JP7733995B2 - Electrode, anode for water electrolysis, electrolytic cell, and method for producing hydrogen - Google Patents

Description

The present invention relates to an electrode, an anode for water electrolysis, a bipolar electrolytic cell using the anode for water electrolysis, and a method for producing hydrogen using the anode for water electrolysis.

In recent years, hydrogen produced using renewable energy has been attracting attention as a clean energy source that can solve problems such as global warming caused by _CO2 and dwindling fossil fuel reserves. Hydrogen production using renewable energy is required to be as inexpensive as hydrogen production by conventional reforming of fossil fuels. Therefore, hydrogen production using renewable energy requires high energy efficiency and inexpensive equipment that cannot be achieved with conventional technologies.

One method of producing hydrogen that can meet the above requirements is the electrolytic decomposition of water (water electrolysis). For example, several concepts have been proposed for producing hydrogen through water electrolysis using power generated by natural energy sources such as wind or solar power, and then storing or transporting the hydrogen.

In water electrolysis, oxygen is generated at the anode and hydrogen is generated at the cathode when an electric current is passed through the water. The main cause of energy loss in electrolysis is the overvoltage at the anode and cathode. Reducing this overvoltage makes it possible to produce hydrogen efficiently. The overvoltage at the anode is particularly high compared to the overvoltage at the cathode, and research and development into reducing the overvoltage at the anode is currently underway.

Among oxides with a perovskite structure, some are known to have high oxygen generation capacity and are attracting attention as anode materials for water electrolysis (Non-Patent Document 1). To use an oxide with a perovskite structure as an anode for alkaline water electrolysis, one method involves forming an oxide layer on the surface of a conductive substrate. For example, Patent Document 1 discloses a water electrolysis anode with low oxygen overvoltage and high durability achieved by forming a metal oxide layer with a high content of perovskite oxide in the crystalline components on the surface of a porous nickel substrate, as well as a water electrolysis device using this anode.

International Publication No. 2018/155503

Science, 2011, 334, 1383

It was found that the anode described in Patent Document 1 has the problem that the oxygen overvoltage does not become sufficiently low when an electrolyte with a high alkaline concentration is used.

The present invention was made in consideration of these problems, and aims to provide an electrode that exhibits a low oxygen evolution overvoltage when an electrolyte with a high alkaline concentration is used, an anode for water electrolysis, a bipolar electrolytic cell using the water electrolysis anode, and a method for producing hydrogen using the water electrolysis anode.

The present inventors have conducted extensive research and experiments to solve the above problems, and as a result have found that an electrode having a LaNi _x M _y O _3-z layer with a specific pore structure formed on a substrate has a low overvoltage for oxygen evolution when an electrolyte solution with a high alkaline concentration is used, and can be used as an anode for water electrolysis, thereby completing the present invention.

That is, the present invention is as follows.
[1] An electrode having _{LaNixNbyO3 -z (} x+y is 0.8 or more and 1.2 or less, y is 0 or more and 0.2 or less, z is -0.5 or more and 0.5 or less) on a substrate, in which the sum of the areas of pores exceeding 0.785 ^μm2 and ²⁰ μm2 or less in a 20 μm square region in _{the LaNixNbyO3 -z} layer is 10 ^μm2 or more and 200 ^μm2 or less.
[2] The electrode according to [1], wherein the average pore area in the 20 μm square region is greater than 0.785 μm ² and less than or equal to 10 μm ² .
[3] The electrode according to [1] or [2], wherein the average number of pores in the 20 μm square region is 20 or more and 150 or less.
[4] The electrode according to any one of [1] to [3], wherein the porosity in the 20 μm square region is more than 29% and not more than 55%.
[5] An electrolytic cell, characterized by using the electrode according to any one of [1] to [4] as an anode.
[6] A method for producing hydrogen by electrolyzing alkali-containing water in an electrolytic cell, the electrolytic cell comprising at least an anode and a cathode, the anode having LaNi _x Nb _y O _3-z (x+y is 0.8 or more and 1.2 or less, y is 0 or more and 0.2 or less, z is -0.5 or more and 0.5 or less) on a substrate, the method being characterized in that in a 20 μm square region in the LaNi _x Nb _y O _3-z layer, the sum of the areas of pores exceeding 0.785 μm ² and 20 μm ² or less is 10 μm ² or more and 200 μm ² or less.

The present invention provides an electrode that exhibits low oxygen overvoltage when using an electrolyte solution with a high alkaline concentration, a water electrolysis anode, and a water electrolysis cell equipped with this water electrolysis anode.

FIG. 1 is a side view showing an example of an entire electrolytic cell including an electrolytic cell equipped with the electrode of the present embodiment as an anode. 2 is a diagram showing a cross section of the inside of an electrolytic cell of an electrolytic cell including an electrode of the present embodiment as an anode, the part enclosed by a dashed square frame in FIG. 1. [0023]FIG. FIG. 1 is a diagram showing an outline of an electrolysis device used in Examples and Comparative Examples. FIG. 1 is a diagram showing an outline of a bipolar electrolytic cell used in an electrolysis test. 1 shows a cross-sectional photograph of an electrode and an example of the setting of a 20 μm square region to be cut out. 6 is an example of an image of a 20 μm square area cut out from FIG. 5. 7 is an example of an image obtained by extracting and binarizing pores from FIG. 6.

The following describes in detail the mode for carrying out the present invention (hereinafter referred to as the "present embodiment"). Note that the following present embodiment is an example for explaining the present invention and is not intended to limit the present invention. Furthermore, the present invention can be modified in various ways without departing from the gist of the invention.

(electrode)
In this embodiment, the electrode is primarily characterized by having at least a substrate.

The substrate is preferably electrically conductive. Examples of conductive substrate materials include nickel, nickel-based materials, titanium, GC (glassy carbon), tantalum, zirconium, gold, platinum, and palladium. Examples of nickel-based materials include nickel-based alloys such as Monel, Inconel, and Hastelloy.

From the viewpoint of heat resistance during the firing process in electrode preparation, it is more preferable that the substrate material be a metal. Furthermore, nickel or a material primarily composed of nickel is even more preferable from the viewpoints of durability, conductivity, and economy, as these metals do not dissolve even at the oxygen generating potential in an alkaline aqueous solution and are more inexpensively available than noble metals.

The conductive substrate of the electrode may be flat or may be a porous body with numerous holes. Specific examples of porous bodies include expanded metal, punched metal, plain weave mesh, foam metal, and similar shapes. Among these, expanded metal is preferred. While there are no particular restrictions on its dimensions, to achieve both an increase in gas generation due to an increase in electrolysis surface area and efficient removal of gas generated by electrolysis from the electrode surface, and from the standpoint of mechanical strength, it is preferable that the center-to-center distance (SW) of the mesh in the short direction be 2 mm to 5 mm, the center-to-center distance (LW) of the mesh in the long direction be 3 mm to 10 mm, the thickness be 0.2 mm to 2 mm, and the aperture ratio be 20% to 80%. More preferably, SW is 3 mm to 4 mm, LW is 4 mm to 6 mm, the thickness be 0.8 mm to 1.5 mm, and the aperture ratio be 40% to 60%.

A second feature of this embodiment is that it has _{LaNixNbyO3 -z} (x+y is 0.8 or more and 1.2 or less, y is 0 or more and 0.2 or less, and z is -0.5 or _more and 0.5 or less).

_{LaNixNbyO3 -z may} be deposited directly on the substrate to form a _{LaNixNbyO3 -z layer} , and an interface may be formed between the substrate and the _{LaNixNbyO3 -z} layer _.
In this embodiment, a high oxygen generating capacity can be achieved by disposing Ni in at least a part of the B site of the metal oxide having a perovskite structure. Furthermore, disposing Nb in the B site together with Ni is preferable because a low oxygen generating overvoltage can be imparted when an electrolyte with a high alkaline concentration is used.
When an electrolyte solution with a high alkaline concentration is used, x+y is 0.8 or more and 1.2 or less from the viewpoint of providing a low oxygen evolution overvoltage, more preferably 0.8 or more and 1.05 or less, and even more preferably 1.0 or more and 1.05 or less.
y is 0 or more and 0.2 or less, preferably 0.005 or more and 0.15 or less, and more preferably 0.01 or more and 0.1 or less.
z is equal to or greater than −0.5 and equal to or less than 0.5.

In the present invention, z is determined from the composition ratio 3-z of O calculated so that the valence balance of the composition formula is balanced, assuming that La is trivalent, Ni is trivalent, Nb is pentavalent, and O is -2valent. For example, in the composition formula LaNi _x Nb _y O _3-z , if x = 0.8 and y = 0.2, then z = -0.2 is determined from the relational formula 3-z = (3 + 3 × 0.8 + 5 × 0.2)/2.

The presence of LaNi _x Nb _y O _3-z (x+y is 0.8 or more and 1.2 or less, y is 0 or more and 0.2 or less, z is −0.5 or more and 0.5 or less) on the substrate can be confirmed by known methods, such as peeling off the LaNi _x Nb _y O _3-z layer on the substrate, dissolving it in aqua regia, and performing composition analysis by ICP-AES (inductively coupled plasma atomic emission spectroscopy), peeling off the LaNi _x Nb _y O _3-z layer on the substrate, and performing composition analysis by an X-ray fluorescence analyzer, or performing SEM-EDX analysis of the LaNi _x Nb _y O _3-z layer on the cross section of the electrode.

A third feature of this embodiment is that the sum of the areas of pores having an area of more than 0.785 _μm ² and not more than 20 μm ² is not less than 10 μm ² and not more than 200 μm ² in a _{20 μm} square region in the LaNixNb yO3-z layer.

When the pores are perfectly circular, the area of a perfectly circular pore with a diameter of 1 μm is 0.5 × 0.5 × π ≈ 0.785 (μm ² ). Similarly, the area of a perfectly circular pore with a diameter of 5 μm is 2.5 × 2.5 × π ≈ 20 (μm ² ). The fact that the sum of the areas of pores with an area of more than 0.785 μm ² and not more than 20 μm ² is in the range of 10 μm ² to 200 μm ² indicates that, when the pores are perfectly circular, pores with a diameter of more than 1 μm and not more than 5 μm are present within a certain area range in the cross section of the catalyst layer.

In this embodiment, the sum of the areas of pores greater than 0.785 μm ² and less than 20 μm ² is calculated by embedding a small piece of the electrode in epoxy resin, cutting and polishing it to create a cross-sectional observation sample, and then using image processing software to cut out an image from the backscattered electron image observed at 500x magnification, setting a 20 μm square region to binarize and extract the pores. Image processing is then performed on the cut-out image to binarize and extract the pores, determining the area of each pore, and calculating the sum of the areas of pores greater than 0.785 μm ² and less than 20 μm ^2. The 20 μm square region from which pores are binarized and extracted from the cross-sectional photograph is set so that it does not include the outer surface or cracks leading to the outer surface. When processing the image, it is preferable to perform appropriate filtering to remove noise. Specific image processing methods will be described later in the examples.

The present inventors have surprisingly discovered that an electrode having LaNi _x Nb _y O _3-z (x+y is 0.8 to 1.2, y is 0 to 0.2, and z is -0.5 to 0.5 ₎ on a substrate, in which the sum of the areas of pores greater than 0.785 μm ² and less than 20 μm ² in a 20 μm square region in the LaNi _x Nb y O _3-z layer is 10 to 200 μm ^{2 ,} exhibits low overvoltage when an electrolyte with a high alkaline concentration is used. The reason for this is not clear, but it is thought that when the sum of the areas of pores greater than 0.785 μm ² and less than 20 μm ² is in the range of 10 to 200 μm ² , ^a decrease in the diffusion rate in the electrode due to the viscosity of the electrolyte is suppressed even when an electrolyte with a high alkaline concentration is used, and the electrode has sufficient stability against an electrolyte with a high alkaline concentration.
From the viewpoint of imparting a lower oxygen evolution overvoltage when an electrolyte solution with a high alkali concentration is used, the total area of the pores having an area of more than 0.785 μm ² and not more than 20 μm ² is more preferably 20 μm ² or more and 160 μm ² or less, and even more preferably 30 μm ² or more and 130 μm ² or less.

In this embodiment, it is preferable that the average pore area in the 20 μm square region is more than 0.785 μm ² and not more than 10 μm ² from the viewpoint of imparting a low oxygen evolution overvoltage when an electrolyte with a high alkaline concentration is used.
The average pore area is the number average of the areas of each pore obtained by binarizing and extracting the pores from the cut-out image by image processing software, and a specific calculation method will be described later in the Examples.
The average pore area is more preferably 1.2 μm ² or more and 6.8 μm ² or less, and even more preferably 3.2 μm ² or more and 4.5 μm ² or less, from the viewpoint of imparting a lower oxygen evolution overvoltage when an electrolyte solution with a high alkaline concentration is used.

In this embodiment, it is preferable that the average number of pores in the 20 μm square region is 20 to 150, from the viewpoint of imparting a low oxygen evolution overvoltage when an electrolyte with a high alkaline concentration is used.
A specific method for calculating the average number of pores will be described later in the Examples.
From the viewpoint of imparting a lower oxygen evolution overvoltage when an electrolyte solution with a high alkaline concentration is used, the average number of pores is more preferably 22 or more and 140 or less, even more preferably 29 or more and 140 or less, and most preferably 29 or more and 51 or less.

In this embodiment, it is preferable that the porosity is more than 29% and not more than 55% in a 20 μm square region from the viewpoint of imparting a low oxygen evolution overvoltage when an electrolyte with a high alkaline concentration is used.
A specific method for calculating the porosity will be described later in the Examples.
The porosity is more preferably 41% or more and 55% or less, even more preferably 42% or more and 55% or less, and most preferably 45% or more and 55% or less.

The electrode of this embodiment can be used practically as an anode for water electrolysis, and this embodiment can provide an electrolytic cell for water electrolysis that uses the electrode of this embodiment as the anode, and a method for producing hydrogen using the anode for water electrolysis. Water containing alkali may be used for water electrolysis.

(Electrode Preparation Method)
The electrode of this embodiment can be prepared by applying an aqueous solution (coating liquid) containing metal salts of La, Ni, and Nb to a substrate, drying and pre-baking the solution, forming a predetermined weight of LaNi _x Nb _y O _3-z precursor on the substrate, and then baking the resulting precursor.

The metal salt may be a water-soluble salt such as a nitrate, oxynitrate, chloride, oxalate, tartrate, acetate, sulfate, etc. The metal salt may be an anhydrous salt or a hydrate salt.
For La, Ni, and Nb, a water-dispersible sol of an oxide or hydroxide may be used instead of a metal salt.
As the metal salt of Nb, niobium oxalate or ammonium niobium oxalate is preferably used from the viewpoint of solubility.

It is preferable to add an organic ligand, such as an amino acid such as glycine, or a carboxylic acid such as oxalic acid or tartaric acid, to the coating solution, from the viewpoint of facilitating the preparation of _{LaNixNbyO3 -z ,} which exhibits a lower oxygen evolution overvoltage when an electrolyte with a high alkaline concentration is used.
Glycine is a preferred organic ligand, because the use of a coating solution containing glycine and a metal nitrate makes it easier to prepare electrodes that exhibit lower oxygen evolution overpotentials when using electrolytes with high alkaline concentrations.

The concentration of the aqueous solution containing metal salts of La, Ni, and Nb is preferably 0.3 mol/kg or more and 4 mol/kg or less in terms of weight molar concentration based on LaNi _x Nb _y O _3-z. At 0.3 mol/kg or more, it is easy to set the total area of pores greater than 0.785 μm ² and less than 20 μm ² to 10 μm ² or more and 200 μm ² or less. At 4 mol/kg or less, dissolution of metal salts and organic ligands is facilitated, which is preferable from the viewpoint of productivity of the coating solution. More preferably, it is 0.35 mol/kg or more and 2.0 mol/kg or less, and even more preferably 0.4 mol/kg or more and 1.0 mol/kg or less.

After applying the coating liquid to the substrate, the drying temperature is preferably 50°C or higher and 200°C or lower. If the temperature is 50°C or higher, drying can be completed in 3 minutes to 1 hour, which is preferable from the perspective of productivity. If the temperature is 200°C or lower, the cooling time of the substrate after drying is shorter, which is preferable from the perspective of productivity.

The method for applying the coating solution to the substrate is preferably a dip coating method in which the substrate is completely immersed in the coating solution. When coating is performed by the dip coating method, it is easy to control the concentration of the coating solution to make the total area of pores greater than 0.785 μm ² and less than or equal to ^{10 μm 2 and} less than or equal to 200 μm ² .
When immersing the substrate in the coating liquid, it is preferable to slowly insert the substrate at an angle, as this allows the substrate to be completely immersed without forming any air bubbles. In addition, the voids in the metal oxide layer (preliminary calcined layer) formed on the surface of the substrate are completely replaced with the coating liquid, making it possible to reproducibly control the total area of pores greater than 0.785 ^μm2 and not greater than 20 ^μm2 .

The temperature at which the dried substrate is pre-baked is preferably 300°C or higher and 500°C or lower. If it is 300°C or higher, pre-baking can be completed in 3 minutes to 1 hour, which is preferable from the perspective of productivity. If it is 500°C or lower, the cooling time for the substrate after drying is shorter, which is preferable from the perspective of productivity.

The coating, drying, and calcination steps may be repeated to prepare an electrode with a desired _{LaNixNbyO3 -z coating} amount. When the coating amount per coating, drying, and calcination step is 10 g/ ^m2 or more and 20 g/m2 or ^less , it is easy to set the total area of pores greater than 0.785 ^μm2 and ²⁰ μm2 or less to 10 μm2 or ^more and 20 ^μm2 or less, which is preferable.

The firing temperature can be between 500°C and 1000°C, but firing at 750°C or higher is preferred. Firing at 750°C or higher is preferred from the standpoint of productivity, as it makes it easier to prepare an electrode that exhibits a lower oxygen evolution overvoltage when using an electrolyte with a high alkaline concentration, with a firing time of between 10 minutes and 24 hours.

Nb may be added by a method in which a coating liquid containing a water-dispersible sol of a metal salt, or an oxide or hydroxide of La or Ni is applied in advance, dried, and pre-baked, and then a coating liquid containing a water-dispersible sol of a metal salt, or an oxide or hydroxide of Nb is applied to the electrode in a topcoat manner, dried, pre-baked, and then baked.
Nb may also be added by a method in which a coating liquid containing a water-dispersible sol of a metal salt, oxide, or hydroxide of La or Ni is applied in advance, followed by drying, pre-baking, and main baking, and then a coating liquid containing a water-dispersible sol of a metal salt, oxide, or hydroxide of Nb is applied to the electrode in a topcoat manner, followed by drying, pre-baking, and main baking.

(electrolytic cell)
FIG. 1 shows a side view of an example of an entire electrolytic cell including an electrolytic cell equipped with the electrode of this embodiment as an anode.
FIG. 2 shows a cross section of the inside of an electrolytic cell in the portion enclosed by the dashed square frame in FIG. 1, of an electrolytic cell including the electrolytic cell equipped with the electrode of this embodiment as an anode.
In the bipolar electrolytic cell 50 (see FIG. 3) of this embodiment, the diaphragm 4 is in contact with the anode 2a and the cathode 2c, forming a zero-gap structure Z (see FIG. 2).
The electrolysis apparatus used in the examples and comparative examples is shown in Fig. 3. The bipolar electrolytic cell used in the electrolysis tests is shown in Fig. 4.

(element)
As shown in FIG. 1 , in a bipolar electrolytic cell 50, a bipolar element 60 is disposed between an anode terminal element 51 a and a cathode terminal element 51 c, and diaphragms 4 are disposed between the anode terminal element 51 a and the bipolar element 60, between adjacent bipolar elements 60, and between the bipolar element 60 and the cathode terminal element 51 c.

In this embodiment, the portion of the bipolar electrolytic cell 50 between the partition walls 1 of two adjacent bipolar elements 60, and the portion between the partition walls 1 of an adjacent bipolar element 60 and a terminal element, are particularly referred to as the electrolytic cell 65. The electrolytic cell 65 includes the partition wall 1, anode chamber 5a, anode 2a, and diaphragm 4 of one element, and the cathode 2c, cathode chamber 5c, and partition wall 1 of the other element.

(electrode chamber)
In the bipolar electrolytic cell 50 of this embodiment, as shown in Fig. 2, electrode chambers 5 through which the electrolytic solution passes are defined by the partition wall 1, the outer frame 3, and the diaphragm 4. Here, the electrode chamber 5 on the anode side across the partition wall 1 is the anode chamber 5a, and the electrode chamber 5 on the cathode side is the cathode chamber 5c.

In this embodiment, the header pipes of the bipolar electrolytic cell can be arranged in either an internal header type or an external header type, and the space occupied by the anode and cathode themselves may also be considered to be the space inside the electrode chamber. Furthermore, particularly when a gas-liquid separation box is provided, the space occupied by the gas-liquid separation box may also be considered to be the space inside the electrode chamber.

(rib)
In the bipolar electrolytic cell 65 for alkaline water electrolysis of this embodiment, the ribs 6 are preferably physically connected to the electrodes 2. With such a configuration, the ribs 6 serve as supports for the electrodes 2, making it easier to maintain the zero-gap structure Z. The ribs 6 are also preferably electrically connected to the partition wall 1. Furthermore, the provision of the ribs 6 can reduce convection that occurs in the electrode chambers 5 due to turbulence in the gas-liquid flow within the electrode chambers 5, thereby suppressing a local increase in the temperature of the electrolyte.

Here, an electrode may be provided on the rib, or a current collector, conductive elastic body, and electrode may be provided on the rib in this order.

In the bipolar electrolytic cell for alkaline water electrolysis described above, the cathode chamber has a structure in which the cathode rib, cathode current collector, conductive elastic body, and cathode are stacked in this order, and the anode chamber has a structure in which the anode rib and anode are stacked in this order.

In the example bipolar electrolytic cell for alkaline water electrolysis described above, the cathode chamber employs the above-mentioned "cathode rib-cathode current collector-conductive elastic body-cathode" structure, and the anode chamber employs the above-mentioned "anode rib-anode" structure, but the present invention is not limited to this, and the anode chamber may also employ an "anode rib-anode current collector-conductive elastic body-anode" structure.

Specifically, in the bipolar electrolytic cell for alkaline water electrolysis of this embodiment, as shown in FIG. 2 , ribs 6 (anode ribs, cathode ribs) are preferably attached to the partition wall 1 .
The ribs (anode ribs, cathode ribs) preferably have a role not only to support the anode or cathode but also to transmit current from the partition wall to the anode or cathode.

In the bipolar electrolytic cell for alkaline water electrolysis of this embodiment, it is preferable that at least a portion of the ribs be conductive, and it is even more preferable that the entire rib be conductive. With this configuration, an increase in cell voltage due to electrode deflection can be suppressed.

The ribs are generally made of a conductive metal. For example, nickel-plated mild steel, stainless steel, or nickel can be used. It is particularly preferable that the ribs be made of the same material as the partition wall, with nickel being most preferred.

The spacing between adjacent anode ribs or adjacent cathode ribs is determined taking into account factors such as the electrolysis pressure and the pressure difference between the anode and cathode chambers.

If the spacing between anode ribs or between adjacent cathode ribs is too narrow, it not only hinders the flow of electrolyte and gas but also increases costs. A rib pitch of 10 mm or more allows for good gas escape to the back surface of the electrode. On the other hand, if the spacing is too wide, the electrodes (anode and cathode) held in place by the slight pressure difference between the anode and cathode chambers may deform, and a reduced number of anode and cathode ribs may increase electrical resistance. A rib pitch of 150 mm or less reduces electrode deflection. The number of ribs, rib length, angle between the rib and the partition wall, number of through holes, and spacing (pitch) of the through holes in a given direction along the partition wall may be determined as appropriate, as long as the effects of the present invention are achieved. It is preferable that the ribs be arranged parallel to a given direction along the partition wall (for example, vertically, or, if the partition wall has a substantially rectangular shape in plan view, parallel to one of two pairs of opposing sides). The rib pitch of the anode ribs and the rib pitch of the cathode ribs may be the same or different, and it is preferable that the rib pitch of the anode ribs and the rib pitch of the cathode ribs both satisfy the above range.

Laser welding or other methods are used to attach the anode ribs and cathode ribs to the bulkhead.

Furthermore, taking into consideration cost, manufacturability, strength, etc., the thickness of the rib may be between 0.5 mm and 5 mm, and it is easier to use a thickness between 1 mm and 2 mm, but there are no particular limitations.

Electrodes and current collectors are usually attached to the ribs by spot welding, but other methods such as laser welding can also be used, or they can be tied together with a wire or string-like member for tight adhesion. The ribs, like the anode and cathode, are fixed to the partition wall by means of spot welding, laser welding, etc.

(Method for producing hydrogen)
Next, a method for producing hydrogen by alkaline water electrolysis using the bipolar electrolytic cell of this embodiment will be described.

In this embodiment, hydrogen is produced at the cathode by applying a current to a bipolar electrolytic cell equipped with an anode and cathode as described above and through which an electrolyte circulates, thereby performing water electrolysis. In this case, for example, a variable power supply can be used as the power source. A variable power supply is a power source derived from a renewable energy power plant whose output fluctuates over a period of several seconds to several minutes, unlike power sources that provide a stable output, such as grid power. There are no particular limitations on the method of renewable energy generation, but examples include solar power generation and wind power generation.

For example, in the case of electrolysis using a bipolar electrolytic cell, cationic electrolytes in the electrolyte move from the anode chamber of one element through the diaphragm to the cathode chamber of the adjacent element, while anionic electrolytes move from the cathode chamber of one element through the diaphragm to the anode chamber of the adjacent element. Therefore, the current during electrolysis flows in the direction in which the elements are connected in series. In other words, the current flows from the anode chamber of one element to the cathode chamber of the adjacent element through the diaphragm. As the electrolysis proceeds, oxygen gas is produced in the anode chamber and hydrogen gas is produced in the cathode chamber.

The bipolar electrolytic cell 65 for alkaline water electrolysis of this embodiment can be used in a bipolar electrolytic cell 50, an electrolysis device 70 for alkaline water electrolysis, etc. An example of the electrolysis device 70 for alkaline water electrolysis is a device including the bipolar electrolytic cell 50 of this embodiment, a liquid feed pump 71 for circulating the electrolyte, a gas-liquid separation tank 72 for separating the electrolyte from hydrogen and/or oxygen, and a water supply device for replenishing water consumed by electrolysis.

The alkaline water electrolysis apparatus may further include a rectifier 74, an oxygen concentration meter 75, a hydrogen concentration meter 76, a flow meter 77, a pressure meter 78, a heat exchanger 79, a pressure control valve 80, etc.

In the alkaline water electrolysis method using the alkaline water electrolysis apparatus, the current density applied to the electrolytic cell is preferably 4 kA/m ² to 20 kA/m ² , and more preferably 6 kA/m ² to 15 kA/m ² .

The electrodes, electrolytic cells, and hydrogen production methods according to embodiments of the present invention have been described above with reference to the drawings. However, the electrodes, electrolytic cells, and hydrogen production methods according to the present invention are not limited to the above examples, and modifications can be made to the above embodiments as appropriate.

Example 1
Solutions A and B were prepared with the compositions of Example 1 in Table 1. Solution B was prepared by dissolving ammonium niobium oxalate in pure water, and the concentration of Nb was confirmed by ICP-AES (inductively coupled plasma atomic emission spectroscopy).
Solution A was placed in a rectangular polyethylene container measuring 6 cm x 10 cm and 11 cm in height, and solution B was slowly mixed in while stirring with a magnetic stirrer to prepare the coating solution of Example 1.
A nickel expand metal having a SW of 3.0 mm, a LW of 4.5 mm, a thickness of 1.2 mm, and an opening ratio of 54% was prepared as a nickel porous substrate. This nickel expand metal was subjected to a blast treatment using a #100 white fused alumina abrasive, and then to an acid treatment in 6N hydrochloric acid at 50°C for 6 hours, followed by rinsing with water and drying to obtain a substrate for application.
The substrate was placed in a container with the coating liquid in a vertical orientation, with the lower half immersed in the coating liquid, and then the substrate was pulled up and turned over, and the upper half was also immersed in the coating liquid, so that the coating liquid was applied to the entire surface of the substrate.The substrate was then placed in a horizontal orientation, and excess coating liquid was blown off using an air gun.Then, the substrate was dried at 60°C for 10 minutes and further baked at 400°C for 10 minutes, forming a metal oxide layer on the surface of the substrate.
This cycle of coating, drying, and firing was repeated 50 times, followed by further firing at 800°C for 1 hour to form a metal oxide layer with a deposition amount of 503 g/ ^m2 on the coating substrate, thereby obtaining an anode for water electrolysis of Example 1.

Example 2
The same procedures as in Example 1 were carried out except that solutions A and B having the compositions of Example 2 in Table 1 were prepared, and a cycle of coating, drying, and firing was repeated 33 times. Thereafter, firing was further carried out at 800°C for 1 hour to form a metal oxide layer on the coating substrate in a deposition amount of 496 g/ ^m2 , thereby obtaining an anode for water electrolysis of Example 2.

Example 3
The same procedures as in Example 1 were carried out except that solutions A and B having the compositions of Example 3 in Table 1 were prepared, and a cycle of application, drying, and firing was repeated 25 times. Thereafter, firing was further carried out at 750°C for 1 hour to form a metal oxide layer on the application base material in a deposition amount of 498 g/ ^m2 , thereby obtaining an anode for water electrolysis of Example 3.

Example 4
34.89 _{g of Ni(NO3)2.6H2O , 51.96} g of La( _NO3 ) _3.6H2O , and 45.04 g _of glycine were dissolved in 400 g of pure water to prepare a coating solution for Example 4. Thereafter, this coating solution was transferred to a polyethylene container having a base dimension of 13 cm x 13 cm and a height of 10 cm.
A substrate was prepared in the same manner as in Example 1. The substrate was completely immersed in the coating solution in a container, then pulled out, turned sideways, and air-blasted with an air gun to remove excess coating solution. The substrate was then dried at 60°C for 10 minutes and then baked at 400°C for 10 minutes to form a metal oxide layer on the substrate surface. This cycle of coating, drying, and baking was repeated 49 times.
Niobium ammonium oxalate was dissolved in pure water to prepare an aqueous solution of niobium ammonium oxalate with an Nb concentration of 157 mmol/kg. This aqueous solution of niobium ammonium oxalate was transferred to a polyethylene container with a base of 13 cm x 13 cm and a height of 10 cm.
The substrate with a metal oxide layer formed on its surface was completely immersed in an aqueous solution of ammonium niobium oxalate and then removed. After the coating solution was applied to the entire surface of the substrate, the substrate was turned sideways and a gentle breeze was applied with an air gun to remove the coating solution clogging the pores of the substrate. The weight of the substrate after the air gun treatment was measured and it was found that 1.3 g of the aqueous solution of ammonium niobium oxalate was applied to the substrate. Since the amount of _LaNiO3 supported on the substrate was 5.0 g and the weight of 1 _mole of LaNiO3 was 245.6 g, it was determined that by applying 1.3 g of an aqueous solution of ammonium niobium oxalate with an Nb concentration of 157 mmol/kg, Nb was applied at a ratio of 0.01 moles per _mole of LaNiO3.
The substrate coated with the aqueous solution of ammonium niobium oxalate was dried at 60°C for 10 minutes, then baked at 400°C for 10 minutes, and further baked at 800°C for 1 hour to form a metal oxide layer with a deposition amount of 512 g/ ^m2 , thereby obtaining an anode for water electrolysis of Example 4.

Example 5
40.71 _{g of Ni(NO3)2.6H2O , 60.62} g of La( _NO3 ) _3.6H2O , and 52.55 g _of glycine were dissolved in 400 g of pure water to prepare a coating solution for Example 5. Thereafter, this coating solution was transferred to a polyethylene container having a base dimension of 13 cm x 13 cm and a height of 10 cm.
As in Example 4, the substrate was completely immersed in the coating solution in the container, then pulled out, turned sideways, and air-blasted with an air gun to remove excess coating solution. The substrate was then dried at 60°C for 10 minutes and baked at 400°C for 10 minutes to form a metal oxide layer on the substrate surface. This cycle of coating, drying, and baking was repeated 32 times.
Niobium ammonium oxalate was dissolved in pure water to prepare an aqueous solution of niobium ammonium oxalate with an Nb concentration of 261 mmol/kg. This aqueous solution of niobium ammonium oxalate was transferred to a polyethylene container with a base of 13 cm x 13 cm and a height of 10 cm.
The substrate having a metal oxide layer formed on its surface was completely immersed in an aqueous niobium ammonium oxalate solution and then pulled out. After the coating solution was applied to the entire surface of the substrate, the substrate was turned sideways and a weak breeze was applied using an air gun to remove the coating solution clogging the pores of the substrate. The weight of the substrate after the air gun treatment was measured, revealing that 1.3 g of the aqueous niobium ammonium oxalate solution had been applied to the substrate. The substrate coated with the aqueous niobium ammonium oxalate solution was dried at 60°C for 10 minutes and then baked at 400°C for 10 minutes. This cycle of applying 1.3 g of the aqueous niobium ammonium oxalate solution, drying at 60°C for 10 minutes, and baking at 400°C for 10 minutes was repeated three times, followed by baking at 800°C for 1 hour to form a metal oxide layer with a coating weight of 492 g/ ^m² , thereby obtaining an anode for water electrolysis of Example 5.
Since the amount of _LaNiO3 supported on the substrate was 5.0 g and the weight of 1 _mole of LaNiO3 was 245.6 g, it was found that by applying 1.3 g of an aqueous solution of ammonium niobium oxalate with an Nb concentration of 261 mmol/kg three times, for a total of 3.9 g, Nb was applied at a ratio of 0.05 moles _per mole of LaNiO3.

Example 6
The same procedures as in Example 1 were carried out except that solutions A and B having the compositions of Example 6 in Table 1 were prepared, and a cycle of coating, drying, and firing was repeated 24 times. Thereafter, firing was further carried out at 750°C for 1 hour to form a metal oxide layer on the coating substrate in a deposition amount of 486 g/ ^m2 , thereby obtaining an anode for water electrolysis of Example 6.

(Comparative Example 1)
A nickel expand metal having a SW of 3.0 mm, a LW of 4.5 mm, a thickness of 1.2 mm, and an opening ratio of 54% was prepared as a nickel porous substrate. This nickel expand metal was subjected to a blast treatment, then to an acid treatment in 6N hydrochloric acid at 50°C for 6 hours, followed by rinsing with water and drying to obtain a substrate for application.
Next, a coating solution was prepared by mixing lanthanum acetate 1.5 hydrate and nickel nitrate hexahydrate to concentrations of 0.20 mol/L and 0.20 mol/L, respectively.
A tray containing the coating solution was placed at the bottom of the coating roll, and the coating solution was soaked into an EPDM coating roll. A roll was placed above it so that the roll and the coating solution were always in contact with each other, and a PVC roller was placed on top of that to apply the coating solution to the substrate (roll method). Before the coating solution dried, the substrate was quickly passed between two EPDM sponge rolls. After that, it was dried at 50°C for 10 minutes, and then baked in a muffle furnace at 400°C for 10 minutes to form a metal oxide layer on the substrate surface.
This cycle of roll coating, drying, and firing was repeated 75 times, and then firing was further performed at 600°C for 1 hour to form a metal oxide layer with a deposition amount of 303 g/ ^m2 , thereby obtaining an anode for water electrolysis of Comparative Example 1.

(Comparative Example 2)
A substrate for application was prepared in the same manner as in Comparative Example 1.
Next, coating solutions were prepared so that the concentrations of lanthanum nitrate hexahydrate, nickel nitrate hexahydrate, ammonium niobium oxalate n-hydrate, and glycine were 0.20 mol/L, 0.16 mol/L, 0.04 mol/L, and 0.36 mol/L, respectively.
As in Comparative Example 1, a cycle of roll coating, drying, and firing was repeated 40 times by the roll method, and then firing was further performed at 700°C for 1 hour to form a metal oxide layer, thereby obtaining an anode for water electrolysis of Comparative Example 2 having a coating weight of 145 g/ ^m2 .

(Method of calculating the sum of the areas of pores greater than 0.785 ^μm2 and less than 20 ^μm2 , the average pore area, the average number of pores, and the porosity)
A small piece of the prepared electrode was embedded in epoxy resin, and then a cut surface was prepared. The cut surface was then processed using a broad-beam argon ion beam processing device (Hitachi High-Technologies Corporation, "E3500") at an acceleration voltage of 6 kV. The cut surface of the electrode to be observed was parallel to the expanded metal strand and passed through the center of the strand.
The cross section was observed under an electron microscope (Hitachi High-Technologies Corporation, "S4800") at an accelerating voltage of 5 kV and a magnification of 500x. A cross-sectional photograph of the backscattered electrons taken using a YAG detector was saved as a BMP file with a data size of 1280 x 960 pixels. The size of one pixel was 198.4375 nm.
Using the image processing software "ImageJ", 20 μm x 20 μm regions were cut out from the cross-sectional photograph according to the following steps 1) to 11), and the pores in each cut-out region were binarized and extracted to measure the area of all pores. Then, the sum of the area of pores greater than 0.785 μm ² and less than 20 μm ² , the average pore area, the number of pores, and the porosity were calculated. Ten 20 μm x 20 μm regions were extracted from the cross-sectional photograph so that they did not overlap, and the sum of the area of pores greater than 0.785 μm ² and less than 20 μm ² , the average pore area, the number of pores, and the average porosity were calculated from each of the 10 regions. The sum of the area of pores greater than 0.785 μm ² and less than 20 μm ² , the average pore area, the average number of pores, and the porosity of the examples and comparative examples were used as the measurement results.
The measurement results for Examples 1 to 6 and Comparative Examples 1 and 2 are shown in Table 2.

1) Open the BMP file of the cross-sectional image and select 8-bit in the Type submenu of the Image menu. Figure 5 shows an example of a cross-sectional image of an electrode.
2) In the Set scale submenu of the Analyze menu, set Distance in Pixels to 1, Known distance to 0.1984375, Pixel aspect ratio to 1.0, and Unit of length to μm, and then click OK.
3) In the Selection submenu of the Edit menu, select the Specify command, check the Scaled units (μm) checkbox, set Width: 20, Height: 20, and further set the X coordinate and Y coordinate values appropriately. Click OK to set a 20 μm square region within the cross-sectional photograph for image processing. Figure 5 shows an example of setting a 20 μm square region to be cut out from a cross-sectional photograph. The 20 μm square region for image processing is set so that it does not include the outer surface or cracks that lead to the outer surface.
4) Execute the Crop command in the Image menu to crop a 20 μm square region from which to extract pores. Figure 6 shows an example of an image of a 20 μm square region cropped from Figure 5.
5) Name and save the cut 20 μm square area.
6) In the Filters submenu of the Process menu, select Median... and set Radius: 0.5, then click OK to perform the filtering process.
7) Select the Threshold command in the Adjust submenu of the Image menu, select Otsu as the automatic threshold setting method, select B&W as the display method, leave the Dark background check box unchecked, click the Auto button, click the Apply button, and perform binarized extraction of the pores. Figure 7 shows an example of an image in which the pores have been binarized and extracted from Figure 6.
8) Select the Set Measurements... command from the Analyze menu and check the Area check box.
9) Select the Analyze Particles... command from the Analyze menu, set Size (Pixel^2): 0-Infinity, Circularity: 0.00-1.00, Show: Outlines, check the Display results, Clear results, and Summarize check boxes, and do not check the Exclude on Edges and Include Holes check boxes, then click OK.
10) The Count displayed in the Summary window is the number of pores (number), the Average Size is the average pore area (μm ² ), and the % Area is the porosity (%).
11) The number of each pore is displayed in the first column on the left side of the Results window, and the pore area (μm ² ) of each pore is displayed in the Area column. The number of each pore can be confirmed from the image of the measured pore outline numbered and displayed in the "Drawing of Image File Name Created in 5)" window. The total area of pores greater than 0.785 μm ² and less than 20 μm ² is calculated by adding up all areas of each pore area value in the Area column that are greater than 0.785 μm ² and less than 20 μm ² .

(Measurement of oxygen overvoltage in 55 wt % potassium hydroxide aqueous solution)
The oxygen overvoltage in a 55 wt % aqueous potassium hydroxide solution was measured by the following procedure.
A 2 cm x 2 cm evaluation electrode was cut from each electrode of the Examples and Comparative Examples and attached to a PTFE-coated nickel rod with a nickel screw. A platinum mesh was used as the counter electrode, and electrolysis was performed at 80°C in a 55 wt% potassium hydroxide aqueous solution at a current density of 6 kA/ ^m² , followed by measurement of the oxygen overvoltage. The oxygen overvoltage was measured by the three-electrode method using a Luggin capillary to eliminate the effects of ohmic loss due to solution resistance. The distance between the tip of the Luggin capillary and the anode was always fixed at 1 mm. A Solartron potentiogalvanostat "1470E System" was used as the oxygen overvoltage measurement device. A silver-silver chloride (Ag/AgCl) electrode was used as the reference electrode for the three-electrode method. The electrolyte resistance that could not be completely eliminated using the three-electrode method was measured using an AC impedance method, and the oxygen overvoltage was corrected based on the measured electrolyte resistance.
A Cole-Cole plot, in which the real and imaginary parts were plotted, was obtained using a frequency response analyzer "1255B" manufactured by Solartron, and then the electrolyte resistance and double layer capacitance were calculated by analyzing the plot using equivalent circuit fitting.
The ohmic loss that could not be completely eliminated by the three-electrode method was measured by the AC impedance method, and the oxygen overvoltage was corrected based on the measured ohmic loss. The ohmic loss was measured using a frequency characteristic analyzer "1255B" manufactured by Solartron.

The evaluation results for Examples 1 to 6 and Comparative Examples 1 and 2 are shown in Table 2.

Example 7
An electrolytic cell for alkaline water electrolysis and a bipolar electrolytic cell were prepared as follows.
-anode-
It was produced in the same manner as in Example 4.
-cathode-
The conductive substrate used was a plain-woven mesh substrate made of nickel thin wires with a diameter of 0.15 mm woven at 40 meshes, on which platinum was supported.
- Partition walls, outer frames -
The bipolar element used had a partition wall separating the anode and cathode and an outer frame surrounding the partition wall. All of the materials used for the partition wall and the frame of the bipolar element, which come into contact with the electrolyte, were nickel.
-Conductive elastic body-
The conductive elastic body used was made by weaving nickel wires with a wire diameter of 0.15 mm and corrugating them to a wave height of 5 mm.
-diaphragm-
A coating solution having the following component composition was obtained using zirconium oxide (trade name "EP Zirconium Oxide", manufactured by Daiichi Kigenso Kagaku Kogyo Co., Ltd.), N-methyl-2-pyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.), polysulfone ("Udel" (registered trademark), manufactured by Solvay Advanced Polymers), and polyvinylpyrrolidone (weight average molecular weight (Mw) 900,000, manufactured by Wako Pure Chemical Industries, Ltd.).
Polysulfone: 15 parts by mass Polyvinylpyrrolidone: 6 parts by mass N-methyl-2-pyrrolidone: 70 parts by mass Zirconium oxide: 45 parts by mass The above coating liquid was applied to both surfaces of a substrate: polyphenylene sulfide mesh (manufactured by Kureha Co., Ltd., film thickness 280 μm, mesh size 358 μm, fiber diameter 150 μm). Immediately after coating, the substrate coated with the coating liquid was exposed to steam and then immersed in a coagulation bath to form a coating film on the substrate surface. The coating film was then thoroughly washed with pure water to obtain a porous membrane.
-gasket-
The gasket used was a square gasket with a thickness of 4.0 mm, a width of 18 mm, and inner dimensions of 504 mm square. It had an opening on the inside that was the same size as the electrode chamber in a plan view, and had a slit structure for inserting and holding a diaphragm.
-Zero-gap type multi-pole element-
The external header type zero gap cell unit 60 was a rectangle measuring 540 mm x 620 mm, and the area of the current-carrying surfaces of the anode 2a and cathode 2c was 500 mm x 500 mm. The cathode side of the zero gap bipolar element 60 was composed of a laminate of the cathode 2c, a conductive elastic body 2e, and a cathode current collector 2r, and was connected to the partition wall 1 via the cathode rib 6, providing a cathode chamber 5c through which the electrolyte flows. The anode side was composed of the anode 2a connected to the partition wall 1 via the anode rib 6, providing an anode chamber 5a through which the electrolyte flows ( FIG. 2 ).
The depth of the anode chamber 5a (anode chamber depth; the distance between the partition wall and the anode in FIG. 2 ) was 25 mm, and the depth of the cathode chamber 5c (cathode chamber depth; the distance between the partition wall and the cathode current collector in FIG. 2 ) was 25 mm. The nickel partition wall 1 to which the nickel anode rib 6 having a height of 25 mm and a thickness of 1.5 mm and the nickel cathode rib 6 having a height of 25 mm and a thickness of 1.5 mm were attached by welding had a thickness of 2 mm.
A nickel expand substrate that had been pre-blasted was used as the cathode current collector 2r. The substrate had a thickness of 1 mm and an aperture ratio of 54%. A conductive elastic body 2e was fixed onto the cathode current collector 2r by spot welding. This zero-gap type bipolar element was stacked via a gasket that held the diaphragm, thereby forming a zero-gap structure Z in which the anode 2a and cathode 2c were pressed against the diaphragm 4.

(Comparative Example 3)
A zero-gap type bipolar element was produced in the same manner as in Example 7, except that an anode produced in the same manner as in Comparative Example 2 was used.

Using the electrolysis devices of Example 7 and Comparative Example 3, water electrolysis was performed by circulating an electrolyte solution of 55 wt % potassium hydroxide aqueous solution at 80°C and continuously applying a positive current for 100 hours so that the current density was 6 kA/ ^m2 . The voltage versus voltage of each cell of Example 7 and Comparative Example 3 was monitored, and the progress of the voltage versus voltage was recorded. The cell voltage of each cell was compared by taking the average value of three cells for each of Example 7 and Comparative Example 3.
In Example 7, the three-cell average overvoltage after 100 hours of energization was a low value of 1.70 V, whereas in Comparative Example 3, the three-cell average overvoltage after 100 hours of energization was a high value of 1.90 V. Therefore, it can be concluded that the anode of Example 7 exhibited a lower oxygen overvoltage in a high-concentration alkaline electrolyte than Comparative Example 3, thereby achieving a low cell voltage even during long-term operation.

The electrode of the present invention has a low overvoltage for oxygen generation in a highly concentrated alkaline electrolyte, making it suitable for use as an anode in a water electrolysis cell for electrolyzing water containing a highly concentrated alkali.

REFERENCE SIGNS LIST 1 Partition wall 2 Electrode 2a Anode 2c Cathode 2e Conductive elastic body 2r Cathode current collector 3 Outer frame 4 Diaphragm 5a Anode chamber 5c Cathode chamber 6 Rib 7 Gasket 50 Bipolar electrolytic cell 51g Fast head, loose head 51i Insulating plate 51a Anode terminal element 51c Cathode terminal element 51r Tie rod 60 Bipolar element 65 Electrolytic cell 70 Electrolytic device 71 Liquid feed pump 72 Gas-liquid separation tank 74 Rectifier 75 Oxygen concentration meter 76 Hydrogen concentration meter 77 Flow meter 78 Pressure gauge 79 Heat exchanger 80 Pressure control valve Z Zero gap structure

Claims (6)

An electrode having LaNi _x Nb _y O _3-z (x+y is 0.8 or more and 1.2 or less, y is 0.005 or more and 0.2 or less, z is -0.5 or more and 0.5 or less) on a substrate, in which the sum of the areas of pores exceeding 0.785 μm ² and 20 μm ² or less is 10 μm ² or more and 200 μm ² or less in a 20 μm square region in the LaNi _x Nb _y O _3-z layer. 2. The electrode of claim 1, wherein the average pore area in the 20 μm square area is greater than 0.785 μm ² and less than or equal to 10 μm ² . The electrode described in claim 1 or 2, wherein the average number of pores in the 20 μm square area is 20 to 150. An electrode according to any one of claims 1 to 3, wherein the porosity in the 20 μm square region is greater than 29% and less than or equal to 55%. An electrolytic cell characterized by using the electrode described in any one of claims 1 to 4 as an anode. A hydrogen production method for producing hydrogen by electrolyzing alkali-containing water in an electrolytic cell, characterized in that the electrolytic cell comprises at least an anode and a cathode, the anode having LaNi _x Nb _y O _3-z (x+y is 0.8 or more and 1.2 or less, y is 0.005 or more and 0.2 or less, z is -0.5 or more and 0.5 or less) on a substrate, and the sum of the areas of pores exceeding 0.785 μm ² and 20 μm ² or less in a 20 μm square region in the LaNi _x Nb _y O _3-z layer is 10 μm ² or more and 200 μm ² or less.