JP5229875B2 - Hydrogen storage alloy - Google Patents

Description

  The present invention is a hydrogen storage alloy having a C14 type Laves structure that absorbs a large amount of hydrogen even in a wide low temperature range including below freezing point, and in particular, maintains a C14 type Laves structure without phase decomposition and generates a metal hydride. The present invention relates to hydrogen storage alloys and novel metal hydrides thereof.

  Hydrogen is clean energy that does not emit greenhouse gases, and is secondary energy produced from various primary energies. The main use of hydrogen energy is fuel for fuel cell vehicles, and it is required to store hydrogen in a lightweight and compact manner.

  In order for a fuel cell vehicle to have a cruising range comparable to that of a gasoline vehicle, it is thought that it is necessary to mount 5 kg of hydrogen required for traveling 500 km. To that end, the New Energy and Industrial Technology Development Organization (NEDO) has set the target storage amount of hydrogen storage materials at 5.5% by mass. Furthermore, it is also set as a goal that hydrogen storage / release of the alloy is performed at 150 ° C. or lower.

However, most of the conventional alloys such as AB type ₅ alloy, which is practically used as an electrode material for Ni-hydrogen secondary batteries, and Ti-based alloys developed for hydrogen storage systems of fuel cell vehicles, have a hydrogen storage capacity. Development of high-capacity hydrogen storage materials is an issue to be cleared because it is not sufficient at 3% by mass or less.
Development of lightweight hydrogen storage materials based on Li, Na, Mg, Al, and Ca has been promoted in order to solve the problem of increasing the hydrogen storage capacity of such hydrogen storage materials, particularly in terms of mass. Has been.

For example, Patent Document 1 discloses hydrogen storage characteristics of a Ca—Al-based hydrogen storage alloy having a C15 type Laves structure. According to this publication, the hydrogen storage capacity of the CaAl ₂ alloy is H / M = 0.09 (about 0.3 mass%) at 0 ° C., and the CaAl _1.8 B _0.1 Si _0.1 alloy is H / M = 0.25 (about 0.8 mass%) at 60 ° C. )Met.

Patent Document 2 discloses the hydrogen storage characteristics of a Ca—Mg—Ni-based hydrogen storage alloy. For example, the hydrogen storage capacity of a Ca _1.36 Mg _0.64 Ni _1.94 alloy was 1.26% by mass at 20 ° C.

Patent Document 3 discloses the hydrogen storage characteristics of a Ca _{x M y} Ni _z hydrogen storage alloy whose main phase is a C14 type Laves phase. The hydrogen storage capacity of Ca ₃₃ Mg ₆₀ Ni ₇ multiphase alloy in which CaMg ₂ phase with C14 type Laves structure and MgNi ₂ phase with C36 type Laves structure coexist is 4.9 mass% at 40 ℃.
However, these conventionally known hydrogen storage alloys have not yet solved the above-mentioned problems relating to the hydrogen storage amount. Further, although Patent Document 3 shows a high hydrogen storage amount, the occlusion of hydrogen requires a temperature of 40 ° C. or more, and the temperature is lower than this, for example, hydrogen is not occluded much at room temperature or below freezing point. was there.

On the other hand, Patent Document 4 discloses a CaLi ₂ alloy, and it is reported that the hydrogen storage amount of this alloy is at least 6.75% by mass at maximum.
However, although this CaLi ₂ alloy occludes hydrogen, the C14 type Laves structure decomposes during the occlusion, and it is difficult to reversibly occlude and release hydrogen, which is a Ca ₁ composed of a mixture of CaH ₂ and LiH. _{+ a} Li _{2 + b} H _{4 + c} Metal hydride is generated.
Therefore, this alloy is decomposed into a plurality of metal hydrides by occlusion of hydrogen and lacks reversibility, so that it can be used in a hydrogen storage system that requires repeated occlusion / release of hydrogen. It was an unsuitable hydrogen storage alloy.

Japanese Patent Laid-Open No. 8-13076 JP 2002-97535 A Japanese Patent Laid-Open No. 2003-96528 Special Table 2005-526678

  The present invention provides a novel hydrogen storage alloy having a C14-type Laves structure that can significantly increase the hydrogen storage capacity even in a wide range of low temperatures including below freezing point, and that is comparable to or surpassing the target value of the aforementioned hydrogen storage capacity. The purpose is to do.

In order to synthesize high-capacity hydrogen storage alloys that solve the above-mentioned problems, the present inventors have made use of basic research on metal-hydrogen systems and experience and achievements in the creation of new metal hydrides. As a result of repeated experiments and examinations, it was surprisingly found that a CaLi _2-x Mg _x (0 <x <2) alloy has excellent hydrogen storage properties, and the present invention has been completed.
That is, this application provides the following invention.
<1> composition represented by the general formula _{CaLi 2-x Mg x (0} <x <2), hydrogen storage alloy having a C14 type Laves structure is created by storing hydrogen, composition formula cali _{2- x} Mg _x H _y (0 <x <2, 0 <y ≦ 4).
<2> Temperature -30～40 ° C. and metal hydrides according to <1> which is generated by absorbing hydrogen under a hydrogen pressure 0.1～35MPa.
<3> Temperature -5 to 25 ° C. and a metal hydride according to <1> which is generated by absorbing hydrogen under a hydrogen pressure 0.1 to 10 MPa.

  Although the hydrogen storage alloy of the present invention is lighter than the conventional one, the hydrogen storage amount is remarkably increased, and a novel metal hydride is generated by hydrogen storage of the alloy. Furthermore, even in the low temperature range including below freezing point, the hydrogen storage amount is comparable to or exceeds the target value set by New Energy and Industrial Technology Development Organization (NEDO), so it is expected to be used in high-capacity hydrogen storage systems. Is done. In particular, the alloy of the present invention has the potential to extend the cruising range of a fuel cell vehicle to that of a gasoline vehicle, and is expected to be used and applied as a high-capacity hydrogen storage alloy that can replace conventional alloys. .

The hydrogen storage alloy of the present invention has a composition represented by the general formula CaLi _2-x Mg _x (0 <x <2) and has a C14 type Laves structure.
The hydrogen storage alloy without decomposing plurality of metal hydride by absorbing hydrogen, holds the C14 type Laves structure _{CaLi 2-x Mg x H y} (0 <x <2, 0 <y ≦ 4) A metal hydride is formed.
Here, C14 type Laves structure is MgZn type ₂ among the structures of a group of intermetallic compounds consisting of atoms A and B (atomic diameter r is r _A > r _B ) and represented by the composition formula of AB ₂ It is a hexagonal crystal represented by the generic name.
This hydrogen storage alloy has excellent hydrogen storage characteristics even in a low temperature range, and can efficiently store hydrogen, for example, at −30 to 40 ° C., preferably −5 to 25 ° C.
Moreover, hydrogen can be occluded in a wide hydrogen pressure range, and for example, hydrogen can be occluded efficiently under a hydrogen pressure of 0.1 to 35 MPa, preferably 0.1 to 10 MPa.
In the above composition formula, x takes a value of 0 <x <2. When x is within this value range, the CaLi _2-x Mg _x alloy does not decompose into multiple metal hydrides, and is 4% by mass that is comparable to or exceeds the hydrogen storage capacity of the conventionally known Ca-based hydrogen storage alloys. The above hydrogen is occluded and the characteristics as a hydrogen storage alloy are improved.
x is a value in the above range, but preferably 0 <x ≦ 1, more preferably 0.2 ≦ x ≦ 0.5, from the viewpoint of increasing hydrogen storage amount.

This CaLi _2-x Mg _x (0 <x <2) alloy itself is known, for example, described in 2002, Thermochimica Acta, Vol. That is, this alloy has a composition between CaLi ₂ and CaMg ₂ having a C14 type Laves structure, as shown in the Ca-Li-Mg system equilibrium diagram by the computational science published in the paper, _2-x Mg _x (0 <x <2) also has a C14-type Laves structure.

However, no attempt has been made to clarify the reaction between the CaLi _2-x Mg _x (0 <x <2) alloy and hydrogen.
That is, the CaLi _2-x Mg _x (0 <x <2) alloy shares the two components, and the C14 type Laves structure is decomposed by the occlusion of hydrogen like the CaLi ₂ and CaMg ₂ alloys that are considered to be similar alloys. Without CaH ₂ and LiH or MgH ₂ , while retaining the C14 type Laves structure and CaLi _2-x Mg _x H _y
(0 <x <2, 0 <y ≦ 4) The production of metal hydrides is something that has never been known before. This is an unexpected finding that has been found for the first time by the present inventors by examining the chemical properties of Ca, Li, Mg and hydrogen.

And this CaLi _2-x Mg _x (0 <x <2) alloy has hydrogen under a temperature of −30 to 40 ° C., preferably −5 to 25 ° C. and a hydrogen pressure of 0.1 to 35 MPa, preferably 0.1 to 10 MPa. Occludes to produce CaLi _2-x Mg _x H _y (0 <x <2, 0 <y ≦ 4) metal hydride with C14 Laves structure.
In the above composition formula, x is 0 <x <2, preferably 0 <x ≦ 1, and more preferably 0.2 ≦ x ≦ 0.5. y takes a value of 0 <y ≦ 4, preferably 2.5 ≦ y ≦ 4. When x and y are within this range, the CaLi _2-x Mg _x alloy does not decompose into multiple metal hydrides, and is at least 4% by mass that is comparable to or surpassing the hydrogen storage capacity of known Ca-based hydrogen storage alloys. The hydrogen storage alloy improves the properties as a hydrogen storage alloy.

Further, when the hydrogen storage amount of these metal hydrides is calculated in mass units, the CaLi _2-x Mg _x H _y (CaLi _2-x Mg _x (x = 0.2) alloy produced by occlusion of hydrogen) x = 0.2, y = 3.8) In the case of metal hydride, it is 6.2% by mass. It is an experimental fact to be emphasized that this value greatly exceeds the target value of 5.5 mass% set by the New Energy and Industrial Technology Development Organization (NEDO).

In addition, this hydrogen storage reaction occurs even at a low temperature of −5 ° C., and CaLi _2-x Mg _x H _y (x = 0.2, y = 3.8) metal hydride is CaLi _2-x Mg _x (x = 0.2) Retaining the C14-type Laves structure of the alloy is an outstanding hydrogen storage property. The characteristic hydrogen storage properties of such CaLi _2-x Mg _x (0 <x <2) alloys have been elucidated by our long-experienced advanced experimental techniques and original attention. This is an innovative matter.

The CaLi _2-x Mg _x H _y (0 <x <2, 0 <y ≦ 4) metal hydride having a C14 type Laves structure obtained in the present invention is a novel substance not described in any literature. Conventionally, metal hydrides having a C14 type Laves structure are known metal hydrides such as Ti-Mn and Zr-Mn alloys. However, lightweight Ca, Li, and Mg, which were first created in the present invention, are expected to be applied to hydrogen storage alloys for hydrogen storage systems of fuel cell vehicles.

  Hereinafter, the present invention will be described in detail with reference to examples.

Examples 1-2
Each metal raw material corresponding to the composition CaLi _2-x Mg _x (x = 0.2) and the composition CaLi _2-x Mg _x (x = 0.5) was weighed and melted in a high-frequency induction melting furnace. The high frequency melting was performed in a short time necessary and sufficient for all the metal to be melted and stirred by the induced current, and then a cast alloy ingot was produced in a copper mold. The prepared alloy ingot was chemically analyzed by inductively coupled plasma (ICP) spectroscopy to confirm that the alloy composition was desired. Thereafter, the alloy ingot was annealed at 135 ° C. for 24 hours in an argon atmosphere. There was no evidence of volatilization of some of the alloy elements during annealing.
FIG. 1 shows powder X-ray diffraction measurement of hydrogen storage alloy samples having the composition CaLi _2-x Mg _x (x = 0.2) (Example 1) and the composition CaLi _2-x Mg _x (x = 0.5) (Example 2). The results are shown.
From this result, it is clear that the CaLi _2-x Mg _x (x = 0.2) alloy (Example 1) and the CaLi _2-x Mg _x (x = 0.5) alloy (Example 2) have a C14 type Laves structure. From the X-ray diffraction peaks, it was found that the lattice constant of each alloy sample was the value shown in Table 1. Since CaLi _2-x Mg _x alloy easily reacts with oxygen in the atmosphere to generate oxides, an alloy sample was handled in an argon atmosphere when performing this measurement.
Moreover, the hydrogen storage amount of these hydrogen storage alloys was measured at each predetermined temperature by a pressure composition isotherm measurement method by a volume method. The results are shown in Table 1 and FIG.
As shown in Table 1, the hydrogen storage capacity _{is, CaLi 2-x Mg x (} x = 0.2) alloy (Example 1) 6.2 wt% at -5 ° C. _{is, CaLi 2-x Mg x (} x = 0.5) The alloy (Example 2) was 4.1% by mass at 10 ° C.
FIG. 3 shows the result of powder X-ray diffraction measurement of the metal hydride obtained after measurement of the hydrogen storage amount of these hydrogen storage alloys. Each metal hydride is mainly composed of C14 type Laves structure of the alloy before hydrogen occlusion, and forms CaLi _2-x Mg _x H _y (0 <x <2, 0 <y ≦ 4) metal hydride not described in the literature. Was shown to do.

Comparative Examples 1-2
Alloy samples having the composition CaLi ₂ (Comparative Example 1) and the composition CaMg ₂ (Comparative Example 2) were prepared by the same method as in Examples 1 and 2, and powder X-ray diffraction measurement was performed. As shown in FIG. 1, it was revealed that both alloys have a C14 type Laves structure.
In the same manner as in Examples 1 and 2, the hydrogen storage amount of the alloys of Comparative Examples 1 and 2 was measured. Table 1 shows the hydrogen storage temperature and the hydrogen storage amount. The hydrogen storage capacity of the CaLi ₂ alloy (Comparative Example 1) was 6.9% by mass at 0 ° C., and the CaMg ₂ alloy (Comparative Example 2) did not store hydrogen at 25 ° C. and was 2.2% by mass at 120 ° C. In addition, in the metal hydride obtained after measuring the hydrogen storage amount of both alloys, as shown in the powder X-ray diffraction measurement result of FIG. 3, the C14 type Laves structure of the alloy before hydrogen storage is not confirmed. It was. Furthermore, from the X-ray diffraction measurement results, it is clear that the CaLi ₂ alloy (Comparative Example 1) is decomposed into CaH ₂ and LiH and the CaMg ₂ alloy (Comparative Example 2) is decomposed into CaH ₂ and Mg after hydrogen storage. became. Therefore, assuming that these alloys are used as hydrogen storage alloys, it has been found that there is a problem that it is difficult to reversibly store and release hydrogen in a low temperature range including below freezing point.

Explanatory drawing of the X-ray-diffraction measurement result of the hydrogen storage alloy obtained in Examples 1-2 and Comparative Examples 1-2. Explanatory drawing of the measurement result of the hydrogen storage amount of the hydrogen storage alloy obtained in Examples 1-2. Explanatory drawing of the X-ray-diffraction measurement result of the metal hydride obtained in Examples 1-2 and Comparative Examples 1-2.

Claims (3)

The composition is represented by the general formula CaLi _2-x Mg _x (0 <x <2), and a hydrogen storage alloy having a C14 type Laves structure is formed by occlusion of hydrogen, and the composition is represented by the general formula CaLi _2-x Mg _x A metal hydride characterized by being represented by H _y (0 <x <2, 0 <y ≦ 4). Temperature -30～40 ° C. and metal hydride according to claim 1 that is generated by absorbing hydrogen under a hydrogen pressure 0.1～35MPa. Temperature -5 to 25 ° C. and a metal hydride according to claim 1 that is generated by absorbing hydrogen under a hydrogen pressure 0.1 to 10 MPa.

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