Electrode and water electrolysis cell, and method for manufacturing the same

Abstract

To provide an electrode capable of achieving high energy conversion efficiency without using expensive elements such as platinum group elements or nickel.SOLUTION: A catalyst layer comprises FexS in the amount of 99.999 mol% or more, where x is greater than 1.00 and less than 1.20, and the catalyst layer has a thickness in the range of 5 to 100 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode and a water electrolysis cell, as well as methods for producing the same. [Background technology]

[0002] In response to growing societal demands for reducing CO2 emissions, interest has recently been turning to the use of hydrogen and methods for producing it.

[0003] Conventionally, hydrogen has been produced by reforming underground resources, but this method of producing hydrogen does not substantially reduce CO2 emissions.

[0004] Therefore, as a cleaner method for producing hydrogen, attention has been drawn to a method of producing hydrogen by electrolyzing water using electricity obtained by wind power generation, solar power generation, or the like, i.e., a method of producing hydrogen by water electrolysis.

[0005] One method for producing hydrogen by water electrolysis is to electrolyze water using a cell (hereinafter also referred to as a water electrolysis cell) in which cations (particularly hydrogen ions) move across a solid polymer electrolyte membrane (ion exchange membrane) that separates a cathode electrode (hereinafter also referred to as a cathode), which is a hydrogen generating electrode, from an anode electrode (hereinafter also referred to as anode), which is an oxygen generating electrode.

[0006] In the above method, a platinum group element is generally used as a catalyst for the cathode.

[0007] For example, Patent Document 1 states: "A water electrolysis cell comprising a solid polymer electrolyte made of a fluororesin-based cation exchange membrane, catalytic electrodes (anode and cathode) provided on both sides of the solid polymer electrolyte, and a power supply body made of a sintered body of metal powder with platinum plating on the surface thereof or a sintered body of metal powder with platinum plating on the surface thereof, (1) The anode of the catalytic electrode has a polymer electrolyte layer. a mixture of inorganic powder whose surface has been previously plated with a platinum group metal and a polymer electrolyte; a mixture of an inorganic powder whose surface has been previously plated with a platinum group metal, an oxide of a platinum group metal, and a polymer electrolyte; or a mixture of an oxide of a platinum group metal and a polymer electrolyte formed on a mixture of an inorganic powder and a polymer electrolyte, the surface of which has been previously plated with a platinum group metal; and (2) The cathode of the catalytic electrode has a polymer electrolyte layer. a mixture of inorganic powder whose surface has been previously plated with a platinum group metal and a polymer electrolyte; a mixture of an inorganic powder whose surface has been previously plated with a platinum group metal, a platinum group metal, and a polymer electrolyte; or a mixture of a platinum group metal and a polymer electrolyte formed on a mixture of an inorganic powder and a polymer electrolyte, the surface of which has been previously plated with a platinum group metal; A water electrolysis cell characterized by being formed from each of the following: has been disclosed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-241880 [Patent Document 2] Patent Publication No. 2021-046585

[0009] Platinum group elements have high catalytic activity for hydrogen generation. However, the use of extremely expensive platinum group elements increases the installation costs of water electrolysis equipment. This is one of the reasons that has hindered the widespread use of hydrogen production methods through water electrolysis. Therefore, it is desirable to minimize the amount of platinum group elements used in water electrolysis equipment, and in particular, to avoid using platinum group elements in the cathode, which is the hydrogen generation electrode.

[0010] In this regard, Patent Document 2 states: "An electrode for water electrolysis characterized by the use of nickel-carbon nano-onion (Ni-CNO) catalyst, which is made by coating the surface of nickel particles with graphene." has been disclosed.

[0011] However, the water electrolysis electrode disclosed in Patent Document 2 does not use any platinum group elements, but does use nickel, which is an expensive element.

[0012] Therefore, there is a need for the development of low-cost electrodes that do not use expensive elements such as platinum group elements or nickel and that can achieve high energy conversion efficiency. The energy conversion efficiency referred to here refers to the efficiency of hydrogen generation when used as the cathode of a water electrolysis cell.

[0013] The present invention has been developed in view of the above-mentioned current situation, and aims to provide an electrode that can achieve high energy conversion efficiency without using expensive elements such as platinum group elements and nickel. Another object of the present invention is to provide a water electrolysis cell having the above electrode. A further object of the present invention is to provide a method for producing the above electrode and water electrolysis cell. Summary of the Invention [Means for solving the problem]

[0014] The inventors have conducted various studies to solve the above problems, and have found that iron sulfide called mackinawite, in other words, Fe in which x is greater than 1.00 and less than 1.20, x The present inventors have found that mackinawite S (hereinafter also referred to simply as mackinawite) is extremely effective in solving the above problems, and have completed the present invention.

[0015] That is, the inventors have found that the overvoltage for hydrogen evolution (hereinafter referred to as hydrogen overvoltage) on the surface of mackinawite is equivalent to that of platinum. From the perspective of improving the efficiency of hydrogen generation, i.e., the energy conversion efficiency, the smaller the hydrogen overvoltage, the better. Furthermore, the hydrogen overvoltage is generally measured based on the hydrogen overvoltage of platinum. Platinum is a platinum group element, a group of metal elements that exhibits the smallest known hydrogen overvoltage. It is not possible to determine an absolute value for hydrogen overvoltage. This is because hydrogen overvoltage varies significantly depending on the environment (generally an aqueous solution), temperature, pressure during measurement, and the current density used to measure it. In other words, hydrogen overvoltage is an index used to compare values obtained under the same environment, temperature, pressure, and current density. If these conditions are not the same, comparing these values is meaningless. In other words, hydrogen overvoltage is an index used to compare values obtained under the same specific conditions.

[0016] First, a water electrolysis cell will be described with reference to FIG. 1. In the figure, reference numeral 10 denotes a solid polymer electrolyte membrane, 20 an electrode (catalytic electrode), 30 a power supply, 40 a separator, and 50 a conducting wire. In a water electrolysis cell, the electrodes where the electrolysis reaction occurs are joined to both sides of the solid polymer electrolyte membrane by hot pressing or electroless plating. Power supply elements are formed on the outer surfaces of the electrodes, and separators are arranged to sandwich them. Water is supplied to the surfaces of the electrodes through the power supply elements. Then, on the surface of the anode side electrode (positive electrode), water is decomposed by an electrolytic reaction to produce oxygen gas and hydrogen ions (protons). The oxygen gas diffuses within the power supply elements and is released outside the water electrolysis cell. Meanwhile, the hydrogen ions diffuse within the solid polymer electrolyte membrane toward the cathode side and receive electrons through a reduction electrolytic reaction on the surface of the cathode side electrode (negative electrode). The hydrogen ions then become hydrogen gas, diffuse within the power supply elements, and are released outside the water electrolysis cell.

[0017] In searching for alternative materials to the platinum group elements used as catalysts in the cathodes of water electrolysis cells, the inventors focused on iron sulfide, particularly mackinawite, and conducted polarization tests to compare the hydrogen overvoltage of mackinawite with that of platinum.

[0018] Specifically, under Condition 1, mackinawite was formed on the surface of an iron plate, and this was used to prepare an electrode for the polarization test. Next, a cathodic polarization test was conducted in a 20 mass% ammonium thiocyanate aqueous solution using this electrode as the working electrode and platinum foil as the counter electrode. The distance between the working electrode and the counter electrode was 10 cm, and the potential of the working electrode was measured using a reference electrode (Ag / AgCl (Sat.KCl)). The solution temperature was 70°C. The gas phase in contact with the aqueous solution was degassed with nitrogen for at least 10 minutes before the start of the polarization test. Degassing with nitrogen continued throughout the polarization test. The pressure was atmospheric pressure. The potential sweep rate was 20 mV / min. The solution pH during the polarization test was 5.09 (at 70°C). The equilibrium potential for hydrogen evolution, derived from the solution pH, and the current density of the current flowing through the working electrode during the polarization test were calculated based on the equilibrium potential of the hydrogen evolution calculated from the solution pH and the current density of the current flowing through the working electrode during the polarization test were calculated based on the equilibrium potential of the hydrogen evolution calculated from the solution pH and the current density of the current flowing through the working electrode during the polarization test. -3 A / cm 2 The hydrogen overpotential (hereinafter also referred to as Mackinawite's hydrogen overpotential) was calculated as the difference between the potential of the working electrode (measured at the reference electrode) at time t and the potential of the reference electrode at time t. The equilibrium potential for hydrogen evolution can be calculated from the solution pH using the following equation (1). [Hydrogen evolution equilibrium potential V vs. NHE]=-1.98×10 -4 ×T × [solution pH] ···(1) In formula (1), T: Solution temperature (unit: Kelvin) is.

[0019] The hydrogen overvoltage (hereinafter also referred to as the hydrogen overvoltage of platinum) was also determined under the same conditions as above, except that platinum was used for the working electrode. Note that a plate material with the same surface area as the above-mentioned mackinawite working electrode was used for the platinum working electrode. The hydrogen overvoltage of mackinawite determined above was then compared with the hydrogen overvoltage of platinum.

[0020] As a result, it was found that the difference in hydrogen overvoltage between mackinawite and platinum is less than 0.05 V in absolute value, that is, mackinawite can be considered to have a hydrogen overvoltage equivalent to that of platinum.

[0021] The hydrogen overvoltage of mackinawite and the hydrogen overvoltage of platinum were also measured under the following conditions 2 to 4, which were obtained by changing the type of solution, solution temperature, and solution pH from the above condition 1. As a result, under all conditions, the difference between the hydrogen overvoltage of mackinawite and the hydrogen overvoltage of platinum was less than 0.05 V in absolute value. ·Condition 2 Solution type: 20 mass% ammonium thiocyanate aqueous solution Solution temperature: 30℃ Solution pH:6.2 ·Condition 3 Solution type: Ion-exchanged water Solution temperature: 70℃ Solution pH:6.8 ·Condition 4 Solution type: Ion-exchanged water Solution temperature: 30℃ Solution pH:6.6

[0022] From the above results, the inventors discovered that in an aqueous solution with a pH of 7.0 or less (a solution in which water is used as a solvent and other solutes are mixed), mackinawite has a hydrogen overvoltage equivalent to that of platinum.

[0023] Although the details of the mechanism by which mackinawite has a hydrogen overvoltage equivalent to that of platinum are not entirely clear, the inventors believe that it is as follows. That is, in the process of hydrogen atoms becoming hydrogen gas, the hydrogen atoms undergo a state of chemisorption on the catalyst surface. The stability of the hydrogen atoms in this state is affected by the d orbitals in the valence band on the catalyst surface. In mackinawite, the spacing between d orbitals is presumably the optimum distance for hydrogen atoms to become hydrogen gas, similar to that of platinum. Furthermore, the optimum spacing between d orbitals in mackinawite is thought to be due to its crystal structure.

[0024] Furthermore, because mackinawite is a single compound, the energy conversion efficiency can be further improved by separating the site where the electrolytic reaction occurs (hereinafter also referred to as the electrolytic reaction site) from the site where hydrogen atoms become hydrogen gas (hereinafter also referred to as the hydrogen gas generation site). That is, mackinawite is an n-type semiconductor. Furthermore, hydrogen atoms can permeate through mackinawite. Therefore, for example, in a water electrolysis cell, the side of mackinawite in contact with a solid polymer electrolyte membrane can be used as an electrolytic reaction site, and the side of mackinawite in contact with a power feeder can be used as a hydrogen gas generation site. This significantly improves the energy conversion efficiency of an electrode using mackinawite as a catalyst layer compared with an electrode in which electrolytic reaction sites and hydrogen gas generation sites are dispersed at points on the particle surface as a three-phase interface between reactants, ionomer, and catalyst, as in Patent Document 2. Furthermore, in an electrode using mackinawite as a catalyst layer, the migration path of hydrogen atoms from the electrolytic reaction site to the hydrogen gas generation site is linear, improving the energy conversion efficiency from the viewpoint of mass transfer as well.

[0025] Furthermore, the inventors have also found that a water electrolysis cell can be produced more advantageously by hot-pressing a structure in which an electrode having mackinawite as a catalyst layer is disposed between a current feeder and a solid polymer electrolyte membrane under predetermined conditions, for example, by applying mackinawite to the current feeder, combining it with the solid polymer electrolyte membrane, and hot-pressing the resulting structure under predetermined conditions. The present invention was completed based on the above findings and further investigations.

[0026] That is, the gist and configuration of the present invention are as follows. 1. An electrode for a water electrolysis cell in which cations move through a solid polymer electrolyte membrane that separates the cathode and anode, the electrode has a catalyst layer; The catalyst layer is Fe x S: 99.999 mol% or more, and x is more than 1.00 and less than 1.20; The electrode, wherein the catalyst layer has a thickness of 5 to 100 μm.

[0027] 2. The electrode according to 1 above, which is a cathode of the water electrolysis cell.

[0028] 3. A water electrolysis cell having the electrode described in 1 or 2 above.

[0029] 4. A stirring step of mixing a 0.05 to 0.50 mol / L NaS aqueous solution and a 0.05 to 0.50 mol / L FeSO aqueous solution to prepare a mixed aqueous solution, and stirring the mixed aqueous solution in an inert gas atmosphere for one hour or more; Next, a standing step of standing the mixed aqueous solution in an inert gas atmosphere for one day or more to obtain a precipitate; Then, a recovery step of recovering the precipitate; a drying step of drying the precipitate in a vacuum for one day or more to form an electrode having a catalyst layer; With The catalyst layer is Fe x S: 99.999 mol% or more, and x is more than 1.00 and less than 1.20; The method for producing an electrode, wherein the catalyst layer has a thickness of 5 to 100 μm.

[0030] 5. A method for producing a water electrolysis cell, comprising: placing the electrode according to 1 or 2 above between a power supplier and a solid polymer electrolyte membrane; and pressing the resulting structure in a vacuum under conditions of a pressure of 10 to 20 MPa, a temperature of 70 to 150°C, and a holding time of 10 minutes or longer. [Effects of the Invention]

[0031] According to the present invention, it is possible to provide an electrode that can achieve high energy conversion efficiency without using expensive elements such as platinum group elements or nickel. Furthermore, by applying the electrode of the present invention, particularly to the cathode of a water electrolysis cell, it becomes possible to produce hydrogen in a clean manner and at low cost. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of the structure of a water electrolysis cell. DETAILED DESCRIPTION OF THE INVENTION

[0033] [1] Electrode First, an electrode according to one embodiment of the present invention will be described. The electrode according to one embodiment of the present invention is suitable as an electrode, particularly a cathode, for a water electrolysis cell in which cations move through a solid polymer electrolyte membrane that separates the cathode and anode.

[0034] An electrode according to one embodiment of the present invention comprises a catalyst layer, The catalyst layer is Fe x S: 99.999 mol% or more, and x is more than 1.00 and less than 1.20; The catalyst layer has a thickness of 5 to 100 μm.

[0035] That is, in the electrode according to one embodiment of the present invention, the catalyst layer contains Fe in which x is greater than 1.00 and less than 1.20. x The use of S is extremely important. In the Fe-S binary phase diagram, there are two types of iron sulfides, with the chemical formulas FeS and FeS2. Of these, mackinawite, which belongs to the FeS group, exhibits a hydrogen overvoltage equivalent to that of platinum, as mentioned above. However, other iron sulfides belonging to the FeS group, such as pyrrhotite, troilite, greigite, and smythite, do not exhibit the same characteristics as mackinawite. Furthermore, as mentioned above, mackinawite is an n-type semiconductor, and therefore energy conversion efficiency can be improved by separating the electrolytic reaction site and the hydrogen gas generation site. Furthermore, mackinawite can also improve energy conversion efficiency from the perspective of mass transfer.

[0036] Mackinawite is an ionic compound, a type of iron sulfide, represented by the chemical formula FeS. However, strictly speaking, mackinawite is not a stoichiometric compound, but rather an Fe-rich compound. That is, mackinawite is a compound having sulfur atoms and iron atoms in a ratio of more than 1.00 and less than 1.20 for 1 sulfur atom, in other words, an Fe-rich compound in which x is more than 1.00 and less than 1.20. x It's S.

[0037] Thus, in mackinawite, excess iron atoms exist as interstitial atoms. Therefore, excess electrons can exist between the lattices to maintain the electrical neutrality of the compound. This is why mackinawite behaves as an n-type semiconductor. In other words, in a water electrolysis cell, electrons supplied from a conductor migrate to the electrolytic reaction site of mackinawite (the surface facing the solid polymer electrolyte membrane) and combine with hydrogen ions to generate hydrogen atoms. Here, the surface of mackinawite has a low hydrogen overvoltage, i.e., the chemical potential of the generated hydrogen atoms is low. Therefore, a driving force acts on the generated hydrogen atoms to diffuse to the hydrogen gas generation site of mackinawite (the surface facing the power supply). Note that, because hydrogen atoms are electrically neutral, their penetration into mackinawite does not affect its electrical neutrality. The hydrogen atoms then migrate to the hydrogen gas generation site of mackinawite and become hydrogen gas. This hydrogen gas then desorbs from mackinawite and is discharged outside the cell.

[0038] Therefore, the catalyst layer is made of mackinawite, i.e., Fe with x greater than 1.00 and less than 1.20. x It is extremely important to use S.

[0039] Fe in the catalyst layer x S content: 99.999 mol% or more As mentioned above, the details of the mechanism by which mackinawite has a hydrogen overvoltage equivalent to that of platinum are not entirely clear, but it is speculated that the spacing between d orbitals in mackinawite, like that of platinum, is a suitable distance for hydrogen atoms to become hydrogen gas. However, if other atoms (impurities) that replace iron atoms or sulfur atoms are mixed into the crystal lattice of mackinawite, the crystal lattice becomes distorted. As a result, the spacing between d orbitals cannot maintain a suitable distance for hydrogen atoms to become hydrogen gas. Therefore, the Fe in the catalyst layer x The S content is 99.999 mol% or more. In other words, the Fe content in the catalyst layer is xThe content of impurities other than S (hereinafter simply referred to as impurities) should be less than 0.001 mol%. Impurities include common metal atoms, i.e., metal atoms classified as transition metals or alkaline earth metals in the periodic table of atoms. These impurities often originate primarily from the aqueous solution used in synthesis. In addition to the purity of the aqueous solution, care must also be taken to always use clean glassware used in synthesis. Hydrogen atoms are very small and can enter the crystal lattice as interstitial elements, so they do not affect the distortion of the mackinawite crystal lattice.

[0040] Fe x x in S: greater than 1.00 and less than 1.20 Troilite, in which sulfur and iron atoms are ionically bonded in the stoichiometric ratio, i.e., iron sulfide with x = 1.00, does not become an n-type semiconductor. Furthermore, if x is less than 1.00, the resulting iron sulfide is different from mackinawite. Iron sulfides different from mackinawite have a crystal structure in which the spacing between d orbitals is not optimal for hydrogen atoms to convert to hydrogen gas. Iron sulfides different from mackinawite do not achieve the same hydrogen overvoltage as platinum. Therefore, x must be greater than 1.00. On the other hand, if x exceeds 1.20, the number of iron atoms between the bond lattices becomes too large. As a result, the crystal lattice becomes distorted, and the spacing between d orbitals cannot maintain the optimal distance for hydrogen atoms to convert to hydrogen gas, preventing the hydrogen overvoltage from being equivalent to that of platinum. Therefore, x must be greater than 1.00 and less than 1.20.

[0041] Catalyst layer thickness: 5 to 100 μm The thickness of the catalyst layer formed from mackinawite is 5 to 100 μm. That is, if the thickness of the catalyst layer is less than 5 μm, the crystalline structure of mackinawite will be distorted by the influence of the power supply and the solid polymer electrolyte membrane bonded to the electrode. As a result, a hydrogen overvoltage equivalent to that of platinum cannot be obtained. Therefore, the thickness of the catalyst layer is 5 μm or more, preferably 8 μm or more. On the other hand, if the thickness of the catalyst layer exceeds 100 μm, the migration distance of hydrogen atoms becomes too large, resulting in a decrease in energy conversion efficiency. Therefore, the thickness of the catalyst layer is 100 μm or less. The thickness of the catalyst layer is preferably 80 μm or less, more preferably 60 μm or less.

[0042] The electrode according to one embodiment of the present invention may be formed of a catalyst layer or may be formed by forming a catalyst layer on the surface of a substrate. The substrate is not particularly limited, and examples thereof include carbon cloth.

[0043] [2] Water electrolysis cell A water electrolysis cell according to one embodiment of the present invention has an electrode according to one embodiment of the present invention described above in [1]. A water electrolysis cell according to one embodiment of the present invention may have, for example, the following structure as shown in FIG. a solid polymer electrolyte membrane; Electrodes disposed adjacent to both sides of the solid polymer electrolyte membrane; and a power supply disposed so as to sandwich the solid polymer electrolyte membrane and the electrode, One of the electrodes, particularly the cathode which is the hydrogen evolution electrode, is an electrode according to one embodiment of the present invention described in [1] above.

[0044] The solid polymer electrolyte membrane, the anode (oxygen generating electrode), the power feeder, and the separator are not particularly limited, and conventional general materials can be used. For example, the solid polymer electrolyte membrane can be suitably made of Nafion (registered trademark) 112, 115, 117, 450, or the like, which are fluororesin-based cation exchange membranes having sulfonic acid groups.

[0045] In a stack of water electrolysis cells, the boundaries between the individual water electrolysis cells (so-called single cells) are defined by separators, such as carbon or metal plates, which form flow paths for hydrogen gas and oxygen gas generated by water electrolysis.

[0046] [3] Electrode manufacturing method A method for manufacturing an electrode according to one embodiment of the present invention comprises the steps of: a stirring step of mixing a 0.05 to 0.50 mol / L NaS aqueous solution with a 0.05 to 0.50 mol / L FeSO aqueous solution to prepare a mixed aqueous solution, and stirring the mixed aqueous solution in an inert gas atmosphere for one hour or more; Next, a standing step of standing the mixed aqueous solution in an inert gas atmosphere for one day or more to obtain a precipitate; Then, a recovery step of recovering the precipitate; a drying step of drying the precipitate in a vacuum for one day or more to form an electrode having a catalyst layer; Provided with. According to this manufacturing method, a catalyst layer is provided, and the catalyst layer is made of Fe x An electrode according to one embodiment of the present invention described above in [1] can be suitably produced, in which S is 99.999 mol % or more, x is more than 1.00 and less than 1.20, and the thickness of the catalyst layer is 5 to 100 μm. Each step will be described below.

[0047] [Stirring process] In the stirring step, a 0.05 to 0.50 mol / L Na2S aqueous solution and a 0.05 to 0.50 mol / L FeSO4 aqueous solution are mixed to prepare a mixed aqueous solution, and the mixed aqueous solution is stirred for one hour or more in an inert gas atmosphere.

[0048] Atmosphere during stirring: inert gas atmosphere When the dissolved oxygen concentration in the mixed aqueous solution increases, the oxygen concentration in the precipitated mackinawite also increases, and the Fe in the catalyst layer xIt is difficult to achieve an S content of 99.999 mol% or more, in other words, an impurity content of less than 0.001 mol%. Therefore, an inert gas atmosphere is used during stirring. To achieve an inert gas atmosphere during stirring, for example, the gas phase of the container containing the mixed aqueous solution may be degassed with an inert gas. In this case, degassing is preferably performed at least one hour before mixing the Na2S aqueous solution and the FeSO4 aqueous solution. It may be necessary to unavoidably interrupt degassing when mixing the Na2S aqueous solution and the FeSO4 aqueous solution. In this case, if the degassing interruption time is limited to 5 minutes or less and degassing is resumed after mixing, the incorporation of impurity elements into the catalyst layer is sufficiently suppressed, and no problem occurs. Examples of inert gases include rare gases such as nitrogen and argon.

[0049] Concentration of Na2S and FeSO4 aqueous solutions: 0.05 to 0.50 mol / L Mackinawite is obtained as a precipitate by mixing an aqueous Na2S solution with an aqueous FeSO4 solution, stirring the resulting mixture, and then allowing it to stand. If the concentrations of the Na2S and FeSO4 solutions are each less than 0.05 mol / L, mackinawite will not be obtained as a precipitate. On the other hand, if the concentrations of the Na2S and FeSO4 solutions are each more than 0.50 mol / L, unreacted cations and anions remain in the iron sulfide, causing it to degrade during the subsequent drying process. Therefore, the concentrations of the Na2S and FeSO4 solutions are both set to 0.05 to 0.50 mol / L.

[0050] It is preferable that the Na2S aqueous solution and the FeSO4 aqueous solution have the same concentration and amount.

[0051] Mixing time: 1 hour or more In order to thoroughly mix the Na2S aqueous solution and the FeSO4 aqueous solution, the stirring time is set to 1 hour or more. Since the Na2S aqueous solution and the FeSO4 aqueous solution are thoroughly mixed after stirring for 1 hour, the stirring time can be selected as an appropriate time from an industrially preferable standpoint, as long as it is 1 hour or more. On the other hand, there are no particular limitations on the upper limit of the stirring time. This is because the purpose of stirring is only to mix the aqueous solutions. If the stirring time is 1 hour or more, it will not affect the properties of the electrode obtained from the precipitate by subsequent processing. Therefore, from a process and economic standpoint, the stirring time should be set to a preferable value of 1 hour or more and then carried out.

[0052] The stirring method is not particularly limited, and may be carried out using, for example, a general commercially available stirrer.

[0053] [Standing process] Next, in the standing step, the mixed aqueous solution is stood for one day or more in an inert gas atmosphere to obtain a precipitate.

[0054] Atmosphere when left standing: inert gas atmosphere As mentioned above, when the dissolved oxygen concentration in the mixed aqueous solution increases, the oxygen concentration in the precipitated mackinawite also increases, and the Fe in the catalyst layer x It becomes difficult to make the S content 99.999 mol% or more, in other words, to make the impurity content less than 0.001 mol%. Therefore, the atmosphere during standing is an inert gas atmosphere. To make the atmosphere during standing an inert gas atmosphere, for example, the gas phase of the container containing the mixed aqueous solution may be degassed with an inert gas, as in the stirring step. Examples of inert gases include rare gases such as nitrogen and argon.

[0055] Standing time: 1 day or more By allowing the mixed aqueous solution of Na2S and FeSO4 to stand after stirring, the chemical reaction between Na2S and FeSO4 can be completed, maximizing the amount of precipitate obtained as a result of the chemical reaction. Therefore, the standing time is set to at least one day (24 hours). Note that the chemical reaction is complete after one day of standing, and further standing is not expected to progress if the solution is left standing for longer. Therefore, the standing time should be set to at least one day, as long as it is an appropriate time that is industrially preferable. On the other hand, there is no particular upper limit to the standing time. This is because the amount of precipitate obtained depends on the concentration of each aqueous solution. Furthermore, a standing time of at least one day does not affect the properties of the electrode obtained by subsequent processing. Therefore, from a process and economic standpoint, a standing time of at least one day is set as preferable.

[0056] [Recovery process] Next, the precipitate obtained in the standing step is collected. For example, the precipitate can be collected by filtering the mixed aqueous solution after the standing step.

[0057] [Drying process] Next, the precipitate is applied to a substrate or a power supply in the form of a film of a predetermined thickness, and dried in a vacuum for at least one day to form an electrode having a catalyst layer.

[0058] Drying time: 1 day or more If the drying time is less than one day, the precipitate will not be sufficiently dried, and during the production of a water electrolysis cell, the electrodes formed from the precipitate will not be sufficiently bonded to the power supply or the solid polymer electrolyte membrane. Therefore, the drying time is set to one day (24 hours) or more. The precipitate will be sufficiently dried within one day. Therefore, the drying time should be selected as an industrially preferable time from one day or more. A drying time of, for example, 10 days (240 hours) or less is preferred. This is because if the drying time exceeds 10 days, the precipitate will become powdery rather than paste-like, making handling of the precipitate difficult in subsequent steps. While the reason for the powdery form is not entirely clear, it is thought that the resulting precipitate needs to contain an appropriate amount of moisture to maintain its paste-like form. The paste-like form may also be due to the precipitate (mackinawite) forming a hydrated compound. Based on the above, a drying time of 10 days or less is preferred.

[0059] Dry atmosphere: vacuum To promote drying of the precipitate, the drying atmosphere is a vacuum (a state in which a space is filled with gas at a pressure lower than normal atmospheric pressure). Furthermore, there is a concern that mackinawite may spontaneously combust in an atmosphere containing oxygen. Therefore, it is preferable that the degree of vacuum in the drying atmosphere be 10 Pa or less. However, mackinawite does not immediately spontaneously combust when it comes into contact with oxygen. In other words, when mackinawite is in the form of a block where heat tends to build up inside, there is a high risk of spontaneous combustion, but when it is in the form of a thin film, the risk of heat being transferred to the surrounding area and resulting in a fire is close to zero. However, from a safety perspective, in addition to the dry atmosphere, it is preferable that the atmosphere during storage of mackinawite also be a vacuum (particularly a vacuum of 10 Pa or less).

[0060] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0061] [4] Water electrolysis cell manufacturing method A method for manufacturing a water electrolysis cell according to one embodiment of the present invention includes the steps of: A structure (hereinafter simply referred to as the structure) in which an electrode according to one embodiment of the present invention described in [1] above is disposed between a power supply and a solid polymer electrolyte membrane is pressed (hot pressed) in a vacuum under the following conditions: a pressure of 10 to 20 MPa, a temperature of 70 to 150°C, and a holding time of 10 minutes or more. This manufacturing method can suitably manufacture the water electrolysis cell according to one embodiment of the present invention described above in [2]. Furthermore, as described above, the electrode according to one embodiment of the present invention described above in [1] can be suitably manufactured by the manufacturing method for an electrode according to one embodiment of the present invention described above in [3].

[0062] Press atmosphere: vacuum As mentioned above, there is a risk that Mackinawite may spontaneously combust in an atmosphere containing oxygen. Therefore, the pressing atmosphere is a vacuum. The degree of vacuum in the pressing atmosphere is preferably 10 Pa or less.

[0063] Press pressure: 10 to 20 MPa A water electrolysis cell according to an embodiment of the present invention (2) having an electrode according to an embodiment of the present invention (1) above can be produced by disposing the electrode according to an embodiment of the present invention (1) above between a power feeder and a solid polymer electrolyte membrane to obtain the above-mentioned structure. The structure can then be pressed under conditions of a pressure of 10 to 20 MPa, a temperature of 70 to 150°C, and a holding time of 10 minutes or more. If the pressing pressure is less than 10 MPa, the mackinawite forming the electrode will not be sufficiently bonded to the power feeder and the solid polymer electrolyte membrane. On the other hand, if the pressing pressure exceeds 20 MPa, components such as the electrode will not be able to withstand the applied pressure, making it difficult to produce a water electrolysis cell. Therefore, the pressing pressure is set to 10 to 20 MPa.

[0064] Temperature: 70~150℃ If the temperature is below 70°C, sufficient heat for thermocompression bonding cannot be obtained. As a result, the mackinawite electrode is not sufficiently bonded to the power feeder and the solid polymer electrolyte membrane. On the other hand, if the temperature exceeds 150°C, the mackinawite is altered, and a hydrogen overvoltage equivalent to that of platinum cannot be obtained. Therefore, the temperature is set to 70 to 150°C. The temperature is preferably 130°C or less, and more preferably 120°C or less. The temperature (pressing temperature) referred to here is the temperature of the indenter of the press that comes into contact with the structure during pressing. During pressing, the structure is sandwiched between the indenters of the press. This applies pressure and heat to the structure. A heating wire is arranged around the indenter, and the indenter is heated by this heating wire. For example, a thermocouple is welded to the indenter, and the temperature of the indenter can be measured and controlled by this thermocouple.

[0065] Holding time: 10 minutes or more If the holding time is less than 10 minutes, the mackinawite electrode will not be sufficiently bonded to the power feeder and the solid polymer electrolyte membrane. On the other hand, if the holding time is 10 minutes, the bonding between the electrode, the power feeder, and the solid polymer electrolyte membrane will be complete. Therefore, the holding time during pressing should be 10 minutes or more, and should be selected as an appropriate time from an industrially preferable viewpoint. The holding time during pressing begins when the pressure measured by the load cell of the pressing machine reaches 95% of the set pressure. From the viewpoint of protecting the solid polymer electrolyte membrane, the holding time is preferably 30 minutes or less. Specifically, if Nafion (registered trademark) 112, 115, 117, 450, and other fluororesin-based cation exchange membranes having sulfonic acid groups, which are suitable as solid polymer electrolyte membranes, are held at the above pressure and temperature for more than 30 minutes, there is a concern that their properties as solid polymer electrolyte membranes may be impaired. The holding time is more preferably 25 minutes or less, and even more preferably 20 minutes or less.

[0066] The above structure has, for example, a solid polymer electrolyte membrane, electrodes arranged adjacent to both sides of the solid polymer electrolyte membrane, and power feeders arranged to sandwich the solid polymer electrolyte membrane and the electrodes, and one of the electrodes, particularly the cathode which is the hydrogen generation electrode, is an electrode according to one embodiment of the present invention described in [1] above.

[0067] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods. [Example]

[0068] Example 1 Under the conditions shown in Table 1, a Na2S aqueous solution and an FeSO4 aqueous solution were mixed in a container to prepare a mixed solution. The mixed solution was stirred and then allowed to stand. The volumes of both the Na2S aqueous solution and the FeSO4 aqueous solution were 100 mL. For No. 1-32, 0.06 mL of a 0.10 mol / L ZnSO4 aqueous solution was simultaneously mixed with the Na2S aqueous solution and the FeSO4 aqueous solution. The mixed solution was then filtered to recover the precipitate. A 20 mm diameter circular carbon cloth (carbon paper manufactured by Toray Industries, Inc.) was used as a substrate, and the recovered precipitate was applied to the substrate and dried in a vacuum under the conditions shown in Table 1. A catalyst layer with the thickness shown in Table 1 was formed on the carbon cloth, and an electrode was obtained. The materials constituting the catalyst layer were then identified by ICP emission spectroscopy. The results are also shown in Table 1.

[0069] In the comparative example No. 1-1, no precipitate was obtained after standing, so the subsequent steps and evaluations were omitted.

[0070] The hydrogen overvoltage of the electrode thus obtained was measured. Specifically, a cathodic polarization test was performed in ion-exchanged water using the obtained electrode as the working electrode and platinum foil as the counter electrode. The distance between the working electrode and the counter electrode was 10 cm, and the potential of the working electrode was measured with a reference electrode (Ag / AgCl (Sat.KCl)). The solution temperature was 30°C. In addition, the gas phase in contact with the solution was degassed with nitrogen for 10 minutes or more before the start of the polarization test. Degassing with nitrogen continued during the polarization test. The pressure was atmospheric pressure. In other words, the nitrogen outlet side for gas phase degassing of the polarization test cell was open to the atmosphere. In the polarization test, a current density of 1.0 × 10 was measured from the immersion potential. -3 A / cm 2 The potential was swept toward the cathode at a sweep rate of 20 mV / min until the equilibrium potential for hydrogen evolution, derived from the solution pH, and the current density of the current flowing through the working electrode in the polarization test was 1.0 × 10 -3 A / cm 2 The hydrogen overvoltage (hereinafter also referred to as the hydrogen overvoltage of the target electrode) was calculated as the difference between the potential of the working electrode (measured at the reference electrode) and the potential of the reference electrode at the time of measurement. The hydrogen overvoltage (hereinafter also referred to as the hydrogen overvoltage of platinum) was also calculated under the same conditions as above, except that platinum was used as the working electrode. The hydrogen overvoltage characteristics were evaluated according to the following criteria. The evaluation results are also shown in Table 1. Pass (has the same hydrogen overvoltage as platinum): The absolute value of the difference between the hydrogen overvoltage of the target electrode and that of platinum (hereinafter referred to as the hydrogen overvoltage difference with platinum) is 0.05 V or less Failed (does not have the same hydrogen overvoltage as platinum): The difference in hydrogen overvoltage from platinum is more than 0.05V

[0071] [Table 1]

[0072] As shown in Table 1, Nos. 1-2, 3, 5-29, and 33-35 all exhibited hydrogen overvoltages equivalent to that of platinum.

[0073] On the other hand, for No. 1-4, the thickness of the catalyst layer was less than 5 μm, so the crystal lattice of mackinawite was distorted by the influence of the power supply and solid polymer electrolyte membrane attached to the electrode, and a hydrogen overvoltage equivalent to that of platinum could not be obtained. In the case of No. 1-30, the concentrations of the Na2S and FeSO4 aqueous solutions were too high, so Na2S and FeSO4 did not react completely. As a result, cations and anions remained in the iron sulfide obtained by subsequent filtration, and the filtered material obtained during drying was altered. As a result, the catalyst layer was composed of iron sulfide (purotite) with x: less than 1.00, and a hydrogen overvoltage equivalent to that of platinum was not obtained. The iron sulfide in the catalyst layer was identified as purothite by X-ray diffraction. For No. 1-31, the stirring time was less than 1 hour (30 minutes), so Na2S and FeSO4 did not completely react. As a result, cations and anions remained in the iron sulfide obtained by subsequent filtration, and the filtered material obtained during drying was altered. As a result, the catalyst layer was composed of iron sulfide (purotite) with x: less than 1.00, and a hydrogen overvoltage equivalent to that of platinum was not obtained. The iron sulfide in the catalyst layer was identified as purothite by X-ray diffraction. For No.1-32, Fe x Because the S concentration was less than 99.999 mol%, the crystal lattice of mackinawite was distorted, and the hydrogen overvoltage equivalent to that of platinum could not be obtained. x The substance contained other than S was zinc, and its concentration was measured by ICP analysis to be 0.004 mol%. Although No. 1-15 had a hydrogen overvoltage equivalent to that of platinum, as shown in No. 2-15 of Example 2 described later, the thickness of the catalyst layer exceeded 100 μm. Therefore, when incorporated into a water electrolysis cell, the migration distance of hydrogen atoms became too long, and sufficient energy conversion efficiency could not be achieved.

[0074] Example 2 A circular sheet of carbon cloth (carbon paper manufactured by Toray Industries, Inc.) with a diameter of 40 mm (area: 12.57 cm 2) was used as a substrate, and each precipitate obtained in Example 1 was applied to the substrate and dried under the same conditions as in Example 1. This carbon cloth corresponded to a current feeder (on the cathode side). Nos. 2-2, 2-3, 2-5 to 29, and 2-33 to 2-35 in Table 2 correspond to Nos. 1-2, 1-3, 1-5 to 1-29, and 1-33 to 1-35 in Table 1, respectively. Note that, in Example 1, for No. 1-1, which did not yield a precipitate after standing, and Nos. 1-4 and 1-30 to 1-32, which did not yield an excellent hydrogen overvoltage difference from platinum, the water electrolysis cells according to Example 2 were not fabricated. As a result, catalyst layers with the thicknesses shown in Table 2 were formed on the carbon cloth, and electrodes were obtained.

[0075] Next, using the obtained electrode as a cathode, a liquid polymer electrolyte (cation exchange resin emulsion: NAFION (registered trademark) emulsion manufactured by Aldrich Chemical Company) was applied to the electrode to a thickness of 1 μm, and the electrode was dried in a vacuum at 100°C for 60 minutes.

[0076] Next, a platinum catalyst was formed as an anode. That is, a circular carbon cloth sheet (carbon paper manufactured by Toray Industries, Inc.) with a diameter of 40 mm (area: 12.57 cm) was placed on the anode. 2 ) was used as the substrate, and FC-R&C 3036Pt-Ru-C catalyst was applied to the substrate and dried at 100°C for 60 minutes to obtain an anode with a thickness of 11±1 μm. This carbon cloth corresponds to the current collector (on the anode side).

[0077] Next, a liquid polymer electrolyte (cation exchange resin emulsion: NAFION (registered trademark) emulsion manufactured by Aldrich Chemical Company) was applied to the obtained anode to a thickness of 1 μm, and dried at 100° C. for 60 minutes.

[0078] Next, a cation exchange membrane (NAFION (registered trademark) 117 manufactured by DuPont, thickness: 210 μm) corresponding to the solid polymer electrolyte membrane was sandwiched between the anode and cathode to obtain a structure with an arrangement of power supplier (carbon cloth)-cathode-solid polymer electrolyte membrane-anode-power supplier (carbon cloth). This structure was then pressed under the conditions shown in Table 2 to obtain a water electrolysis cell.

[0079] The obtained water electrolysis cell was energized under the following conditions to perform water electrolysis. (Electrification conditions) Electrolytic bath: deionized water Bath temperature: 80℃ Applied voltage: 2.0V

[0080] The energy conversion efficiency η (%) was calculated using the following formula (2), and a value of η of 82% or more was evaluated as pass (excellent), and a value of η of less than 82% was evaluated as fail (poor). The evaluation results are also shown in Table 2. η={(ΔG 〇 × N) / (V×i)}×100 (2) In formula (2), ΔG 〇 : Standard free energy change of hydrogen gas formation (J / mol) N: Hydrogen generation rate (mol / s) V: Applied voltage (V) i: Measured current value (A) In addition, in an aqueous solution, ΔG 〇 can be considered to be 17.6 kJ / mol.

[0081] [Table 2]

[0082] As shown in Table 2, all of the inventive examples achieved excellent energy conversion efficiency.

[0083] On the other hand, in Comparative Example No. 2-15, the thickness of the cathode catalyst layer was more than 100 μm, and the migration distance of hydrogen atoms was too long, so sufficient energy conversion efficiency was not obtained. In Comparative Example No. 2-19, the pressing pressure was less than 10 MPa, so the electrodes and the solid polymer electrolyte membrane were not bonded together, and a water electrolysis cell was not formed. Therefore, evaluation of the energy conversion efficiency was omitted. In the comparative example No. 2-22, the pressing pressure was more than 20 MPa, so cracks were found in the solid polymer electrolyte membrane and a water electrolysis cell was not formed, so evaluation of the energy conversion efficiency was omitted. In Comparative Example No. 2-23, the electrodes and the solid polymer electrolyte membrane were not bonded together and a water electrolysis cell was not formed because the pressing temperature was less than 70° C. Therefore, evaluation of the energy conversion efficiency was omitted. In the comparative example No. 2-28, the pressing temperature was over 150°C, so the mackinawite in the cathode catalyst layer was Fe with x=1.00. x The catalyst was transformed into S, which did not generate hydrogen, and excellent energy conversion efficiency was not achieved. [Industrial Applicability]

[0084] The electrode of the present invention can achieve high energy conversion efficiency without using expensive elements such as platinum group elements or nickel, and when applied to the cathode of a water electrolysis cell in particular, it becomes possible to produce clean hydrogen at low cost. Therefore, the industrial utility value of the present invention is extremely great. [Explanation of symbols]

[0085] 10 Solid polymer electrolyte membrane 20 electrodes (catalyst electrodes) 30 Power feeder 40 Separator 50 conductor

Claims

1. An electrode for a water electrolysis cell in which cations move through a solid polymer electrolyte membrane that separates a cathode and an anode, the electrode has a catalyst layer; The catalyst layer is Fe ｘ S: 99.999 mol% or more, and x is more than 1.00 and less than 1.20; The electrode, wherein the catalyst layer has a thickness of 5 to 100 μm.

2. 10. The electrode of claim 1, wherein the electrode is a cathode of the water electrolysis cell.

3. A water electrolysis cell comprising the electrode according to claim 1 or 2.

4. 0.05 to 0.50 mol / L Na ２ S aqueous solution and 0.05 to 0.50 mol / L FeSO ４ an agitation step of mixing the aqueous solution with the aqueous solution to form a mixed aqueous solution, and agitating the mixed aqueous solution in an inert gas atmosphere for one hour or more; Next, a standing step of standing the mixed aqueous solution in an inert gas atmosphere for one day or more to obtain a precipitate; Then, a recovery step of recovering the precipitate; a drying step of drying the precipitate in a vacuum for one day or more to form an electrode having a catalyst layer; With The catalyst layer is Fe ｘ S: 99.999 mol% or more, and x is more than 1.00 and less than 1.20; The method for producing an electrode, wherein the catalyst layer has a thickness of 5 to 100 μm.

5. A method for producing a water electrolysis cell, comprising: placing the electrode according to claim 1 or 2 between a current feeder and a solid polymer electrolyte membrane; and pressing the structure in vacuum under conditions of a pressure of 10 to 20 MPa, a temperature of 70 to 150°C, and a holding time of 10 minutes or more.

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