurement point. The calculation results were plotted on a graph with "electrode thickness (nm)" on the horizontal axis and "Fe ratio (%)" on the vertical axis. The results are shown in Figs. 9 to 11. Fig. 9 is a graph showing the variation of Fe ratio in the thickness direction in Example 1. Fig. 10 is a graph showing the variation of Fe ratio in the thickness direction in Example 2. Fig. 11 is a graph showing the variation of Fe ratio in the thickness direction in Example 3.

[0091] In addition, in this analysis, the thickness (nm), average Fe ratio (%), and increase rate of Fe ratio (% / nm) of each layer were measured based on the above-mentioned procedure. The results are shown in Table 2.

[0092] [Table 2]

[0093] As shown in Figs. 9 to 11 and Table 2, in Examples 1 to 3, a first layer having a concentration gradient in which the Fe ratio increases upward was formed on the conductive substrate. It is presumed that in this first layer, the amount of metallic Ni increases toward the bottom layer, and the amount of Fe-Ni oxide increases toward the top layer. In addition, a second layer having a high average Fe ratio and a low Fe ratio increase rate was formed on the first layer. It is presumed that a large amount of Fe-Ni oxide is uniformly present in this second layer. Here, it was found that the second layer in Example 3 was extremely thin compared to Examples 1 and 2. It is presumed that this point is the reason for the improvement of the electrode performance. Specifically, it is presumed that the electrode performance was significantly improved by making the second layer, which has low electronic conductivity, thinner, which facilitates the transfer of electrons between the surface layer of the catalyst layer and the conductive substrate.

[0094] (3)XRD analysis In this test, XRD analysis was performed to analyze the crystal structure of the catalyst layer in Examples 1 to 5. The conditions of the XRD analysis are as follows. Then, the peak appearing in the vicinity of 2θ=52° in the XRD chart after the measurement was regarded as a "peak originating from the 200 plane of metallic Ni (Ni originating from the conductive substrate) with an FCC structure", and the peak intensity was measured. Also, the peak appearing in the vicinity of 2θ=65° was regarded as a "peak originating from the 200 plane of metallic Fe with a BCC structure", and the peak intensity was measured. Then, the ratio (BCC / FCC) of the peak intensity of metallic Ni with an FCC structure to the peak intensity of metallic Fe with a BCC structure was calculated. The measurement results are shown in Table 3.

[0095] [Measurement conditions] Measurement equipment: Rigaku Corporation fully automated multipurpose X-ray diffraction device SmartLab Scan speed: 2.00℃ / min Step width: 0.01° Scan range: 2θ(5~80°) Incident angle: ω=1.0°

[0096] [Table 3]

[0097] As shown in Table 3, in Examples 1 to 3, the amount of metallic Fe with the BCC structure was significantly reduced. In particular, in Examples 2 and 3, the BCC / FCC peak intensity ratio was 0%. That is, in Examples 2 and 3, metallic Fe with the BCC structure was substantially absent. On the other hand, in Example 4, metallic Fe with the BCC structure was present in large amounts. This point may also affect the difference in electrode performance between Examples 1 to 3 and Example 4.

[0098] (4) XPS measurement In this test, the electrodes of Examples 3 and 5 were cut to 5 mm x 5 mm to prepare measurement samples. Then, an XPS measurement device (ULVAC-PHI, Inc., PHI-4700V) was used to analyze the oxidation states of Ni and Fe in the catalyst layers of Examples 3 and 5. The results are shown in Figs. 12 to 15. Fig. 12 shows the measurement results indicating the oxidation state of Ni element in Example 3. Fig. 13 shows the measurement results indicating the oxidation state of Fe element in Example 3. Fig. 14 shows the measurement results indicating the oxidation state of Ni element in Example 5. Fig. 15 shows the measurement results indicating the oxidation state of Fe element in Example 5.

[0099] As shown in Fig. 12 and Fig. 14, zero-valent, +2-valent, and +3-valent Ni were confirmed in the catalyst layers of Example 3 and Example 5. However, it was confirmed that the number of counts of +3-valent Ni was smaller in Example 5 than in Example 3. Furthermore, as shown in Fig. 13 and Fig. 15, zero-valent, +2-valent, and +3-valent Fe were confirmed in the catalyst layers of Example 3 and Example 5. Here, it was confirmed that the number of counts of zero-valent Fe was larger in Example 5 than in Example 3. Considering the above-mentioned XRD analysis and the results of this XPS measurement, it is presumed that a large amount of metallic Fe with a BCC structure was generated in Example 5.

[0100] (5) ICP optical emission spectrometry In this test, the electrodes of Examples 1 to 5 were cut to 5 mm x 5 mm and dissolved in an acidic solution to prepare measurement samples. The iron content (%) in the catalyst layer of each example was measured using an ICP measurement device (Agilent 5800 ICP-OES). The measurement results are shown in Table 4.

[0101] [Table 4]

[0102] As shown in Table 4, the Fe content was significantly lower in Examples 1 to 3 than in Example 4. This is presumably because, in Examples 1 to 3, the Fe electrodeposition treatment time was shortened and the Fe ion concentration in the electrolyte for Fe electrodeposition was reduced.

[0103] Although the technology disclosed herein has been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples exemplified above. That is, the technology disclosed herein includes the embodiments described in items 1 to 10 below.

[0104] <Item 1> A conductive substrate; a catalytic layer formed on the surface of the conductive substrate, the catalytic layer including at least Ni-Fe oxide and metallic Ni; Equipped with The catalyst layer is a first layer formed on the conductive substrate and including a high Ni region having a Fe element content lower than a Ni element content; a second layer formed on the first layer and having a higher Fe element content than the first layer; It is equipped with When the ratio of the count number of the Fe element to the total count number of the Ni element and the Fe element in a TEM-EDX line scan from the conductive base material toward the surface of the catalytic layer is defined as the Fe ratio (%), the first layer has an increase rate of the Fe ratio along the TEM-EDX line scan of 0.17% / nm or more; the second layer has an increase in the Fe fraction along the TEM-EDX line scan of less than 0.17% / nm; An electrode, wherein a ratio of a thickness of the second layer to a thickness of the first layer is 0.9 or less.

[0105] <Item 2> 2. The electrode according to item 1, wherein the first layer has an increasing rate of the Fe fraction along the TEM-EDX line scan of 3.17% / nm or more.

[0106] <Item 3> 3. The electrode according to item 1 or 2, wherein the average Fe ratio in the first layer is 5% or more and 45% or less.

[0107] <Item 4> 4. The electrode according to any one of items 1 to 3, wherein the average Fe ratio in the second layer is 45% or more and 85% or less.

[0108] <Item 5> 5. The electrode according to any one of items 1 to 4, wherein the density of the second layer based on electron microscope observation of a cross section of the catalyst layer is 50% or less.

[0109] <Item 6> 6. The electrode according to any one of items 1 to 5, wherein the density of the first layer based on electron microscope observation of a cross section of the catalyst layer is 50% or more.

[0110] <Item 7> 7. The electrode according to any one of items 1 to 6, wherein the catalyst layer is substantially free of metallic Fe having a BCC structure.

[0111] <Item 8> 8. The electrode according to any one of items 1 to 7, wherein the catalyst layer is substantially free of NiO.

[0112] <Item 9> 9. The electrode according to any one of items 1 to 8, wherein the conductive substrate is a Ni substrate.

[0113] <Item 10> an oxygen evolution electrode having a first catalyst layer on a surface of a first substrate; a hydrogen generating electrode having a second catalyst layer on a surface of a second substrate; an anion exchange membrane interposed between the oxygen evolution electrode and the hydrogen evolution electrode; a water supply pipe attached near the surface of the first catalyst layer; a conductive line electrically connecting the first substrate and the second substrate; Equipped with 10. A water electrolysis device, wherein the oxygen evolution electrode is the electrode according to any one of items 1 to 9. [Explanation of symbols]

[0114] 1 electrode 10 Conductive substrate 20 Catalyst layer 21 1st layer 22 2nd layer 100 Water electrolysis equipment 110 Oxygen evolution electrode (anode) 111 First base material 112 1st catalyst layer 120 Hydrogen generation electrode (cathode) 121 Second base material 122 2nd catalyst layer 130 Anion exchange membrane 140 Conductive Line 150 Power supply 162 Water supply channel 164 Oxygen recovery pipe 166 Hydrogen recovery pipe

Claims

1. A conductive substrate; a catalyst layer formed on the surface of the conductive substrate and containing at least Ni—Fe oxide and metallic Ni; Equipped with The catalyst layer is a first layer formed on the conductive substrate and including a high Ni region having a Fe element content lower than a Ni element content; a second layer formed on the first layer and having a higher Fe element content than the first layer; It is equipped with When the ratio of the count number of the Fe element to the total count number of the Ni element and the Fe element in a TEM-EDX line scan from the conductive substrate toward the surface of the catalytic layer is defined as the Fe rate (%), the first layer has an increasing rate of the Fe fraction along the TEM-EDX line scan of 0.17% / nm or more; the second layer has an increase in the Fe fraction along the TEM-EDX line scan of less than 0.17% / nm; An electrode, wherein a ratio of a thickness of the second layer to a thickness of the first layer is 0.9 or less.

2. 2. The electrode of claim 1, wherein the first layer has an increase in the Fe fraction along the TEM-EDX line scan of 3.17% / nm or more.

3. The electrode according to claim 1 , wherein the average Fe percentage in the first layer is 5% or more and 45% or less.

4. The electrode according to claim 1 , wherein the average Fe percentage in the second layer is 40% or more and 85% or less.

5. 2. The electrode according to claim 1, wherein the density of the second layer based on observation of a cross section of the catalyst layer with an electron microscope is 50% or less.

6. 2. The electrode according to claim 1, wherein the density of the first layer based on observation of a cross section of the catalyst layer with an electron microscope is 50% or more.

7. 2. The electrode of claim 1, wherein the catalyst layer is substantially free of metallic Fe in a BCC structure.

8. The electrode of claim 1 , wherein the catalyst layer is substantially free of NiO.

9. The electrode of claim 1 , wherein the conductive substrate is a Ni substrate.

10. an oxygen evolution electrode having a first catalyst layer on a surface of a first substrate; a hydrogen generating electrode having a second catalyst layer on a surface of a second substrate; an anion exchange membrane interposed between the oxygen evolution electrode and the hydrogen evolution electrode; a water supply pipe attached near the surface of the first catalyst layer; a conductive line electrically connecting the first substrate and the second substrate; It is equipped with A water electrolysis device, characterized in that the oxygen evolution electrode is the electrode according to any one of claims 1 to 9.

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