Oxygen evolution catalyst, method for producing oxygen evolution catalyst, hydrogen production apparatus, metal-air secondary battery, and artificial photosynthesis system

Abstract

The present invention provides an oxygen-evolving catalyst that can achieve high catalytic activity, or furthermore, high durability, under alkaline conditions, and a method for producing the same. [Solution] The oxygen evolution catalyst is a boride containing at least boron and having an icosahedral boron cluster made up of 12 boron atoms. x Al y B 14+z (A = alkali metal or alkaline earth metal, 0 ≤ x ≤ 1.0, 0.7 ≤ y ≤ 1.0, -1.0 ≤ z ≤ 1.0) or A x B 29+y (0≦x<2.0, -1.0≦y≦1.0). This oxygen-evolving catalyst contains at least alkali metal or alkaline earth metal cations and boron, and is obtained by removing at least some of the cations from a boride having an icosahedral boron cluster consisting of 12 boron atoms. Alkali metals include Na, Li, K, etc., and alkaline earth metals include Mg, Be, Ca, etc.

Description

[Technical Field]

[0001] This invention relates to an oxygen evolution catalyst, a method for producing an oxygen evolution catalyst, a hydrogen production apparatus, a metal-air secondary battery, and an artificial photosynthesis system, and more particularly to an oxygen evolution catalyst suitable for application to hydrogen production involving an oxygen evolution reaction (OER), a method for producing the same, and a hydrogen production apparatus, a metal-air secondary battery, and an artificial photosynthesis system using this oxygen evolution catalyst as the anode, positive electrode, and light-reactive electrode, respectively. [Background technology]

[0002] Water electrolysis is an effective method for producing high-purity hydrogen using surplus electricity, and conventionally, oxygen evolution catalysts (OER catalysts) containing ir, such as IrO2, have been used as the anode catalyst. However, in alkaline water electrolysis, OER catalysts containing ir have durability issues, and research and development of OER catalysts containing 3d transition metals that are resistant to OER under alkaline conditions has been actively pursued, especially since 2011. Among these, Co3O4 nanoparticles have relatively high catalytic activity and durability among OER catalysts containing 3d transition metals, and have been used as standard samples in many papers.

[0003] However, in order to put it into practical use as an anode for alkaline water electrolysis, an OER catalyst is needed that possesses both high catalytic activity and high durability.

[0004] Furthermore, using a pressure cell for high-pressure solid-state electrochemistry, NaAlB 14 By diffusing and removing only Na ions from a disc-shaped bulk polycrystalline material obtained by sintering pellets formed from powder, Na x AlB 14 A method for obtaining (x=0.06) is known (see Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-33707 [Non-patent literature]

[0006] [Non-Patent Document 1] M.Fujioka et al. Chem. Mater. 35, 3008(2023) [Non-Patent Document 2] S.Iwasaki et al. Solid State Sci. 144 107308(2023) [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide an oxygen-evolving catalyst and a method for producing the same that can achieve high catalytic activity, or furthermore, high durability, under alkaline conditions.

[0008] Another problem that this invention aims to solve is to provide a hydrogen production apparatus that can efficiently produce hydrogen by alkaline water electrolysis.

[0009] Another problem that this invention aims to solve is to provide a metal-air secondary battery that can be charged at high speed.

[0010] Another problem that this invention aims to solve is to provide an artificial photosynthesis system that can improve the efficiency of artificial photosynthesis. [Means for solving the problem]

[0011] The present inventors have diligently conducted research with the aim of developing a technology to solve the above problems, and as a result, have found that NaAlB described in Patent Document 1 and Non-Patent Document 1 14 We discovered that it exhibits high catalytic activity and high durability as an OER catalyst, and also found that the novel material Na x B 29It has been found that (0 < x < 2.0) also exhibits high catalytic activity as an OER catalyst. Furthermore, as a result of repeated studies based on these findings, it is considered that the same effect can be obtained when using other alkali metals or alkaline earth metals such as Mg instead of Na in these substances. Furthermore, since all of these substances are borides having a regular icosahedral boron cluster (B12 cluster) composed of 12 boron atoms, more generally, it is considered that the same effect can be obtained even with an oxygen generation catalyst composed of a boride containing at least boron and having a regular icosahedral boron cluster composed of 12 boron atoms, and thus the present invention has been devised.

[0012] That is, in order to solve the above problems, the present invention provides an oxygen generation catalyst comprising a boride containing at least boron and having a regular icosahedral boron cluster composed of 12 boron atoms.

[0013] The boride having the above boron cluster is typically A x Al y B 14+z (A = alkali metal or alkaline earth metal, 0 ≦ x ≦ 1.0, 0.7 ≦ y ≦ 1.0, -1.0 ≦ z ≦ 1.0) or A x B 29+y (A = alkali metal or alkaline earth metal, 0 ≦ x < 2.0, -1.0 ≦ y ≦ 1.0). The alkali metal is Na, Li, K, etc. The alkaline earth metal is Mg, Be, Ca, etc. A x Al y B 14+z is most typically Na x Al y B 14+z and, for example, 0.06 ≦ x ≦ 1.0, but is not limited thereto. A x B 29+y is most typically Na x B 29+y and, for example, 0.33 ≦ x ≦ 1.90 or 0.52 ≦ x ≦ 1.90, but is not limited thereto. These borides may be single crystals or polycrystals. This boride is generally used in the form of particles.

[0014] Furthermore, this invention, This is a method for producing an oxygen evolution catalyst, which involves producing an oxygen evolution catalyst using a boride obtained by removing at least a portion of the cations from a boride that contains at least alkali metal or alkaline earth metal cations and boron, and has an icosahedral boron cluster consisting of 12 borons.

[0015] In this invention of a method for producing an oxygen-evolving catalyst, the methods disclosed in Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2 can be used as methods for removing cations. Other aspects are the same as those of the above-described invention of the oxygen-evolving catalyst.

[0016] Furthermore, this invention, Na x Al y B 14+z (0≦x≦1.0, 0.7≦y≦1.0, -1.0≦z≦1.0) or Na x B 29+y This is a hydrogen production apparatus using alkaline water electrolysis, which has an anode containing an oxygen-evolving catalyst with the following properties: (0≦x<2.0, -1.0≦y≦1.0).

[0017] The cathode of this hydrogen production apparatus and the configuration of this hydrogen production apparatus are not particularly limited and can be selected as necessary from among those that are conventionally known. In this invention of the hydrogen production apparatus, matters other than those described above are considered to have been explained in relation to the invention of the oxygen generation catalyst described above.

[0018] Furthermore, this invention, Na x Al y B 14+z (0≦x≦1.0, 0.7≦y≦1.0, -1.0≦z≦1.0) or Na x B 29+y This is a metal-air secondary battery having a positive electrode containing an oxygen-evolving catalyst with the following properties: (0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.0).

[0019] The negative electrode of this metal-air secondary battery and the configuration of this metal-air secondary battery are not particularly limited and are selected as necessary from among those that are conventionally known. In this invention of the metal-air secondary battery, matters other than those described above are considered to have been explained in relation to the invention of the oxygen-evolving catalyst described above.

[0020] Furthermore, this invention, Na x Al y B 14+z (0≦x≦1.0, 0.7≦y≦1.0, -1.0≦z≦1.0) or Na x B 29+y This is an artificial photosynthesis system having a light-dependent electrode containing an oxygen-evolving catalyst with the following properties: (0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.0).

[0021] The dark reaction electrode and the configuration of this artificial photosynthesis system are not particularly limited and can be selected as needed. In this invention of the artificial photosynthesis system, matters other than those described above are considered to have been explained in connection with the invention of the oxygen evolution catalyst described above. [Effects of the Invention]

[0022] According to this invention, Na x Al y B 14+z or Na x B 29+y Borides having 12 boron icosahedral boron clusters, such as those mentioned above, can achieve high catalytic activity and even high durability as oxygen evolution catalysts under alkaline conditions. By using this oxygen evolution catalyst as a catalyst at the anode of a hydrogen production device, the efficiency of the oxygen evolution reaction can be improved, and consequently, hydrogen can be produced efficiently. Furthermore, by using this oxygen evolution catalyst as a catalyst at the positive electrode of a metal-air secondary battery, the efficiency of the oxygen evolution reaction at the positive electrode during charging can be improved, and consequently, charging can be accelerated. Moreover, by using this oxygen evolution catalyst as a catalyst at the light-dependent electrode in an artificial photosynthesis system, the efficiency of the oxygen evolution reaction at the light-dependent electrode can be improved, and consequently, oxygen can be produced efficiently. [Brief explanation of the drawing]

[0023] [Figure 1A] This is a schematic diagram showing the structure of NaAlB14 in Nax Aly B14+z, which constitutes the oxygen evolution catalyst according to the first embodiment, with x=1, y=1, and z=0. [Figure 1B] This is a schematic diagram showing the structure of AlB14 in Nax Aly B14+z, which constitutes the oxygen evolution catalyst according to the first embodiment, with x=0, y=1, and z=0. [Figure 2A] This is a cross-sectional view showing a method for producing an oxygen-evolving catalyst according to the first embodiment. [Figure 2B] This is a cross-sectional view showing a method for producing an oxygen-evolving catalyst according to the first embodiment. [Figure 3] This is a schematic diagram showing the pressure cell for high-pressure solid electrochemistry used in the method for producing Nax AlB14 with Na ions removed in Example 1. [Figure 4A] This is a photograph in lieu of a diagram showing NaAlB14 at the start of Na ion removal by high-pressure diffusion control in Example 1. [Figure 4B] This is a photograph in lieu of a drawing showing Nax AlB14 1 hour after the start of Na ion removal by high-pressure diffusion control in Example 1. [Figure 4C] This is a photograph in lieu of a drawing showing Nax AlB14 6 hours after the start of Na ion removal by high-pressure diffusion control in Example 1. [Figure 4D] This is a photograph in lieu of a drawing showing Nax AlB14 23 hours after the start of Na ion removal by high-pressure diffusion control in Example 1. [Figure 5A] This is a photograph serving as a substitute for a drawing, showing the state of Nax AlB14 after heat treatment, 1 hour after the start of Na ion removal by high-pressure diffusion control method in Example 1. [Figure 5B] This is a photograph serving as a substitute for a drawing, showing the state of Nax AlB14 after heat treatment 6 hours after the start of Na ion removal by high-pressure diffusion control in Example 1. [Figure 6] This is a schematic diagram showing the thickness-direction distribution of the Na composition ratio of Nax AlB14 after Na ion removal by high-pressure diffusion control method in Example 1. [Figure 7] This is a schematic diagram showing the measurement results of the X-ray diffraction pattern of Nax AlB14 prepared in Example 1. [Figure 8] This is a schematic diagram showing the relationship between the Na composition ratio and lattice constant measurement results of Nax AlB14 prepared in Example 1. [Figure 9] This is a schematic diagram showing the measurement results of the electronic conductivity properties of Nax AlB14 prepared in Example 1. [Figure 10] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Nax AlB14 prepared in Example 1. [Figure 11] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Nax AlB14 prepared in Example 1. [Figure 12] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Na0.4 AlB14 prepared in Example 1. [Figure 13] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Na0.06AlB14 prepared in Example 1. [Figure 14] This is a schematic diagram showing the results of amperometric measurements of Na0.06AlB14N prepared in Example 1. [Figure 15] This is a photograph in lieu of a drawing showing Nax B29 after the removal of Na ions by the high-pressure diffusion control method in Example 2. [Figure 16]This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Nax B29 prepared in Example 2. [Figure 17] This is a photograph used as a substitute for a drawing, showing Nax B29 after the removal of Na ions by the high-pressure diffusion control method in Example 3. [Figure 18] This is a schematic diagram showing the results of elemental analysis of the carbon-side portion and the zeolite-side portion of Nax B29 prepared in Example 3. [Figure 19] This is a schematic diagram showing the measurement results of the X-ray diffraction pattern of Nax B29 prepared in Example 3. [Figure 20] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Nax B29 prepared in Example 3. [Figure 21] This schematic diagram shows the results of measuring OER activity using an electrode prepared by coating the surface with a catalyst support solution containing catalyst particles obtained by pulverizing Nax B29 prepared in Example 3. [Figure 22] This is a schematic diagram showing a hydrogen production apparatus according to a third embodiment of the present invention. [Figure 23] This is a schematic diagram showing a metal-air secondary battery according to a fourth embodiment of the present invention. [Figure 24] This is a schematic diagram showing an artificial photosynthesis system according to a fifth embodiment of the present invention. [Modes for carrying out the invention]

[0024] The embodiments for carrying out the invention (hereinafter referred to as "embodiments") will be described below with reference to the drawings.

[0025] <First Embodiment> [Oxygen-evolving catalyst] The oxygen evolution catalyst according to the first embodiment is Na x Al y B14+z (0 ≦ x ≦ 1.0, 0.7 ≦ y ≦ 1.0, -1.0 ≦ z ≦ 1.0). This Na x Al y B 14+z has a B12 cluster. Figure 1A shows the structure of NaAlB 14 (when x = 1.0, y = 1.0, z = 0), and Figure 1B shows the structure of AlB 14 (when x = 0, y = 1.0, z = 0). When 0 < x < 1.0, some of the Na at the Na sites shown in Figure 1A are missing. Also, when 0 < y < 1.0, some of the Al at the Al sites shown in Figure 1A are missing. Furthermore, when -1.0 ≦ z < 0, B is missing, and when 0 < z ≦ 1.0, B is in excess.

[0026] [Method for manufacturing an oxygen generation catalyst] Na x Al y B 14+z An example of a method for manufacturing an oxygen generation catalyst composed of will be described. Here, z = 0 is assumed.

[0027] As shown in Figure 2A, first, a Na ion conductor 20 is brought into contact with a NaAl y B 14 crystal 10 containing Na ions 11. The Na ion conductor 20 is a solid electrolyte through which Na ions can diffuse. Next, as shown in Figure 2B, in this state, a voltage of a polarity such that the Na ions 11 of this NaAl y B 14 crystal 10 move to the Na ion conductor 20, specifically, a voltage V is applied while heat treatment is performed under high pressure such that the potential of the NaAl y B 14 crystal 10 is higher than that of the Na ion conductor 20. By doing so, the Na ions 11 are diffused from the NaAl y B 14 crystal 10 to the Na ion conductor 20 and removed. The temperature of this heat treatment is, for example, 300°C or higher and 1000°C or lower, typically 450°C or higher and 800°C or lower. Also, the time of the heat treatment is such that for NaAl y B 14 crystal 10 y B 14It is appropriately selected in consideration of the size of crystal 10, the temperature of heat treatment, etc., and generally NaAl y B 14 As the size of crystal 10 increases, it needs to be longer, for example, 5 hours or more and 10 days or less, typically 10 hours or more and 2 days or less. The pressure during heat treatment is, for example, 0.5 GPa or more and 10 GPa or less. The atmosphere for heat treatment is preferably an inert gas atmosphere such as N2 or Ar. Thus, by removing Na ions 11 from NaAl y B 14 crystal 10, Na x Al y B 14 crystal 30 (0 < x ≤ 1.0) can be produced.

[0028] This Na x Al y B 14 For the implementation of the production method of, for example, the high-pressure solid electrochemical device described in Patent Document 1 and Non-Patent Document 1 can be used. This high-pressure solid electrochemical device has a pressure cell for high-pressure solid electrochemistry.

[0029] Example 1 will be described.

[0030] A pressure cell for high-pressure solid electrochemistry shown in Figure 3 was fabricated, and using this pressure cell for high-pressure solid electrochemistry, Na x Al y B 14Crystal 30 was prepared. As shown in Figure 3, this high-pressure solid-state electrochemical pressure cell comprises a cylindrical sample chamber 110, a carbon heater 120 provided to surround the sample chamber 110, a square prism-shaped support 130 having a through hole 131 that houses the sample chamber 110 and the carbon heater 120, a pair of current introduction terminals 140 and 150 provided on the side of the support 130, facing each other across the through hole 131 and electrically connected to the carbon heater 120, a lid 160 provided in contact with the upper end of the sample chamber 110 so as to close the upper end of the through hole 131 of the support 130, a lid 170 provided in contact with the lower end of the sample chamber 110 so as to close the lower end of the through hole 131, an anode 180 provided on the lid 160, and a cathode 190 provided on the lid 170. The sample chamber 110 was formed from a BN cylinder with an inner diameter of 4.4 mm, an outer diameter of 5.5 mm, and a height of 9 mm. The carbon heater 120 was 0.5 mm thick and 7 mm high. The support 130 was made of pyroferrite with a side length of 12.5 mm, a height of 12.5 mm, and a through hole with a diameter of 6.5 mm in the center. The current introduction terminals 140 and 150 were formed from cylindrical Mo electrodes with a diameter of 2.0 mm and a length of 3.0 mm. The lids 160 and 170, the anode 180, and the cathode 190 were made of Mo. The carbon heater 120 was sandwiched above and below by rings 111 and 112 fitted to the outer surface of the sample chamber 110. Rings 111 and 112 were made of BN. The current input terminals 140 and 150 are provided through through holes located on a pair of opposing sides of the support 130, with their respective tips in contact with the carbon heater 120. Pyroferrite 181 and 191 are embedded inside the anode 180 and cathode 190, respectively. The current input terminals 140 and 150 are connected to an externally provided temperature control circuit (not shown). The anode 180 and cathode 190 are connected to an externally provided voltage application circuit (not shown). Pressure is applied to this high-voltage solid electrochemical pressure cell from six directions by pressing each of the six sides of the support 130 with a high-voltage anvil.

[0031] NaAlB 14A disc-shaped bulk polycrystalline material with a diameter of 4.3 mm and a thickness of 2.0 mm was fabricated by sintering pellets formed from the powder. As shown in Figure 3, a composite layer 210 of zeolite (Na ion conductor) and graphite and a zeolite layer 220 were sequentially filled from the bottom on top of the lid 170 inside the sample chamber 110, and the bulk polycrystalline NaAlB was placed on top of that. 14 A 230 was placed on top, a graphite layer 240 was filled on top of it, and the lid 160 was closed. Then, a voltage V was applied to the anode 180 and cathode 190 using a high-voltage solid-state electrochemical power supply, and while heating was performed by a carbon heater 120 by passing current between current input terminals 140 and 150 using a heating power supply, high pressure was applied by pressing each of the six sides of the support 130 with a high-voltage anvil. The voltage V was 50V, the heating temperature was 550℃, and the pressure was 1GPa. As a result, bulk polycrystalline NaAlB 14 Na ions diffused from layer 230 into the zeolite layer 220, resulting in the removal of Na ions. This process is called High-Pressure Diffusion Control (HPDC).

[0032] Figure 4A shows NaAlB at the start of Na ion removal by high-pressure diffusion control. 14 Figures 4B, 4C, and 5D show the NaAlB levels after 1 hour, 6 hours, and 23 hours, respectively, following the start of removal. 14 This is shown in Figures 4B, 4C, and 4D. Over time, NaAlB is released from the cathode side. 14 The gradual removal of Na ions is clearly visible. As shown in Figure 4D, after 23 hours, NaAlB 14 Na ions are removed from almost the entire sample. In this way, by controlling the time of the Na ion removal process, the amount of Na ions removed can be controlled, thereby reducing the amount of Na AlB 14 The Na composition ratio can be controlled.

[0033] Figure 5A shows the NaAlB content one hour after the start of removal shown in Figure 4B. 14 NaAlB after heat treatment at 800°C for 48 hours 14is shown. In the state shown in FIG. 4B, there is a region with a low Na composition ratio on the cathode side in NaAlB 14 and the distribution of Na ions was non-uniform. However, in the state of FIG. 5A after heat treatment, the Na ion distribution in NaAlB 14 has become uniform. Similarly, FIG. 5B shows NaAlB 14 after heat treatment at 800 °C for 48 hours of NaAlB 14 is shown. In the state shown in FIG. 4C, there is a region with a low composition ratio on the cathode side in NaAlB 14 and the distribution of Na ions was non-uniform. However, in the state of FIG. 5B after heat treatment, the Na ion distribution in NaAlB 14 has become uniform.

[0034] FIG. 6 shows the change in the Na composition ratio in the thickness direction (the direction from the anode side to the cathode side) of NaAlB 14 shown in FIGS. 5A, 5B, 4D, and 4A. From FIG. 6, NaAlB 14 shown in FIGS. 5A, 5B, 4D, and 4A are respectively Na x AlB 14 with an average x of 0.06, Na x AlB 14 with an average x of 0.44, Na x AlB 14 with an average x of 0.89, Na x AlB 14 with an average x of 1.0, that is, NaAlB 14 it can be seen that.

[0035] Before the removal of Na ions, NaAlB 14 and the results of powder X-ray diffraction measurement of Na x AlB 14 from which Na ions were removed are shown in FIG. 7. In FIG. 7, for comparison, the X-ray diffraction pattern of NaAlB 14 (x = 1) obtained by simulation is shown. From FIG. 7, from the X-ray diffraction patterns of Na x AlB 14 (x = 0.06, 0.44, 0.89) from which Na ions were removed, Nax AlB 14 (x=0.06, 0.44, 0.89) is the matrix phase (NaAlB 14 It was confirmed that it retains the structure of ).

[0036] Figure 8 shows Na x AlB 14 The relationship between the lattice constants (a, b, c) and the Na composition ratio x is shown. As shown in Figure 8, it can be seen that a, b, and c all decrease as x decreases.

[0037] Na x AlB 14 The electronic conductivity characteristics were measured for (x=0.06, 0.44, 0.89, 1). The results are shown in Figure 9. As shown in Figure 9, Na x AlB 14 It can be seen that the smaller x is, the better the electronic conductivity. This is true for NaAlB. 14 This is thought to be due to hole doping resulting from the removal of Na ions.

[0038] Na x AlB 14 The OER activity of the oxygen evolution catalyst was evaluated. The evaluation samples were prepared as follows.

[0039] Polycrystalline NaAlB 14 Synthesize Na by high-pressure diffusion control. x AlB 14 (x=0.4, 0.06) was prepared. Polycrystalline NaAlB 14 and Na x AlB 14 and together Na x AlB 14 These are denoted as (x=1.0, 0.4, 0.06). x AlB 14 (x=1.0, 0.4, 0.06) is crushed and Na x AlB 14 Catalyst particles were prepared. The Na prepared in this way x AlB 14Catalyst particles and acetylene black were dispersed in a solution of Nafion dispersion, tetrohydrofuran, and KOH aqueous solution to prepare catalyst-supported solutions. Catalyst-supported solutions with Na composition ratios of x = 1.0, 0.4, and 0.06 were prepared in this manner, coated onto the surface of glassy carbon (GC) electrodes, and dried in vacuum. The OER activity and durability in alkaline electrolytes were then evaluated using these electrodes.

[0040] Figure 10 shows Na x AlB 14 The measurement results for OER activity at (x=1.0, 0.4, 0.06) are shown. In Figure 10, the horizontal axis represents the overpotential (voltage relative to the Reversible Hydrogen Electrode (RHE)), and the vertical axis represents the OER current density. For comparison, Figure 10 also shows the measurement results for the OER activity of Co3O4. As shown in Figure 10, Na x AlB 14 The OER activity for all of the following values ​​(x=1.0, 0.4, 0.06) is higher than that of Co3O4. x AlB 14 The smaller x is, the higher the OER activity tends to be.

[0041] Figure 11 shows the durability measurement results. Figure 11 shows Na for x=0.06 and x=0.4. x AlB 14 For this, after performing a voltage sweep for 1000 cycles, Na with x=1.0 x AlB 14 The results of measuring OER activity after 50 voltage sweeps are shown below. Figure 11 also shows the results of measuring OER activity for Co3O4 for comparison. As shown in Figure 11, Na at x=0.06 and x=0.4 x AlB 14 Regarding this, it can be seen that OER activity is significantly enhanced after 1000 cycles. Also, Na at x=1.0 x AlB 14 Regarding this, it can be seen that the OER activity is significantly enhanced after 50 cycles. Furthermore, the presence of boron clusters is important for OER activity, and NaAlB 14LiAlB instead 14 KAlB 14 It is believed that similar results can be obtained using [another method].

[0042] Figure 12 shows Na at x=0.4. x AlB 14 The results of measuring OER activity during the initial cycle and after 1000 cycles are shown. Also, Figure 13 shows the Na at x=0.06. x AlB 14 The results of measuring OER activity during the initial cycle and after 1000 cycles are shown. As shown in Figure 13, Na x AlB 14 At (x=0.06), we can see that the OER activity after 1000 cycles is higher than the OER activity after the initial cycles.

[0043] Figure 14 shows Na at x=0.06 under 1.67V vs. RHE. x AlB 14 The results of the amperometric measurement are shown. As shown in Figure 14, 10 mAcm was measured for more than 3 hours (10800 seconds). -2 It was confirmed that it can maintain a current density exceeding [a certain value], demonstrating high durability.

[0044] As described above, according to the first embodiment, Na has high catalytic activity and high durability for the oxygen evolution reaction under alkaline conditions. x Al y B 14+z This enables the realization of an oxygen evolution catalyst consisting of Na. x Al y B 14+z The Na composition ratio can be easily controlled. x Al y B 14+z From a chemical composition standpoint, it does not contain rare metals and is therefore low-cost.

[0045] <Second Embodiment> [Oxygen-evolving catalyst] The oxygen evolution catalyst according to the second embodiment is Na x B 29+yIt consists of (0 ≦ x < 2.0, -1.0 ≦ y ≦ 1.0). Na x B 29+y In this case, typically 0 < x < 2.0, for example 0.33 ≦ x ≦ 1.90 or 0.52 ≦ x ≦ 1.90. This Na x B 29+y has a B12 cluster. When 0 < x < 2.0, some of the Na at the Na sites is missing. Also, when -1.0 ≦ y < 0, B is missing, and when 0 < y ≦ 1.0, B is in excess.

[0046] [Method for manufacturing an oxygen generation catalyst] Na x B 29+y The oxygen generation catalyst composed of Na x Al y B 14+z can be manufactured in the same manner as the method for manufacturing the oxygen generation catalyst composed of Na x B 29+y However, here y = 0 for Na 29 Contact a Na ion conductor with Na2B 29 crystals containing Na ions, and apply a voltage such that the potential of the Na2B 29 crystals is higher than that of the Na ion conductor while performing heat treatment under high pressure to diffuse and remove Na ions from the Na2B 29 crystals to the Na ion conductor. Thus, by removing Na ions from the Na2B 29 crystals, Na x B 29 (0 < x < 2.0) can be manufactured.

[0047] Example 2 will be described.

[0048] Similar to Example 1, Na x B 29 crystals were prepared using the pressure cell for high-pressure solid electrochemistry shown in Fig. 3.

[0049] Fig. 15 shows Na2B by the high-pressure diffusion control method 29The sample after removing Na ions is shown. The left side of Figure 15 shows a scanning electron microscope image, and the right side shows a scanning electron microscope energy-dispersive X-ray spectroscopy (SEM-EDX) image. In Figure 15, the upper side is the anode and the lower side is the cathode. As shown in Figure 15, Na2B is visible from the cathode side. 29 From the chemical composition, Na ions are gradually removed, and x decreases from the anode to the cathode side, as shown in the values ​​x = 1.90, 1.48, 0.98, and 0.52. This sample is Na2B 29 From B 29 It consists of a mixed powder containing particles of each of the following compositions.

[0050] Na x B 29 The OER activity of the oxygen evolution catalyst was evaluated. The evaluation samples were prepared as follows.

[0051] The sample shown in Figure 15 was ground and Na x B 29 Catalyst particles were prepared. The Na prepared in this way x B 29 Catalyst particles and acetylene black were dispersed in a solution of Nafion dispersion, tetrohydrofuran, and KOH aqueous solution to prepare a catalyst support solution. This catalyst support solution was applied to the surface of a GC electrode and dried in a vacuum. The OER activity in an alkaline electrolyte was then evaluated using the electrode thus prepared.

[0052] Figure 16 shows Na x B 29 The results of measuring OER activity are shown. Figure 16 shows Na2B for comparison. 29 The results of measuring the OER activity of Co3O4 are also shown. As shown in Figure 16, Na2B 29 Despite showing almost no OER activity, Na-deficient Na x B 29 The OER activity of [component name] was about an order of magnitude higher than that of Co3O4.

[0053] Example 3 will now be described.

[0054] Similar to Example 1, using the high-pressure solid-state electrochemical pressure cell shown in Figure 3, Na x B 29 Crystals were fabricated. However, in this high-pressure solid-state electrochemical pressure cell, the current input terminals 140 and 150 were formed using cylindrical Cu electrodes with a diameter of 2.0 mm and a length of 3.0 mm.

[0055] Figure 17 shows Na2B obtained by high-pressure diffusion control method. 29 The sample after removing Na ions is shown. However, the voltage V applied to the cathode 190 at the anode 180 was 5V, the heating temperature by the carbon heater 120 was 550°C, the processing time was 237 hours, and the pressure applied by the high-pressure anvil was 0.5 GPa. The left side of Figure 17 shows a scanning electron microscope image, and the right side shows a SEM-EDX image. In Figure 17, the upper side is the carbon (graphite) side (anode side), and the lower side is the zeolite side (cathode side). Similar to Example 2, Na2B is removed from the cathode side. 29 Na ions were gradually removed from the chemical composition. Figure 18 shows Na x B 29 The results of elemental analysis of the carbon side and the zeolite side are shown. As shown in Figure 18, the Na of the carbon side x B 29 x = 0.59, Na on the zeolite side x B 29 x = 0.33. Also, Na x B 29 Overall, the average x was 0.46. As shown in Figure 18, Al and Si contained in the aluminosilicate that makes up the zeolite were not detected, therefore, Na was detected due to the zeolite. x B 29 It was confirmed that no contamination had occurred.

[0056] Sodium with Sodium ions removed x B 29 Figure 19 shows the results of powder X-ray diffraction measurements performed on the carbon side and the zeolite side of the sample. For comparison, Figure 19 also shows the results of Na2B obtained by simulation. 29 The X-ray diffraction pattern at (x=1) is shown. From Figure 19, the Na ions have been removed.x B 29 From the X-ray diffraction pattern at (x=0.59, 0.33), the Na ions were removed. x B 29 (x=0.59, 0.33) is the matrix phase (Na2B) 29 It was confirmed that it retains the structure of ).

[0057] Na x B 29 The OER activity of the oxygen evolution catalyst was evaluated. The evaluation samples were prepared as follows.

[0058] The sample shown in Figure 17 was pulverized to obtain Na at x=0.46. x B 29 Catalyst particles were prepared. The Na prepared in this way x B 29 Catalyst particles and acetylene black were dispersed in a solution of Nafion dispersion, tetrohydrofuran, and KOH aqueous solution to prepare a catalyst support solution. This catalyst support solution was applied to the surface of a GC electrode and dried in a vacuum. The OER activity in an alkaline electrolyte was then evaluated using the electrode thus prepared.

[0059] Figure 20 shows Na at x=0.46. x B 29 The results of measuring the OER activity are shown. Figure 20 shows the Na with x=1.2 prepared in the same manner as in Example 3 for comparison. x B 29 Na2B 29 The results of measuring the OER activity of Co3O4 are also shown. Figure 20 further shows the Na at x=0.46. x B 29 and Na at x=1.2 x B 29 The results of measuring OER activity during the initial cycle and after 1000 cycles are also shown. As shown in Figure 20, Na2B 29 Despite showing almost no OER activity, Na at x=0.46 x B 29 The OER activity of is more than 15 times higher than that of Co3O4, and x=1.2 Na x B 29The OER activity of was about an order of magnitude higher than that of Co3O4. Also, Na at x=0.46 x B 29 Then, the OER activity after 1000 cycles is slightly lower than the OER activity after the initial cycle, and x=1.2 Na x B 29 Therefore, the OER activity after 1000 cycles is considerably lower than the OER activity after the initial cycles.

[0060] Figure 21 shows Na at x=0.46. x B 29 OER activity after the initial cycle and after 1000 cycles is Na x AlB 14 The results are shown in comparison with the OER activity at (x=0.06). From Figure 21, the Na at x=0.46 x B 29 Furthermore, it not only exhibits high durability even after 1000 cycles of OER, but its OER activity after the initial cycle and after 1000 cycles is Na x AlB 14 It can be seen that the OER activity is higher than that of (x=0.06). This is because Na x AlB 14 Guidelines for designing highly active and durable catalysts in Na x B 29 This result clearly demonstrates that it can also be applied to other applications, suggesting that it can be broadly applied to boron compounds.

[0061] As described above, according to the second embodiment, Na has high catalytic activity for the oxygen evolution reaction under alkaline conditions. x B 29+y This enables the realization of an oxygen evolution catalyst consisting of Na. x B 29+y The Na composition ratio can be easily controlled. x B 29+y From a chemical composition standpoint, it does not contain rare metals and is therefore low-cost.

[0062] <Third Embodiment> [Hydrogen production equipment] In the hydrogen production apparatus according to the third embodiment, hydrogen is produced by alkaline water electrolysis.

[0063] Figure 22 shows this hydrogen production apparatus. As shown in Figure 22, in this hydrogen production apparatus, the space inside the electrolytic cell 300 is filled with hydroxide ions (OH - The space is divided into two by a permeable membrane 310, with an anode 320 provided in one space and a cathode 330 provided in the other space. The anode 320 is Na according to the first embodiment. x Al y B 14+z Oxygen evolution catalyst or Na according to the second embodiment x B 29+y It includes an oxygen-evolving catalyst. Specifically, the anode 320 is, for example, particulate Na x Al y B 14+z or Na x B 29+y The electrode is manufactured by supporting conductive particles such as carbon on the electrode surface. The electrode may be porous. An alkaline aqueous solution is supplied from the bottom of Figure 22 through piping (not shown) connected to the electrolytic cell 300 into the space on the anode 320 side and the space on the cathode 330 side inside the electrolytic cell 300, and is discharged to the outside of the electrolytic cell 300 through another pipe (not shown) connected to the electrolytic cell 300. The alkaline aqueous solution is not particularly limited, but for example, an aqueous KOH solution is used. Oxygen (O2) generated at the anode 320 and hydrogen (H2) generated at the cathode 330 are recovered through piping connected to the electrolytic cell 300, respectively.

[0064] [Operation Method of Hydrogen Production Equipment] In this hydrogen production apparatus, a predetermined DC current is passed between the anode 320 and the cathode 330. As shown in Figure 22, an alkaline aqueous solution is supplied to the space on the anode 320 side and the space on the cathode 230 side of the electrolytic cell 300. At this time, 4OH is present at the anode 320. - →O2+2H2O+4e - The reaction occurs, generating oxygen (O2), and at cathode 330, 2H2O + 2e - →H2+2OH -A reaction occurs, producing hydrogen (H2). This is how hydrogen is produced.

[0065] According to the third embodiment, Na has high OER activity. x Al y B 14+z Oxygen evolution catalyst or Na x B 29+y By using anode 320 containing an oxygen evolution catalyst, the efficiency of the oxygen evolution reaction can be significantly improved, and consequently, hydrogen can be produced efficiently.

[0066] <Fourth Embodiment> [Metal-air rechargeable battery] A metal-air secondary battery according to a fourth embodiment will be described.

[0067] Figure 23 shows a metal-air secondary battery. As shown in Figure 23, in the metal-air secondary battery, the internal space of the container 400 is divided into two spaces by a separator 410. A negative electrode 420 is provided in one space, and a positive electrode (air electrode) 430 is provided in the other space. A gas diffusion layer 440 is provided above the positive electrode 430 to allow oxygen to pass through to the positive electrode 430. The internal space of the container 400 is filled with an alkaline water electrolyte 450.

[0068] The positive electrode 430 is Na according to the first embodiment. x Al y B 14+z Oxygen evolution catalyst or Na according to the second embodiment x B 29+y It includes an oxygen-evolving catalyst. The material for the positive electrode 430 is selected as needed, but various nanocarbon materials are available, specifically carbon black, graphene, carbon gel, carbon fiber, and carbon nanotubes. These materials have a high specific surface area, making it possible to realize a positive electrode 430 with a large discharge capacity per unit weight of carbon. The material for the negative electrode 420 is selected as needed, but examples include zinc (Zn), iron (Fe), cadmium (Cd), and lead (Pb). The alkaline water electrolyte 440 is, for example, an aqueous solution of potassium hydroxide (KOH).

[0069] [Operation of a metal-air secondary battery] During the discharge of a metal-air secondary battery, the following reaction proceeds at the positive electrode 430. O2 + 2H2O + 4e - →4OH -

[0070] The discharge reaction of the negative electrode 420 differs depending on the type of metal that makes up the negative electrode 420. For example, if the negative electrode 420 is made of Zn, 2Zn + 4OH - →2ZnO+2H2O+4e - That is the case.

[0071] The charging reaction of the positive electrode 430 is the reverse of the discharging reaction, and is as follows: 4OH - →O2+2H2O+4e -

[0072] The charging reaction of the negative electrode 420 differs depending on the type of metal that makes up the negative electrode 420. For example, if the negative electrode 420 is made of Zn, 2ZnO + 2H2O + 4e - →2Zn+4OH - That is the case.

[0073] According to the fourth embodiment, Na has high OER activity. x Al y B 14+z Oxygen evolution catalyst or Na x B 29+y By using a positive electrode 430 containing an oxygen evolution catalyst, the efficiency of the oxygen evolution reaction in the positive electrode 430 can be improved, which in turn enables high-speed charging of the metal-air secondary battery.

[0074] <Fifth Embodiment> [Artificial photosynthesis system] A fifth embodiment of an artificial photosynthesis system will be described.

[0075] Figure 24 shows an artificial photosynthesis system. As shown in Figure 24, in the artificial photosynthesis system, a reaction vessel 500 in which the light-dependent reaction takes place and a reaction vessel 600 in which the dark-dependent reaction takes place are connected on the side via a proton-conducting membrane 700. The reaction vessel 500 contains water 510, and a light-dependent electrode 520 is inserted into the water 510. The light-dependent electrode 520 has, for example, an electrode 521 and a visible light-responsive photoexcitation material layer 522 provided on the portion of the electrode 521 that is immersed in water 510. The surface of the visible light-responsive photoexcitation material layer 522 is Na according to the first embodiment. x Al y B 14+z Oxygen evolution catalyst or Na according to the second embodiment x B 29+y An oxygen-evolving catalyst is supported. The reaction vessel 600 contains water 610 in which carbon dioxide (CO2) is dissolved, and the dark reaction electrode 620 is inserted into this water 610. The dark reaction electrode 620 has an electrode 621 and a catalyst layer 622 provided on the portion of the electrode 621 that is immersed in the water 610. The light reaction electrode 520 and the dark reaction electrode 620 are connected to each other by a wire 800.

[0076] [How to operate an artificial photosynthesis system] In the reaction vessel 500, when sunlight is incident on the visible light-responsive photoexcitation material layer 522 provided on the electrode 521 of the light-reactive electrode 520, electrons (e - )-Holes (h + ) pairs are generated. Of these, holes (h + ) by 2H2O+4h + →O2+4H + Oxygen is produced in this reaction. Protons (H) are produced in this reaction. + The electrons move through the proton-conducting film 700 to the reaction vessel 600. Meanwhile, the electrons from the electron-hole pairs generated in the visible light-responsive photo-excitation material layer 522 flow through the electrode 521 and the wire 800 to the dark reaction electrode 620. Then, in the reaction vessel 600, - +H + Carbohydrates are produced by the reaction with +CO2. Note that if CO2 is not dissolved in water 610, e - +H +→Hydrogen can be generated through the reaction of H2.

[0077] According to the fifth embodiment, Na has high OER activity. x Al y B 14+z Oxygen evolution catalyst or Na x B 29+y By using the light-dependent electrode 520 containing an oxygen-evolving catalyst, the oxygen-evolving reaction at the light-dependent electrode 520 can be accelerated, allowing for efficient oxygen generation. This enables efficient artificial photosynthesis.

[0078] Although embodiments and examples of this invention have been described in detail above, this invention is not limited to the embodiments and examples described above, and various modifications based on the technical idea of ​​this invention are possible.

[0079] For example, the numerical values, materials, shapes, and arrangements mentioned in the above embodiments and examples are merely examples, and different numerical values, materials, shapes, and arrangements may be used as needed. [Explanation of Symbols]

[0080] 10…NaAl y B 14 Crystal, 20...Na ion conductor, 30...Na x AlB 14 Crystal, 110... Sample chamber, 120... Carbon heater, 130... Support, 140, 150... Current introduction terminal, 160, 170... Lid, 180... Anode, 190... Cathode, 220... Zeolite layer, 230... NaAlB 14 240...Graphite layer, 300...Electrolytic cell, 310...Diaphragm, 320...Anode, 330...Cathode, 400...Container, 410...Separator, 420...Negative electrode, 430...Positive electrode, 440...Gas diffusion layer, 450...Alkaline water electrolyte, 500, 600...Reaction vessel, 520...Light reaction electrode, 521...Electrode, 522...Visible light-responsive photoexcitation material layer, 620...Dark reaction electrode, 621...Electrode, 622...Catalyst layer

Claims

1. An oxygen evolution catalyst comprising a boride containing at least boron and having an icosahedral boron cluster composed of 12 boron atoms.

2. The above boride is A x Al y B 14+z The oxygen-evolving catalyst according to claim 1, wherein A = alkali metal or alkaline earth metal, 0 ≤ x ≤ 1.0, 0.7 ≤ y ≤ 1.0, -1.0 ≤ z ≤ 1.

0.

3. The above A x Al y B 14+z is Na x Al y B 14+z The oxygen generation catalyst according to claim 2, which is such.

4. The oxygen-evolving catalyst according to claim 3, wherein 0.06 ≤ x ≤ 1.

0.

5. The above boride is A x B 29+y The oxygen-evolving catalyst according to claim 1, wherein A = alkali metal or alkaline earth metal, 0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.

0.

6. A above x B 29+y is Na x B 29+y The oxygen-evolving catalyst according to claim 5.

7. The oxygen-evolving catalyst according to claim 6, wherein 0.33 ≤ x ≤ 1.

90.

8. A method for producing an oxygen evolution catalyst, comprising producing an oxygen evolution catalyst using a boride obtained by removing at least a portion of the cations from a boride that contains at least alkali metal or alkaline earth metal cations and boron, and has an icosahedral boron cluster consisting of 12 borons.

9. Na x Al y B 14+z (0 ≤ x ≤ 1.0, 0.7 ≤ y ≤ 1.0, -1.0 ≤ z ≤ 1.0) or Na x B 29+y A hydrogen production apparatus by alkaline water electrolysis having an anode containing an oxygen-evolving catalyst (0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.0).

10. Na x Al y B 14+z (0 ≤ x ≤ 1.0, 0.7 ≤ y ≤ 1.0, -1.0 ≤ z ≤ 1.0) or Na x B 29+y A metal-air secondary battery having a positive electrode containing an oxygen-evolving catalyst (0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.0).

11. Na x Al y B 14+z (0 ≤ x ≤ 1.0, 0.7 ≤ y ≤ 1.0, -1.0 ≤ z ≤ 1.0) or Na x B 29+y An artificial photosynthesis system having a light-dependent electrode containing an oxygen-evolving catalyst (0 ≤ x < 2.0, -1.0 ≤ y ≤ 1.0).

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