Hydride ion conductor

Abstract

The hydride ion conductor has the general formula represented by formula (1): Ba2-x-mAxMg1-y-nByH6-x-y-2m-2n where A and B are each selected from at least one or more species in the group consisting of Li, Na, K, Rb, and Cs, and 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ m ≤ 0.2, and 0 ≤ n ≤ 0.2, with the proviso that x = y = m = n = 0 is excluded.

Description

[Technical Field]

[0001] This invention relates to a hydride ion conductor. [Background technology]

[0002] A hydride ion (H) is composed of a hydrogen atom and two electrons. - ) is lighter than lithium ions, and its ionic radius is smaller than that of fluoride ions (F - It possesses many characteristics, including being equivalent to ), making it an electrochemically attractive charge carrier.

[0003] For example, in electrochemical devices such as fuel cells and secondary batteries, conventional protons (H + ) and lithium ions (Li + If hydride ion conductors are used as an alternative to ion conductors, it may be possible to realize novel energy devices.

[0004] Several hydride ion conductors exhibiting high ionic conductivity have been reported to date (e.g., Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Keiga Fukui,etal.,“Characteristic fast H- ion conduction in oxygen-substituted lanthanum hysride”,nature communications,(2019)10:2578 [Non-Patent Document 2] Maarten C. Verbraeken, etal., “High H- ionic conductivity in barium hydride”, nature materials, vol.14, p.95-p.100, January, 2015 [Overview of the project]

Problems to be Solved by the Invention

[0006] In order to apply a hydride ion conductor to an electrochemical device, high hydride ion conductivity is required, and in this regard, conventional hydride ion conductors are still considered insufficient.

[0007] The present invention has been made in view of such a background, and an object of the present invention is to provide a hydride ion conductor having higher ion conductivity.

Means for Solving the Problems

[0008] In the present invention, a hydride ion conductor, the general formula is Ba 2-x-m A x Mg 1-y-n B y H 6-x-y-2m-2n (1) formula Here, A and B are each selected from at least one or more of the group consisting of Li, Na, K, Rb, and Cs, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ m ≦ 0.2, 0 ≦ n ≦ 0.2, provided that x = y = m = n = 0 is excluded, A hydride ion conductor represented by is provided.

[0009] Also, in the present invention, a hydride ion conductor, A hydride ion conductor having an (NH4)SiF6 type structure is provided.

[0010] Furthermore, in the present invention, a hydride ion conductor, when 1000 times the reciprocal of the temperature T (K) is X and the common logarithm log(σ) of the conductivity (S / cm) of the hydride ion conductor is Y, (a) X < 1.6 and (b) 0 ≥ Y ≥ -3X + 3 A hydride ion conductor that satisfies the above is provided.

Advantages of the Invention

[0011] In the present invention, a hydride ion conductor having higher ionic conductivity can be provided.

Brief Description of the Drawings

[0012] [Figure 1] It is a diagram schematically showing the crystal structure of a hydride ion conductor (Ba2MgH6) according to an embodiment of the present invention. [Figure 2] It is a graph showing the temperature dependence of the conductivity of a hydride ion conductor (Ba2MgH6) according to an embodiment of the present invention together with the temperature dependence of a conventional hydride ion conductor. [Figure 3] It is a diagram schematically showing the flow of a method for manufacturing a hydride ion conductor according to an embodiment of the present invention. [Figure 4] It is a diagram showing the X-ray diffraction results of a hydride ion conductor (Sample A and Sample B) according to an embodiment of the present invention. [Figure 5] It is a graph showing the measurement results of the conductivity in a hydride ion conductor (Sample A and Sample B) according to an embodiment of the present invention. [Figure 6] It is a diagram collectively showing the X-ray diffraction results at each temperature in a hydride ion conductor (Sample C) according to an embodiment of the present invention. [Figure 7] It is a diagram collectively showing the X-ray diffraction results of a hydride ion conductor (Sample A to Sample C) according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] (Hydride ion conductor according to one embodiment of the present invention) In one embodiment of the present invention, A hydride ion conductor, The general formula is Ba 2-x-m A x Mg 1-y-n B y H 6-x-y-2m-2n (1) Formula Here, A and B are each selected from at least one of the groups consisting of Li, Na, K, Rb, and Cs. 0≦x≦1, 0≦y≦1, 0≦m≦0.2, 0≦n≦0.2, except x=y=m=n=0. A hydride ion conductor represented by [formula] is provided.

[0015] Furthermore, in one embodiment of the present invention, A hydride ion conductor, A hydride ion conductor having a (NH4)SiF6 type structure is provided.

[0016] A hydride ion conductor according to one embodiment of the present invention is, for example, Ba2MgH6 or Ba 1.9 K 0.1 MgH 5.9 These compounds all have a crystal structure of the (NH4)SiF6 type.

[0017] Figure 1 schematically shows the crystal structure of Ba2MgH6, a type of hydride ion conductor according to one embodiment of the present invention.

[0018] In Figure 1, the largest atom is the Ba atom, the next largest is the Mg atom, and the smallest atom is the hydrogen atom.

[0019] Furthermore, the right side of Figure 1 shows the transfer energies for each diffusion pathway of hydrogen (H) atoms assumed in the crystal structure of Ba2MgH6.

[0020] In the crystal structure of Ba2MgH6, the following four types of diffusion pathways for H atoms are possible: (I) Hopping between MgH6-MgH6 in the ab-plane (i-iii pathway); (II) Rotation of the MgH6 octahedron H in the ab plane (paths i-iv); (III) Hopping between MgH6-MgH6 in the c-axis direction (i-ii pathway); (IV) Rotation of the MgH6 octahedron H in the c-axis direction (path iv).

[0021] As shown on the right side of Figure 1, the Nudged Elastic Band calculation using VASP showed that the H atom migration energy was smallest in case (IV). However, in all cases, the H atom migration energy in the Ba2MgH6 crystal lattice was found to be sufficiently small, at a maximum of approximately 0.45 eV.

[0022] Thus, in the crystal structure of Ba2MgH6, the migration barrier for H atoms is sufficiently small, and therefore, Ba2MgH6 is expected to exhibit significantly high hydride ion conductivity.

[0023] Figure 2 shows the temperature dependence of the conductivity of a hydride ion conductor according to one embodiment of the present invention.

[0024] In Figure 2, the horizontal axis represents 1000 times the reciprocal of temperature T (1 / T), and the vertical axis represents conductivity σ (logarithmic scale). In Figure 2, (i) shows the conductivity characteristics of Ba2MgH6 according to one embodiment of the present invention. (ii) and (iii) show the conductivity characteristics of the hydride ion conductor (LaHO) described in Non-Patent Document 1 and the hydride ion conductor (BaH2) described in Non-Patent Document 2, respectively.

[0025] Figure 2 shows that Ba2MgH6 exhibits superior ionic conductivity compared to conventional LaHO hydride ion conductors and BaH2 hydride ion conductors in the temperature range of approximately 350°C to 500°C.

[0026] From the above considerations, it is expected that the conduction of Ba2MgH6 is due to hydride ion conduction.

[0027] Here, in FIG. 2, the thin dot region is a region called the "Norby gap", and it has been reported that it is difficult for proton (H + ) conduction to occur in this region (for details, see Solid State Ionics 125 1-11 (1999)).

[0028] The left boundary line L1 of this Norby gap region is represented by the following equation: Y = -3X + 3 (Equation (2)) Here, X is represented by 1000 times the reciprocal of the temperature T (K), and Y is the common logarithm log(σ) of the conductivity (S / cm).

[0029] The hydride ion conductor according to an embodiment of the present invention is characterized in that the conductivity belongs to the region shaded in FIG. 2.

[0030] Therefore, in one embodiment of the present invention, a hydride ion conductor, when 1000 times the reciprocal of the temperature T (K) is X and the common logarithm log(σ) of the conductivity (S / cm) of the hydride ion conductor is Y, (a) X < 1.6, and (b) 0 ≥ Y ≥ -3X + 3 A hydride ion conductor that satisfies the above is provided.

[0031] In particular, it is preferable that X is 1.3 < X < 1.5.

[0032] The hydride ion conductor according to an embodiment of the present invention may have a conductivity of 5 × 10 -2 S / cm or more within the Norby gap region. The hydride ion conductor according to an embodiment of the present invention may have a conductivity of 1 × 10 -1It is preferable that the conductivity be greater than or equal to S / cm.

[0033] Thus, the hydride ion conductor according to one embodiment of the present invention exhibits significantly higher ionic conductivity in a predetermined temperature range compared to conventional hydride ion conductors.

[0034] Therefore, it is expected that an electrochemical device with good properties can be realized when a hydride ion conductor according to one embodiment of the present invention is used.

[0035] (Method for producing a hydride ion conductor according to one embodiment of the present invention) A method for producing a hydride ion conductor according to one embodiment of the present invention will be briefly described below with reference to Figure 3.

[0036] Figure 3 schematically shows a flow chart of a method for manufacturing a hydride ion conductor according to one embodiment of the present invention.

[0037] As shown in Figure 3, the method for producing a hydride ion conductor according to one embodiment of the present invention is as follows: (i) A step of preparing a mixed powder by mixing predetermined raw material powders (step S110), (ii) A step of synthesizing a hydride ion conductor by calcining the mixed powder in a high-temperature, high-pressure environment (step S120), It has.

[0038] Furthermore, because the hydride ion conductor according to one embodiment of the present invention is highly reactive, each step is carried out under an argon environment.

[0039] The following describes each step.

[0040] (Step S110) First, the powder for the raw materials is prepared.

[0041] The raw materials may include hydrides of each metal, namely BaH2, AH, MgH2, and BH (where A and B are each selected from at least one of the group consisting of Li, Na, K, Rb, and Cs, and B may be the same as or different from A).

[0042] For example, if the hydride ion conductor is Ba2MgH6, BaH2 and MgH2 may be used as raw materials.

[0043] Each raw material may be thoroughly mixed using a ball mill or similar device.

[0044] (Process S120) Next, the mixed powder is calcined under high temperature and high pressure conditions to synthesize a hydride ion conductor.

[0045] A cubic anvil high-pressure device may be used for synthesis.

[0046] When using this device, a cubic cell called a pyrophyllite cell is used, and a mixed powder is filled inside this cell. Then, by generating ultra-high hydrostatic pressure using a cubic anvil high-pressure device, the six sides of the pyrophyllite cell installed inside can be isotropically pressurized.

[0047] The pressure applied to the pyrophyllite cell is, for example, in the range of 2 GPa to 6 GPa.

[0048] The firing temperature is, for example, in the range of 700°C to 1000°C.

[0049] Through the above process, a hydride ion conductor with the high ionic conductivity described above can be manufactured.

[0050] The above manufacturing method is merely an example, and the hydride ion conductor according to one embodiment of the present invention may be manufactured by a different manufacturing method. [Examples]

[0051] Next, embodiments of the present invention will be described.

[0052] Hydride ion conductor samples were prepared using the following method. The properties of the prepared samples were also evaluated.

[0053] (Example 1) (Sample preparation) Samples for evaluation were prepared using the following method.

[0054] (Preparation of Sample A) Under an Ar atmosphere, 1.827 g of BaH2 powder (manufactured by Sigma-Aldrich) and 0.173 g of MgH2 powder (manufactured by Sigma-Aldrich) were weighed to prepare a mixed powder.

[0055] The average particle size of the BaH2 powder was 10 μm, and the average particle size of the MgH2 powder was also 10 μm. Furthermore, the molar ratio of BaH2:MgH2 was set to 2:1. That is, the target composition of the mixed powder was the stoichiometric ratio of Ba2MgH6.

[0056] The resulting mixed powder was placed in a planetary ball mill and ground and mixed at room temperature. The rotation speed was set to 200 rpm, and the processing time was 12 hours.

[0057] This resulted in the creation of sample A.

[0058] (Preparation of Sample B) Sample B was prepared using the same method as Sample A.

[0059] However, in Sample B, the raw materials used were 1.794 g of BaH2 powder (manufactured by Sigma-Aldrich), 0.178 g of MgH2 powder (manufactured by Sigma-Aldrich), and 0.027 g of KH powder (manufactured by Sigma-Aldrich).

[0060] The average particle size of the KH powder is 10 μm. The molar ratio of BaH2:MgH2:KH was set to 1.9:1:0.1. That is, the composition of the mixed powder is Ba 1.9 K 0.1 MgH 5.9 That was the goal.

[0061] (evaluation) (X-ray diffraction analysis) The crystalline phases of samples A and B were evaluated using a benchtop X-ray diffraction analyzer (MiniFlex600; manufactured by RIGAKU Corporation).

[0062] (AC impedance measurement) Samples A and B were molded to produce molded bodies with a diameter of approximately 6 mm and a thickness of approximately 2 mm, respectively. Gold electrodes were placed in contact with both bottom surfaces of these molded bodies, and AC impedance measurements were performed using an atmosphere-controlled measurement cell.

[0063] A VSP-300 (Biologic Co., Ltd.) was used as the measuring device. The measurement frequency was set to 1 Hz to 7 MHz, and the applied AC voltage was set to 50 to 500 mV. The measurements were performed in a hydrogen atmosphere.

[0064] Figure 4 shows the X-ray diffraction results for samples A and B. In Figure 4, the peaks marked with circles correspond to the Ba2MgH6 crystalline phase.

[0065] Figure 4 confirms that the dominant phase in all samples is the Ba2MgH6 phase. Furthermore, sample B shows a narrower diffraction peak width compared to sample A, indicating higher crystallinity.

[0066] Figure 5 shows the conductivity results obtained from AC impedance measurements at each temperature.

[0067] Figure 5 shows that in all samples, the logarithm of conductivity changes linearly with respect to the reciprocal of temperature, indicating that the ion conduction mechanism follows the Arrhenius equation.

[0068] In sample B, Ba 2+ Ions are K + Ions are substituted, and hydrogen vacancies are actively introduced. Sample A showed higher conductivity than Sample B in the temperature range from room temperature to 200°C. Furthermore, Figure 4 shows that Sample A has lower crystallinity than Sample B. From this, it can be concluded that in Sample A, Schottky defects of one Ba atom and two H atoms, or one Mg atom and two H atoms, have been generated, meaning that in equation (1), there is no mixing of A and B, i.e., x=y=0 and m≠0 or n≠0, making it an ionic conductor.

[0069] Sample A showed high conductivity values ​​in the temperature range from room temperature to 200°C. Therefore, measurements were not performed at temperatures higher than 200°C.

[0070] (Experiment 2) (Sample preparation) A sample for evaluation (hereinafter referred to as "Sample C") was prepared using the following method.

[0071] BaH2 powder and MgH2 powder were weighed in a molar ratio of BaH2:MgH2=2:1.1, then ground in a mortar for 20 minutes and mixed to prepare a mixed powder.

[0072] The reason for including a slight excess of Mg in the mixed powder is to avoid deviations in the composition of the final sample due to reactions with the boron nitride tubes used later.

[0073] Next, the resulting mixed powder was compressed and sealed in a boron nitride tube. Furthermore, this tube was assembled into a pyrophyllite cell, and the mixed powder was calcined using a cubic anvil high-pressure device.

[0074] The firing conditions were 5 GPa and 900°C, and the product was fired for 30 minutes.

[0075] This yielded sample C.

[0076] (evaluation) (X-ray diffraction analysis) X-ray diffraction analysis of sample C was performed using the BL19B2 line at the SPring-8 synchrotron radiation facility (wavelength: 0.5 Å).

[0077] Measurements were performed in a quartz glass capillary with an inner diameter of 0.1 mm under an argon atmosphere, at temperatures ranging from room temperature to 500°C. Specifically, X-ray diffraction analysis was performed at room temperature, followed by heating the sample to a predetermined temperature and performing the same measurement. This process was repeated up to 500°C.

[0078] (AC impedance measurement) Sample C was polished under an argon atmosphere to prepare a cylindrical sample with a diameter of approximately 4 mm and a thickness of 1 mm.

[0079] Gold electrodes were placed in contact with both bottom surfaces of the obtained sample, and AC impedance measurements were performed using an atmosphere-controlled measurement cell.

[0080] A VSP-300 (Biologic Co., Ltd.) was used as the measuring device. The measurement frequency was set to 1 Hz to 7 MHz, and the applied AC voltage was set to 50 to 500 mV. The measurements were performed in a hydrogen atmosphere. Conductivity was calculated from the measurement results (cole-cole plot).

[0081] Figure 6 summarizes the X-ray diffraction results for sample C at various temperatures. In Figure 6, the peaks marked with circles correspond to the Ba2MgH6 crystalline phase.

[0082] Figure 6 shows that the dominant phase in sample C was Ba2MgH6 at all measurement temperatures.

[0083] Furthermore, as the temperature of sample C increased, peaks of a phase different from the main phase, indicated by △, appeared. These correspond to the BaH2 phase.

[0084] Furthermore, these results indicate that the position of the main phase peak tends to shift to a lower angle as the temperature increases. This trend corresponds to the expansion of the crystal lattice as the temperature rises.

[0085] Figure 7 shows the X-ray diffraction results (Figure 4) again. However, this Figure 7 also shows the measurement results for sample C at room temperature using a benchtop X-ray diffraction analyzer (MiniFlex600; RIGAKU Corporation), in addition to the measurement results for sample A and sample B.

[0086] A comparison of sample A (or sample B) with sample C reveals that in sample C, the peak of the main phase, Ba2MgH6, is sharper and has a smaller full width at half maximum. This suggests that sample C contains Mg2MgH6 with higher crystallinity.

[0087] Figure 2(i) above shows the relationship between temperature and ionic conductivity obtained for sample C.

[0088] Figure 2 shows that in sample C, the conductivity in the high-temperature range falls within the Norbigap region, and a significantly higher conductivity is obtained. In particular, around 450°C, 10 -1 High conductivity exceeding S / cm was achieved.

[0089] To the best of the applicant's knowledge, no hydride ion conductor exhibiting such high conductivity at around 450°C has been previously identified.

[0090] Furthermore, Figure 2 shows that the conductivity of sample C increases significantly in the temperature range above approximately 350°C.

[0091] Figure 6 shows the synchrotron XRD results at various temperatures, confirming the precipitation of the BaH2 phase (△) above 370°C. Despite the precipitation of BaH2, the Ba2MgH6 phase has not decomposed, suggesting that Ba and H vacancies have been introduced into Ba2MgH6. Rietveld analysis of the synchrotron XRD results confirmed that the Ba2MgH6 phase above 370°C has Ba vacancies corresponding to the mass fraction of precipitated BaH2. In sample C, it is thought that the H vacancies were introduced not by the introduction of foreign elements, but by the desorption of BaH2 due to the increase in temperature. 2-x―m A x Mg 1-y―n B y H 6-x-y―2m-2n (x=y=n=0) was generated.

[0092] Based on these considerations, it is thought that in sample C, the conductivity jumped at approximately 350°C, resulting in high ionic conductivity.

[0093] This application claims priority based on Japanese Patent Application No. 2021-130270, filed on 6 August 2021, and the entire contents of the said Japanese application are incorporated herein by reference.

Claims

1. A hydride ion conductor, The general formula is Ba ２－ｘ－ｍ A ｘ Mg １－ｙ－ｎ B ｙ H ６－ｘ－ｙ－２ｍ－２ｎ Formula (1) Here, A and B are each selected from at least one of the group consisting of Li, Na, K, Rb, and Cs. Unlike A, B is 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ m ≤ 0.2, 0 ≤ n ≤ 0.2, except x = y = m = n = 0. A hydride ion conductor represented by [formula].

2. (NH ４ ) SiF ６ A hydride ion conductor according to claim 1, having a type structure.

3. When X is 1000 times the reciprocal of the temperature T (K), and Y is the common logarithm log(σ) of the conductivity (S / cm) of the hydride ion conductor, (a) X < 1.6, and (b) 0≧Y≧-3X+3 A hydride ion conductor according to claim 1, satisfying the following conditions.

4. The general formula is Ba₂₋ₘMg₁₋ₙH₆₋₂ₘ₋₂ₙ (excluding m = n = 0) or Ba １．９ K ０．１ MgH ５．９ The hydride ion conductor according to any one of claims 1 to 3, represented by

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