Electrode, manufacturing method thereof, water electrolysis device and fuel cell

Abstract

An electrode 10 comprises, as a catalyst, an alloy which is composed of at least three base metal elements, wherein the atomic composition percentages of the at least three base metal elements are approximately equal, and the at least three base metal elements form a solid solution. In addition, an electrode 12 has: a carbon fiber; and a catalyst in which at least a portion of elements is chemically bonded to the carbon fiber and which comprises a base metal. In addition, a water electrolyzer comprises an anode, a cathode, and a solid polymer electrolyte membrane provided between the anode and the cathode, wherein the anode is the electrode 10, and / or the cathode is the electrode 12.　

Description

[Technical Field]

[0001] The present invention relates to an electrode, a method for producing the same, a water electrolysis device, and a fuel cell. [Background technology]

[0002] Precious metals such as platinum are used as catalysts in the electrodes of water electrolysis devices, fuel cells, secondary batteries, and electric double-layer capacitors. A base metal alloy in which a graphene film is formed on a NiMo (nickel-molybdenum) alloy is known (e.g., Non-Patent Document 1). It is known that an alloy containing Cr, Mn, Fe, Co, and Ni is used as a catalyst for the oxygen reduction reaction (ORR) (e.g., Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] "Development of a corrosion-resistant base metal electrode with high hydrogen generation efficiency", [online], March 30, 2018, University of Tsukuba, Osaka University, Tohoku University, [Retrieved September 16, 2020], Internet<URL:http: / / www.tsukuba.ac.jp / wp-content / uploads / 180330ito-1.pdf> [Non-patent document 2] Adv. Energy Mater. 2018, 8, 1802269 Summary of the Invention [Problem to be solved by the invention]

[0004] Precious metals are expensive, and using them for electrodes makes the device expensive. Base metals are inexpensive but have poor chemical stability. Therefore, using base metals for electrodes results in poor durability and poor long-term use. Using graphene films, as in Non-Patent Document 1, increases manufacturing costs. It is common knowledge among those skilled in the art that the required catalytic properties differ depending on the type of reaction. Therefore, ORR catalysts are not used as catalysts for oxygen evolution reactions (OER) or hydrogen evolution reactions (HER). OER catalysts are exposed to high potentials, which can easily deteriorate due to oxidation, in order to achieve the desired current density for OER. For this reason, ORR catalysts are not typically used as OER catalysts. HER catalysts are also easily oxidized and easily deteriorate depending on the device used. For this reason, ORR catalysts are not typically used as HER catalysts. Furthermore, hydrogen oxidation reaction (HOR) catalysts suffer from the problem of CO poisoning. For this reason, ORR catalysts are not usually used as HOR catalysts.

[0005] The present invention has been made in view of the above-mentioned problems, and has an object to provide an inexpensive electrode with improved durability, a method for manufacturing the same, and a water electrolysis device. [Means for solving the problem]

[0006] The present invention provides an electrode containing, as a catalyst for an oxygen generation reaction or a hydrogen generation reaction, an alloy consisting of three or more base metal elements, wherein the atomic composition ratios of the three or more base metal elements are approximately equal and the three or more base metal elements form a solid solution.

[0007] In the above configuration, the three or more base metal elements may be at least three of Fe, Cu, Ni, Al, Pb, Zn, Sn, W, Mo, Ta, Mg, Co, Bi, Cd, Ti, Zr, Sb, Mn, Be, Cr, Ge, V, Ga, Hf, In, Nb, Re, and Tl.

[0008] In the above configuration, the three or more base metal elements may be five or more base metal elements.

[0009] In the above configuration, the three or more base metal elements may be six or more base metal elements.

[0010] In the above configuration, the catalyst may be a catalyst for an oxygen generating reaction.

[0011] The present invention provides an electrode containing, as a catalyst for a hydrogen oxidation reaction, an alloy consisting of three or more base metal elements, wherein the atomic composition ratios of the three or more base metal elements are approximately equal and the three or more base metal elements form a solid solution.

[0012] In the above configuration, the three or more base metal elements may be at least three of Fe, Cu, Ni, Al, Pb, Zn, Sn, W, Mo, Ta, Mg, Co, Bi, Cd, Ti, Zr, Sb, Mn, Be, Cr, Ge, V, Ga, Hf, In, Nb, Re, and Tl.

[0013] In the above configuration, the three or more base metal elements may be five or more base metal elements.

[0014] In the above configuration, the three or more base metal elements may be six or more base metal elements.

[0015] The present invention provides an electrode comprising carbon fibers and a catalyst made of a base metal, at least some of the elements of which are chemically bonded to the carbon fibers.

[0016] In the above configuration, the catalyst may be an alloy made of at least two elements selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Mo.

[0017] In the above configuration, the catalyst may be a NiMo alloy.

[0018] The present invention provides a water electrolysis device comprising: an anode, which is an electrode containing, as a catalyst for an oxygen generation reaction or a hydrogen generation reaction, an alloy consisting of three or more base metal elements, the three or more base metal elements having approximately equal atomic composition ratios and forming a solid solution; a cathode; and a solid polymer electrolyte membrane provided between the anode and the cathode.

[0019] In the above configuration, the cathode may be an electrode containing an alloy as a catalyst for the oxygen generation reaction or the hydrogen generation reaction, the alloy being made of three or more base metal elements, the atomic composition ratios of the three or more base metal elements being approximately equal, and the three or more base metal elements forming a solid solution.

[0020] In the above configuration, the cathode may be an electrode including carbon fibers and a catalyst made of a base metal, at least some elements of which are chemically bonded to the carbon fibers.

[0021] In the above configuration, the anode may be an electrode that contains, as a catalyst, an alloy consisting of at least five elements selected from the group consisting of Fe, Cu, Ni, Al, Pb, Zn, Sn, W, Mo, Ta, Mg, Co, Bi, Cd, Ti, Zr, Sb, Mn, Be, Cr, Ge, V, Ga, Hf, In, Nb, Re, and Tl, in which the atomic composition ratios of the at least five elements are approximately equal and the at least five elements form a solid solution.

[0022] The present invention provides a water electrolysis device comprising: an anode; a cathode which is an electrode comprising: carbon fibers; and a catalyst made of a base metal, at least a portion of elements of which are chemically bonded to the carbon fibers; and a solid polymer electrolyte membrane provided between the anode and the cathode.

[0023] The present invention provides a fuel cell comprising: an anode, which is an electrode containing, as a catalyst for a hydrogen oxidation reaction, an alloy consisting of three or more base metal elements, wherein the three or more base metal elements have approximately equal atomic composition ratios and form a solid solution; a cathode; and an electrolyte membrane provided between the anode and the cathode.

[0024] The present invention provides a method for manufacturing an electrode, which includes the steps of: attaching a base metal oxide to the surface of a carbon fiber; and heat-treating the carbon fiber in a reducing gas atmosphere to form a catalyst that is chemically bonded to the carbon fiber and contains a base metal.

[0025] In the above configuration, the step of adhering the oxides may include a step of placing the carbon fibers in an aqueous solution containing a plurality of base metals and using a hydrothermal synthesis method to adhere oxides of the plurality of base metals to the surface of the carbon fibers, and the step of forming the catalyst may include a step of forming an alloy of the plurality of base metals. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide an inexpensive electrode with improved durability, a method for producing the same, and a water electrolysis device. [Brief explanation of the drawings]

[0027] [Figure 1] 1(a) to 1(d) are schematic diagrams showing a method for producing an electrode according to the second embodiment. [Figure 2] FIG. 2 is a cross-sectional view of the device according to the third embodiment. [Figure 3] FIG. 3 is a cross-sectional view of a PEM water electrolysis apparatus according to a third embodiment. [Figure 4] 4(a) and 4(b) show the X-ray diffraction patterns of the pentad HEA and hexadental HEA in the examples. [Figure 5] FIG. 5 is an SEM image of a 5-component HEA supported on carbon black in an example. [Figure 6] FIG. 6 is a diagram showing an X-ray diffraction pattern of the NiMo alloy in the example. [Figure 7] 7(a) to 7(c) are SEM images of NiMo alloys supported on carbon fibers in the examples. [Figure 8] FIG. 8 shows the current-voltage characteristics of cells A, C, and D. [Figure 9] FIG. 9 is a diagram showing the current-voltage characteristics of cell B. [Figure 10] FIG. 10 is a graph showing the constant voltage characteristics of the cell C. [Figure 11] FIG. 11 is a graph showing the constant voltage characteristics of cells D and E. [Figure 12] FIG. 12 is a diagram showing the current-voltage characteristics of cells C, D, and F. [Figure 13] 13(a) to 13(c) show the impedance test results for cells C, D, and F, respectively. [Figure 14] 14(a) and 14(b) are diagrams showing the current-voltage characteristics of cells using 5-component HEA to 9-component HEA in the anode or cathode. [Figure 15] FIG. 15 shows the current-voltage characteristics of a cell using a nine-component HEA for the anode and cathode. [Figure 16] FIG. 16 shows the constant voltage characteristics of a nine-component HEA-nine-component HEA cell. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, an embodiment of the present invention will be described.

[0029] (Embodiment 1) Base metals are metals that have a lower ionization tendency than hydrogen and are inexpensive. Therefore, the use of base metals as electrode catalysts was investigated. Note that Cu has a higher ionization tendency than hydrogen but is inexpensive, so it is included in the base metals in this specification. In embodiment 1, attention was focused on high-entropy alloys as highly durable catalysts using base metals. High-entropy alloys are solid solutions in which the entropy of mixing is increased by making five or more constituent elements have approximately equal atomic compositions. A solid solution is a state in which the constituent elements are fused together to form a uniform solid phase, and has a single crystal structure such as a face-centered cubic (FCC) structure or a body-centered cubic (BCC) structure.

[0030] High-entropy alloys have high strength. Using high-entropy alloys for electrodes is expected to improve durability. From the perspective of increasing entropy, the constituent elements do not need to be five or more, but using three or more base metal elements can increase entropy.

[0031] From the above viewpoints, the electrode according to the first embodiment includes a catalyst containing an alloy containing three or more base metal elements, the atomic composition ratios of the three or more base metal elements being approximately equal and forming a solid solution, thereby providing a highly durable electrode using inexpensive base metals.

[0032] "The atomic composition ratios of three or more base metal elements are approximately equal" means that the element composition ratios are equal to the extent that entropy is high, and allow for differences on the order of manufacturing error. For example, when the atomic composition ratio of the element with the highest atomic composition ratio among the three or more base metal elements is Cmax and the atomic composition ratio of the element with the lowest atomic composition ratio is Cmin, (Cmax-Cmin) / (Cmax+Cmin)≦0.2, and preferably (Cmax-Cmin) / (Cmax+Cmin)≦0.1.

[0033] Examples of base metal elements include Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel), Mo (molybdenum), Zr (zirconium), Ti (titanium), Nb (niobium), Zn (zinc), Al (aluminum), Cu (copper), Sb (antimony), Be (beryllium), Bi (bismuth), Cd (cadmium), Ga (gallium), Ge (germanium), Hf (hafnium), In (indium), Pb (lead), Mg (magnesium), Re (rhenium), Ta (tantalum), Tl (thallium), Sn (tin), W (tungsten), V (vanadium), Sc (scandium), Y (yttrium), and lanthanoids.

[0034] As base metal elements that can be easily used to prepare an alloy, the three or more base metal elements preferably include at least three of Cr, Mn, Fe, Co, Ni, Mo, Zr, Ti, Nb, Zn, Al, and Cu.

[0035] From the viewpoint of increasing the entropy of the alloy, the number of base metal elements contained in the alloy is preferably 4 or more, more preferably 5 or more. From the viewpoint of functioning as a catalyst and increasing the entropy of the alloy, the 5 or more base metal elements preferably include at least five elements of Cr, Mn, Fe, Co, Ni, and Mo.

[0036] An alloy containing three or more base metals may be supported on carbon such as carbon black or carbon fiber. The alloy is preferably porous, which improves the catalytic performance of the alloy and improves electrode performance. The average pore size of the porous body is, for example, 1 nm to 100 μm, and the average ligament size is, for example, 1 nm to 100 μm. The porous body is formed, for example, by a dealloying method.

[0037] (Embodiment 2) In Non-Patent Document 1, covering a NiMo alloy with a graphene film can suppress corrosion of the NiMo alloy by the electrolyte. However, forming the graphene film is costly. In addition, the graphene film hinders the movement of ions such as protons in the electrolyte, which reduces the electrode characteristics.

[0038] In embodiment 2, the electrode includes carbon fibers and a catalyst containing a base metal, at least some of whose elements are chemically bonded to the carbon fibers. The electrode further includes carbon fibers and a catalyst made of a base metal, at least some of whose elements are chemically bonded to the carbon fibers. Because at least some of the elements of the catalyst are chemically bonded to the carbon fibers, the contact resistance between the carbon fibers and the catalyst is low, improving the performance of the electrode. Furthermore, because the catalyst is firmly bonded to the carbon fibers, the durability of the electrode is improved. Furthermore, because a graphene film is not required, manufacturing costs can be reduced. Furthermore, because a graphene film that hinders ion movement in the electrolyte is not required, electrode performance can be further improved.

[0039] Carbon fibers are fibers obtained by subjecting organic fibers to a thermal carbonization treatment, and are fibers composed of 90% carbon by mass. The diameter of the carbon fibers is, for example, 0.1 μm to 100 μm, and preferably 1 μm to 20 μm. The carbon fibers may be carbon cloth woven from a plurality of carbon fibers or carbon paper made from a plurality of carbon fibers in a paper form. From the viewpoint of improving the permeability of liquids such as water or electrolytes, the porosity of the carbon cloth or carbon paper is preferably 50% or more, and more preferably 70% or more.

[0040] The catalyst containing a base metal is Cr, Mn, Fe, Co, Ni, Mo, Zr, Ti, Nb, Zn, Al, Cu, Sb, Be, Bi, Cd, Ga, Ge, Hf, In, Pb, Mg, Re, Ta, Tl, Sn, W, V, Se (selenium), Te (tellurium), Si (silicon), P (phosphorus), Sc, Y, or an alloy containing at least two lanthanides, or a compound of these base metals (e.g., carbide, nitride, sulfide, selenium compound, tellurium compound, chalcogenide alloy). Examples of carbides include TiC, ZrC, VC, NbC, TaC, MoC, WC, V2C, Ta2C, Mo2C, WC, Mn3C, Te3C, Co3C, Ni3C, and Cr3C2. The nitride is, for example, Si3N4, Ge3N4, GaN, P2N3, TiN, ZrN, NbN, TaN, Cr2N, CrN, VN, V2N, Mn2N, Mn3N, Mn4N, Mn6N, FeN, Fe2N, Fe3N, Fe4N, CoN, Co2N, Co3N, Co4N, Ni2N, Ni3N, Ni4N, Cu3N, Zn3N2, MoN, Mo2N, InN, Sn3N4 or Sn3N2.

[0041] Examples of sulfides include chalcogenide-based layered materials such as MoS2, WS2, ReS2, SnS2, SeS2, ZrS2, NiS2, HfS2, PdS2, TaS2, TiS2, VS2, and NbS2. Examples of selenium compounds include chalcogenide-based layered materials such as MoSe2, WSe2, ReSe2, SnSe2, ZrSe2, NiSe2, HfSe2, PdSe2, TaSe2, TiSe2, VSe2, and NbSe2. Examples of tellurium compounds include chalcogenide-based layered materials such as MoTe2, WTe2, ReTe2, SnTe2, SeTe2, ZrTe2, NiTe2, HfTe2, PdTe2, TaTe2, TiTe2, VTe2, and NbTe2. Chalcogenide alloys are alloys in which two or more metal elements other than S, Se, and Te are mixed, or two or more of S, Se, and Te are mixed, such as MoWS2, MoWSe2, MoWSeS, or SnSeS.

[0042] To improve catalytic performance, the catalyst containing a base metal preferably contains at least one element selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Mo. The catalyst is also preferably an alloy containing at least two elements selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Mo. The catalyst is also preferably an alloy consisting of at least two elements selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Mo. The catalyst is also preferably a NiMo alloy.

[0043] The composition ratio of the NiMo alloy is, for example, a 2:1 to 1:2 atomic ratio of Ni:Mo. The NiMo alloy is preferably porous, which improves catalytic performance and electrode performance. The average pore size of the porous body is, for example, 1 nm to 100 μm, and the average ligament size is, for example, 1 nm to 100 μm.

[0044] 1(a) to 1(d) are schematic diagrams showing a method for producing an electrode according to embodiment 2. As shown in FIG. 1(a), carbon fibers 50, such as carbon cloth or carbon paper, are immersed in an aqueous solution 54 of a compound containing a base metal in a container 55. The compound containing a base metal is, for example, nickel chloride hexahydrate NiCl2·6H2O, and the compound containing Mo is, for example, sodium molybdate Na2MoO4·2H2O. An oxide of the base metal is produced from the compound containing the base metal using a hydrothermal synthesis method.

[0045] As shown in FIG. 1(b), a base metal oxide 51 is impregnated into carbon fiber 50. The oxide 51 is, for example, NiMoO4. As shown in FIG. 1(c), the carbon fiber 50 impregnated with the oxide 51 is placed in a cylindrical furnace 56. The carbon fiber 50 is heat-treated in an atmosphere of a reducing gas 57. The reducing gas 57 is, for example, hydrogen gas or a mixture of hydrogen gas and an inert gas such as a rare gas. As shown in FIG. 1(d), the base metal oxide 51 is reduced, and a catalyst 52 containing the base metal is directly bonded to the carbon fiber 50. The catalyst 52 is, for example, a NiMo alloy. The catalyst 52 is not only physically bonded to the carbon fiber 50, but at least some of the elements of the catalyst 52 are chemically bonded to the carbon of the carbon fiber 50.

[0046] As shown in Figures 1(a) and 1(b), a base metal oxide 51 is deposited on the surfaces of a plurality of carbon fibers 50. Then, as shown in Figures 1(c) and 1(d), the carbon fibers 50 are heat-treated in a reducing gas atmosphere to form a catalyst 52 containing a base metal, at least some of whose elements are chemically bonded directly to the carbon fibers 50. The electrode produced in this manner has improved performance and durability because the catalyst 52 is firmly fixed to the carbon fibers 50.

[0047] In FIG. 1(a), the step of depositing oxide 51 involves placing carbon fiber 50 in an aqueous solution 54 containing multiple base metals and using hydrothermal synthesis to deposit oxides of the multiple base metals on the surface of carbon fiber 50. Heat treatment in a reducing gas atmosphere forms an alloy of the multiple base metals. This allows the formation of an electrode with excellent performance and durability. In the step of depositing oxide 51 on the surfaces of multiple carbon fibers 50, it is preferable to impregnate the surfaces of carbon fiber 50 with oxide 51. Note that impregnation means allowing oxide 51 to penetrate deep into the structure of the multiple carbon fibers.

[0048] (Embodiment 3) Embodiment 3 is an example of a water electrolysis device, a fuel cell, a secondary battery, and an electric double layer capacitor using the electrodes of Embodiments 1 and 2. FIG. 2 is a cross-sectional view of the device according to Embodiment 3. As shown in FIG. 2, a device 100 such as a water electrolysis device, a fuel cell, a secondary battery, or an electric double layer capacitor includes electrodes 10 and 12 and an electrolyte 11 disposed between the electrodes 10 and 12. The electrodes 10 and 12 are, for example, an anode and a cathode. The electrode of Embodiments 1 and 2 is used for at least one of the electrodes 10 and 12. This improves the durability of the electrodes 10 and 12. Furthermore, the use of a base metal allows for an inexpensive device to be provided.

[0049] (Embodiment 4) Embodiment 4 is an example of a polymer electrolyte membrane (PEM) water electrolysis device using the electrodes of Embodiments 1 and 2. Water electrolysis, which involves electrolyzing water, has attracted attention because it can produce hydrogen gas (H2) without emitting CO2. Known water electrolysis methods include alkaline water electrolysis and PEM water electrolysis. Alkaline water electrolysis is a method that uses an alkaline aqueous solution. Although alkaline water electrolysis can use base metal electrodes, it has problems such as poor energy efficiency and the generation of harmful alkaline waste liquid. PEM water electrolysis uses a solid polymer electrolyte and generates 2H + +2e -→H2 reaction. PEM water electrolysis is energy efficient and does not produce alkaline waste liquid. However, when base metals are used for the electrodes, there is an issue of poor electrode durability.

[0050] FIG. 3 is a cross-sectional view of a PEM water electrolysis device according to a third embodiment. As shown in FIG. 3, in a water electrolysis device 102, a solid polymer electrolyte membrane 14 is sandwiched between catalysts 16 and 18. The solid polymer electrolyte membrane 14 is a polymer membrane having ion (proton) conductivity, such as a fluorine-based polymer having sulfonic acid groups (e.g., Nafion). The thickness of the solid polymer electrolyte membrane 14 is, for example, 0.1 mm to 1 mm. The catalyst 16 is, for example, the alloy described in the first embodiment, and the catalyst 18 is, for example, the catalyst containing a base metal bonded to carbon fibers described in the second embodiment.

[0051] Gas diffusion layers 20 and 21 are provided to sandwich catalysts 16 and 18. Gas diffusion layer 20 supplies water to electrode 10, and gas diffusion layer 21 discharges generated hydrogen gas. Furthermore, gas diffusion layers 20 and 21 are electrically conductive. Gas diffusion layers 20 and 21 are, for example, carbon paper or carbon cloth. The thickness of gas diffusion layers 20 and 21 is, for example, 0.01 mm to 1 mm each. Gas diffusion layers 20 and 21 are provided in openings 22a and 23a formed in insulating gaskets 22 and 23, respectively.

[0052] The electrode 10 includes a catalyst 16 and a gas diffusion layer 20, and the electrode 12 includes a catalyst 18 and a gas diffusion layer 21. The catalysts 16 and / or 18 may be supported on the surface of the electrolyte membrane 14, or may be supported on the gas diffusion layers 20 and / or 21. The method of coating the catalysts 16 and / or 18 on the surface of the electrolyte membrane 14 is called the CCM (Catalyst Coated Membrane) method. The method of coating the catalysts 16 and / or 18 on the surfaces of the gas diffusion layers 20 and / or 21 is called the CCE (Catalyst Coated Electrode) method.

[0053] Current collectors 24 and 25 are disposed on either side of gas diffusion layers 20 and 21. Current collectors 24 and 25 supply current to electrodes 10 and 12 via gas diffusion layers 20 and 21, respectively. Current collectors 24 and 25 are technically current feeders because they supply current, but are conventionally referred to as current collectors here. Water channels 24a and 25a may be provided on the surfaces of current collectors 24 and 25. Current collectors 24 and 25 are conductors, such as Ti. Insulating gaskets 26 and 27 are disposed on either side of current collectors 24 and 25, and housing 28 is disposed on either side of gaskets 26 and 27.

[0054] Terminals 30 and 31 are electrically connected to current collectors 24 and 25, respectively. When a voltage is applied between terminals 30 and 31 such that terminal 30 is positive with respect to terminal 31, hydrogen gas 36 is generated at electrode 12. An inlet channel 32 and an outlet channel 33 are provided so as to penetrate through casing 28, gasket 26, and current collector 24. Water 35 is introduced into water channel 24a via inlet channel 32, and discharged from water channel 24a via outlet channel 33. An outlet channel 34 is provided so as to penetrate casing 28, gasket 27, and current collector 25. Hydrogen gas 36 is discharged from water channel 25a via outlet channel 34.

[0055] The water electrolysis device 102 can be designed with any appropriate configuration as long as it includes the anode catalyst 16, the cathode catalyst 18, and the solid polymer electrolyte membrane 14. By using the electrodes of embodiments 1 and 2 in the PEM water electrolysis device 102, the durability of the electrodes 10 and 12 can be improved even when a base metal is used. This makes it possible to provide an inexpensive, highly durable water electrolysis device 102. At least one of the electrodes 10 and 12 may be the electrode of embodiment 1 or 2.

[0056] The catalysts 16 and 18 may be disposed using either the CCM method or the CCE method, but it is preferable to use the CCM method when the catalyst of embodiment 1 is used in the anode, and the CCE method when the catalyst of embodiment 2 is used in the cathode, which can reduce the contact electrical resistance between the electrolyte membrane 14 and the electrodes 10 and 12. [Example]

[0057] As an example, a PEM type water electrolysis device was fabricated.

[0058] (anode) We prepared a quinary high-entropy alloy (HEA) wire containing Mn, Co, Ni, Cr, and Fe. The atomic composition ratios of Mn, Co, Ni, Cr, and Fe were approximately 20 atomic %. We also prepared a hexa-component HEA wire containing Mo, Mn, Co, Ni, Cr, and Fe. The atomic composition ratios of Mo, Mn, Co, Ni, Cr, and Fe were approximately 16.7 atomic %. The HEA wire was ground in a planetary ball mill at 600 rpm for 3 hours. The ground HEA and carbon black (CB) were mixed in a 1:1 weight ratio, and the HEA was physically pressed onto the CB in a planetary ball mill. The carbon black used was Cabot Vulcan XC72R, Lion Specialty Chemicals Ketjenblack EC600JD, or Sigma-Aldrich 05-1530. Other carbon blacks may also be used. The planetary ball mill rotation speed was 600 rpm, and the milling time was 25 hours. This produced a mixture of HEA and CB, HEA-CB. 30 mg of HEA-CB was dispersed in 0.6 ml of a mixture of 1-propanol and water (1-PrOH:HO = 4.56:0.55 weight ratio). Nafion (Nafion 115) was added and ultrasonic treatment was performed. The Nafion concentration was 5 wt %, and the C:Nafion ratio was 1:0.8 weight ratio. The solution containing HEA-CB and Nafion was applied to an electrolyte membrane 14 (CCM method). The electrolyte membrane 14 was Nafion. As a result, the HEA was supported on the CB, and the CB was supported on the electrolyte membrane 14.

[0059] Figures 4(a) and 4(b) show the X-ray diffraction (XRD) patterns of the pentad and hexadental HEAs in the examples. Figures 4(a) and 4(b) show the XRD patterns of the HEA wire and the HEA powder after grinding. 1 h, 3 h, and 6 h indicate the grinding times for the HEA (without CB). The peaks at 2θ between 43° and 52° indicate the FCC crystal structure.

[0060] As shown in Figure 4(a), the quinary HEA wire has an FCC crystal structure. The HEA powder obtained by milling the HEA wire has a broader peak than the HEA wire, but still has an FCC crystal structure. This indicates that all quinary HEA powders are solid solutions of quinary base metals, forming high-entropy alloys. The FCC crystal structure is maintained even after milling for 6 hours. The peaks at 2θ of 24° to 37° are zirconia peaks. The balls in the ball mill are made of zirconia, and it is thought that zirconia is mixed into the HEA powder as an impurity. It is thought that the surface area of ​​the HEA increases with the milling time.

[0061] As shown in Figure 4(b), the hexanion HEA wire has an FCC crystal structure. The HEA powder milled from the HEA wire retains the FCC crystal structure, although the peaks become broader as the milling time increases. This indicates that all hexanion HEA powders are high-entropy alloys. The amount of zirconia increases with increasing milling time.

[0062] Figure 5 is a scanning electron microscope (SEM) image of a 5-component HEA supported on carbon black in this example. As shown in Figure 5, the HEA is supported on carbon black CB. The particle size of the HEA is approximately 1 μm to 10 μm.

[0063] (cathode) As shown in Figure 1(a), 1.5 mM NiCl2·H2O and 1.5 mM Na2MoO4·2H2O are dissolved in 30 ml of water to prepare aqueous solution 54. A 2 cm x 2 cm piece of carbon paper (Toray TGP-H-090) is immersed in aqueous solution 54. Hydrothermal synthesis is performed at 150°C for 6 hours. After drying, carbon paper impregnated with NiMoO4 is formed, as shown in Figure 1(b). There are cases where the amount of NiMo is insufficient using only the NiMoO4-impregnated carbon paper. In this case, NiMoO4 solution that has been prepared in advance or that has not adhered to the carbon paper in aqueous solution 54 is placed on the carbon paper. This adjusts the amount of NiMo alloy. The amount of NiMoO4 solution to be added later is determined so that the area of ​​the carbon paper is 4 cm. 2 The volume is, for example, 1 ml to 10 ml. As shown in Figure 1(c), heat treatment is performed in a tubular furnace 56 in a mixed atmosphere of Ar gas and H2 gas at a temperature of 950 °C for 20 minutes. This reduces NiMoO4, forming a NiMo alloy supported on carbon paper as shown in Figure 1(d). A solution containing NiMo alloy and Nafion (NiMO:Nafion in a weight ratio of 1:0.22) is applied to the gas diffusion layer 21 (CCE method).

[0064] FIG. 6 shows the X-ray diffraction pattern of the NiMo alloy in the example. As shown in FIG. 6, the NiMo alloy before current is applied contains NiMo and Ni4Mo. The ratio of NiMo is greater than the ratio of Ni4Mo. It also contains a small amount of Mo2C. Thus, at least a portion of the element Mo in NiMo is chemically bonded to the carbon of the carbon fiber 50. The Ni:Mo ratio of the produced NiMo alloy was Ni:Mo = 1:1.5 by weight, and Ni:Mo = 1.09:1 by atomic ratio.

[0065] 7(a) to 7(c) are SEM images of NiMo alloys supported on carbon fibers in the examples. As shown in FIG. 7(a), multiple carbon fibers 40 are arranged in a mesh pattern on the carbon paper. The diameter of the carbon fibers 40 is several μm. As shown in FIG. 7(b), NiMo alloys 42 are supported so as to surround each of the multiple carbon fibers 40. As shown in FIG. 7(c), the NiMo alloys 42 are directly bonded to the carbon fibers 40 and grow directly from the carbon fibers 40. The NiMo alloys 42 are porous. The size of the pores and ligaments in the NiMo alloys 42 is 0.1 μm to 3 μm.

[0066] (Water electrolysis cell) The electrolyte membrane 14 is sandwiched between gas diffusion layers 20 and 21. The catalysts 16 and 18 are formed by the CCM method or the CCE method. The electrolyte membrane 14 and gas diffusion layers 20 and 21 are pressed together using a hot press at a temperature of 130°C and a load of 250 kg for 3 minutes. The gas diffusion layers 20 and 21 are then sandwiched between current collectors 24 and 25 and gaskets 26 and 27. The gaskets 26 and 27 are then sandwiched between the plates of a housing 28 and tightened with a force of approximately 4 N to produce a water electrolysis cell as shown in Figure 3.

[0067] The following four cells were fabricated: Cell A Cathode catalyst: Pt / C, Anode catalyst: IrO2 Cell B Cathode catalyst: NiMo alloy only, Anode catalyst: IrO2 Cell C Cathode catalyst: NiMo alloy, Anode catalyst: IrO2 Cell D Cathode catalyst: Pt / C, Anode catalyst: 5-way HEA Cell E Cathode catalyst: Pt / C, Anode catalyst: 6-component HEA Cell F Cathode catalyst: NiMo alloy, Anode catalyst: 5-way HEA Weight density per unit area Pt / C: 1 mg / cm 2 IrO2: 1 mg / cm2 NiMo alloy: 3mg / cm 2 5 yuan HEA: 3mg / cm 2 6-yuan HEA: 3mg / cm 2 Other ingredients Electrolyte membrane 14: Nafion (registered trademark) 115 or 117 manufactured by Merck Gas diffusion layers 20 and 21: Toray carbon paper TGP-H-090

[0068] Cell A is a comparative example in which both the cathode catalyst and the anode catalyst are made of precious metals. Cell B corresponds to a comparative example in which a porous NiMo alloy is pressed against the gas diffusion layer 21. In Cell B, the NiMo alloy and the carbon fibers in the gas diffusion layer 21 are only physically bonded, not chemically bonded. Cell C uses the NiMo alloy of the example in the cathode. Cells D and E use the 5-component HEA and 6-component HEA of the example in the anode. Cell F uses the NiMo alloy of the example in the cathode and the 5-component HEA of the example in the anode.

[0069] For cells A, B, and C, water electrolysis cells with an active area of ​​2 cm × 2 cm were fabricated and the current-voltage characteristics were measured. The measurement conditions were a cell temperature of 80°C, a water flow rate of 5 ml / min, and a current of 4 A / cm. 2 After aging for 25 hours at a current density of 1000 kJ / s, the current-voltage characteristics were measured.

[0070] Figure 8 shows the current-voltage characteristics of cells A, C, and D. The horizontal axis represents current density, and the vertical axis represents cell voltage. As shown in Figure 8, compared to cell A, which is a comparative example, the cell voltage of cell C is almost the same, and the cell voltage of cell D is slightly higher. When the current density is 1 A / cm 2 The cell voltage is used as an indicator of electrolytic properties. A lower cell voltage indicates better efficiency. Cell A, a comparative example using precious metals, had a cell voltage of 1 A / cm 2 The cell voltage of cell C is 1.75V, which is almost the same as cell A. The cell voltage of cell D is 1.95V, which is slightly higher than cell A.

[0071] As a comparison example of Cell C, Cell B has an active area of ​​4cm 2 The water electrolysis cells were fabricated and the current-voltage characteristics were measured. The measurement conditions were a cell temperature of 25°C and a water flow rate of 1 ml / min. Figure 9 shows the current-voltage characteristics of Cell B. As shown in Figure 9, although the measurement conditions were different, the cell voltage of Cell B was much higher than that of Cell C. Thus, Cell B, in which the NiMo alloy was physically pressed against the carbon fiber, had poor electrode performance. This is thought to be because physical contact alone would result in high contact resistance.

[0072] The change in current density over time was measured for Cell C. The measurement conditions were a cell voltage of 2 V, a cell temperature of 80°C, and a water flow rate of 20 ml / min. Figure 10 shows the constant voltage characteristics for Cell C. As shown in Figure 10, the current density did not decrease even after 250 hours, but rather increased. No decrease in current density was observed even after 250 hours. Under these conditions, the area around the cathode catalyst is thought to be strongly acidic with a pH of 1 to 2. The fact that no decrease in current density was observed even after 250 hours in such a strongly acidic environment demonstrates the remarkable durability of the NiMo alloy. As shown in Figure 6 after 250 hours of current application, the XRD pattern indicates no deterioration in the crystalline structure of the NiMo alloy.

[0073] The change in current density over time was measured for cells D and E. The measurement conditions were a cell voltage of 2.4 V, a cell temperature of 80°C, and a water flow rate of 5 ml / min. Figure 11 shows the constant voltage characteristics for cells D and E. As shown in Figure 11, the increase in current density is a general phenomenon in which the electrolyte membrane 14 swells and the electrolysis performance improves. The maximum current density for cell D was 3.0 A / cm 2 After 150 hours of current flow, the current density was 2.72 A / cm 2 The current density after 150 hours of current application is 90.6% of the maximum current density.

[0074] For cell E, the cell voltage was turned off after about 75 hours and then reapplied. By turning the voltage off and then on again, the current density recovered by 6%. The maximum current density was 4.15 A / cm. 2 After 130 hours of current flow, the current density was 3.7 A / cm 2 The current density after 150 hours of current application is 89% of the maximum current density. For example, if the NiMo alloy used as the cathode catalyst in cell C is used as the anode catalyst, the NiMo alloy will dissolve within one minute. In contrast, both cells D and E are extremely durable.

[0075] For cells C, D, and F, water electrolysis cells with an active area of ​​1 cm × 1 cm were fabricated and the current-voltage characteristics were measured. The measurement conditions were a cell temperature of 80°C, a water flow rate of 5 ml / min, and a current of 4 A / cm. 2 After aging for 25 hours at a current density of 1000 kJ / s, the current-voltage characteristics were measured.

[0076] Figure 12 shows the current-voltage characteristics of cells C, D, and F. As shown in Figure 12, the active areas of cells C and D are different from those in Figure 8, so the cell voltages of cells C and D are higher than those in Figure 8. Comparison within Figure 12 is possible. 2 The cell voltages are 2.0 V for cell C, 2.6 V for cell D, and 2.8 V for cell F. Cell F has a slightly higher cell voltage than cell D. Thus, water electrolysis is possible even in cell F, where both the cathode catalyst and anode catalyst are made of base metals.

[0077] Figures 13(a) to 13(c) show the impedance test results for cells C, D, and F, respectively. The -Z'' intercepts in Figures 13(a) to 13(c) represent the contact resistance from terminals 30 and 31 to catalysts 16 and 18. As shown in Figure 13(a), the contact resistance for cell C is estimated to be approximately 200 mΩ to 650 mΩ. As shown in Figure 13(b), the contact resistance for cell D is estimated to be approximately 300 mΩ. As shown in Figure 13(c), the contact resistance for cell F is estimated to be 350 mΩ to 1.6 Ω. As shown, cells C, D, and F have high contact resistance. However, it should be possible to reduce the contact resistance by adjusting cell fabrication conditions, and with optimization, it is thought that the performance of cells C, D, and F could be improved to the same level as cell A, which uses precious metals.

[0078] As described above, by using cells C to F, the performance and durability of the electrodes are improved, and the performance and durability of the water electrolysis device are improved.

[0079] We fabricated water electrolysis cells with an active area of ​​2 cm x 2 cm using 5- to 9-component high-entropy alloys (HEAs) as the anode and / or cathode catalysts. The elements used in each HEA are as follows: 5 element HEA: Co, Cr, Mn, Ni, Fe 6 element HEA: Co, Cr, Mn, Ni, Fe, Mo 7 element HEA: Co, Cr, Mn, Ni, Fe, Mo, Nb 8-component HEA: Co, Cr, Mn, Ni, Fe, Mo, Nb, Zr 9-component HEA: Co, Cr, Mn, Ni, Fe, Mo, Nb, Zr, Ti

[0080] We fabricated cells with anode catalysts of 5-way to 9-way HEAs and cathode catalysts of IrO2. We also fabricated cells with anode catalysts of Pt / C and cathode catalysts of 5-way to 9-way HEAs. We measured the current-voltage characteristics of these cells. The measurement conditions were a cell temperature of 80°C, a water flow rate of 5 ml / min, and a current of 4 A / cm. 2 After aging for 25 hours at a current density of 1000 kJ / s, the current-voltage characteristics were measured.

[0081] Figures 14(a) and 14(b) show the current-voltage characteristics of cells using five-component to nine-component HEAs as the anode or cathode. Figure 14(a) shows the current-voltage characteristics of a Pt / C-five-component to nine-component HEA cell, which uses Pt / C as the cathode and five-component to nine-component HEAs as the anode. As a comparative example, the current-voltage characteristics of a Pt / C-IrO2 cell, which uses a Pt / C cathode and an IrO2 anode, are also shown. As shown in Figure 14(a), the Pt / C-five-component to nine-component HEA cells have a slightly higher cell voltage than the Pt / C-IrO2 cell. For example, when the current density is 1 A / cm 2 The cell voltage at this temperature is 1.60 V for the Pt / C-IrO2 cell and 2.20 V for the Pt / C-9-element HEA cell. As the number of elements increases from five to nine, the cell voltage decreases. This is especially true for cells using eight or more element HEAs.

[0082] Figure 14(b) shows the current-voltage characteristics of a cell (5- to 9-component HEA-IrO2) using a 5-component to 9-component HEA as the cathode and IrO2 as the anode. As a comparative example, the current-voltage characteristics of a Pt / C-IrO2 cell are also shown. As shown in Figure 14(b), the 5- to 9-component HEA-IrO2 cell has a slightly higher cell voltage than the Pt / C-IrO2 cell. For example, when the current density is 1 A / cm 2 The cell voltage of the Pt / C-IrO2 cell is 1.60 V, while that of the 5- to 9-element HEA-IrO2 cell is 2.06 V. As the number of elements increases from 5 to 9, the cell voltage decreases. This is especially true for cells using 8-element or more HEAs.

[0083] Figure 15 shows the current-voltage characteristics of a cell using a 9-component HEA for both the anode and cathode. The 9-component HEA-9-component HEA cell uses a 9-component HEA for both the anode and cathode. As a comparative example, the current-voltage characteristics of a Pt / C-IrO2 cell are shown. For each cell, the current-voltage characteristics at the first cycle and at a current density of 0 A / cm are also shown. 2 to 1A / cm 2The graph shows the current-voltage characteristics after 10,000 cycles of sweeping from 0.01 to 0.01. As shown in Figure 15, the cell voltage of the 9-component HEA-9-component HEA cell is higher than that of the Pt / C-IrO2 cell. For the Pt / C-IrO2 cell, the current-voltage characteristics after 10,000 cycles are almost unchanged from the first cycle. For the 9-component HEA-9-component HEA cell, the current-voltage characteristics after 10,000 cycles are slightly higher than the first cycle.

[0084] The time-dependent change in current density was measured for the nine-component HEA-nine-component HEA cell. Figure 16 shows the constant-voltage characteristics of the nine-component HEA-nine-component HEA cell. As a comparative example, the characteristics of a Pt / C-IrO2 cell are shown. The cell voltage of the nine-component HEA-nine-component HEA cell is 3 V, and the cell voltage of the Pt / C-IrO2 cell is 2.5 V. As shown in Figure 16, the current density hardly changes for both the nine-component HEA-nine-component HEA cell and the Pt / C-IrO2 cell up to 130 to 140 hours of current flow. Even when current flow was stopped for about 10 hours and then resumed, the current density hardly changed at all.

[0085] The reaction at the anode of a water electrolysis device is OER, while the reaction at the cathode is HER. To achieve the desired current density for OER, an overvoltage is applied to the anode, which is exposed to a high potential, causing the OER catalyst to oxidize and easily deteriorate. Therefore, if a catalyst with low oxidation resistance is used at the anode, the anode catalyst will dissolve instantly when the water electrolysis device is energized. For this reason, no practical catalyst exists for the anode of a PEM water electrolysis device other than IrO2. As shown in Figures 11 and 16, a durable OER catalyst can be achieved by using an HEA at the anode. Although the cell voltage is higher when an HEA is used as the anode catalyst compared to IrO2, HEAs made from base metals are less expensive and more practical than Ir. Furthermore, in water electrolysis devices, the cathode is oxidized when the current is stopped. If a catalyst with low oxidation resistance is used at the cathode, the cathode catalyst will dissolve after the current is stopped. To prevent this, for example, alkaline water electrolysis devices using base metal catalysts continue to run a low current when the water electrolysis device is stopped. As shown in Figure 16, by using an HEA for the cathode, there was no deterioration of the cathode even when the current was stopped and then restarted.

[0086] The durability of an electrode can be improved by using an alloy consisting of three or more base metal elements, in which the atomic composition ratios of the three or more base metal elements are approximately equal and the three or more base metal elements form a solid solution, as a catalyst for the oxygen evolution reaction (OER) or the hydrogen evolution reaction (HER).

[0087] Catalysts for HOR are used in the anode of fuel cells. When hydrogen is produced from hydrocarbons such as methanol or natural gas as fuel gas for fuel cells, the hydrogen contains carbon monoxide (CO). For example, when Pt is used as the catalyst for the anode of a fuel cell, the Pt is poisoned by CO. Pt-Ru alloys are used as anode catalysts to enhance CO resistance, but over long periods of use, the CO resistance decreases due to Ru dissolution, etc. In this way, HOR catalysts encounter problems that ORR catalysts do not encounter. For this reason, ORR catalysts are not usually used as HOR catalysts.

[0088] The durability of the electrode can be improved by using an alloy consisting of three or more base metal elements, in which the atomic composition ratios of the three or more base metal elements are approximately equal and the three or more base metal elements form a solid solution, as a catalyst for the hydrogen oxidation reaction (HOR). In particular, the durability of the anode can be improved by using the catalyst of embodiment 1 in the anode of a fuel cell.

[0089] The three or more base metal elements are preferably at least three of the following: Fe, Cu, Ni, Al, Pb, Zn, Sn, W, Mo, Ta, Mg, Co, Bi, Cd, Ti, Zr, Sb, Mn, Be, Cr, Ge, V, Ga, Hf, In, Nb, Re, and Tl. The number of base metal elements is preferably five or more, more preferably six or more. As shown in Figures 14(a) and 14(b), eight or more is even more preferable.

[0090] Here, "a catalyst or alloy consisting of a plurality of base metal elements" means that the catalyst or alloy does not intentionally contain any elements other than the plurality of base metal elements. In other words, the catalyst or alloy may contain unavoidable elements other than the plurality of base metal elements. The total atomic composition ratio of unavoidable non-metallic elements such as oxygen, carbon, and hydrogen in the catalyst or alloy is preferably Cat / 2 or less, and more preferably Cat / 5 or less, where Cat is the atomic composition ratio of the element with the smallest atomic composition ratio among the plurality of base metal elements. The total atomic composition ratio of unavoidable metal elements other than the plurality of base metal elements in the catalyst or alloy is preferably Cat / 10 or less, and more preferably Cat / 100 or less. Furthermore, the total atomic composition ratio of unavoidable elements in the catalyst or alloy is preferably 10 atomic % or less, and more preferably 1 atomic % or less.

[0091] Although the preferred embodiment of the invention has been described in detail above, the present invention is not limited to such a specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims. [Explanation of symbols]

[0092] 10, 12 electrodes 11 Electrolytes 14 Electrolyte membrane 16, 18 Catalyst 20, 21 Gas diffusion layer 24, 25 Current collector 22, 23, 26, 27 Gaskets 50 carbon fiber 51 Oxides 52 Catalyst 54 Aqueous solution 57 Reducing Gas

Claims

1. An electrode comprising, as a catalyst for an oxygen evolution reaction or a hydrogen evolution reaction, an alloy consisting of eight elements, Co, Cr, Mn, Ni, Fe, Mo, Nb, and Zr, in which the atomic composition ratios of the eight elements are approximately equal and the eight elements form a solid solution, or an alloy consisting of nine elements, Co, Cr, Mn, Ni, Fe, Mo, Nb, Zr, and Ti, in which the atomic composition ratios of the nine elements are approximately equal and the nine elements form a solid solution.

2. 10. The electrode of claim 1, wherein the catalyst is a catalyst for an oxygen evolution reaction.

3. An electrode containing, as a catalyst for a hydrogen oxidation reaction, an alloy consisting of eight elements, Co, Cr, Mn, Ni, Fe, Mo, Nb, and Zr, wherein the atomic composition ratios of the eight elements are approximately equal and the eight elements form a solid solution, or an alloy consisting of nine elements, Co, Cr, Mn, Ni, Fe, Mo, Nb, Zr, and Ti, wherein the atomic composition ratios of the nine elements are approximately equal and the nine elements form a solid solution.

4. an anode which is the electrode according to claim 1 or 2; a cathode; a solid polymer electrolyte membrane provided between the anode and the cathode; A water electrolysis device comprising:

5. 5. The water electrolysis apparatus according to claim 4, wherein the cathode is the electrode according to claim 1.

6. The cathode is Carbon fiber and a catalyst which is a NiMo alloy having a composition ratio of Ni to Mo of 2:1 to 1:2 atomic ratio, and in which at least Mn is chemically bonded to the carbon fiber; The water electrolysis device according to claim 4, wherein the electrode comprises:

7. an anode which is the electrode according to claim 3; a cathode; an electrolyte membrane provided between the anode and the cathode; A fuel cell comprising:

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