Water electrolysis method for buffer solution and system therefor

Abstract

[Problem] To provide a water electrolysis method and a water electrolysis system with which water electrolysis can be highly efficiently performed under non-extreme pH conditions. [Solution] Provided is a water electrolysis method in a buffer solution having a pH of 7 to 12, wherein the buffer solution is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions.

Description

Buffer solution water electrolysis method and system

[0001] The present invention relates to a method, a system and a water electrolysis cell for efficiently performing water electrolysis.

[0002] With the widespread use of renewable energy, hydrogen generation by water electrolysis has attracted attention. Commercialized water electrolysis systems operate at temperatures around 80°C using a strong alkaline solution (30 wt% KOH) or an acidic solid electrolyte membrane, and require materials that can withstand this harsh environment. By adjusting the pH of the electrolyte to a near-neutral range (pH 5-12), the range of materials that can be used increases (Non-Patent Document 1). However, compared to commercialized water electrolysis systems, this method results in greater energy loss, and the development of efficient electrode catalysts and electrolytes is required.

[0003] The use of seawater as a medium for the electrolyte in water electrolysis has also been considered. However, to avoid deterioration of the entire system (consisting of the anode, cathode, and electrolyte membrane), actual electrolysis equipment requires purifying seawater containing various impurities to produce a highly pure water reactant, which may lead to reduced cost efficiency. Therefore, a seawater splitting process that directly uses seawater has attracted much attention because it has the potential to reduce hydrogen production costs. However, impurities in seawater, especially chloride ions (Cl), － The presence of Cl accelerates the degradation of the electrocatalyst and impairs the selectivity of the oxygen evolution reaction (OER). ２ Gas and hypochlorite (ClO － ) Cl to produce the compound － This is due to competition from oxidation reactions.

[0004] Nocera, DG, Inorg. Chem. 48, 21, (2009).

[0005] An object of the present invention is to provide a water electrolysis method and a water electrolysis system that can perform water electrolysis with high efficiency under non-polar pH conditions.

[0006] The inventors focused on non-polar pH (also called non-extreme pH) buffer solutions, i.e., pH ranges that are neither strongly alkaline nor strongly acidic. Iron is known to exist stably at a pH of approximately 7 to 13 according to its potential-pH diagram (Pourbaix diagram). They conceived that if a water electrolysis system operating in this pH range could be constructed, it would be possible to replace expensive materials such as nickel currently used in practical water electrolysis devices with iron. They then focused on carbonate and boric acid as electrolytes that provide a pH range of approximately 7 to 13. Furthermore, they discovered that the above-mentioned problems could be solved by adding a small amount of phosphate ions as an additive to the electrolyte solution, leading to the completion of the present invention.

[0007] That is, the present invention has the following features. [1] A method for electrolyzing water in a buffer solution having a pH of 7 to 12, wherein the buffer solution is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions. [2] The method according to [1], wherein the cation species is a lithium, sodium, potassium, or cesium cation. [3] The method according to [1] or [2], wherein the concentration of the anion species is 0.1 to 8.0 mol / kg. [4] A system for electrolyzing water in a buffer solution having a pH of 7 to 12, wherein the buffer solution is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions. [5] A water electrolysis cell comprising an anode gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, and a separator, wherein the electrolyte membrane contains a buffer solution having a pH of 7 to 12, the buffer solution being composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution containing 0.1 to 6.0 mol / kg of phosphate ions. [6] A water electrolysis cell stack comprising the water electrolysis cells according to [5] stacked together.

[0008] The present invention provides a water electrolysis method and a water electrolysis system that can operate water electrolysis with high efficiency under non-polar pH conditions. ｘ In systems using electrocatalysts, it is believed that adding phosphate to the electrolyte solution stabilizes the Ni site, and chloride ions (Cl －Even in the presence of ions, it is possible to suppress the corrosion of Ni. Furthermore, when using a hydrogen evolution reaction (HER) electrode catalyst, adding phosphate ions to a carbonate electrolyte can reduce the overpotential for hydrogen generation, and highly efficient hydrogen generation is possible through interface design that takes into account the interaction between the electrode catalyst and electrolyte ions.

[0009] The results of the investigation of OER performance in a borate / phosphate mixed buffer solution are shown. NiFeO in borate only and borate / phosphate mixed electrolytes. ｘ The CV profile of Cl － The graph shows the CV profiles recorded by exchanging the electrolyte. NiFeO in borate / phosphate + KCl mixed buffer. ｘ The results of a long-term stability test of NiFeO / NF at 298 K for 1 day are shown. ｘ The OER performance for NiFeO / NF is shown. ｘ The application to electrode stabilization is shown. － CV in the presence of iodine ions (I － ) in different electrolytes at commercial current densities. ｘ The results of the stability test are shown below. －１ MoO in K-carbonate electrolyte ｘ Cyclic voltammetry (CV) profiles at the (Cu) / Ni foam electrode (Fig. 10a) and 0.5 mol kg －１ 3.0 mol kg with and without the addition of K-phosphate －１ A comparison of Tafel plots (Fig. 10b) at steady state for K-carbonate electrolytes is shown. ｘ (Cu) and NiMoO ｘ The catalysts with different carbonate / phosphate concentrations (1.5 or 3.0 mol kg －１ / 0.5 or 1.5 mol kg －１ ) are shown. In Example 2, the results of HER measurements were performed using Na-, K-, and Cs-carbonate electrolytes. －１K-carbonate electrolyte and 0.5 mol kg －１ The CV profiles of K-phosphate, K-borate, and K-sulfate are compared. To confirm that phosphate anions are adsorbed under HER conditions, MoO was measured in a K-carbonate electrolyte with added phosphate. ｘ The results of operandra manifold spectroscopy analysis of the (Cu) surface are shown.

[0010] 1. Method for Electrolyzing Water One embodiment of the present invention is a method for electrolyzing water in a buffer solution having a pH of 7 to 12, wherein the buffer solution is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions (hereinafter also referred to as the "water electrolysis method of the present invention").

[0011] The buffer solution used in the present invention (hereinafter also referred to as the "buffer solution of the present invention") is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borate and carbonate, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions. That is, it is important that the buffer solution of the present invention contains at least one anion species selected from the group consisting of borate and carbonate, and also contains phosphate ions (phosphate salts) as an additive. While not intending to be bound by theory, the inventors have found that borate and carbonate salts function as primary buffers that maintain local pH stability, and the addition of phosphate salts functions as a primary stabilizer for the metal components of the electrode. Furthermore, as will be described in detail in the Examples, NiFeO ｘ Electrocatalysis has been widely studied due to its high intrinsic activity and earth-abundant components. The addition of phosphate to the electrolyte solution is believed to stabilize the Ni sites, and the addition of phosphate to the electrolyte solution also stabilizes the chloride ions (Cl). －), the corrosion of Ni could be suppressed and the potential was very stable. Therefore, not only pure or purified water but also seawater or river water can be used as the water to be electrolyzed. Furthermore, the inventors have found that when using a hydrogen evolution reaction (HER) electrode catalyst, adding phosphate ions to a carbonate electrolyte can increase the current density (up to 1 A cm －２ ) region, it was found that the overpotential for hydrogen generation can be reduced. Furthermore, this effect was not observed when anions other than phosphate ions (borate ions, sulfate ions) were used as additives, indicating that this is a phenomenon specific to phosphate ions. Furthermore, as shown in the examples, the results of operando Raman spectroscopy suggested that phosphate ions were adsorbed on the surface of MoOx(Cu), the HER electrode catalyst.

[0012] The concentration of phosphate ions is 0.1 to 6.0 mol / kg, preferably 0.1 to 5.0 mol / kg, and more preferably 0.1 to 3.0 mol / kg. By setting the concentration of phosphate ions within this range, the solubility in the buffer solution is good and the above-mentioned effects of adding phosphate ions can be obtained.

[0013] The phosphate ion is H ２ P.O. ４ － , H.P.O. ４ ２－ , P.O. ４ ３－ Examples include:

[0014] The buffer solution of the present invention contains at least one anion species selected from the group consisting of borate and carbonate. The pH of an aqueous solution of carbonate and boric acid is usually about 7 to 13, making it suitable as an electrolyte that provides a nonpolar pH range.

[0015] The at least one anion species selected from the group consisting of borates and carbonates contained in the buffer solution of the present invention is specifically an anion of boric acid (B(OH) ４－ , B ４ O ７ ２－ ) and the anion of carbonate (HCO ３ － , CO ３２－ ) at least one selected from the group consisting of

[0016] With regard to the concentration of the anion species, the higher the concentration, the higher the conductivity and the greater the maximum value of the diffusion constant. Therefore, the optimum value of the concentration of the anion species may vary depending on the operating conditions, but is preferably 0.1 to 8.0 mol / kg, more preferably 0.1 to 6.0 mol / kg, and even more preferably 0.1 to 5.0 mol / kg.

[0017] The cationic species is preferably a lithium, sodium, potassium or cesium cation, more preferably a potassium or cesium cation. The potassium or cesium cation is weakly hydrated compared to sodium etc., and the presence of potassium or cesium cation makes Na ＋ This is more preferable because the hydrogen production activity is higher in the presence of a catalyst than in the presence of a catalyst.

[0018] The buffer solution of the present invention may contain one type of salt composed of the above-mentioned cation species and anion species, or may contain two or more types of salts.

[0019] Examples of phosphates include KH ２ P.O. ４ , K. ２ HPO ４ , LiH ２ P.O. ４ , Li ２ HPO ４ , NaH ２ P.O. ４ , Na ２ HPO ４ , CsH ２ P.O. ４ , Cs ２ HPO ４ Preferably, KH ２ P.O. ４ , K. ２ HPO ４ , CsH ２ P.O. ４ , Cs ２ HPO ４ is.

[0020] In addition, examples of salts composed of the above cation species and anion species include Na ２ B４ O ７ , K. ２ B ４ O ７ , LiHCO ３ , NaHCO ３ , KHCO ３ , and CsHCO ３ Examples include:

[0021] In addition to the salts composed of the above-mentioned cationic and anionic species, the buffer solution of the present invention may contain other components that do not adversely affect the method of the present invention, such as sulfate, hydrochloride, and perchlorate.

[0022] The buffers of the present invention have a non-polar pH, between pH 7 and 12, preferably between pH 9 and 11.

[0023] The concentration of the electrolyte solution is preferably 1.0 M or more, and more preferably 3.0 M or more.

[0024] The water electrolysis method of the present invention is carried out at a temperature in the range of 60° C. to 120° C., preferably 60 to 110° C., and more preferably 80 to 100° C. The water electrolysis method of the present invention can also be carried out under high pressure conditions, thereby enabling operation at a temperature of about 120° C.

[0025] The electrode material (electrocatalyst) used in the water electrolysis method of the present invention can be any electrode material used in conventional water electrolysis methods. The electrode catalyst can include, but is not limited to, one or more abundant elements such as nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), copper (Cu), titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo), and / or tungsten (W). In some embodiments of the present invention, the electrode catalyst can include one or more metals such as noble metals (e.g., ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au)), aluminum (Al), zinc (Zn), cadmium (Cd), gallium (Ga), indium (In), tin (Sn), and bismuth (Bi) and / or other transition metals. Additionally, in some embodiments of the present invention, the electrocatalyst may include one or more non-metals, such as boron (B), carbon (C), nitrogen (N), oxygen (O), phosphine (P), sulfur (S), etc., that may form metal oxides, metal carbides, metal nitrides, metal sulfides, and / or metal phosphides.

[0026] In the water electrolysis method of the present invention, a conventionally known cathode is used as the cathode. Examples of the material for the cathode include platinum group metals such as Pt, Rh, and Ir, Ni, Fe, Mo, Cu, and alloys thereof. Examples of the cathode form include a flat plate, a mesh, and a film formed by sputtering or the like.

[0027] In the water electrolysis method of the present invention, a conventionally known anode is used as the anode. Examples of the material for the anode include Ni, Ru, Ir, Ti, Sn, Mo, Ta, Nb, V, Fe, Mn, and alloys and oxides thereof. Examples of the form of the anode include a flat plate, a mesh, and a film formed by sputtering or the like.

[0028] In one preferred embodiment of the water electrolysis method of the present invention, the electrode catalyst of the anode (OER) is NiFeO ｘ NiFeO ｘmay be doped with one or more selected from Mn, Ni, Fe, and Cu.

[0029] In one preferred embodiment of the water electrolysis method of the present invention, the electrode catalyst of the cathode (HER) is NiMoCu or MoCu. In another preferred embodiment of the water electrolysis method of the present invention, the electrode catalyst of the cathode (HER) is MoO ｘ or NiMoO ｘ and MoO ｘ , NiMoO ｘ may be doped with one or more selected from the group consisting of Cu (copper), W (tungsten), Ag (silver), Au (gold), In (indium), and Sn (tin).

[0030] 2. Water Electrolysis System Water electrolysis methods are generally broadly divided into alkaline water electrolysis, solid polymer water electrolysis, and high-temperature water electrolysis. The water electrolysis method of the present invention can be suitably applied to conventional alkaline water electrolysis methods.

[0031] That is, another embodiment of the present invention is a system for electrolyzing water in a buffer solution having a pH of 7 to 12, wherein the buffer solution comprises an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions (hereinafter also referred to as the "water electrolysis system of the present invention"). Details of the buffer solution used in the water electrolysis system of the present invention are the same as those of the buffer solution of the present invention described in the water electrolysis method of the present invention. Furthermore, details of the cation species, anion species, phosphate ions, electrolyte solution, electrode materials (electrode catalysts), cathode, anode, operating temperature, etc. that can be used in the water electrolysis system of the present invention are the same as those described in detail in the water electrolysis method of the present invention.

[0032] 3. Water Electrolysis Apparatus Another embodiment of the present invention is a water electrolysis apparatus comprising an electrolytic cell, a power source, an anode, and a cathode, wherein an aqueous electrolyte solution is stored in the electrolytic cell, the anode and the cathode are electrically connected to the power source, and the anode and the cathode are in contact with the aqueous electrolyte solution, the aqueous electrolyte solution has a pH of 7 to 12, and the aqueous electrolyte solution contains at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions. Hereinafter, the water electrolysis apparatus of the present invention will also be referred to as the "water electrolysis apparatus of the present invention."

[0033] In the water electrolysis apparatus of the present invention, the electrolytic cell may further include a diaphragm. Providing a diaphragm in the electrolytic cell divides the interior of the electrolytic cell into a first chamber having an anode and a second chamber having a cathode, thereby preventing the generated hydrogen gas and oxygen gas from mixing. When a diaphragm is provided, a porous ceramic plate such as a biscuit plate, a porous polymer membrane such as a polypropylene film or Zilphon (registered trademark), an ion exchange membrane such as Nafion (registered trademark), or a mixture thereof can be used.

[0034] An aqueous electrolyte solution is stored inside the electrolytic cell.

[0035] The concentration of phosphate ions in the aqueous electrolyte solution is 0.1 to 6.0 mol / kg, preferably 0.1 to 5.0 mol / kg, and more preferably 0.1 to 3.0 mol / kg.

[0036] The phosphate ion is H ２ P.O. ４ － , H.P.O. ４ ２－ , P.O. ４ ３－ Examples include:

[0037] The aqueous electrolyte solution contains at least one anion species selected from the group consisting of borates and carbonates. Specific examples of the at least one anion species selected from the group consisting of borates and carbonates contained in the aqueous electrolyte solution include an anion of boric acid (B(OH) ４－ , B ４ O ７ ２－ ) and the anion of carbonate (HCO ３ － , CO ３ ２－ ) at least one selected from the group consisting of

[0038] With regard to the concentration of the anion species, the higher the concentration, the higher the conductivity and the greater the maximum value of the diffusion constant. Therefore, the optimum value of the concentration of the anion species may vary depending on the operating conditions, but is preferably 0.1 to 8.0 mol / kg, more preferably 0.1 to 6.0 mol / kg, and even more preferably 0.1 to 5.0 mol / kg.

[0039] The cationic species is preferably a lithium, sodium, potassium or cesium cation, more preferably a potassium or cesium cation. The potassium or cesium cation is weakly hydrated compared to sodium etc., and the presence of potassium or cesium cation makes Na ＋ This is more preferable because the hydrogen production activity is higher in the presence of a catalyst than in the presence of a catalyst.

[0040] The aqueous electrolyte solution may contain one type of salt composed of the above-mentioned cation species and anion species, or may contain two or more types of salts.

[0041] Examples of phosphates include KH ２ P.O. ４ , K. ２ HPO ４ , LiH ２ P.O. ４ , Li ２ HPO ４ , NaH ２ P.O. ４ , Na ２ HPO ４ , CsH ２ P.O. ４ , Cs ２HPO ４ Preferably, KH ２ P.O. ４ , K. ２ HPO ４ , CsH ２ P.O. ４ , Cs ２ HPO ４ is.

[0042] In addition, examples of salts composed of the above cation species and anion species include Na ２ B ４ O ７ , K. ２ B ４ O ７ , LiHCO ３ , NaHCO ３ , KHCO ３ , and CsHCO ３ Examples include:

[0043] In addition to the salts composed of the above-mentioned cationic and anionic species, the aqueous electrolyte solution may contain other components that do not adversely affect the method of the present invention, such as sulfates, hydrochlorides, and perchlorates.

[0044] The aqueous electrolyte solution has a non-polar pH, between pH 7 and 12, preferably between pH 9 and 11.

[0045] The anode and cathode are disposed inside the electrolytic cell so as to be in contact with the aqueous electrolyte solution. The anode and cathode are electrically connected to a power source, which will be described later. Oxygen is generated on the anode. Hydrogen is generated on the cathode.

[0046] In the water electrolysis apparatus of the present invention, a conventionally known cathode is used as the cathode. Examples of the material for the cathode include platinum group metals such as Pt, Rh, and Ir, Ni, Fe, Mo, Cu, and alloys thereof. Examples of the cathode form include a flat plate, a mesh, and a film formed by sputtering or the like.

[0047] In the water electrolysis apparatus of the present invention, a conventionally known anode is used as the anode. Examples of the material for the anode include Ni, Ru, Ir, Ti, Sn, Mo, Ta, Nb, V, Fe, Mn, and alloys and oxides thereof. Examples of the form of the anode include a flat plate, a mesh, and a film formed by sputtering or the like.

[0048] In one preferred embodiment of the water electrolysis device of the present invention, the electrode catalyst of the anode (OER) is NiFeO ｘ NiFeO ｘ may be doped with one or more selected from Mn, Ni, Fe, and Cu.

[0049] In one preferred embodiment of the water electrolysis apparatus of the present invention, the electrode catalyst of the cathode (HER) is NiMoCu or MoCu. In another preferred embodiment of the water electrolysis method of the present invention, the electrode catalyst of the cathode (HER) is MoO ｘ or NiMoO ｘ and MoO ｘ , NiMoO ｘ may be doped with one or more selected from the group consisting of Cu (copper), W (tungsten), Ag (silver), Au (gold), In (indium), and Sn (tin).

[0050] The anode and cathode can be formed by supporting the above-mentioned metals, their alloys, or their oxides on a conductive substrate. The conductive substrate can have various shapes such as a plate, a rod, or a mesh. Examples of materials that can be used for the conductive substrate include conventionally known materials such as titanium, aluminum, chromium, nickel, copper, or alloys thereof, and carbon.

[0051] The anode is in contact with the aqueous electrolyte solution, and the cathode is in contact with the aqueous electrolyte solution.

[0052] The power supply is used to apply a predetermined potential difference between the anode and the cathode. The predetermined potential difference is applied between the anode and the cathode using the power supply, and water contained in the electrolyte aqueous solution is electrolyzed. It is desirable to apply a potential difference of 1.2 volts or more and 4.0 volts or less. Examples of the power supply are a potentiostat or a battery.

[0053] The power source may also be a solar cell. When the power source is a solar cell, the solar cell may include, for example, three sets of silicon cells.

[0054] When the water electrolysis apparatus of the present invention is used for operation, it can be carried out at a temperature in the range of 60 to 120°C, preferably 60 to 110°C, and more preferably 80 to 100°C.

[0055] The water electrolysis apparatus of the present invention can also be used to perform water electrolysis under high pressure conditions. In this case, operation is possible even at a temperature of about 120° C. When operating under high pressure conditions, conventionally known materials suitable for high pressures can be used.

[0056] 4. Water Electrolysis Cell Another embodiment of the present invention is a water electrolysis cell comprising an anode gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, and a separator, wherein the electrolyte membrane contains a buffer solution having a pH of 7 to 12, the buffer solution comprising an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 6.0 mol / kg of phosphate ions (hereinafter also referred to as the "water electrolysis cell of the present invention").

[0057] The anode gas diffusion layer, electrolyte membrane, cathode catalyst layer, cathode gas diffusion layer, and separator used in the water electrolysis cell of the present invention may be materials used in conventionally known water electrolysis cells.

[0058] The water electrolysis cell may further include other components selected from known components of water electrolysis cells, such as gaskets and sealing materials.

[0059] For example, the anode gas diffusion layer and the cathode gas diffusion layer can each independently be made of a material that allows fluid to flow through the layer, such as a porous body, a powder sintered body, a fiber sintered body, a metal mesh, or felt.

[0060] The anode gas diffusion layer may be coated with a corrosion-resistant conductive material to prevent the resistance from increasing due to oxidation, such as platinum, gold, silver, titanium nitride, titanium carbide, or titanium carbonitride.

[0061] The electrolyte membrane used in the water electrolysis cell of the present invention contains a buffer solution having a pH of 7 to 12. The buffer solution is composed of an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and at least one anion species selected from the group consisting of borates and carbonates. The electrolyte solution contains 0.1 to 6.0 mol / kg, preferably 0.1 to 5.0 mol / kg, and more preferably 0.1 to 3.0 mol / kg of phosphate ions. Details of the buffer solution and electrolyte solution (including cation species, anion species, phosphate ions, etc.) contained in the electrolyte membrane used in the water electrolysis cell of the present invention are as described in detail in the water electrolysis method of the present invention. The water electrolysis cell of the present invention uses a buffer solution having a pH of 7 to 12, which may enable the stable use of inexpensive iron as a component of the electrolysis cell.

[0062] The electrolyte membrane used in the water electrolysis cell of the present invention can be obtained by, for example, impregnating the above-mentioned buffer solution into a known electrolyte membrane (which may be an ion exchange membrane) used in water electrolysis.

[0063] The electrolyte membrane may be selected from known electrolyte membranes (which may be ion exchange membranes) used in water electrolysis. The electrolyte membrane preferably has the property of selectively permeating buffer ions and suppressing crossover of the produced gas. Examples of the electrolyte membrane include a polymer electrolyte membrane (PEM) and a gas diaphragm. Examples of the gas diaphragm include a porous polymer thin film such as polysulfone, and examples of the polymer electrolyte membrane include a perfluorocarbon membrane having sulfonic acid groups. Examples of the perfluorocarbon membrane having sulfonic acid groups include a Nafion membrane.

[0064] The electrolyte membrane is a polymer having proton conductivity due to the presence of ionic groups, and may be, for example, either a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte.

[0065] Here, the fluorine-based polymer electrolyte refers to a polymer in which most or all of the hydrogen atoms in the alkyl and / or alkylene groups in the polymer have been substituted with fluorine atoms. Representative examples of fluorine-based polymer electrolytes having ionic groups include commercially available products such as Nafion (registered trademark) (manufactured by Chemours Inc.), Aquivion (registered trademark) (manufactured by Solvay), Flemion (registered trademark) (manufactured by AGC Inc.), and Aciplex (registered trademark) (manufactured by Asahi Kasei Corporation).

[0066] The hydrocarbon electrolyte is preferably an aromatic hydrocarbon polymer having an aromatic ring in the main chain. Here, the aromatic ring may include not only a hydrocarbon aromatic ring consisting only of carbon atoms and hydrogen atoms, such as a benzene ring or a naphthalene skeleton, but also a heterocycle such as a pyridine ring, an imidazole ring, or a thiol ring. In addition, the polymer may also include aliphatic units in addition to the aromatic ring units.

[0067] Specific examples of aromatic hydrocarbon polymers include polymers having a structure selected from polysulfone, polyethersulfone, polyphenylene oxide, polyarylene ether, polyphenylene sulfide, polyphenylene sulfide sulfone, polyparaphenylene, polyarylene, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, polyether phosphine oxide, polybenzoxazole, polybenzothiazole, polybenzimidazole, polyamide, polyimide, polyetherimide, and polyimide sulfone in the main chain together with an aromatic ring. Note that the terms "polysulfone," "polyethersulfone," and "polyether ketone" are generic terms for structures having a sulfone bond, an ether bond, a ketone bond, or the like in the molecular chain, and include polyether ketone ketone, polyether ether ketone, polyether ether ketone ketone, polyether ketone ether ketone ketone, and polyether ketone sulfone. The aromatic hydrocarbon polymer may have a plurality of these structures. Among these, polymers having a polyether ketone skeleton, that is, polyether ketone polymers, are particularly preferred as aromatic hydrocarbon polymers.

[0068] The electrolyte membrane may be combined with a reinforcing material, which can reduce the risk of gas leakage and short circuits within the electrodes due to membrane damage when the electrolyte membrane and electrodes are joined by hot pressing, for example.

[0069] Specific examples of reinforcing materials include homogeneous porous films made of fluorine-based polymers such as PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), PVDF (polyvinylidene fluoride), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer), or thermoplastic resins such as PE (polyethylene) and PP (polypropylene), and engineering plastics such as PI (polyimide), PSF (polysulfone), PES (polyethersulfone), PEEK (polyetheretherketone), PPSS (polyphenylene sulfide sulfone), PPO (polyphenylene oxide), PEK (polyetherketone), PBI (polybenzimidazole), PPS (polyphenylene sulfide), PPP (polyparaphenylene), PPQ (polyphenylquinoxaline), polybenzoxazole (PBO), polybenzothiazole (PBT), and polyparaphenylene terephthalamide (PPTA).

[0070] The cathode catalyst in the cathode catalyst layer may be selected from known catalysts used in water electrolysis. Examples of catalyst components include platinum group metals such as Pt, Rh, and Ir, Ni, Fe, Mo, Cu, and alloys thereof. The cathode catalyst may include a catalyst supported on a carrier. Examples of the carrier include carbon black. The cathode catalyst layer may be in the form of a flat plate, a mesh, or a film formed by sputtering or the like.

[0071] The anode catalyst in the anode catalyst layer may be selected from known catalysts used in water electrolysis. Examples of catalyst components include Ni, Ru, Ir, Ti, Sn, Mo, Ta, Nb, V, Fe, Mn, and alloys and oxides thereof. The anode catalyst layer may be in the form of a flat plate, a mesh, a film formed by sputtering, or the like.

[0072] In one preferred embodiment of the water electrolysis device of the present invention, the electrode catalyst of the anode (OER) is NiFeO ｘ NiFeO ｘ may be doped with one or more selected from Mn, Ni, Fe, and Cu.

[0073] In one preferred embodiment of the water electrolysis apparatus of the present invention, the electrode catalyst of the cathode (HER) is NiMoCu or MoCu. In another preferred embodiment of the water electrolysis method of the present invention, the electrode catalyst of the cathode (HER) is MoO ｘ or NiMoO ｘ and MoO ｘ , NiMoO ｘ may be doped with one or more selected from the group consisting of Cu (copper), W (tungsten), Ag (silver), Au (gold), In (indium), and Sn (tin).

[0074] The separator may be an anode separator disposed on the anode gas diffusion layer side, or a cathode separator disposed on the cathode gas diffusion layer side. Examples of separator materials include titanium, stainless steel, and carbon. From the viewpoint of suppressing oxidation due to oxygen generated on the anode side, the anode separator preferably contains titanium.

[0075] The anode separator may be coated with a corrosion-resistant conductive material to prevent high resistance due to oxidation, such as platinum, gold, silver, titanium nitride, titanium carbide, or titanium carbonitride.

[0076] The arrangement of each component in the water electrolysis cell may be determined with reference to known water electrolysis cells. In the water electrolysis cell, the electrolyte membrane is preferably located between the anode catalyst layer and the cathode catalyst layer. In the water electrolysis cell, the electrolyte membrane, the anode catalyst layer, and the cathode catalyst layer are preferably located between the anode gas diffusion layer and the cathode gas diffusion layer. In the water electrolysis cell, the electrolyte membrane, the anode catalyst layer, the cathode catalyst layer, and the anode gas diffusion layer and the cathode gas diffusion layer are preferably located between two separators.

[0077] Another embodiment of the present invention is a water electrolysis cell stack in which a plurality of water electrolysis cells of the present invention are stacked (hereinafter also referred to as the "cell stack of the present invention").

[0078] The cell stack of the present invention may be a device including the water electrolysis cell of the present invention and other components. The other components may be selected from components of known water electrolysis devices. Examples of the other components include auxiliary devices such as a power conditioner, a water pump, an ion exchange resin, a heat exchanger, and a dehumidifier.

[0079] The present invention will be described below using examples, but the present invention is not limited to these examples.

[0080] Materials and Chemicals Ni foam, Ni plate, and Pt wire were purchased from Nilaco Corporation. Ni felt was purchased from Nikko Techno Co., Ltd. Carbon paper 060 was purchased from Fuel Cell Store. KCl saturated Ag / AgCl and Hg / HgO (1 M NaOH) were purchased from BAS Inc. Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich with the following purity: NaOH (99.99%), KOH (99.99%), boric acid (≥99.5%), phosphoric acid (≥85 wt%), potassium sulfate (≥99.0%), HCl (35-37%, Fujifilm Wako), HNO ３(≒60%, Fujifilm Wako), KBr (>99%, PIKE Technologies), KI (>99.5%, TCI), KCl (>99%), NaCl (≥99%), EtOH (≥99.5%), i-PrOH (99.7%, Fujifilm Wako), K ４ [Fe(CN) ６ ]・3H ２ O (>98.5%), formic acid (≥98.0%), NaClO (available chlorine >5%, Fujifilm Wako), Ni(NO ３ ) ２ ・6H ２ O (98%, Fujifilm Wako), Fe(NO ３ ) ３ ・9H ２ O (99.9%, Fujifilm Wako), CO(NH ２ ) ２ (>99%), Ni standard solution (Ni 1000 ppm, Fujifilm Wako), and Fe standard solution (Fe 1000 ppm, Fujifilm Wako) were used. N,N-diethyl-p-phenylenediamine (DPD) reagent was purchased from Kazusa Corporation. Ar (99.9999%) and O ２ (99.99995%) was used. Ultrapure water was used to prepare all the aqueous solutions.

[0081] Preparation of electrolyte First, the desired molar concentration (mol kg －１ The pH of the solution was then adjusted by adding KOH pellets until the target pH was reached.

[0082] Electrode preparation: NiFeO ｘ / Ni substrate and carbon paper 569.9 mg of Ni(NO) was dissolved in 152 mL of Milli-Q water. ３ ) ２ ・6H ２ O, 759.5 mg of Fe(NO ３ ) ３ ・9H ２ O, 570.6 mg CO(NH ２ ) ２The precursor solution was prepared by mixing the above and stirring at room temperature. The solution was then transferred to a 190 mL Teflon-lined stainless steel autoclave, and the nickel substrate was washed with EtOH. For deposition on Ni substrates, the substrate was washed with neat HCl until bubbles emerged from the surface. For deposition on carbon paper, the substrate was washed with neat HNO. ３ The sealed autoclave was then transferred to an oven and heated at 393 K for 12 hours. After hydrothermal synthesis, the prepared electrode was rinsed with a large amount of water and dried in the air. ｘ / The double layer capacitance (C) calculated by CV of Ni ｄｌ ) value is NiFeO ｘ / Ni foam, NiFeO ｘ / Ni felt and NiFeOx / Ni plate were 0.876, 0.166, and 0.091 mFcm, respectively. －２ It was NiFeO ｘ The / Ni foam was used to compare the OER performance in various electrolytes, while other foams were used for specific property characterization (e.g., XPS and cross-sectional SEM).

[0083] Electrochemical measurements were performed using a three-electrode system. A Pt wire was used as the counter electrode. Ag / AgCl (saturated KCl) and Hg / HgO (1 M NaOH) reference electrodes were used as reference electrodes at mild and alkaline pH, respectively. During the measurements, the designated gases were: O for OER; ２ , Faraday efficiency and C ｄｌ Ar was supplied to the cell for measurements. All electrochemical measurements were performed using a 16-channel research potentiostat system (VMP3; Biologic Science Instruments). The reference electrode was maintained at 25 °C using a long jacket (BS Corporation). For high-temperature experiments, a water-jacketed glass cell (BS Corporation) was heated with warm water. All working electrodes were sealed with PTFE tape to ensure effective geometric electrode area. All current-potential relationships described herein were iR-corrected using measured impedance values ​​(≥100 kHz, amplitude 10 mV) unless otherwise specified.

[0084] Faraday efficiency measurement２ A single-chamber sealed cell was used for the Faraday efficiency. Ar gas was supplied unidirectionally as the carrier gas. The cell was connected to a gas chromatograph (GC-8A; Shimadzu Corporation) equipped with a thermal conductivity detector (TCD) and a molecular sieve 5A column. ２ The Faraday efficiency of the measured O ２ The calculation was based on the production rate (mol). The faradaic efficiency of hypochlorite was determined by taking 0.1 mL of the spent electrolyte and diluting it to 10 mL with water. N,N-diethyl-p-phenylenediamine (DPD) powder was then introduced. The UV-Vis spectrum was recorded within 3 minutes. The amount of hypochlorite was calculated based on the recorded intensity with reference to a calibration curve.

[0085] Evaluation: X-ray photoelectron spectroscopy (XPS) was performed using a JEOL JPS-9030 microscope with a Mg anode at 300 W, 12 kV, and 25 mA. Data were calibrated using 284.6 eV C1s as an internal standard. Inductively coupled plasma (ICP) measurements were performed using a Thermo iCAP PRO series microscope. Ultraviolet-visible absorption spectroscopy (UV-Vis) spectra were recorded using a JASCO V-770 microscope. Solution viscosity was evaluated using an Anton Paar SVM 2001 microscope. Scanning electron microscopy (SEM) and elemental beam scanning images were taken using a JEOL JSM IT-800 microscope equipped with an ULTIMMAX microscope. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2010HC microscope. Operandra manometry was carried out using a JASCO RMP-510 equipped with a CCD camera.

[0086] Example 1: Investigation of OER performance in borate / phosphate mixed buffer solution (1) Contribution of phosphate additive to OER performance in borate buffer solution. Ni-Fe(hydr) oxide (NiFeO) was deposited on a conductive Ni foam substrate using a hydrothermal method. ｘ ) was precipitated. Figure 1a shows the 1.0 mol kg －１ K-borate alone, 1.0 mol kg －１The OER performance of K-phosphate alone and K-borate / 1.0 phosphate mixed electrolytes is shown. At a specific pH of 9.2, the OER performance of the borate-only electrolyte is superior to that of the phosphate-only electrolyte due to its higher buffer capacity than the phosphate-only electrolyte, demonstrating that utilizing an appropriate pKa is important for efficient OER. This trend in concentration overpotential versus the identity of the electrolyte pKa is consistent with the IrO ｘ and NiFeO ｘ In the mixed buffer electrolyte, the current density (j) region (31 mA cm) was significantly higher than that in the mixed buffer electrolyte. －２ The potentials are in good agreement with those for the borate-only condition, indicating that the presence of phosphate does not significantly degrade performance. The double layer capacitance (C ｄｌ ) showed comparable values ​​even in the presence of phosphate. This also indicates that phosphate does not affect the OER performance. In the high j region, the potential reached lower values ​​than in the borate-only system. Figure 1b shows the monotonic potential decrease with increasing phosphate molar concentration at fixed j. Figure 1c shows the potential of NiFeO in two electrolytes. ｘ The observed redox peaks are mainly Ni ２＋ From Ni ３＋ The peaks due to Ni redox species up to 10 ... ２＋ is Ni(OH) ２ It has been shown that phosphate forms a more stable complex than phosphate. This phenomenon can be explained by the thermodynamic stability of the complex based on the solubility product.

[0087] (2) OER performance in chloride ions containing borate / phosphate mixed buffer solution. Next, NiFeO ｘ / NF OER performance － Figure 2 shows the results of NiFeO in borate only and borate / phosphate mixed electrolytes. ｘ The CV profile of Cl －In the presence of NiFeO ｘ is borate + Cl － It underwent significant electrochemical corrosion in the electrolyte. This deterioration was attributed to Ni(OH) ２ This is caused by the formation of a green solid, as has been previously shown in previous studies, including our report (H. Komiya, T. Shinagawa and K. Takanabe, ChemSusChem, 2022, 15, e202201088.). Interestingly, in electrolytes containing phosphate, Cl － The CVs were in good agreement regardless of the presence or absence of Cl － (No profile shown.) This improvement in CV profile stability was observed only when phosphate was present in the electrolyte.

[0088] FIG. 2 shows NiFeO in an electrolyte containing halide ions. ｘ The OER performance for KCl is shown, and the CV profiles of K-borate and K-borate / phosphate electrolytes containing KCl at 298 K and pH 9.2 are shown. The molar concentration of K-borate is 1.0 mol kg －１ , the molar concentration of K-borate / phosphate is 1.0 mol kg －１ K-borate + 1.0 mol kg －１ K-phosphate, molar concentration of halide salt is 0.5 mol kg －１ All CVs were 1 mVs －１ The recording was performed at a scan rate of 100 s.

[0089] Figure 3 shows in detail the CV profiles recorded by exchanging the electrolyte. ｘ was activated in a K-borate / phosphate mixed buffer solution, and the CV was shown after activation. After that, the electrode was washed with water and then washed with phosphate-free Cl － The electrode was immersed in a borate electrolyte containing Cl. CV was then recorded. The initial anodic current was very small. －Although the CV showed corrosion, it was unstable from the first cathodic scan. These results suggest that a kind of Ni passivation layer was formed in situ on the electrode surface or that phosphate acted as a charge-repelling layer, revealing that phosphate in the electrolyte plays an important role in protecting the electrode. Figure 4 shows the NiFeO in a borate / phosphate + KCl mixed buffer solution. ｘ The results of a long-term stability test of 1-day 298K solution of 1 / NF were shown in Table 1. The potential, CV, and metal dissolution amount were in good agreement with the values ​​measured without KCl. － It was confirmed that [Ni(H ２ O) ５ Cl] ＋ Considering the formation constant of Ni-phosphate, Ni ions tend to form Ni-phosphate complexes. － Although Cl is present on the electrode surface, － It was confirmed by XPS that does not corrode the in-situ formed Ni passivated electrode.

[0090] Next, Cl － Counter cation of K ＋ From Na ＋ The OER performance was not affected by changing the NiFeO ｘ The OER performance for Na / K-Cl / NF is shown. The potential at different j as a function of the molar concentration of added phosphate is shown. The molar concentrations of K-borate and Na / K-Cl are 1.0 mol kg －１ and 0.5 mol kg －１ The potential was recorded using steady-state CP. All electrolytes were O ２ and fixed at pH 9.2 and 298K.

[0091] We also tried adding sulfate instead of phosphate, which is the main phosphate species in this study, HPO ４ ２－ Similarly, Cl － This is because a charge repulsion effect that blocks the charge is expected (Fig. 6a). － In a borate electrolyte containing NiFeOｘ The CV profile and photograph after CV using / NF are shown. A sharp corrosion current was observed, and the electrode formed a green solid. This unstable trend can be explained by the fact that the solubility product of Ni-sulfate is higher than that of phosphate, suggesting that the interaction between Ni and sulfate is weak.

[0092] Figure 6 shows the relationship between charge repulsion ability and NiFeO ｘ Figure 6(a) shows a charge repulsion capability diagram, where the X-axis represents the ionic potential, which is the ionic charge (Z) divided by the Stokes radius (r), and the Y-axis is Z × r. Figure 6(b) shows the NiFeO ｘ / 1.0 mol kg of NF －１ K-borate + 0.5 mol kg －１ K-sulfate solution (pH 9.2, 298K, 0.5 molkg －１ The CV profiles are shown for the samples with and without KCl. The scan rate was 1 mV / s. －１ The photo shows NiFeO ｘ / NF to Cl － The reaction was carried out in the presence of

[0093] Since previous literature (T. Ma et al., Angew. Chem., Int. Ed., 2021, 60, 22740-22744) has claimed that the improved stability is due to a charge-repulsion layer formed on the surface in alkaline solutions containing sulfate, we next investigated the charge-repulsion theory at non-extreme pH levels by adding a redox probe.

[0094] (3) Borate / phosphate mixed buffer with various redox probes. To investigate the various effects of phosphate additives on selectivity and stability, OER performance was also evaluated in the presence of various redox probes. Figures 7 and 8 show the effect of bromide ion (Br － ) and iodine ion (I － ) are shown, respectively. For both ions, after applying a positive potential to the borate-only electrolyte, a green solid is generated to form NiFeO. ｘ On the other hand, stable CV was observed in the borate / phosphate mixed electrolyte, which is believed to be due to the formation of a Ni passivation layer.

[0095] 7 and 8 show NiFeO in an electrolyte containing halide ions. ｘ The OER performance for KBr (Fig. 7), KI (Fig. 8) and K-borate / phosphate electrolytes containing KBr and KI at 298 K and pH 9.2 are shown. The molar concentration of K-borate is 1.0 mol kg －１ , the molar concentration of K-borate / phosphate is 1.0 mol kg －１ K-borate + 1.0 mol kg －１ K-phosphate, molar concentration of halide salt is 0.5 mol kg －１ All CVs were 1 mVs －１ The recording was performed at a scan rate of 100 s.

[0096] Table 1 below shows the FE recorded for various halide ions. Ｏ２ The following has been summarized.

[0097]

[0098] Table 1 shows the O in various electrolytes supplemented with halide ions. ２ (FE Ｏ２ The table shows the trend of the Faraday efficiency (FE) of the ZnSe film and its Stokes radius (a / Å). FE measurements were performed at 50 mA cm unless otherwise specified in the table. －２ The CP method was used, and the FE value was calculated based on the product gas detected by gas chromatography (GC).

[0099] The potential distribution during FE measurement was stable regardless of the type of halide ion. － and I － In the presence of Cl, the charge was below the detection limit of the gas chromatograph, suggesting that most of the charge was consumed in the oxidation of halide ions. － In the presence of FE Ｏ２ was almost 1. This difference in selectivity was explained by the redox potential. － Assuming that the reaction proceeds with the generation of (X = Cl, Br, I), Br － Oxidation reaction (1.59V ＲＨＥ ) and the redox potential (E０ ) is FE Ｏ２ The potential is lower than that at the time of measurement, and the chloride ions are preferentially oxidized. ＲＨＥ ) E ０ Only FE Ｏ２ 50mAcm at the time of measurement －２ The measured potential of Br is slightly lower than that of Br, which partially induces the HCFR. A comparison of these halides reveals the primary function of phosphate under extreme pH conditions. If phosphate acts as a charge repulsion layer at the surface, then Br － and I － is not oxidized considering the chemical similarity such as Stokes radius (Table 1) and ionic charge (-1). However, FE Ｏ２ is very low, indicating that halides adsorb onto the catalyst surface and inner-sphere charge transfer occurs, indicating that the primary role of phosphate is to stabilize Ni sites rather than to act as a charge-repulsion layer.

[0100] Figure 9 shows the results of commercial NiFeO in different electrolytes at different current densities. ｘ The results of the stability test are shown in Fig. 9(a). －２ The multi-on-off stability test at a high temperature of 353 K was performed in the presence and absence of Cl-. The molar concentration of each electrolyte was 1.5 mol kg －１ K-boric acid + 0.5 molkg －１ KCl and 1.0 mol kg －１ K-boric acid + 1.0 molkg －１ K-phosphate (i.e., K-borate / phosphate + 0.5 mol kg-1 KCl. The test consisted of six on / off cycles (1 cycle: 500 mA cm －２ 1.5 hours CP and 5 minutes open circuit (OC) at Cl － In K-borate containing electrolytes, Cl － A rapid potential overshoot was observed due to Ni corrosion caused by Cl. － The potential was very stable even in the presence of additives. － The CVs after stability testing in the presence and absence of benzodiazepine are shown. All CVs are within 1 mVs.－１ All OER measurements were recorded at a scan rate of 0. ２ The analysis was carried out at pH 9.2 using bubbling. The CVs were nearly overlapping, and Cl － It was confirmed that there was no significant corrosion due to erosion.

[0101] From these data, Cl － In the presence of NiFeO ｘ It has been confirmed that adding phosphate ions to the electrolyte is very effective in maintaining the OER performance of / NF.

[0102] Example 2 Improvement of HER by Addition of Phosphate Next, the present inventors investigated the improvement of HER (hydrogen evolution reaction) by adding phosphate anions to a dense K-carbonate electrolyte (pH 10.5) using a copper molybdenum electrocatalyst.

[0103] Electrode preparation: MoO ｘ (Cu), NiMoO ｘ (Cu), NiMoO ｘ The electrocatalysts were prepared by co-electrodeposition of Cu, Mo, and Ni metal species onto Ni foam or Ni plate substrates. ｘ The precipitation bath for (Cu) was 0.06M CuSO ４ -5H ２ 0, 0.2 M Na ２ MoO ４ -2H ２ O, 0.35M C ６ H ５ Na ３ O ７ -2H ２ O, pH is NH ４ OH was added to adjust the pH to 10.5. ｘ (Cu) bath is MoO ｘ (Cu) bath with 0.3M NiSO ４ -6H ２ O was added. ｘ The bath was 0.06 M CuSO ４ -5H ２ NiMoO containing no O ｘ Each electrode catalyst was placed in a copper (Cu) bath at −160 mA cm －２The deposition was carried out on the substrate by applying a constant current density of 1000 kJ / cm2 at room temperature for 1 hour.

[0104] Figure 10a shows pH 10.5, 3.0 mol kg －１ MoO in K-carbonate electrolyte ｘ Figure 10a shows the cyclic voltammetry (CV) profile at a (Cu) / Ni foam electrode. Specifically, Figure 10a shows the cyclic voltammetry (CV) profile at a 3.0 mol kg －１ K-carbonate solution and 0.1, 0.3, or 0.5 mol kg －１ 3.0 mol kg containing K-phosphate －１ 5 mVs in K-carbonate solution －１ In our previous work, we found that this non-precious metal catalyst exhibited HER in carbonate solutions comparable to benchmark catalysts under highly alkaline conditions. While HER occurs at the Mo oxide sites, the Cu oxide sites uniquely maintain this less extreme pH, resulting in the formation of HCO ３ - aids in the dissociation of the proton source. Addition of phosphate shifted the onset potential of HER to lower potentials while maintaining the redox peak behavior at positive potentials. This onset potential was 0.3 mol kg －１ The results showed that there was a sufficient amount of phosphate anions in the electrolyte.

[0105] Figure 10b shows 0.5 mol kg －１ 3.0 mol kg with and without the addition of K-phosphate －１ These data compare the steady-state Tafel plots for K carbonate electrolytes at pH 10.5 and 353 K. ｘ The HER2 improvement by adding phosphate was observed at several hundred mAcm. ―２ It can also be seen that the overvoltage is 168±4 mV and -1 A cm －２The overpotential was reduced by approximately 30 mV compared to when no phosphate was added. This overpotential was comparable to that of a previously reported extremely alkaline HER (T. Nishimoto, K. Obata, H. Komiya, T. Naito, K. Harada, M. Yoshida, K. Takanabe, ACS Catal. 2023, 13, 14725-14736), demonstrating sufficient performance for practical use.

[0106] MoO ｘ (Cu) and NiMoO ｘ The catalysts with different carbonate / phosphate concentrations (1.5 or 3.0 mol kg －１ / 0.5 or 1.5 mol kg －１ Tafel plots were also obtained for different combinations of phosphate and phosphate (Figure 11). In all cases, phosphate addition showed similar HER improvement and a decrease in the Tafel slope. From these results, (1) a phosphate concentration of 0.5 mol kg －１ It was revealed that (1) the HER performance improved with increasing the amount of Ni in the catalyst, and (2) the Ni element in the catalyst was not necessary to cause the phosphate effect of HER improvement.

[0107] (2) Cation Contribution To gain insight into the contribution of cations, HER measurements were performed in Na-, K-, and Cs-carbonate electrolytes. The CV profiles in Figure 12(a) show almost the same current density in K- and Cs-carbonate electrolytes, but the HER onset potential was more negative in Na-carbonate electrolyte. As the magnitude of the current density increased, Cs-carbonate showed a lower overpotential than K-carbonate, but at -100 mA cm －２ The same overpotential and apparent Tafel slope were observed at the following current densities (Fig. 12(b)). This observation is similar to that in Fig. 10 and illustrates the effect of electrolyte cations on the HER.

[0108] The data in Figures 12(a) and (b) show that the 1.5 mol kg －１ MoO in Na-carbonate, K-carbonate, and Cs-carbonate solutions ｘThe data for K-carbonate were obtained on (Cu) / Ni foams. The data for K-carbonate were taken from the aforementioned paper (T. Nishimoto, K. Obata, H. Komiya, T. Naito, K. Harada, M. Yoshida, K. Takanabe, ACS Catal. 2023, 13, 14725-14736).

[0109] (3) Investigation of other anions To confirm whether the improvement in HER performance is specific to the phosphate anion, similar electrochemical tests were performed on other anions. －１ K-carbonate electrolyte and 0.5 mol kg －１ The CV profiles of K-phosphate, K-borate, and K-sulfate were compared. An increase in cathodic current density was observed only with the addition of phosphate. This trend persisted even at high current densities, as shown in Figure 13b, indicating that phosphate anions are necessary for improved HER performance.

[0110] The data in Figures 13a and b show that at pH 10.5 and 353 K, 1.5 mol kg －１ K-carbonate, and 0.5 mol kg －１ 1.5 mol kg containing K-phosphate, K-borate, and K-sulfate －１ MoO in K-carbonate ｘ Data obtained on (Cu) / Ni foam for K-carbonate without other anions are taken from the aforementioned paper (T. Nishimoto, K. Obata, H. Komiya, T. Naito, K. Harada, M. Yoshida, K. Takanabe, ACS Catal. 2023, 13, 14725-14736).

[0111] Cation accumulation near the electrode surface is HCO ３－ When phosphate anions are adsorbed on the electrode surface, they interact with weakly hydrated cations, forming HCO3 at the reaction interface. ３－In fact, it has been reported that under weakly alkaline conditions of pH 13 or less, phosphate anions are adsorbed onto a copper electrode even at negative potentials on the reversible hydrogen electrode (RHE) scale.

[0112] Therefore, to confirm that phosphate anions are adsorbed under HER conditions, we performed a phosphate-added K-carbonate electrolyte test using MoO ｘ The (Cu) surface was analyzed by operandraman spectroscopy (Fig. 14). To obtain a clear Raman signal, MoO ｘ (Cu) was deposited on a Ni plate, and the electrode was subjected to the HER process in the same electrolyte before Raman analysis. The chronoamperometry (CA) profile during Raman analysis showed a reversible cathodic current density, and MoO ｘ It was confirmed that Raman signals were recorded under HER conditions without significant degradation of MoO (Cu). ｘ The operandraman spectrum on the (Cu) electrode is shown in Figure 14a, and the operandraman spectrum in the bulk electrolyte is shown in Figure 14b. －１ Peak and 1370cm －１ The shoulder peak at 930 cm is observed only at the electrode and is attributed to the electrode surface species. －１ The peak of ＊ P.O. ４ ２－ It is derived from the P-O bond of the anion and has a peak at 1370 cm －１ The peak is Cu ２ CO ３ (OH) ２ It is related to the carbonate-center mode of 930 cm －１ The peak was also observed when only the pH 14 electrolyte was measured. ４ ３－ is dominant, so it is likely to be derived from that.

[0113] These results suggest that phosphate anions are adsorbed in the initial HER process and remain on MoO during Raman analysis and high-current HER. ｘ This suggests that some of the (Cu) remains on the surface. ＊ P.O. ４ ２－The negative charge on the cations creates more hydrated cations and HCO than without it. ３－ Attracts HCO ３－ It helps to accumulate reactants. From the above, the phosphate adsorbent ( ＊ P.O. ４ ２－ ) is a cation and HCO in HER ３－ This may act as an attractant for ions and increase the ionic strength at the electrode / electrolyte interface.

Claims

1. A method for electrolyzing water in a buffer solution with a pH of 9 to 12, The buffer solution comprises an electrolyte solution containing at least one cationic species selected from the group consisting of alkali metal cations, and at least one anionic species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 3.0 mol / kg of phosphate ions. The method wherein the concentration of the anion species is 0.1 to 8.0 mol / kg.

2. The method according to claim 1, wherein the cation species is a lithium, sodium, potassium, or cesium cation.

3. A system for electrolyzing water in a buffer solution with a pH of 9 to 12, The buffer solution comprises an electrolyte solution containing at least one cationic species selected from the group consisting of alkali metal cations, and at least one anionic species selected from the group consisting of borates and carbonates, and the electrolyte solution contains 0.1 to 3.0 mol / kg of phosphate ions. The system wherein the concentration of the anion species is 0.1 to 8.0 mol / kg.

4. A water electrolysis cell comprising an anode gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, and a separator, The electrolyte membrane contains a buffer solution with a pH of 9 to 12. The buffer solution comprises an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations, and at least one anion species selected from the group consisting of borates and carbonates. The electrolyte solution contains 0.1 to 3.0 mol / kg of phosphate ions. The water electrolysis cell wherein the concentration of the aforementioned anion species is 0.1 to 8.0 mol / kg.

5. A water electrolysis cell stack comprising water electrolysis cells according to claim 4.