Patent Publication Number: US-2023143685-A1

Title: Electrode catalyst for water electrolysis cell, water electrolysis cells, and water electrolysis devices

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electrode catalyst for a water electrolysis cell and also relates to water electrolysis cells and water electrolysis devices. 
     2. Description of the Related Art 
     In recent years, the development of a catalyst material that is used in a water electrolysis device has been expected. 
     Seyeong Lee et al., “Operational durability of three-dimensional Ni—Fe layered double hydroxide electrocatalyst for water oxidation,” Electrochimica Acta, 2019, Vol. 315, pp. 94-101 (Non-Patent Literature 1) discloses a Ni—Fe layered double hydroxide (Ni—Fe LDH). 
     Jiande Chen et al., “Interfacial Interaction between FeOOH and Ni—Fe LDH to Modulate the Local Electronic Structure for Enhanced OER Electrocatalysis,” ACS Catalysis, 2018, Vol. 8, pp. 11342-11351 (Non-Patent Literature 2) discloses a composite of FeOOH nanoparticles and a Ni—Fe LDH. 
     CITATION LIST 
     Summary 
     In one general aspect, the techniques disclosed here feature an electrode catalyst for a water electrolysis cell. The electrode catalyst includes a catalyst, a support, and an organic compound. The catalyst is a layered double hydroxide that contains a chelating agent. The support contains a transition metal. The organic compound has an anionic functional group. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an electrode catalyst for a water electrolysis cell, according to a first embodiment; 
         FIG.  2    is a schematic diagram of an exemplary crystal structure of a portion of an LDH; 
         FIG.  3    is a schematic cross-sectional view of an exemplary water electrolysis cell, according to a second embodiment; 
         FIG.  4    is a schematic cross-sectional view of an exemplary water electrolysis device, according to a third embodiment; 
         FIG.  5    is a schematic cross-sectional view of another exemplary water electrolysis cell, according to a fourth embodiment; and 
         FIG.  6    is a schematic cross-sectional view of another exemplary water electrolysis device, according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Underlying Knowledge Forming Basis of the Present Disclosure 
     A step for combating global warming that is attracting attention is the utilization of renewable energy, such as solar power or wind power. However, power generation that uses renewable energy presents a problem in that the excess electrical power goes to waste. That is, the renewable energy is not necessarily utilized with sufficient efficiency. Correspondingly, studies are being conducted regarding methods for producing hydrogen from excess electrical power and storing the hydrogen. 
     A typical method for producing hydrogen from excess electrical power may be the electrolysis of water. The electrolysis of water is also referred to as water electrolysis. It is desired that a highly efficient and long-lasting water electrolysis device be developed so that hydrogen can be produced inexpensively and consistently. Principal constituent elements of a water electrolysis device include a membrane electrode assembly (MEA), which is formed of a gas diffusion layer, a catalyst, and an electrolyte membrane. 
     Providing a highly efficient and long-lasting water electrolysis device requires, in particular, improvements in the performance and durability of the catalyst. Layered double hydroxides (LDHs) that contain a transition metal exhibit excellent catalytic activity. Accordingly, utilization of an LDH as a catalyst material has attracted attention in recent years. However, as indicated in Non-Patent Literature 1, a problem is presented in that application of voltage to the catalyst decreases catalytic performance, which increases overvoltage. This is believed to be because the application of voltage changes a shape of the Ni—Fe LDH, which causes the Ni—Fe LDH to be separated from a support. An important point for ensuring that a Ni—Fe LDH has high durability is to inhibit the separation of the Ni—Fe LDH from a support. 
     The present inventors performed studies and newly discovered that in instances where an LDH that contains a chelating agent is used as the catalyst, catalytic activity is enhanced. However, even with such a catalyst, the catalytic performance may decrease if the catalyst is separated from the support. In view of this, the present inventors diligently performed studies to develop a technology for inhibiting a catalyst that is an LDH that contains a chelating agent form being separated from a support. As a result, it was newly discovered that using a specific organic compound is advantageous. Based on the finding, the present inventors discovered an electrode catalyst for a water electrolysis cell, which is disclosed in the present disclosure. 
     Overview of Aspects of Present Disclosure 
     According to a first aspect of the present disclosure, an electrode catalyst for a water electrolysis cell includes: 
     a catalyst, the catalyst being a layered double hydroxide that contains a chelating agent; 
     a support that contains a transition metal; and 
     an organic compound that has an anionic functional group. 
     With regard to the first aspect, an electrode catalyst for a water electrolysis cell can be provided, with the electrode catalyst having high durability. 
     In a second aspect of the present disclosure, for example, in the electrode catalyst for a water electrolysis cell according to the first aspect, the anionic functional group may include a sulfonic acid ionic group. 
     In a third aspect of the present disclosure, for example, in the electrode catalyst for a water electrolysis cell according to the first aspect, the anionic functional group may include a carboxylic acid ionic group. 
     With regard to the second and third aspects, the organic compound enables a desired attractive force to be generated between the organic compound and the LDH and between the organic compound and the support. 
     In a fourth aspect of the present disclosure, for example, in the electrode catalyst for a water electrolysis cell according to the first aspect, the organic compound may include a perfluorocarbon polymer having a sulfonic acid ionic group. With this configuration, a withstand voltage of the electrode catalyst is further improved. 
     In a fifth aspect of the present disclosure, for example, in the electrode catalyst for a water electrolysis cell according to any one of the first to fourth aspects, the layered double hydroxide may contain Ni and Fe. With this configuration, the LDH can have higher catalytic activity. 
     In a sixth aspect of the present disclosure, for example, in the electrode catalyst for a water electrolysis cell according to any one of the first to fifth aspects, the transition metal may include Ni. With this configuration, the electrode catalyst can have high durability against alkalis. 
     According to a seventh aspect of the present disclosure, a water electrolysis cell includes: 
     an anode; 
     a cathode; and 
     an electrolyte membrane disposed between the anode and the cathode. 
     At least one selected from the group consisting of the anode and the cathode includes the electrode catalyst according to any one of the first to sixth aspects. 
     With regard to the seventh aspect, the water electrolysis cell can have high durability. 
     In an eighth aspect of the present disclosure, for example, in the water electrolysis cell according to the seventh aspect, the electrolyte membrane may include a proton exchange membrane. 
     In a ninth aspect of the present disclosure, for example, in the water electrolysis cell according to the seventh aspect, the electrolyte membrane may include an anion exchange membrane. 
     With regard to the eighth and ninth aspects, oxygen gas generated at the anode and hydrogen gas generated at the cathode do not easily mix with each other. 
     According to a tenth aspect of the present disclosure, a water electrolysis cell includes: 
     a diaphragm that separates a first space from a second space; 
     an anode provided in the first space; and 
     a cathode provided in the second space. 
     At least one selected from the group consisting of the anode and the cathode includes the electrode catalyst according to any one of the first to sixth aspects. 
     With regard to the tenth aspect, the water electrolysis cell can have high durability. 
     According to an eleventh aspect of the present disclosure, a water electrolysis device includes: 
     the water electrolysis cell according to any one of the seventh to tenth aspects; and 
     a voltage applicator that applies a voltage between the anode and the cathode, the voltage applicator being connected to the anode and the cathode. 
     With regard to the eleventh aspect, the water electrolysis device can have high durability. 
     Embodiments of the present disclosure will now be described with reference to the drawings. The present disclosure is not limited to the embodiments described below. 
     First Embodiment 
       FIG.  1    is a schematic diagram of an electrode catalyst for a water electrolysis cell, according to the present embodiment. An electrode catalyst  1 , according to the present embodiment, includes a catalyst  10 , a support  11 , and an organic compound  12 . The catalyst  10  is an LDH that contains a chelating agent. The support  11  contains a transition metal. For example, the organic compound  12  has an anionic functional group and is present on a surface of the catalyst  10 , which is supported on the support  11 . With this configuration, the electrode catalyst  1  can have high durability. 
     Catalyst 
     For example, the catalyst  10  contains two or more transition metals and a chelating agent. The transition metals include at least two selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru, for example. 
     For example, the catalyst  10  is an LDH having a composition represented by composition formula (1) below, with which a chelating agent coordinates. 
       [M 1   2+   1-   x M 2   3+   x (OH) 2 ][ y A n−   .m H 2 O]  Composition formula (1)
 
     In composition formula (1), M 1   2+  is an ion of a divalent transition metal. M 2   3+  is an ion of a trivalent transition metal. A n−  is an anion present between layers. x is a rational number satisfying the condition of 0&lt;x&lt;1. y is a number corresponding to an amount necessary for a charge balance. n is an integer. m is an appropriate rational number. 
     The LDH may contain Ni and Fe. In composition formula (1), M 1  may be Ni, and M 2  may be Fe. That is, the transition metal elements included in the catalyst  10  may be Ni and Fe. With this configuration, the electrode catalyst  1  can have a higher catalytic activity than, for example, a Co—Fe LDH. 
     The catalyst  10  is an LDH that contains a chelating agent. Consequently, the catalyst  10  has improved dispersion stability. Furthermore, the presence of a chelating agent in the catalyst  10  may enable the synthesized catalyst  10  to have a small particle diameter. As a result, the catalyst  10  can have an improved surface area, which in turn can improve catalytic activity. The catalyst  10  may have an average particle diameter of less than or equal to 100 nm, less than or equal to 50 nm, or less than or equal to 10 nm. The average particle diameter of the catalyst  10  is a value determined as follows. A particle size distribution of the catalyst  10 , which is obtained by small angle X-ray scattering (SAXS), is represented in a two-dimensional distribution graph that shows a relationship between the particle diameter and the distribution, and the area in the two-dimensional distribution graph is divided by the total number of the particles. The distribution indicates numerical values proportional to the total volume of the particles of a corresponding particle diameter. The area in the two-dimensional distribution graph is, for example, the product of a particle diameter and the number of particles corresponding to the particle diameter. 
     The chelating agent is not limited to any particular chelating agent. For example, the chelating agent is an organic compound that coordinates with a transition metal in the LDH. The chelating agent may be at least one selected from bidentate organic ligands and tridentate organic ligands. Examples of the chelating agent include β-diketones, β-ketoesters, and hydroxycarboxylic acids. Examples of the β-diketones include acetylacetone (ACAC), trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, thenoyltrifluoroacetone, dipilovaylmethane, dibenzoylmethane, and ascorbic acid. Examples of the β-ketoesters include methyl acetoacetate, ethyl acetoacetate, allyl acetoacetate, benzyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, n-butyl acetoacetate, isobutyl acetoacetate, tert-butyl acetoacetate, 2-methoxyethyl acetoacetate, and methyl 3-oxopentanoate. Examples of the hydroxycarboxylic acids and salts thereof include tartaric acid, citric acid, malic acid, gluconic acid, ferulic acid, lactic acid, glucuronic acid, and salts thereof. The chelating agent may include at least one selected from the group consisting of acetylacetone and trisodium citrate. The chelating agent may be at least one selected from acetylacetone and trisodium citrate. 
     A n−  is an interlayer anion. A n−  is an inorganic ion or an organic ion. Examples of the inorganic ion include CO 3   2− , NO 3   − , Cl − , SO 4   2− , Br − , OH − , F − , I − , Si 2 O 5   2− , B 4 O 5 (OH) 4   2− , and PO 4   3− . Examples of the organic ion include CH 3 (CH 2 ) n SO 4− , CH 3 (CH 2 )COO − , CH 3 (CH 2 ) n PO 4− , and CH 3 (CH 2 ) n NO 3− . A n−  is an anion intercalated between the layers of the metal hydroxide together with water molecules. A n−  has a charge and an ionic size, which are not limited to any particular value. The catalyst  10  may include one type of A n−  or two or more types of A n− . 
       FIG.  2    is a schematic diagram of an exemplary crystal structure of a portion of an LDH represented by composition formula (1). As illustrated in  FIG.  2   , a crystal structure  20  includes an OH −  ion on each of the vertices of octahedra, in which M 1   2+  or M 2   3+  is the center. The metal hydroxide is represented by [M 1   2+   1-   x M 2   3+   x (OH) 2 ] x+ . The metal hydroxide has a layered structure in which hydroxide octahedra are two-dimensionally connected to one another with edges thereof being shared. The anion and water molecules are positioned between the layers of the metal hydroxide. The layers of the metal hydroxide serve as host layers  21 , between which the anion and water molecules, which serve as guest layers  22 , are intercalated. That is, the crystal structure  20 , as a whole, has a sheet structure in which the host layers  21 , which are formed of the metal hydroxide, and the guest layers  22 , which are formed of the anion and water molecules, are alternately layered. The crystal structure  20  has a structure in which some of the M 1   2+  present in the layers of the metal hydroxide is replaced by the M 2   3+ . Accordingly, a surface of the crystal structure  20  is typically positively charged. 
     Organic Compound that has Anionic Functional Group 
     The organic compound  12 , which has an anionic functional group, is, for example, an organic compound that can be present on a surface of the catalyst  10  supported on the support  11 . Furthermore, the organic compound  12  is, for example, an organic compound that can be present between the catalyst  10  and the support  11  and between the particles of the support  11 . The organic compound  12  can be an organic binder. In the organic compound  12 , the anionic functional group may be present in a main chain or a side chain. The presence of the anionic functional group in the organic compound  12  enables, for example, an attractive force due to electrostatic interaction to be generated with respect to the catalyst  10  and the support  11 . Consequently, even in instances where a voltage is applied to the electrode catalyst  1 , the catalyst  10  is not easily separated from the support  11 . As a result, an increase in the overvoltage is inhibited, and, therefore, the electrode catalyst  1  has high durability. 
     The attractive force that is generated between the organic compound  12  and the catalyst  10  and between the organic compound  12  and the support  11  is a Coulomb force, for example. The use of the organic compound  12  enables a Coulomb force to be generated between the catalyst  10  and the organic compound  12  and between the support  11  and the organic compound  12 . The Coulomb force can inhibit the separation between the catalyst  10  and the support  11 . The Coulomb force, denoted as ΔE, is represented by the following equation. 
     
       
         
           
             
               Δ 
               ⁢ 
               E 
             
             = 
             
               
                 1 
                 
                   4 
                   ⁢ 
                   
                     πε 
                     0 
                   
                 
               
               * 
               
                 
                   
                     q 
                     + 
                   
                   ⁢ 
                   
                     q 
                     - 
                   
                 
                 r 
               
             
           
         
       
     
     In the equation, ε 0  represents the dielectric constant of a vacuum. q +  and q −  represent an amount of charge of charged particles. r represents a distance between charged particles. 
     The Coulomb force provides an attractive force between the support  11  and the organic compound  12  and between the catalyst  10  and the organic compound  12 . Consequently, even in instances where a voltage is applied to the electrode catalyst  1 , the catalyst  10  is not easily separated from the support  11 . Thus, the electrode catalyst  1  can have higher durability. 
     The anionic functional group is not limited to any particular type. Examples of the anionic functional group include sulfonic acid ionic groups, carboxylic acid ionic groups, acetic acid ionic groups, citric acid ionic groups, lactic acid ionic groups, glycolic acid ionic groups, phosphoric acid ionic groups, nitric acid ionic groups, and alkyl sulfonic acid ionic groups. At least one selected from the group consisting of sulfonic acid ionic groups and carboxylic acid ionic groups may be included as the anionic functional group. The anionic functional group may include a sulfonic acid ionic group. The anionic functional group may be a sulfonic acid ionic group. The anionic functional group may include a carboxylic acid ionic group. The anionic functional group may be a carboxylic acid ionic group. Consequently, it is likely that a desired attractive force is generated between the organic compound  12  and the catalyst  10  and between the organic compound  12  and the support  11 . 
     The organic compound  12  may be an organic compound containing fluorine. The organic compound  12  may be an organic polymer having fluorine and a sulfonic acid ionic group. The organic compound  12  may include a perfluorocarbon polymer having a sulfonic acid ionic group. The organic compound  12  may be a perfluorocarbon polymer having a sulfonic acid ionic group. Since fluorine has an inductive effect, the presence of fluorine in the organic compound  12  improves the thermal stability and electrochemical stability of the organic compound  12 . In addition, the presence of fluorine in the organic compound further improves the withstand voltage of the electrode catalyst  1 . Examples of the perfluorocarbon polymer having a sulfonic acid ionic group include Nafion (registered trademark). Nafion has the following chemical formula. 
     
       
         
         
             
             
         
       
     
     In the chemical formula, y/x satisfies 1≤y/x≤9. n is an integer greater than or equal to 0 and less than or equal to 2. 
     Support 
     Typically, the support  11  has a conductivity. The support  11  is not limited to any particular material provided that the support  11  contains a transition metal. The support  11  can support the catalyst  10 . For example, the organic compound  12  binds to the support  11  and the catalyst  10  via a Coulomb force. The support  11  may contain at least one selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. The support  11  may be at least one selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. A surface of the support  11  is positively charged under neutral conditions, for example. Accordingly, the support  11  and the organic compound  12  can bind to each other via a Coulomb force. In addition, the catalyst  10  and the organic compound  12  can bind to each other via a Coulomb force. Consequently, even in instances where a voltage is applied, the support  11  is not easily separated from the catalyst  10 , and, therefore, degradation in catalytic performance can be further inhibited. 
     The support  11  may contain Ni. The support  11  may be Ni. With this configuration, the electrode catalyst  1  can have high durability against alkalis. 
     A shape of the support  11  is not limited to any particular shape. The shape of the support  11  may be a foam shape or a particle shape. When the shape of the support  11  is a particle shape, a particle diameter of the support  11  is not limited to any particular value. The particle diameter of the support  11  may be less than or equal to 100 nm or less than or equal to 50 nm. In these cases, a desired attractive force tends to be generated between the organic compound  12  and the support  11 . In addition, even in instances where a voltage is applied, the support  11  is not easily separated from the catalyst  10 , and, therefore, degradation in catalytic performance can be further inhibited. The particle diameter of the support  11  is a particle diameter measured by using a transmission electron microscope (TEM). 
     For example, the electrode catalyst  1  according to the present embodiment is used in a proton exchange membrane type water electrolysis device, an anion exchange membrane type water electrolysis device, or an alkali diaphragm type water electrolysis device. The electrode catalyst  1  may be used in at least one selected from an anode and a cathode in any of these water electrolysis devices. 
     Second Embodiment 
       FIG.  3    is a schematic cross-sectional view of an exemplary water electrolysis cell, according to the present embodiment. 
     A water electrolysis cell  2  includes an electrolyte membrane  31 , an anode  100 , and a cathode  200 . The electrolyte membrane  31  is disposed between the anode  100  and the cathode  200 , for example. At least one selected from the anode  100  and the cathode  200  includes the electrode catalyst  1 , which is described in the first embodiment. 
     The electrolyte membrane  31  may be an electrolyte membrane having ionic conductivity. The electrolyte membrane  31  is not limited to any particular type. The electrolyte membrane  31  may include a proton exchange membrane. The electrolyte membrane  31  may be a proton exchange membrane. The electrolyte membrane  31  may include an anion exchange membrane. The electrolyte membrane  31  may be an anion exchange membrane. The electrolyte membrane  31  is configured such that oxygen gas generated at the anode  100  and hydrogen gas generated at the cathode  200  do not easily mix with each other. 
     The anode  100  includes a catalyst layer  30 , for example. The catalyst layer  30  may be provided on one of major surfaces of the electrolyte membrane  31 . The “major surfaces” refers to surfaces having the largest area of the electrolyte membrane  31 . The catalyst layer  30  includes an electrode catalyst, which may be the electrode catalyst  1  of the first embodiment. The anode  100  may further include a gas diffusion layer  33 , which is porous and conductive and may be provided on the catalyst layer  30 . 
     The cathode  200  includes a catalyst layer  32 , for example. The catalyst layer  32  may be provided on the other of the major surfaces of the electrolyte membrane  31 . That is, the catalyst layer  32  may be provided on the major surface of the electrolyte membrane  31  opposite to the major surface on which the catalyst layer  30  is provided. The electrode catalyst that may be used in the catalyst layer  32  is not limited to any particular type. The electrode catalyst may be platinum or the electrode catalyst  1 . The cathode  200  may further include a gas diffusion layer  34 , which is porous and conductive and may be provided on the catalyst layer  32 . 
     With any of the configurations described above, the water electrolysis cell  2  can have high durability; this is because at least one selected from the anode  100  and the cathode  200  includes the electrode catalyst  1 . 
     Third Embodiment 
       FIG.  4    is a schematic cross-sectional view of an exemplary water electrolysis device, according to the present embodiment. 
     A water electrolysis device  3  includes a water electrolysis cell  2  and a voltage applicator  40 . The water electrolysis cell  2  is similar to the water electrolysis cell  2  of the second embodiment, and, therefore, descriptions thereof will be omitted. 
     The voltage applicator  40  is connected to an anode  100  and a cathode  200  of the water electrolysis cell  2 . The voltage applicator  40  is a device that applies a voltage between the anode  100  and the cathode  200  in the water electrolysis cell  2 . 
     The voltage applicator  40  increases the potential of the anode  100  and decreases the potential of the cathode  200 . The voltage applicator  40  is not limited to any particular type provided that the voltage applicator  40  can apply a voltage between the anode  100  and the cathode  200 . The voltage applicator  40  may be a device that adjusts the voltage to be applied between the anode  100  and the cathode  200 . Specifically, in instances where the voltage applicator  40  is connected to a DC power supply, such as a battery, a solar cell, or a fuel cell, the voltage applicator  40  includes a DC/DC converter. In instances where the voltage applicator  40  is connected to an AC power supply, such as a commercial power supply, the voltage applicator  40  includes an AC/DC converter. The voltage applicator  40  may be a power-type power supply that adjusts the voltage that is applied between the anode  100  and the cathode  200  and adjusts the current that flows between the anode  100  and the cathode  200 , such that the power to be supplied to the water electrolysis device  3  is adjusted to a predetermined set value. 
     With any of the configurations described above, the water electrolysis device  3  can have high durability. 
     Fourth Embodiment 
       FIG.  5    is a schematic cross-sectional view of another exemplary water electrolysis cell, according to the present embodiment. 
     The water electrolysis cell according to the present embodiment is, for example, an alkaline water electrolysis cell  4 , which utilizes an alkaline aqueous solution. In alkaline water electrolysis, an alkaline aqueous solution is used. Examples of the alkaline aqueous solution include aqueous potassium hydroxide solutions and aqueous sodium hydroxide solutions. 
     The alkaline water electrolysis cell  4  includes an anode  300  and a cathode  400 . The alkaline water electrolysis cell  4  further includes an electrolysis chamber  70 , a first space  50 , and a second space  60 . The anode  300  is provided in the first space  50 . The cathode  400  is provided in the second space  60 . The alkaline water electrolysis cell  4  includes a diaphragm  41 . The diaphragm  41  is provided within the electrolysis chamber  70  and separates the first space  50  from the second space  60 . At least one selected from the anode  300  and the cathode  400  includes the electrode catalyst  1 . 
     The anode  300  may include the electrode catalyst  1 . For example, the anode  300  may include a catalyst layer, and the electrode catalyst  1  may be included in the catalyst layer. 
     The cathode  400  may include the electrode catalyst  1 . For example, the cathode  400  may include a catalyst layer, and the electrode catalyst  1  may be included in the catalyst layer. 
     The diaphragm  41  is a diaphragm for alkaline water electrolysis, for example. 
     The anode  300  may be disposed in contact with the diaphragm  41 , or the anode  300  may be spaced apart from the diaphragm  41 . The cathode  400  may be disposed in contact with the diaphragm  41 , or the cathode  400  may be spaced apart from the diaphragm  41 . 
     The alkaline water electrolysis cell  4  produces hydrogen and oxygen by electrolyzing an alkaline aqueous solution. An aqueous solution containing a hydroxide of an alkali metal or an alkaline earth metal may be supplied to the first space  50  of the alkaline water electrolysis cell  4 . An alkaline aqueous solution may be supplied to the second space  60  of the alkaline water electrolysis cell  4 . The production of hydrogen and oxygen is carried out by performing electrolysis while discharging an alkaline aqueous solution having a predetermined concentration from the first space  50  and the second space  60 . 
     With any of the configurations described above, the alkaline water electrolysis cell  4  can have high durability; this is because at least one selected from the anode  300  and the cathode  400  includes the electrode catalyst  1 . 
     Fifth Embodiment 
       FIG.  6    is a schematic cross-sectional view of another exemplary water electrolysis device, according to the present embodiment. 
     The water electrolysis device according to the present embodiment is, for example, an alkaline water electrolysis device  5 , which utilizes an alkaline aqueous solution. The alkaline water electrolysis device  5  includes an alkaline water electrolysis cell  4  and a voltage applicator  40 . The alkaline water electrolysis cell  4  is similar to the alkaline water electrolysis cell  4  of the fourth embodiment, and, therefore, descriptions thereof will be omitted. 
     The voltage applicator  40  is connected to an anode  300  and a cathode  400  of the alkaline water electrolysis cell  4 . The voltage applicator  40  is a device that applies a voltage between the anode  300  and the cathode  400  in the alkaline water electrolysis cell  4 . 
     With this configuration, the alkaline water electrolysis device  5  can have high durability. 
     EXAMPLES 
     The present disclosure will now be described in more detail with reference to examples. Note that the examples described below are merely illustrative of the present disclosure and are not to be construed as limiting the present disclosure. 
     Sample 1 
     Preparation of Ni—Fe LDH 
     A mixture including a Ni—Fe LDH and Ni particles was prepared in the following manner. First, a mixed solvent of water and ethanol (special grade reagent, manufactured by FUJIFILM Wako Pure Chemical Corporation) was prepared. A volume ratio between the water and the ethanol was 2:3. Nickel chloride hexahydrate (manufactured by FUJIFILM Wako Pure Chemical Corporation) and iron chloride hexahydrate (manufactured by FUJIFILM Wako Pure Chemical Corporation) were dissolved in the mixed solvent such that a total concentration of the Ni ions and Fe ions became 1.0 M, and the ratio of the amount of substance of the Fe ions to the total amount of substance of the Ni ions and the Fe ions became 0.33. Note that “M” denotes “mol/dm 3 ”. Furthermore, acetylacetone (ACAC), which was used as a chelating agent, was added such that the amount of substance of the acetylacetone became one-third of the total amount of substance of the Ni ions and the Fe ions. The obtained solution was stirred for 30 minutes. 
     Ni particles (manufactured by US Research Nanomaterials, Inc., a particle diameter of 40 nm) were added to the solution, with the mass of the Ni particles being equal to the mass of a Ni—Fe LDH that would be produced if all the Ni and Fe present in the solution ideally reacted. Next, propylene oxide (POX), which was used as a pH raising agent, was added to the solution containing the Ni—Fe LDH and the Ni particles such that the amount of substance of the POX became twice the amount of substance of the chloride ions present in the solution. The obtained solution was stirred for 1 minute. In such an instance, POX gradually scavenges hydrogen ions in the solution, and, therefore, the pH of the solution gradually increases. Accordingly, the obtained solution was allowed to stand for approximately three days, and subsequently, the mixture of the Ni—Fe LDH and the Ni particles, which was the target sample, was collected. 
     Preparation of Sample for Evaluation of Catalytic Activity 
     Nafion (registered trademark) was used as an organic binder. Nafion (a 5 wt % dispersion, manufactured by Aldrich Corporation) was mixed with the mixture of the Ni—Fe LDH and the Ni particles such that the mass ratio between the Ni—Fe LDH and Nafion became 5:1, and the total mass became 21 mg. 1.03 mL of a mixed solvent of N,N-dimethylformamide (special grade reagent, manufactured by FUJIFILM Wako Pure Chemical Corporation) and ethanol (special grade reagent, manufactured by FUJIFILM Wako Pure Chemical Corporation) was added to the resulting mixture. Accordingly, a catalyst ink solution was prepared. A volume ratio between the N,N-dimethylformamide and the ethanol was 3:1. The catalyst ink solution was sonicated with an ultrasonic homogenizer for 30 minutes. In this manner, a catalyst ink of Sample 1 was prepared. 10 μL of the catalyst ink of Sample 1 was added dropwise onto a rotating disk electrode, and the resultant was dried at room temperature. In this manner, a sample for evaluation of catalytic activity of Sample 1 was obtained. 
     Sample 2 
     A sample for evaluation of catalytic activity of Sample 2 was obtained in a manner similar to that for Sample 1, except that FAA-3 (registered trademark), manufactured by Fumatech, was used as an organic binder instead of Nafion, and 1.05 mL of the mixed solvent of N,N-dimethylformamide and ethanol was added. A volume ratio between the N,N-dimethylformamide and the ethanol was 3:1. FAA-3 is an organic binder having a cationic functional group. 
     Sample 3 
     A sample for evaluation of catalytic activity of Sample 3 was obtained in a manner similar to that for Sample 1, except that Sustainion (registered trademark), manufactured by Dioxide Materials, was used as an organic binder instead of Nafion, and 1.05 mL of the mixed solvent of N,N-dimethylformamide and ethanol was added. A volume ratio between the N,N-dimethylformamide and the ethanol was 3:1. Sustainion is an organic binder having a cationic functional group. 
     Sample 4 
     A sample for evaluation of catalytic activity of Sample 4 was obtained in a manner similar to that for Sample 1, except that Ketjen Black EC600JD, manufactured by Lion Specialty Chemicals Co., Ltd., was used as the support instead of Ni particles; the Ni—Fe LDH and the Ketjen black were mixed together such that the mass ratio between the Ni—Fe LDH and the Ketjen black became 2:1, and the total mass became 7.5 mg; and 1.03 mL of the mixed solvent of N,N-dimethylformamide and ethanol was added. A volume ratio between the N,N-dimethylformamide and the ethanol was 3:1. Ketjen black (KB) has a conductivity, and a surface thereof is negatively charged. 
     Sample 5 
     A sample for evaluation of catalytic activity of Sample 5 was obtained in a manner similar to that for Sample 4, except that FAA-3, manufactured by Fumatech, was used as an organic binder, and 1.05 mL of the mixed solvent of N,N-dimethylformamide and ethanol was added. A volume ratio between the N,N-dimethylformamide and the ethanol was 3:1. 
     Evaluation of Overvoltage of Catalyst 
     The overvoltage of the sample for evaluation of catalytic activity of each of Samples 1 to 5 was measured. The measurement was carried out by using a potentiostat VersaSTAT 4, manufactured by Princeton Applied Research, and a rotating electrode AFE3T050GC, manufactured by Pine Research. The currents produced by the anode reaction in the water electrolysis cell in the 1st cycle and the 120th cycle were measured by using a rotating disk electrode (RDE) method under the following measurement conditions. The anode reaction is an oxygen evolution reaction. The results are shown in Table 1. 
     Measurement Conditions 
     
         
         
           
             Solution: a 1 M KOH solution 
             Potential: 1.0 V to 1.65 V (vs. reversible hydrogen electrode (RHE)) 
             Potential sweep rate: 10 mV/sec 
             Electrode rotation speed: 1500 revolutions per minute (rpm) 
           
         
       
    
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Overvoltage 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Electrode catalyst 
                 Overvoltage V 1   
                 Overvoltage V 2   
                 difference 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Organic 
                 1st cycle 
                 120th cycle 
                 (ΔV = V 2  − V 1 ) 
               
               
                   
                 LDH 
                 Support 
                 binder 
                 (mV) 
                 (mV) 
                 (mV) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sample 1 
                 Ni—Fe LDH 
                 Ni 
                 Nafion 
                 262 
                 265 
                 3 
               
               
                 Sample 2 
                 Ni—Fe LDH 
                 Ni 
                 Sustainion 
                 324 
                 394 
                 70 
               
               
                 Sample 3 
                 Ni—Fe LDH 
                 Ni 
                 FAA-3 
                 337 
                 390 
                 53 
               
               
                 Sample 4 
                 Ni—Fe LDH 
                 KB 
                 Nafion 
                 266 
                 299 
                 33 
               
               
                 Sample 5 
                 Ni—Fe LDH 
                 KB 
                 FAA-3 
                 254 
                 268 
                 14 
               
               
                   
               
            
           
         
       
     
     Table 1 shows the results of the measurement performed on the samples for evaluation of catalytic activity of Samples 1 to 5 to measure overvoltages. Table 1 shows an overvoltage V 1 , an overvoltage V 2 , and an overvoltage difference (ΔV=V 2 −V 1 ). The overvoltage V 1  is an overvoltage in the 1st cycle of the redox reaction. The overvoltage V 2  is an overvoltage in the 120th cycle of the redox reaction. The overvoltage difference is the difference between the overvoltage of the 1st cycle and the overvoltage of the 120th cycle. 
     The catalyst ink of Sample 1 had an overvoltage difference of 3 mV in the redox cycle. Thus, an increase in the overvoltage was inhibited. In the catalyst ink of Sample 1, degradation in catalytic performance was inhibited. That is, the catalyst ink of Sample 1 had high durability. 
     On the other hand, in the catalyst inks of Samples 2 to 5, the overvoltage increased. This is believed to be because, in the catalyst inks of Samples 2 to 5, a Coulomb force between the support and the organic compound and/or between the LDH and the organic compound was low, and, consequently, the LDH was separated from the support as a result of the application of voltage. 
     The results described above indicate that when an electrode catalyst for a water electrolysis cell includes a Ni—Fe LDH that contains a chelating agent, a support that contains a transition metal, and an organic compound that has an anionic functional group, the electrode catalyst can have high durability against the application of voltage. 
     According to the present disclosure, the electrode catalyst for a water electrolysis cell can be used in water electrolysis devices.