JP7810360B2 - Alkaline water electrolysis method and anode for alkaline water electrolysis - Google Patents

Description

The present invention relates to an alkaline water electrolysis method and an anode for alkaline water electrolysis. Specifically, the present invention relates to a technology that enables the formation of a large amount of catalyst in a short period of time by simply supplying, as needed, an electrolyte solution containing a dispersed catalyst with a unique composition that exhibits good dispersibility to at least the anode chamber that constitutes an electrolytic cell, thereby maintaining the catalytic activity of the oxygen generating anode for a long period of time. This technology makes it possible to perform alkaline water electrolysis more stably over a long period of time, with less degradation of electrolysis performance, even when using electricity with large output fluctuations, such as renewable energy, as a power source.

Hydrogen is a secondary energy source that is suitable for storage and transportation, and has a low environmental impact, so interest is growing in hydrogen energy systems that use hydrogen as an energy carrier. Currently, hydrogen is mainly produced by methods such as steam reforming of fossil fuels. However, in light of issues such as global warming and fossil fuel depletion, hydrogen production by water electrolysis using renewable energy sources such as solar and wind power is becoming increasingly important as a fundamental technology. Water electrolysis is low-cost and suitable for large-scale production, making it a promising technology for hydrogen production.

Currently, practical water electrolysis can be broadly divided into two types. One is alkaline water electrolysis, which uses a highly concentrated alkaline aqueous solution as the electrolyte. The other is solid polymer water electrolysis, which uses a solid polymer membrane (SPE) as the electrolyte. When using water electrolysis for large-scale hydrogen production, alkaline water electrolysis, which uses inexpensive materials such as iron-based metals like nickel, is said to be more suitable than solid polymer water electrolysis, which requires electrodes that use large amounts of expensive precious metals.

High-concentration alkaline aqueous solutions become more conductive as the temperature rises, but also more corrosive. For this reason, the upper limit of the operating temperature for alkaline water electrolysis is limited to approximately 80 to 90°C. In recent years, the development of electrolytic cell components and various piping materials that can withstand high temperatures and high-concentration alkaline aqueous solutions, as well as the development of low-resistance diaphragms and electrodes with increased surface area and catalysts, has improved the electrolytic cell voltage to 2 V or less at a current density of 0.6 A cm ⁻² .

Nickel-based materials, which are stable in highly concentrated alkaline aqueous solutions, are used as anodes for alkaline water electrolysis, and it has been reported that nickel-based anodes have a lifespan of several decades or more when alkaline water electrolysis is performed using a stable power source (Non-Patent Documents 1 and 2). However, when renewable energy is used as the power source, harsh conditions such as frequent start-stops and load fluctuations are often encountered, and degradation of the performance of nickel-based anodes is an issue (Non-Patent Document 3).

Both the nickel oxide production reaction and the reduction reaction of the produced nickel oxide proceed on the metal surface. Therefore, these reactions promote the detachment of the electrode catalyst formed on the metal surface. When the supply of power for electrolysis is discontinued, electrolysis stops, and the nickel-based anode is maintained at a potential lower than the oxygen evolution potential (1.23 V vs. RHE) and higher than the counter electrode, the cathode for hydrogen generation (0.00 V vs. RHE). Within the electrolytic cell, various chemical species generate electromotive forces, and as the cell reaction progresses, the anode potential is maintained low, promoting the reduction reaction of nickel oxide.

In the case of an electrolytic cell that combines multiple cells, such as an anode chamber and a cathode chamber, the current generated by the cell reaction leaks through the piping connecting the cells. One way to prevent this current leakage is to continue to flow a small current when the cell is shut down. However, continuing to flow a small current when the cell is shut down requires special power supply control and constantly generates oxygen and hydrogen, which requires excessive effort in terms of operational management. Furthermore, while it is possible to intentionally avoid a reverse current state by draining the solution immediately after shutting down the cell to prevent the cell reaction, this is hardly an appropriate measure when operating on electricity with large output fluctuations, such as renewable energy.

Conventionally, catalysts (anode catalysts) for oxygen generation used in alkaline water electrolysis include platinum group metals, platinum group metal oxides, valve metal oxides, iron group oxides, lanthanide group metal oxides, etc. Other known anode catalysts include nickel-based alloys such as Ni-Co and Ni-Fe, nickel with increased surface area, conductive oxides (ceramic materials) such as spinel-based Co ₃ O ₄ and NiCo ₂ O ₄ and perovskite-based LaCoO ₃ and LaNiO ₃ , noble metal oxides, and oxides composed of lanthanide group metals and noble metals (Non-Patent Document 3).

In recent years, various configurations of oxygen-generating anodes for use in high-concentration alkaline water electrolysis have been proposed. For example, an anode for alkaline water electrolysis (Patent Document 1) has been proposed in which a lithium-containing nickel oxide catalyst layer containing lithium and nickel at a specific molar ratio is formed on the surface of a nickel substrate, and an anode for alkaline water electrolysis (Patent Document 2) has been proposed in which a catalyst layer containing a nickel-cobalt oxide and iridium oxide or ruthenium oxide is formed on the surface of a nickel substrate.

The inventors have already proposed an oxygen generating anode with a novel configuration as a technology that solves the problems of the prior art proposed above. Specifically, they proposed an oxygen generating anode comprising a catalytic layer containing hybrid cobalt hydroxide nanosheets (Co-ns), a composite of a metal hydroxide and an organic substance, provided on the surface of a conductive substrate made of nickel or a nickel-based alloy. Furthermore, they proposed an alkaline water electrolysis method using this oxygen generating anode, in which an electrolyte containing dispersed hybrid cobalt hydroxide nanosheets (Co-ns), a component of the catalytic layer, is supplied to the anode and cathode chambers that make up the electrolysis cell, and is used commonly for electrolysis in each chamber (Patent Document 3, Non-Patent Document 4).

Furthermore, because there was room for improvement in the performance of the electrodes formed by the above electrolysis method, further research was conducted on the nanosheet components, and the following technology was proposed. Specifically, it was discovered that an oxygen generating anode equipped with a catalyst layer containing hybrid nickel-iron hydroxide nanosheets (NiFe-ns), a composite of metal hydroxide and organic matter, had superior electrode performance, and an alkaline water electrolysis method was proposed in which an electrolyte solution with NiFe-ns dispersed therein is supplied to the anode and cathode chambers that make up the electrolysis cell, and is used commonly for electrolysis in each chamber (Patent Document 4).

JP 2015-86420 A Japanese Patent Application Laid-Open No. 2017-190476 Re-table 2020/184607 publication Japanese Patent Application Laid-Open No. 2021-139027

P.W.T.Lu, S.Srinivasan, J.Electrochem.Soc.,125, 1416 (1978) C.T.Bowen, Int.J.Hydrogen Energy,9,59 (1984) S. Mitsushima et al., Electrocatalysis, 8, 422 (2017) Y. Kuroda, T. Nishimoto, S. Mitsushima, Electrochim. Acta, 323, Article 134812 (2019)

However, according to the inventors' investigations, the technology of Patent Document 4 presented a new problem: the poor dispersibility of NiFe-ns in alkaline aqueous solutions made it difficult to form a sufficient catalytic layer in an electrolysis cell in a short period of time. This poses a concern for practical application and is an important issue that must be resolved. Both the Co-ns-based technology of Patent Document 3 and the NiFe-ns-based technology of Patent Document 4, which is an extension of this technology, achieve excellent electrocatalytic activity. However, according to the inventors' investigations, even more effective alkaline water electrolysis anodes using NiFe-ns as a catalyst require further development. Specifically, the conventional alkaline water electrolysis anodes targeted by these technologies are prone to degradation (deterioration) of electrolytic performance when powered by renewable energy or other sources with large output fluctuations, making stable long-term use difficult. However, these technologies have yet to be fully addressed. That is, to fully solve the above technical problems, there is a need for high durability that enables the electrolysis performance to be more quickly restored and improved when the performance of the oxygen generating anode used deteriorates due to potential fluctuations caused by frequent start-stops or potential load fluctuations. In the specification of the present application, the alkaline water electrolysis anode that generates oxygen is also referred to as the "oxygen generating anode".

The present invention was made in consideration of the above-mentioned conventional technology, and its objective is to provide a useful electrode for electrolysis that is highly durable and maintains excellent catalytic activity more stably over a long period of time, even when powered by electricity with large output fluctuations, such as renewable energy. The ultimate objective of the present invention is to provide an operating method for alkaline water electrolysis that, by using the above-mentioned excellent electrode for electrolysis, is able to perform alkaline water electrolysis more stably over a long period of time, and is less likely to deteriorate in electrolysis performance, even when powered by electricity with large output fluctuations.

The objective of the present invention is therefore to further advance the technology developed by the inventors mentioned above, making it more industrially applicable. Specifically, the objective is to develop an oxygen generating anode that is more resistant to degradation of electrolytic performance when powered by electricity with large output fluctuations, can efficiently recover degraded electrolytic performance (i.e., the catalytic layer self-repairs through electrolysis), and maintains excellent catalytic activity stably for a longer period of time, compared to conventional technologies that use Co-ns or NiFe-ns catalysts. Furthermore, the objective is to develop technology that enables the catalytic layer of an oxygen generating anode that achieves such excellent effects to be formed efficiently over a long period of time using more versatile materials and a simple electrolysis method.

The above object can be achieved by the present invention described below. That is, the present invention provides the following alkaline water electrolysis method. Note that the abbreviation "hmh" used in the present invention is an abbreviation for "hybrid metal hydroxide."
[1] A method for alkaline water electrolysis, characterized in that an electrolyte solution containing a dispersed catalyst comprising a hybrid nickel- _iron -cobalt hydroxide ( _{NixFeyCoz -} hmh), which is a composite of a metal hydroxide and an organic substance, is supplied to an anode chamber and a cathode chamber constituting an electrolytic cell, and is used in common for electrolysis in each chamber.

[2] A method for alkaline water electrolysis, comprising: supplying an electrolyte solution having dispersed therein a catalyst comprising a hybrid _nickel -iron- _cobalt hydroxide ( _{NixFeyCoz -} hmh), which is a composite of a metal hydroxide and an organic substance, to at least an anode chamber constituting an electrolytic cell; electrolytic deposition of the _NixFeyCoz - _hmh in the electrolytic cell during operation; and electrolytic deposition of the _NixFeyCoz - _hmh on the surface of a _conductive substrate having a catalyst layer formed on its surface, which constitutes an oxygen generating anode, thereby recovering and improving electrolysis performance.

Preferred embodiments of the alkaline water electrolysis method described above include the following.
[3] The alkaline water electrolysis method according to [1] or [2], wherein the electrolytic solution is supplied intermittently.
[4] The alkaline water electrolysis method according to any one of [1] to [3], wherein the Ni _x Fe _y Co _z -hmh comprises at least one of a sheet-like substance having a layered molecular structure, a needle-shaped substance having a tunnel structure, and a particulate substance having an amorphous structure, all of which are substances with a size within the range of 1 to 200 nm.
[5] The alkaline water electrolysis method according to any one of [2] to [4], wherein the condition for electrolytically depositing Ni _x Fe _y Co _z -hmh on the surface of the conductive substrate is to maintain the conductive substrate in a potential range of 1.2 V to 1.8 V vs. RHE.
[6] The alkaline water electrolysis method according to any one of [1] to [5], wherein the electrolyte solution in which _{NixFeyCoz - hmh} is dispersed is a _{NixFeyCoz - hmh} dispersion solution having a concentration of 5 to 100 g/L, and the concentration of the _{NixFeyCoz - hmh} dispersion solution added to the electrolyte solution is adjusted to be within the range of 0.1 to 8 mL/L.
[7] The alkaline water electrolysis method according to any one of [1] to [6], wherein the Ni _x Fe _y Co _z -hmh has an atomic ratio of Ni/Fe/Co of 0.1-0.9/0.1-0.9/0.1-0.9.

In another embodiment, the present invention provides the following anode for alkaline water electrolysis, which is useful when applied to the alkaline water electrolysis method described above.
[8] An anode for alkaline water electrolysis that generates oxygen, comprising: a conductive substrate having a surface made of nickel or a nickel-based alloy; and a catalytic layer formed on the surface of the conductive substrate, the catalytic layer comprising a hybrid _nickel -iron- _cobalt hydroxide ( _NixFeyCoz -hmh) that is a composite of a metal hydroxide and an organic substance.
[9] An anode for alkaline water electrolysis that generates oxygen, comprising: a conductive substrate having a surface made of nickel or a nickel-based alloy; an intermediate layer formed on the surface of the conductive substrate and made of a lithium-containing nickel oxide represented by the composition formula Li _x Ni _2-x O ₂ (0.02≦x≦0.5); and a catalyst layer formed on the surface of the intermediate layer and containing a hybrid nickel-iron-cobalt hydroxide ( _{NixFe y} Co _z -hmh) that is a composite of a metal hydroxide and an organic substance.

The present invention makes it possible to provide an alkaline water electrolysis anode (oxygen generating anode) that generates oxygen and is more durable, in which electrolytic performance is less likely to deteriorate during electrolysis operation and excellent catalytic activity is more stably and efficiently maintained over a long period of time, even when using electricity with large output fluctuations, such as renewable energy, as a power source. Furthermore, the present invention makes it possible to more efficiently maintain the catalytic activity of the oxygen generating anode stably over a long period of time by the simple means of supplying a common electrolytic solution to the anode chamber and the cathode chamber, or by the simple means of supplying an electrolytic solution in which a specific catalyst is dispersed, as needed, to the anode chamber. The present invention makes it possible to provide an industrially useful alkaline water electrolysis method that is less likely to deteriorate in electrolytic performance of the oxygen generating anode and that enables more stable alkaline water electrolysis over a long period of time, particularly when using electricity with large output fluctuations as a power source. Furthermore, the material constituting the catalytic layer of the alkaline water electrolysis anode used in the present invention, which provides the above-mentioned excellent effects, is highly versatile and allows for the simple formation (deposition) of a large amount of catalyst in a short period of time by constant-current electrolysis, thereby enabling more efficient formation of the catalytic layer, particularly stable and efficient self-repair of the catalytic layer, and therefore has excellent industrial applicability and extremely high practical value. The above-mentioned excellent effects can be easily obtained by the oxygen generating anode provided with a catalytic layer containing a hybrid of a metal hydroxide and an organic substance, nickel-iron _{- cobalt} hydroxide ( _NixFeyCoz - _hmh ), which the inventors have newly proposed, and a new alkaline water electrolysis method using the anode and performing electrolysis in an electrolyte solution in which _{NixFeyCoz -} hmh is dispersed. Furthermore, it was confirmed that the above-mentioned excellent effects can also be obtained using an oxygen generating anode provided with a catalytic layer containing a hybrid of an extremely versatile iron ion and an organic substance (Fe-hmh), compared to conventional techniques. Alternatively, it was confirmed that by configuring an oxygen generating anode having a catalyst layer containing not only Fe-hmh but also Fe-hmh and Co-ns coexisting therein, better performance was exhibited than either a catalyst layer containing Fe-hmh alone or a catalyst layer containing Co-ns alone.

FIG. 1 is a cross-sectional view schematically illustrating one embodiment of an oxygen generating anode used in the alkaline water electrolysis method of the present invention. FIG. 1 is a diagram showing a schematic molecular structure of a layered Ni _x Fe _y Co _z -Tris-NH ₂ having a tripod ligand, which is an example of a catalyst component used in the present invention. FIG. 1 is a diagram showing a schematic diagram of a needle-shaped material with a tunnel structure, which is an example of a catalyst component used in the present invention, that is, Ni _x Fe _y Co _z -Tris-NH ₂ having a tripod ligand. FIG. 1 is a diagram schematically illustrating an example of a method for producing a catalyst according to the present invention and an example of the resulting reaction product. 1 is a graph showing current-potential changes (activity changes) in the potential cycle of Study Example 1 when a catalyst of the present invention, Ni _11.5 Fe _70.5 Co ₁₈ -hmh, is used. 1 is a graph showing current-potential change (activity change) when Ni _61.5 Fe _38.5 -ns is used as a catalyst for comparison in potential cycling in Study Example 1. 4C is a graph showing the relationship between catalyst deposition time and catalyst deposition amount (accumulation amount) obtained using the peak areas of FIGS. 4A and 4B. 1 is a graph showing the change in electrolytic characteristics with respect to deposition time in Study Example 1, when Ni _11.5 Fe _70.5 Co ₁₈ -hmh is used as the catalyst and when Ni _61.5 Fe _38.5 -ns is used as the catalyst. 1 is a graph showing changes in electrolysis characteristics when each catalyst component is used in an accelerated test performed in Study Example 1. 1 is a graph showing changes in electrolysis characteristics when each catalyst component was used in a test of continuous electrolysis at a constant current conducted in Study Example 2. 1 is a graph showing changes in electrolysis characteristics when each catalyst component is used in an accelerated durability test conducted in Study Example 3. FIG. 1 is a transmission electron microscope photograph showing the properties of Fe-hmh particles in a liquid, which is an example of a catalyst component that provides the effects of the present invention. FIG. 1 is a transmission electron microscope photograph showing a state in which Fe-hmh particles, which are an example of a catalyst component that can provide the effects of the present invention, are dispersed in an electrolyte and precipitated on the surface of a Ni substrate by electrolysis. This is a field emission scanning electron microscope (FE-SEM) image showing the state in which Fe-hmh particles, which are an example of a catalyst component that can provide the effects of the present invention, are dispersed in an electrolyte solution together with Co-ns particles and precipitated on the surface of a Ni substrate by electrolysis. 11B is a field emission scanning electron microscope (FE-SEM) image showing the precipitated state in a different field of view from that of FIG. 11A. 10 is a graph showing the change in electrolytic characteristics when an electrolyte solution in which Fe-hmh particles are dispersed and an electrolyte solution in which a catalyst component in which Fe-hmh particles and Co-ns particles coexist are dispersed, as conducted in Study Example 4, are used. 10 is a graph showing the change in the amount of Fe deposition and electrolysis characteristics relative to the amount of Co deposition obtained after electrolysis in an electrolytic solution in which catalyst components in which Fe-hmh particles and Co-ns particles are coexisted at different ratios, as performed in Study Example 5. 10 is a graph showing the amounts of Co and Fe precipitates obtained after electrolysis in each electrolyte solution in which catalyst components, Fe-hmh particles and Co-ns particles, were dispersed in the presence of each other at different ratios, as performed in Study Example 5.

The present invention will be described in detail below with reference to preferred embodiments. The aforementioned Non-Patent Document 4 reported for the first time that an electrolyte solution containing a self-repairing catalyst Co-ns for the anode improved anode performance while having almost no effect on the cathode electrode. However, as mentioned above, although the inventors have previously proposed new self-repairing catalysts for the anode, they have found that there is still room for improvement in anode performance, and that there are issues that need to be resolved, which are particularly important for practical application. Specifically, the NiFe-ns proposed so far have poor dispersibility in alkaline aqueous solutions, and therefore, when used in an electrolyte solution, it is difficult to form a sufficient catalyst layer in an electrolytic cell in a short period of time. Therefore, the development of a useful technology for more efficiently and stably recovering (repairing) electrolytic performance has been desired. As a result of intensive research into solving the above problems, the present inventors have found that, firstly, hybrid nickel-iron- _cobalt hydroxide ( _{NixFeyCoz -} hmh) can function more effectively as the highly durable self-repairing electrode catalyst targeted in the present invention, and that the use of this composite can solve the above problems in the conventional technology to a higher level, thereby completing the present invention.

Specifically, the inventors discovered that by dispersing a hybrid nickel-iron-cobalt hydroxide ( _NixFeyCoz - _hmh ), a composite of a metal hydroxide and an organic _substance , in an electrolyte and using it as a self-repairing electrode catalyst, _{NixFeyCoz - hmh} acts as a catalyst and a corrosion-resistant coating, significantly improving the durability of Ni-based anodes against potential fluctuations compared to the previously proposed anodes using Co-ns as the catalyst. Furthermore, the use of _{NixFeyCoz - hmh} as the catalyst allows a larger amount of catalyst to be formed (deposited) on the anode surface during electrolysis in a shorter time than the previously proposed case where _NiFe -ns was used as the catalyst, and _{NixFeyCoz -} hmh is a useful material that can more efficiently and stably restore electrolysis performance.

Furthermore, during the course of the above-mentioned research, the inventors discovered that even in the case of an anode using Fe-hmh as a catalyst, in which the only metal ion is the commonly used metal iron, the durability of a Ni-based anode against potential fluctuations can be improved compared to anodes using the previously proposed Co-ns as a catalyst. Furthermore, as a result of extensive research aimed at improving the practicality of using the above-mentioned Fe-hmh as a catalyst, the inventors discovered that even greater effects can be achieved by using the following configuration. Specifically, they discovered that even in the case of an anode configured to use a catalyst in which Fe-hmh and Co-ns are prepared separately and a precipitate in which these coexist is used, the durability of a Ni-based anode against potential fluctuations can be improved compared to anodes using Fe-hmh alone as a catalyst.

[anode]
Figure 1 is a cross-sectional view schematically illustrating one embodiment of an alkaline water electrolysis anode for generating oxygen, used in the alkaline water electrolysis method of the present invention. The oxygen generating anode 10 of the embodiment shown in Figure 1 comprises a conductive substrate 2, an intermediate layer 4 formed on the surface of the conductive substrate 2, and a catalytic layer 6 formed on the surface of the intermediate layer 4. Details of the oxygen generating anode used in the alkaline water electrolysis method of the present invention will now be described with reference to the drawings. In the following description, the alkaline water electrolysis anode is configured to include the intermediate layer 4 shown in Figure 1 . However, in the alkaline water electrolysis anode of the present invention, the intermediate layer 4 is formed between the conductive substrate 2 and the catalytic layer 6 as needed and is not an essential component.

<Conductive substrate>
The conductive substrate 2 is a conductor for conducting electricity for electrolysis and functions as a carrier for depositing the intermediate layer 4 and catalyst layer 6. At least the surface of the conductive substrate 2 (the surface on which the intermediate layer 4 is formed) is formed of nickel or a nickel-based alloy. That is, the entire conductive substrate 2 may be formed of nickel or a nickel-based alloy, or only the surface may be formed of nickel or a nickel-based alloy. Specifically, the conductive substrate 2 may be formed by coating the surface of a metal material such as iron, stainless steel, aluminum, or titanium with nickel or a nickel-based alloy by plating or the like.

The thickness of the conductive substrate 2 is preferably approximately 0.05 to 5 mm. The conductive substrate preferably has openings to remove bubbles of oxygen, hydrogen, etc. that are generated. For example, expanded mesh or porous expanded mesh can be used as the conductive substrate 2. If the conductive substrate has openings, the opening ratio of the conductive substrate is preferably 10 to 95%.

The oxygen generating anode used in the water electrolysis method of the present invention can be obtained, for example, by forming an intermediate layer 4 and a catalyst layer 6 on the surface of the above-mentioned conductive substrate 2 in the following manner.
(Pretreatment process)
Prior to the steps of forming the intermediate layer 4 and the catalyst layer 6, the conductive substrate 2 is preferably subjected to a chemical etching treatment in advance to remove contaminating particles such as metals and organic substances from the surface. The amount of wear of the conductive substrate due to the chemical etching treatment is preferably about 30 g/ ^m2 or more and 400 g/ ^m2 or less. Furthermore, in order to increase adhesion to the intermediate layer, it is preferable to roughen the surface of the conductive substrate in advance. Examples of roughening methods include blasting, which involves spraying powder, etching using an acid that is soluble in the substrate, and plasma spraying.

<Middle class>
The intermediate layer 4 is a layer formed on the surface of the conductive substrate 2. The intermediate layer inhibits corrosion of the conductive substrate and stably fixes the catalyst layer 6 to the conductive substrate. The intermediate layer also serves to rapidly supply current to the catalyst layer. The intermediate layer may be formed, for example, from a lithium-containing nickel oxide represented by the composition formula Li _x Ni _2-x O ₂ (0.02≦x≦0.5). If x in the composition formula is less than 0.02, the conductivity becomes insufficient. On the other hand, if x exceeds 0.5, the physical strength and chemical stability decrease. The intermediate layer 4 formed from the lithium-containing nickel oxide represented by the composition formula has sufficient conductivity for electrolysis and exhibits excellent physical strength and chemical stability even when used for a long period of time.

The thickness of the intermediate layer is preferably 0.01 μm or more and 100 μm or less, and more preferably 0.1 μm or more and 10 μm or less. If the thickness of the intermediate layer is less than 0.01 μm, the above-mentioned functions will not be realized. On the other hand, if the thickness of the intermediate layer is more than 100 μm, the voltage loss due to resistance in the intermediate layer will be large, and the above-mentioned functions will not be realized and there may be some disadvantages in terms of manufacturing costs, etc.

(Coating process for forming intermediate layer)
In the coating process, an aqueous precursor solution containing lithium ions and nickel ions is applied to the surface of the conductive substrate 2. The intermediate layer is formed by a so-called pyrolysis method. When forming the intermediate layer by pyrolysis, first, an aqueous precursor solution for the intermediate layer is prepared. Known precursors containing a lithium component, such as lithium nitrate, lithium carbonate, lithium chloride, lithium hydroxide, and lithium carboxylate, can be used. Examples of lithium carboxylates include lithium formate and lithium acetate. Known precursors containing a nickel component, such as nickel nitrate, nickel carbonate, nickel chloride, and nickel carboxylate, can be used. Examples of nickel carboxylates include nickel formate and nickel acetate. In particular, using at least one of a lithium carboxylate and a nickel carboxylate as the precursor is particularly preferred, as it allows for the formation of a dense intermediate layer even when fired at a low temperature, as described below.

The heat treatment temperature when forming the intermediate layer by thermal decomposition can be set appropriately. Considering the decomposition temperature of the precursor and production costs, the heat treatment temperature is preferably 450°C or higher and 600°C or lower, and more preferably 450°C or higher and 550°C or lower. For example, the decomposition temperature of lithium nitrate is approximately 430°C, and the decomposition temperature of nickel acetate is approximately 373°C. By setting the heat treatment temperature at 450°C or higher, the decomposition of each component can be more reliably achieved. Heat treatment temperatures above 600°C can easily promote oxidation of the conductive substrate, increasing electrode resistance and potentially resulting in increased voltage loss. The heat treatment time can be set appropriately, taking into account factors such as reaction rate, productivity, and the oxidation resistance of the catalyst layer surface.

The thickness of the intermediate layer formed can be controlled by appropriately setting the number of times the aqueous solution is applied in the aforementioned coating process. The application and drying of the aqueous solution may be repeated for each layer, and the entire structure may be heat-treated after the top layer is formed. Alternatively, the application and heat treatment (pretreatment) of the aqueous solution may be repeated for each layer, and the entire structure may be heat-treated after the top layer is formed. The pretreatment temperature and the overall heat treatment temperature may be the same or different. It is preferable that the pretreatment time be shorter than the overall heat treatment time.

<Catalyst layer>
The oxygen generating anode used in the alkaline water electrolysis method of the present invention has a catalytic layer 6 made of a specific catalytic component formed on the outermost surface of the conductive substrate 2, thereby achieving the remarkable effects of the present invention when applied to alkaline water electrolysis. The catalytic layer, which is important for achieving the remarkable effects of the present invention, is described below.

(Catalyst Component)
The hybrid nickel-iron-cobalt ( _NixFeyCoz - _hmh ) composite of a metal hydroxide and an organic substance, which is the catalyst component used in and characterizes the present invention, can be easily produced, for example, as follows. _{NixFeyCoz - hmh} can be synthesized, for example, by mixing an aqueous solution of the tripodal ligand tris(hydroxymethyl)aminomethane (Tris- _{NH2 )} with an aqueous solution of _NiCl2 , _FeCl2 , and _CoCl2 , and allowing the mixture to react at 90°C for 24 hours. The reaction product synthesized above is then separated as a gel by filtration and washing with pure water, and this _is subjected to ultrasonic treatment in pure water, thereby easily obtaining a _NixFeyCoz - _hmh dispersion to be used as the electrolyte in the alkaline water electrolysis method of the present invention. The concentration of Ni _x Fe _y Co _z -hmh in the dispersion is preferably about 5 to 100 g/L. In the test examples of the present invention described below, a dispersion with a concentration of 5 g/L was used. Hereinafter, the dispersion in which the Ni _x Fe _y Co _z -hmh catalyst used in preparing the electrolyte is referred to as the "NiFeCo-hmh dispersion." The "NiFeCo-hmh dispersion" obtained by the above-mentioned production method was used as the electrolyte in which the Ni _x Fe _y Co _z -hmh catalyst used in the present invention is dispersed. When preparing the electrolyte, the catalyst was adjusted to an appropriate addition concentration. In the description of the present invention, Tris-NH ₂ shown in FIG. 2A was used as a representative example of a tripodal ligand, but the tripodal ligand is not particularly limited, and any ligand having a molecular structure similar to Tris-NH ₂ may be used.

_{NixFeyCoz -} hmh has a composite structure in which a metal hydroxide and a tripodal ligand are covalently fixed. According to the inventors' research, _NixFeyCoz -hmh can be of a layered structure, a tunnel structure, or an amorphous structure. For example, as shown in Figure 2A, _NixFeyCoz - _hmh with a layered structure has a layered _{NixFeyCoz - Tris} - _NH2 molecular structure with a _tripodal ligand, and is composed of a brucite layer to which Tris molecules are covalently fixed. Modification with Tris- _NH2 enhances the exfoliation and dispersion capabilities of layered nickel-iron-cobalt hydroxide in an electrolyte. TEM images confirmed that the molecular structure of the _{NixFeyCoz -hmh} obtained above is a _nanosheet -like substance with a layered molecular structure, approximately 1.3 nm thick and 10 to 100 nm in lateral size, similar to the previously proposed "_{NiFe-ns." XRD also confirmed that NixFeyCoz - hmh} has a layered structure with an expanded basal spacing.

Further investigation was conducted on Ni _x Fe _y Co _z -hmh, a reaction product easily obtained by heating a mixture of raw materials to cause a reaction. As a result, it was discovered that the dispersed substances constituting the "NiFeCo-hmh dispersion" contained needle-shaped crystals characteristic of a tunnel structure, in addition to the nanosheet-like substance shown in Figure 2A, and thus contained "materials with needle-shaped tunnel structures." Figure 9 shows a transmission electron microscope image of Fe-hmh as an example of a material with such a structure. As mentioned above, whether Fe-hmh is used alone as a catalyst or in combination with Co-ns, the durability of Ni-based anodes against potential fluctuations can be improved compared to the previously proposed anodes using Co-ns alone as a catalyst. The material with this tunnel-shaped structure was a needle-like crystal with a width of about 5 nm and a length of mostly 100 nm or less, and although some were longer, the majority were 200 nm or less. Furthermore, irregularly shaped fine particles consistent with an amorphous structure were also observed. Although primary particles of this fine particle material were not clearly observed, they were expected to be about 1 to 5 nm in size, and in their actual state, primary particles of this size aggregated to form secondary particles on the order of microns. Therefore, it can be said that the catalyst Ni _x Fe _y Co _z -hmh constituting the present invention is a material with a size in the range of 1 to 200 nm.

Furthermore, as a result of the inventors' investigations, it was found that the amount of the above-mentioned substance present in the reaction product is affected by the atomic ratio of Ni/Fe/Co. For example, when the Fe content is 70 mol% or less, the substance tends to become a nanosheet-like substance with a layered structure. Furthermore, when the Fe content is greater than 70 mol%, the substance tends to become a particulate substance with an amorphous structure. Furthermore, as will be described later, the electrolytic performance of the oxygen generating anode of the present invention using Ni _x Fe _y Co _z -hmh as the catalyst is less affected by differences in the atomic ratio of the constituent Ni/Fe/Co, compared to the case of the prior art using Ni _x Fe _y -ns as the catalyst, and it was observed that the electrolytic performance tends to be stable.

The resulting reaction product was also dispersed in water and ultrasonically treated to test its water dispersibility. First, when the previously proposed Ni _x Fe _y -ns catalyst was dispersed in an electrolyte, it did not disperse highly in water in the dry, fine powder state. For this reason, the dispersion used in the electrolyte was prepared from the hydrogel obtained by filtration. On the other hand, in the case of Ni _x Fe _y Co _z -hmh constituting the present invention, a dispersion could also be prepared from the dry powder. Of course, the NiFeCo-hmh dispersion used in the electrolyte can also be prepared from the hydrogel obtained by filtration, as in the case of using the Ni _x Fe _y -ns catalyst. The inventors believe that this difference in dispersibility is the cause of the difference in the amount of catalyst layer formed (deposited) on the surface of the oxygen generating anode by constant electrolysis, as described below. In other words, it is believed that the use of a catalyst that exhibits good dispersibility increases the amount of material that can contribute to (effectively utilize) the formation (deposition) of the catalyst layer in the electrolyte solution in which the catalyst is dispersed, and that this is why the present invention, which uses Ni _x Fe _y Co _z -hmh as the catalyst, enables the formation (deposition) of a larger amount of catalyst layer over a longer period of time compared to the use of the prior art Ni _x Fe _y -ns as the catalyst. It is believed that the use of an electrolyte solution in which a Ni _x Fe _y Co _z -hmh catalyst with excellent dispersibility is thus able to achieve the object of the present invention, i.e., the provision of a superior oxygen generating anode that is less susceptible to deterioration in electrolysis performance and enables more stable alkaline water electrolysis over a longer period of time, and an alkaline water electrolysis method using the same, has been achieved.

Examples of Ni _x Fe _y Co _z -hmh that can be suitably used for the above-described object of the present invention include those having a Ni/Fe/Co atomic ratio of (0.1 to 0.9)/(0.1 to 0.9)/(0.1 to 0.9). Furthermore, the Ni/Fe/Co atomic ratio is more preferably (0.1 to 0.7)/(0.3 to 0.8)/(0.05 to 0.2) (see FIG. 7 ). As described above, when used in the alkaline water electrolysis method of the present invention, the size of the Ni _x Fe _y Co _z -hmh substance preferably has a length (major axis) in the range of 1 to 200 nm. According to the studies of the present inventors, a size exceeding this range is undesirable because it tends to decrease the efficiency of electrolytic deposition and make it difficult to improve overvoltage and achieve the repair effect. Ni _x Fe _y Co _z -hmh that can be used as a catalyst suitable for the purpose of the present invention includes at least one of a sheet-like substance having a layered molecular structure, a needle-shaped substance having a tunnel structure, and a particulate substance having an amorphous structure.

Examples of Ni _x Fe _y Co _z -hmh that can obtain the effects of the present invention and can be suitably used for the purposes of the present invention include those having a structure made of extremely general-purpose materials that do not contain metal ions other than Fe and have a Ni/Fe/Co atomic ratio of (0.0)/(1.0)/(0.0). As mentioned above, Fe-hmh has the geometric characteristic of having a "tunnel-structured needle-shaped substance." Other examples include those having a structure made of a material containing Fe and Co metal ions and having a Ni/Fe/Co atomic ratio of (0.0)/(0.1-0.9)/(0.1-0.9). According to the inventors' studies, it has been found that the precipitation occurring when Fe-hmh and Co-ns are used in combination as a catalyst, in the presence of a Co component, precipitates more highly active Fe-hmh than when Fe-hmh is used alone as a catalyst. This point will be discussed later.

According to the inventors' investigations, the present invention is characterized by the use of a hybrid nickel-iron-cobalt ( _{NixFeyCoz - hmh} ) composite of a metal hydroxide and an organic substance as the catalyst component used in forming the catalytic layer of an alkaline water electrolysis anode and incorporating it into the electrolyte solution. This results in superior effects compared to the technology previously proposed by the inventors that utilizes hybrid cobalt hydroxide nanosheets (Co-ns). Specifically, the anode catalytic layer was formed using each of the above composites, and the resulting anode was used to supply electrolytes containing the different composites as catalysts to the anode chamber. An accelerated degradation test of electrolysis performance was then conducted to examine the dependence of the oxygen evolution overpotential on potential fluctuation cycles. As a result, when _NixFeyCoz -hmh was used as the _catalytic component, a clear and significant decrease in the initial overpotential was observed, confirming superior durability compared to when Co- _ns was used. Details will be described later. Furthermore, Ni _x Fe _y Co _z -hmh used as the catalyst component in the present invention can be synthesized from general-purpose materials by a simple method, and as described above, has excellent dispersibility, so it has an extremely important industrial advantage in that it can be easily used as a dispersion liquid in which the catalyst is dispersed, which is required in the present invention, and as an electrolyte solution prepared using the dispersion liquid.

(Method of forming catalyst layer)
The method for forming the catalyst layer 6 will now be described. A 1.0 M KOH aqueous solution was used as the electrolyte. To clean the surface of the conductive substrate 2, it is preferable to perform potential manipulation in the electrolyte. For example, potential cyclic manipulation (-0.5 to 0.5 V vs. RHE, 200 mV/s, 200 cycles) is performed. Then, a 1.0 M KOH aqueous solution containing the NiFeCo-hmh dispersion obtained as described above at an additive concentration of 1 mL/L was prepared and used as the electrolyte. Then, to deposit Ni _x Fe _y Co _z -hmh on the Ni substrate surface, constant-current electrolysis was performed eight times for 30 minutes at 800 mA/cm ² . This electrolysis operation reduced the dispersibility of Ni _x Fe _y Co _z -hmh on the electrode surface due to oxidation of the hydroxide layer and oxidative decomposition of the surface organic groups, resulting in the deposition of Ni _x Fe _y Co _z -hmh on the electrode surface.

In the above, the concentration of the "NiFeCo-hmh dispersion" added to the electrolyte is preferably within the range of 0.1 to 10 mL/L, more preferably 0.1 to 8 mL/L. According to the inventors' studies, if the concentration is higher than this, the dispersion of Ni _x Fe _y Co _z -hmh in the electrolyte becomes insufficient, and homogeneous deposition may not be obtained during electrolysis, which is undesirable. Furthermore, if the concentration is lower than this, a sufficient amount cannot be obtained during electrolytic deposition within a practical time period. Furthermore, as the electrolysis conditions for deposition, it is preferable to maintain the conductive substrate in a potential range of 1.2 V to 1.8 V vs. RHE. The deposition reaction does not proceed below 1.2 V, and if it exceeds 1.8 V, oxygen generation simultaneously proceeds, inhibiting deposition, which is undesirable.

A transmission electron microscope photograph is shown below as an example of the deposit formed when a catalyst was deposited on a Ni substrate using an electrolyte solution containing dispersed Fe-hmh and electrolysis under the above conditions for 4 hours. As shown in Figure 10, in this case, the Ni substrate surface was found to be covered with a network of fibrous material in which Fe-hmh was bundled.

Figures 11A and 11B show field-emission scanning electron microscope (FE-SEM) images of the precipitate formed on a Ni substrate surface by electrolysis after dispersing Fe-hmh particles and Co-ns particles, which are examples of catalytic components, in an electrolyte solution. The electrode surface is uniformly covered with a catalytic layer, and Figure 11B and its enlarged image (not shown) reveal that the microstructure is an aggregate of nanosheets (Co-ns). Furthermore, as shown in Figure 11A, from a different perspective, it was confirmed that the Fe-hmh particles are elongated particles and are incorporated into the aggregates. Analysis results showed that the amount of Fe-hmh deposited was significantly increased by co-deposition (estimated to be approximately 60 times the amount obtained when Fe-hmh is used alone), and that the use of Co-ns particles in combination increases the amount of Fe-hmh deposited in the formed catalytic layer. Although the mechanism is not yet clear, it is thought that by dispersing both Fe-hmh particles and Co-ns particles in the electrolyte, the amount of Fe-hmh precipitated increases significantly due to the precipitation of the Co-ns component, which is the reason for the improved catalytic activity.

The alkaline water electrolysis method of the present invention requires the use of an electrode having the above-described specific catalyst layer as the oxygen generating anode. On the other hand, there are no particular restrictions on the cathode (negative electrode) or diaphragm, and any suitable material used in conventional alkaline water electrolysis may be used. These are described below.

[Cathode]
As the cathode, it is preferable to select and use a substrate made of a material that can withstand alkaline water electrolysis and a catalyst with a small cathode overvoltage. As the cathode substrate, a nickel substrate or a nickel substrate coated with an active cathode can be used. The shape of the cathode substrate can be a plate, an expanded mesh, a porous expanded mesh, or the like.

Cathode materials include porous nickel with a large surface area and Ni-Mo-based materials. Other examples include Raney nickel-based materials such as Ni-Al, Ni-Zn, and Ni-Co-Zn; sulfide-based materials such as Ni-S; and hydrogen storage alloy-based materials such as Ti ₂ Ni. Preferred catalysts have properties such as low hydrogen overvoltage, high short-circuit stability, and high poisoning resistance. Other preferred catalysts include metals such as platinum, palladium, ruthenium, and iridium, and oxides of these metals.

[diaphragm]
Any of the conventionally known electrolytic diaphragms can be used, such as asbestos, nonwoven fabrics, ion exchange membranes, porous polymer membranes, and composite membranes of inorganic materials and organic polymers. Specifically, an ion-permeable diaphragm can be used, which comprises a mixture of a hydrophilic inorganic material, such as a calcium phosphate compound or calcium fluoride, and an organic binder, such as polysulfone, polypropylene, or polyvinylidene fluoride, and an organic fiber cloth embedded therein. Alternatively, an ion-permeable diaphragm can be used, which comprises a film-forming mixture of a granular inorganic hydrophilic material, such as an oxide or hydroxide of antimony or zirconium, and an organic binder, such as a fluorocarbon polymer, polysulfone, polypropylene, polyvinyl chloride, or polyvinyl butyral, and an expanded organic fiber cloth embedded therein.

In the alkaline water electrolysis method of the present invention, a high-concentration alkaline aqueous solution can be electrolyzed by using an alkaline water electrolysis cell including the oxygen generating anode that characterizes the present invention. The alkaline aqueous solution used as the electrolyte is preferably an aqueous solution of an alkali metal hydroxide such as potassium hydroxide (KOH) or sodium hydroxide (NaOH). The concentration of the alkaline aqueous solution is preferably 1.5% by mass or more and 40% by mass or less. Furthermore, a concentration of 15% by mass or more and 40% by mass or less is preferable because it provides high electrical conductivity and reduces power consumption. Furthermore, considering cost, corrosiveness, viscosity, operability, etc., the concentration of the alkaline aqueous solution is preferably 20% by mass or more and 30% by mass or less.

[Operation method]
The catalytic layer 6 constituting the alkaline water electrolysis anode described above can be formed before assembly into an electrolytic cell by electrolysis as described below. In the alkaline water electrolysis method of the present invention, for example, Ni _x Fe _y Co _z -hm, which is a component forming the catalytic layer 6 characterizing the present invention, is suspended in a common electrolytic solution supplied to the anode chamber and cathode chamber constituting the electrolytic cell, and electrolysis is initiated in this state, thereby allowing the catalytic component to be deposited in large quantities on the anode surface and accumulated in a short period of time to form a catalytic layer, as described above. Therefore, the alkaline water electrolysis technology of the present invention makes it possible to restore the performance of an electrolytic cell whose performance has deteriorated during operation without the effort of dismantling the electrolytic cell, and to maintain the performance of the catalytic layer stably over a long period of time. Therefore, the alkaline water electrolysis technology of the present invention is practical and has significant industrial benefits.

Next, the present invention will be explained in more detail with reference to examples, studies, and comparative examples. First, the state of deposition on the electrolytic surface and its effects were investigated when Ni _x Fe _y Co _z -hmh, the catalyst component that characterizes the present invention, was dispersed in an electrolyte and electrolysis was performed. For comparison, a similar test was also performed when Ni _x Fe _y -ns was used as the catalyst component.

(Study example 1)
The electrolysis was performed using a three-electrode cell made of PFA, a fluororesin. A Ni wire etched with boiling hydrochloric acid for 6 minutes was used as the working electrode, a reversible hydrogen electrode (RHE) as the reference electrode, a Ni coil as the counter electrode, and 250 mL of 1.0 M KOH aqueous solution as the electrolyte. The experiment was carried out at 30±1°C. First, as a pretreatment, cyclic voltammetry (0.5 to 1.5 V vs. RHE, 200 mV/s, 200 cycles) was performed without adding the NiFeCo-hmh dispersion to the electrolyte. The electrolyte used in this study was prepared as follows. Specifically, a NiFeCo-hmh dispersion with a concentration of 5 g/L obtained by the same method as described above was used, and this dispersion was mixed with the electrolyte used in the pretreatment. The concentration of the dispersion added was adjusted to 8 mL/L to obtain an electrolyte containing a dispersed catalyst. Then, using the above pretreated three-electrode cell, electrolysis was carried out at a constant current of 800 mA/cm ^for 30 minutes with this electrolyte. Electrolysis in this manner oxidized the catalyst component Ni _x Fe _y Co _z -hmh on the electrode surface, reducing dispersibility through oxidation of the hydroxide layer of Ni _x Fe _y Co _z -hmh and oxidative decomposition of the surface organic groups, resulting in the deposition of Ni _x Fe _y Co _z -hmh on the electrode surface, resulting in an anode with a unique catalyst layer.

Figure 4A shows the change in cyclic voltammetry during the catalyst layer formation process for Ni _11.5 Fe _70.5 Co ₁₈ -hmh. After 30 minutes of electrolysis, oxidation peaks were observed at 1.33 V and 1.40 V vs. RHE. These can be attributed to the reactions of Co ²⁺ /Co ³⁺ and Ni ²⁺ /Ni ³⁺ , respectively, and are thought to be derived from the deposited catalyst. As the electrolysis time increased to 240 minutes and 600 minutes, each of these peaks shifted to a higher potential and the peak area increased. These observations suggest that the amount of catalyst deposited on the electrode increased with increasing electrolysis time.

For comparative purposes, constant-current electrolysis was performed in the same manner as above, except that an electrolyte containing a conventional Ni _61.5 Fe _38.5 -ns catalyst dispersed therein was used to obtain an anode with a catalytic layer formed thereon. Figure 4B shows the change in cyclic voltammetry during the catalytic layer formation process using Ni _61.5 Fe _38.5 -ns. Oxidation peaks were observed at 1.40 V vs. RHE after 30 and 240 minutes of electrolysis, but unlike the case in Figure 4A, the peak height did not increase further. This suggests that the amount of catalyst deposited on the electrode when using the NiFe-ns catalyst is small compared to when using the NiFeCo-hmh catalyst constituting the present invention. That is, with the Ni _11.5 Fe _70.5 Co ₁₈ -hmh catalyst used in the present invention, as shown in Figure 4A, the peak area increased significantly compared to the case in Figure 4B, which used a catalyst without Co in its composition. The Ni _11.5 Fe _70.5 Co ₁₈ -hmh catalyst used in the present invention has improved dispersibility compared to the NiFe-ns catalyst. It is believed that the inclusion of Co in the composition improves the dispersibility of the catalyst, which in turn facilitates the formation (deposition) of the catalyst layer on the electrode. Figure 4C shows a graph of the relationship between catalyst deposition time and catalyst deposition amount, obtained using the peak areas in Figures 4A and 4B. As shown in Figure 4C, it was confirmed that a larger catalyst layer could be deposited by using the NiFeCo-hmh catalyst constituting the present invention as a catalyst, compared to when the conventional Ni _x Fe _y -ns catalyst was used.

Figure 5 shows the change in overpotential at a current density of 100 mA/ ^cm2 versus catalyst deposition time in tests using Ni _11.5 Fe _70.5 Co ₁₈ -hmh and Ni _61.5 Fe _38.5 -ns. As shown in Figure 5, when Ni _11.5 Fe _70.5 Co ₁₈ -hmh was used as the catalyst, a smaller overpotential than Ni _61.5 Fe _38.5 -ns was obtained in the range of small deposition amounts.

Next, an accelerated test for potential fluctuation was performed under the following conditions using electrolytes in which various catalysts were dispersed. The accelerated test for potential fluctuation consisted of 2000 cycles of cyclic voltammetry at 0.5 to 1.7 V vs. RHE and 500 mV/s, and electrode performance measurements were performed using cyclic voltammetry at 0.5 to 1.8 V vs. RHE and 5 mV/s for two cycles, and at 0.5 to 1.5 V vs. RHE and 50 mV/s for two cycles. The catalysts dispersed in the electrolyte were Co-ns, Fe-hmh, Ni _11.5 Fe _70.5 Co ₁₈ -hmh, and Ni _61.5 Fe _38.5 -ns, respectively. For comparison, a test was performed using an electrolyte without a dispersed catalyst, which was labeled "Bare Ni." The above-mentioned operation was repeated 20 times for a total of 40,000 cycles.

FIG. 6 shows the results of the accelerated durability test against the above-mentioned potential fluctuation. As shown in FIG. 6, an increase in overvoltage was observed after 10,000 cycles with bare Ni, whereas the increase in overvoltage was suppressed in all cases using an electrolyte with a dispersed catalyst. This is thought to be due to redeposition (self-repair) of the catalyst from the electrolyte by constant current electrolysis every 2,000 cycles. In all cases, examples using Ni _61.5 Fe _38.5 -ns, Ni _11.5 Fe _70.5 Co ₁₈ -hmh, and Fe-hmh as the catalyst maintained a lower overvoltage than bare Ni or when Co-ns was used as the catalyst. In particular, when Ni _11.5 Fe _70.5 Co ₁₈ -hmh constituting the present invention was used as the catalyst, the initial activity was highest. Although a slight increase in overvoltage was observed between 4000 and 8000 cycles, the increase in overvoltage was stably suppressed thereafter. Although the performance was inferior to that of the catalysts Ni _61.5 Fe _38.5 -ns and Ni _11.5 Fe _70.5 Co ₁₈ -hmh, good performance was also obtained when Fe-hmh was used.

(Study Example 2)
Figure 7 shows the change in overpotential when various catalyst components dispersed in the electrolyte were used and continuous electrolysis was performed at a current density of 100 mA/ ^cm2 . Electrolysis was performed using the following catalysts dispersed in the electrolyte and an electrolyte obtained in the same manner as in Study Example 1. Specifically, _Ni83.6Fe16.3 -ns, _{Ni76.6Fe23.5 -} ns, _Ni61.5Fe38.5 -ns, and _{Ni58.8Fe41.2 -} ns were used as Ni-Fe _binary samples (catalysts) _. Furthermore, Ni _65.6 Fe _33.7 Co _0.6 -hmh, Ni _11.5 Fe _70.5 Co ₁₈ -hmh, and Ni _8.2 Fe ₈₅ Co _6.8 -hmh were used as Ni-Fe-Co ternary samples (catalysts) constituting the present invention. Tests were also conducted when Co-ns and Fe-hmh were used as catalysts.

As shown in FIG. 7 , the smallest overvoltage was obtained when Ni _11.5 Fe _70.5 Co ₁₈ -hmh, one of the Ni—Fe—Co ternary samples constituting the present invention, was used as the catalyst. Furthermore, when the Ni—Fe—Co ternary samples constituting the present invention were used, all samples exhibited stable low overvoltages, and most samples exhibited lower overvoltages than those using Co-ns. Among the Ni—Fe—Co ternary samples, those with a low cobalt content of 0.6% exhibited slightly higher overvoltages than those using Co-ns, but the results were comparable. In contrast, when Ni—Fe binary samples were used, there were cases where the overvoltages were stable at lower levels than those using Co-ns as the catalyst, but there was a tendency for the resulting overvoltages to fluctuate significantly depending on the sample. This suggests that effective continuous electrolysis that provides a stable and small overvoltage can be more easily achieved by using the Ni-Fe-Co ternary composite catalyst newly discovered in the present invention than by using a Ni-Fe binary catalyst.

(Study Example 3)
Next, a shutdown durability test was performed using the following procedure. First, the anode under test was (1) subjected to oxygen generation at 0.6 A/ ^cm² for 1 minute, followed by (2) shifting the anode potential to 0.5 V vs. RHE at a rate of 500 mV/s, and then (3) holding the anode potential at 0.5 V vs. RHE for 1 minute. The steps (1) to (3) were repeated, and the change in 0.1 A/ ^cm² oxygen generation potential was measured as a characteristic value. The number of repetitions of the above steps is referred to as an ADT (Advanced Durability Test) cycle. It is known that this accelerated life test leads to degradation in fewer cycles than the accelerated durability test for potential fluctuations, the results of which are shown in Figure 6.

Durability tests were conducted using the above procedure using four types of anodes: a Ni anode and three types of anodes with different catalytic layers formed on a Ni substrate. The results are shown in Figure 8. Specifically, four types of anodes were used: a Ni anode, a Ni anode with a Co-ns catalytic layer, an anode with a Ni-Co spinel oxide catalytic layer prepared by pyrolysis, and a Ni anode with an Fe-hmh catalytic layer (an example in which the effects of the present invention were achieved). As shown in Figure 8, an increase in oxygen evolving potential was observed with the Ni anode after only a few ADT cycles. Even with the anode with the Ni-Co spinel oxide catalytic layer, an increase in oxygen evolving potential was observed after 1,000 cycles. Furthermore, the Ni anode with the Co-ns catalytic layer showed a slight tendency for potential to increase up to 2,700 cycles. In contrast to the above results, in an example using an anode (Ni anode) with an Fe-hmh catalyst layer formed on a Ni substrate, a stable potential was observed even after 4,000 repetitions, confirming its effectiveness as an anode.

(Study Example 4)
As the component dispersed in the electrolyte, we investigated the case where Fe-hmh was used alone, and the case where the Fe-hmh component and the Co-ns component were added to the electrolyte at a mass ratio of 1:1. When preparing an electrolyte using both the Fe-hmh component and the Co-ns component, the total amount of these components added was adjusted to a dispersion concentration of 8 mL/L, as in Study Example 1. Then, using two types of electrolytes with different dispersed catalyst components, 10 cycles of constant-current electrolysis at 800 mA/cm ² for 30 minutes were performed according to the method described in Study Example 1. The composition of the latter electrode was determined to be Fe ₅₅ Co ₄₅ -hmh based on ICP analysis of the aqueous solution in which these components were dissolved. Figure 12 shows the change in overvoltage when electrodes were fabricated with the catalyst layers of the different compositions described above deposited on each electrode and continuously electrolyzed at a current density of 100 mA/cm ² . As shown in Figure 12, the electrode fabricated by coexisting the Fe-hmh component and the Co-ns component exhibited a smaller overvoltage than the electrode fabricated with only Fe-hmh. In addition, Figure 12 also shows the test results for the case where the Co-ns component was dispersed alone in the electrolyte to demonstrate the effect of the above-mentioned configuration.

(Study Example 5)
As components to be dispersed in the electrolyte, Fe-hmh particles and Co-ns particles were used in the following different ratios, and electrolytes were prepared in which catalyst components coexisting at the same concentration as in Study Example 4 were dispersed. Electrolysis was performed with each of the obtained electrolytes, and the amount of Fe-hmh component (hereinafter also referred to as Fe) precipitated relative to the amount of Co-ns component (hereinafter also referred to as Co) obtained after electrolysis and the electrolytic characteristics were examined. Specifically, electrolytes to which the Fe-hmh component and the Co-ns component were added were prepared so that Fe and Co coexisted in mass ratios of Fe:Co = 5:1 (Co/Fe = 0.2), Fe:Co = 10:1 (Co/Fe = 0.1), Fe:Co = 1:5 (Co/Fe = 5), and Fe:Co = 1:10 (Co/Fe = 10), respectively. Then, using a plurality of electrolyte solutions each having a different composition ratio of the dispersed catalyst component, constant current electrolysis was performed 10 times at 800 mA/ ^cm² for 30 minutes according to the method described in Study Example 1. As a result, as shown in Figure 14, it was confirmed that the use of an electrolyte solution in which a catalyst component containing coexisting Fe and Co was dispersed resulted in an increase in the amounts of Fe and Co deposited. Figure 14 also shows the amounts of deposition when electrolysis was performed using an electrolyte solution in which the catalyst components were dispersed at an addition ratio of Fe:Co = 1:1 (Co/Fe = 1), as used in Study Example 4.

FIG. 13 shows electrodes prepared by depositing catalyst layers of different compositions using electrolytes in which the two different catalyst components described above were dispersed at different addition ratios, and shows the amount of Fe deposited relative to the amount of Co deposited after electrolysis, as well as the change in overvoltage when electrolyzed at a current density of 100 mA/cm ^2. The "○" in FIG. 13 indicates the amount of Fe deposited relative to the amount of Co deposited, and the "□" in FIG. 13 indicates the change in overvoltage. As shown in FIG. 13, the electrode with an increased amount of Co-ns component deposition had an increased amount of Fe-hmh deposition, and at the same time, the overvoltage decreased.

Example 1
The anode substrate used was a nickel expanded mesh (10 cm × 10 cm, LW × 3.7 SW × 0.9 ST × 0.8 T) that had been chemically etched by immersion in 17.5% by mass hydrochloric acid near its boiling point for 6 minutes. This expanded mesh was then blasted with 60-mesh alumina particles (0.3 MPa), immersed in 20% by mass hydrochloric acid, and chemically etched near its boiling point for 6 minutes. An aqueous solution containing a component that would become a precursor of lithium-containing nickel oxide was then applied to the surface of the chemically etched anode substrate with a brush, followed by drying at 80°C for 15 minutes. The anode substrate was then heat-treated at 600°C for 15 minutes in an air atmosphere. The process from application of the aqueous solution to the heat treatment was repeated 20 times to obtain an intermediate having an intermediate layer (composition: Li _0.5 Ni _1.5 O ₂ ) formed on the surface of the anode substrate.

Next, a NiFeCo-hmh dispersion similar to that described in Study Example 1 was used, and the dispersion was adjusted so that the dispersion concentration relative to the electrolyte described in Study Example 1 was 1 mL/L to prepare an electrolyte containing a dispersed catalyst for use in this example. Then, using this electrolyte, an electrolysis operation similar to that described in Study Example 1 was performed to obtain a Ni anode (oxygen generating anode) in which a catalytic layer made of Ni _x Fe _y Co _z -hmh was formed on the surface of the intermediate formed as described above. A small-sized zero-gap electrolytic cell using a neutral diaphragm was then fabricated using the obtained Ni anode, a diaphragm Zirfon (trade name, manufactured by AGFA), and an active cathode in which a catalytic layer made of Ru and Pr oxides was formed. The electrode area was 19 cm ² .

Alkaline water electrolysis was performed using the zero-gap electrolytic cell obtained as described above. A 25 mass% KOH aqueous solution was used as the electrolyte, to which the NiFeCo-hmh dispersion used to form the catalytic layer of the Ni anode had been added at a concentration of 1 mL/L. The electrolyte was then supplied to the anode and cathode chambers constituting the electrolytic cell, and electrolysis was performed for 6 hours at a current density of 6 kA/m ² . The anode and cathode were then short-circuited (0 kA/m ² ), and the electrolysis was stopped for 15 hours. A shutdown test was performed, with the above operation from electrolysis to shutdown being one cycle. As a result, it was confirmed that the voltage was maintained stably over 20 shutdown tests.

(Comparative Example 1)
Alkaline water electrolysis was performed in the same manner as in Example 1, except that an electrolyte containing no Ni _x Fe _y Co _z -hmh was used as the electrolyte supplied to each of the anode and cathode chambers constituting the zero-gap electrolytic cell prepared in Example 1. Specifically, a shutdown test similar to that performed in Example 1 was performed using the same electrolytic cell as used in Example 1. As a result, it was confirmed that the cell voltage gradually increased with an increase in the number of shutdowns. This confirmed the superiority of the alkaline water electrolysis method using the electrolyte in which Ni _x Fe _y Co _z -hmh was dispersed in Example 1.

An example of the use of the present invention is an oxygen generating anode with a unique configuration that uses hybrid nickel-iron _- cobalt hydroxide ( _{NixFeyCoz -hmh} ) in the catalytic layer. This oxygen generating anode is used, and an electrolyte solution in which _NixFeyCoz - _hmh is dispersed is supplied to at least the anode chamber, and electrolysis is performed using a conventional method _. This extremely simple method allows a large amount of catalyst to be easily deposited in a short period of time, thereby making it possible to effectively restore the catalytic activity of the catalytic layer. This is expected to lead to the realization of an alkaline water electrolysis method with high industrial practical value, in which electrolysis performance is less likely to deteriorate and can be maintained more stably for a longer period of time, even when powered by electricity with large output fluctuations, such as renewable energy. Another example of the application of the present invention is an oxygen generating anode in which a catalyst layer is formed using an electrolyte in which Fe-hmh, which is made of an extremely general-purpose material, is dispersed, or an electrolyte in which Fe-hmh and Co-ns are dispersed and coexist, and in which it has been confirmed that the amount of Fe-hmh deposited is large.

2: Conductive substrate 4: Intermediate layer 6: Catalyst layer 10: Anode for alkaline water electrolysis

Claims (11)

An alkaline water electrolysis method comprising supplying an electrolyte to an anode chamber and a cathode _chamber constituting an electrolytic cell, and using the electrolyte in common for electrolysis in each chamber, the electrolyte containing dispersed therein a catalyst comprising a hybrid nickel-iron-cobalt hydroxide (Ni _x Fe _y Co _z -hmh, in which the atomic ratio x/y/z of Ni/Fe/Co is 8.2-65.6/33.7-85/0.6-18 ), which is a composite of a metal hydroxide and an organic substance and has a composite structure in which a tripodal Tris-NH 2 ligand is covalently fixed to the metal hydroxide. An alkaline water electrolysis method comprising: supplying an electrolyte solution to at least an anode _chamber constituting an electrolytic cell; and dispersing a catalyst comprising a metal hydroxide and a hybrid nickel-iron-cobalt hydroxide ( _{NixFeyCoz - hmh} , in which the atomic ratio x/y/z of Ni/Fe/Co is 8.2-65.6/33.7-85/0.6-18 ), which is a composite of a metal hydroxide and an organic substance and has a composite structure in which a Tris- _NH2 tripod ligand is covalently fixed. During operation, electrolytic deposition of the _{NixFeyCoz - hmh} is carried out in the electrolytic cell, and the _NixFeyCoz -hmh is electrolytically deposited on the surface of a conductive substrate having a _catalyst layer formed on its surface, which constitutes an oxygen generating anode. The alkaline water electrolysis method according to claim 2, wherein the electrolytic solution is supplied to the anode chamber when performance of the oxygen generating anode deteriorates due to operation, and Ni _x Fe _y Co _z -hmh is electrolytically deposited on the surface of the conductive substrate, thereby recovering and improving electrolysis performance. The alkaline water electrolysis method according to claim 1 or 2, wherein the Ni _x Fe _y Co _z -hmh comprises at least one of a sheet-like substance having a layered molecular structure, a needle-shaped substance having a tunnel structure, and a particulate substance having an amorphous structure. The alkaline water electrolysis method according to claim 3, wherein the condition for electrolytically depositing Ni _x Fe _y Co _z -hmh on the surface of the conductive substrate is to maintain the conductive substrate in a potential range of 1.2 V to 1.8 V vs. RHE. The alkaline water electrolysis method according to claim 1 or 2, wherein the electrolyte solution in which Ni _x Fe _y Co _z -hmh is dispersed is a Ni _x Fe _y Co _z -hmh dispersion solution having a concentration of 5 to 100 g/L, and the concentration of the Ni _x Fe _y Co _z -hmh dispersion solution added to the electrolyte solution is adjusted to be within the range of 0.1 to 8 mL/L. The alkaline water electrolysis method according to claim 1 or 2, wherein the Ni _x Fe _y Co _z -hmh is at least one selected from the group consisting of Ni _11.5 Fe _70.5 Co ₁₈ -hmh, Ni _65.6 Fe _33.7 Co _0.6 -hmh, and Ni _8.2 Fe ₈₅ Co _6.8 -hmh. a conductive substrate having a surface made of nickel or a nickel-based alloy;
a catalyst layer comprising a metal hydroxide, which is an electrolytic deposit formed on the surface of the conductive substrate, and a hybrid nickel-iron- _cobalt hydroxide ( _NixFeyCoz - _hmh , in which the atomic ratio x/ _y /z of Ni/Fe/Co is 8.2-65.6/33.7-85/0.6-18), which is a composite of a metal hydroxide and an organic substance and has a composite structure in which a tripodal ligand of Tris-NH2 is covalently fixed;
An anode for alkaline water electrolysis that generates oxygen, comprising: The Ni _x Fe _y Co _z -hmh is Ni _11.5 Fe _70.5 Co ₁₈ -hmh, Ni _65.6 Fe _33.7 Co _0.6 -hmh and Ni _8.2 Fe ₈₅ Co _6.8 The anode for alkaline water electrolysis for generating oxygen according to claim 8, wherein the anode is at least one selected from: a conductive substrate having a surface made of nickel or a nickel-based alloy;
an intermediate layer formed on the surface of the conductive substrate and made of a lithium-containing nickel oxide represented by the composition formula Li _x Ni _2-x O ₂ (0.02≦x≦0.5);
a catalyst layer formed on the surface of the intermediate layer as an electrolytic deposit , the catalyst layer being made of a hybrid nickel-iron-cobalt hydroxide (Ni _x Fe _y Co _z -hmh , where the atomic ratio x/y/z of Ni/Fe/Co is 8.2-65.6/33.7-85/0.6-18 ) which is a _composite of a metal hydroxide and an organic substance and has a composite structure in which a tripodal ligand of Tris-NH 2 is covalently fixed;
An anode for alkaline water electrolysis that generates oxygen, comprising: The Ni _x Fe _y Co _z -hmh is Ni _11.5 Fe _70.5 Co ₁₈ -hmh, Ni _65.6 Fe _33.7 Co _0.6 -hmh and Ni _8.2 Fe ₈₅ Co _6.8 The anode for alkaline water electrolysis for generating oxygen according to claim 10, wherein the anode is at least one selected from: