US20090035175A1

US20090035175A1 - Hydrogen storage material

Info

Publication number: US20090035175A1
Application number: US12/183,807
Authority: US
Inventors: Ryoji Takenawa; Takanori Suzuki; Norio Komura; Izuru Kanoya; Mitsuya Hosoe
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2007-08-01
Filing date: 2008-07-31
Publication date: 2009-02-05
Also published as: JP4925962B2; JP2009035772A

Abstract

An unactivated, poorly activatable hydrogen storage component and an activated hydrogen storage component are mixed to prepare a hydrogen storage material. When the hydrogen storage material is activated, the poorly activatable hydrogen storage component is converted to a hydrogen storable state in a remarkably short time. The poorly activatable hydrogen storage component may be a V—Cr—Ti hydrogen storage alloy having a body-centered cubic (BCC) crystal structure. The activated hydrogen storage component preferably is MgH_x(0.1≦x≦2) doped with a nanoparticle of at least one atom selected from the group of Ni, Fe, Ti, Mn, and V.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a hydrogen storage material, which can store hydrogen when activated.
2. Description of the Related Art
Fuel-cell cars, which use fuel cells as a power source, have recently been attracting much attention from the viewpoint of environmental protection. Fuel-cell cars do not emit greenhouse gases such as NO_X, SO_X, CO₂, and hydrocarbon gases, and discharge only H₂O, which is generated by a reaction between hydrogen and oxygen that are contained within the fuel gas supplied to an anode and the oxidant gas supplied to a cathode in the fuel cell.
In general, the fuel gas and the oxidant gas are hydrogen gas and air, respectively. Therefore, a fuel-cell car includes a vessel therein for storing hydrogen.
When a larger amount of hydrogen can be stored in the hydrogen storage vessel, the fuel cell can be operated for a longer period, and thus the fuel-cell car can be driven a longer distance. From this viewpoint, various methods for increasing the hydrogen storage capacity of the hydrogen storage vessel have been studied. In one of these methods, a hydrogen storage material such as a hydrogen storage alloy is placed inside the hydrogen storage vessel.
The hydrogen storage material is capable of storing hydrogen at a volume that is larger than the material's own volume. Thus, by using such a hydrogen storage material, the hydrogen storage capacity of the hydrogen storage vessel can be increased. Further, hydrogen is reversibly stored within the hydrogen storage material, so that the required amount of hydrogen can be released from the material in order to operate the fuel cell.
The surface of the hydrogen storage material (particularly the hydrogen storage alloy) is covered with an oxide layer initially, and the material cannot store hydrogen in this state. Thus, the hydrogen storage material is subjected to an activation treatment in order to reduce and remove the oxide layer at a predetermined hydrogen pressure and temperature. The hydrogen storage material is made capable of reversibly storing and releasing hydrogen as a result of the activation treatment.
However, it is difficult to activate a hydrogen storage material having a body-centered cubic (BCC) crystal structure, such as a V—Ti—Cr hydrogen storage alloy. For example, such a material is activated by repeating evacuation to 10⁻⁴torr at 500° C. and pressurization to a hydrogen pressure of 50 atm four times (see Japanese Laid-Open Patent Publication No. 10-110225), or by repeatedly evacuating, maintaining the hydrogen storage material at 400° C. at a hydrogen pressure of 8 MPa for one hour, and then cooling to room temperature three times.
In the above method, the hydrogen storage material may be activated after introducing the material into the hydrogen storage vessel. In this case, the hydrogen pressure and temperature required for activation must be controlled within an allowable pressure and temperature range for the hydrogen storage vessel. When a component of the hydrogen storage vessel, such as a liner or a sealant, is composed of a resin, the vessel has an upper allowable pressure limit of 10 MPa and an upper allowable temperature limit of 100° C. Activation of a V—Ti—Cr hydrogen storage alloy requires a long time, i.e., about 75 hours, under such pressure and temperature conditions. Thus, disadvantageously, it takes several tens of hours to activate the hydrogen storage material at such a relatively low pressure and temperature.
A hydrogen storage alloy, which has a mixed phase of a BCC alloy phase and a Laves phase, thus enabling the hydrogen storage alloy to be easily activated, is described in Japanese Laid-Open Patent Publication No. 10-245653. In paragraph [0008] of Japanese Laid-Open Patent Publication No. 10-245653, it is conjectured that the Laves phase is easily activated, hydrogenated, and pulverized, so that fractures are propagated to the poorly activatable BCC alloy phase. Then, the BCC alloy phase, which is not poisoned with air, is exposed at the fracture surface, and the BCC alloy phase becomes hydrogenated and pulverized from the exposed surface, whereby activation of the alloy is accelerated.
In the production of a BCC hydrogen storage alloy, the mixed phase of the BCC alloy phase and the Laves phase can be formed by adding a small amount of Zr during a step of melting a starting material powder. Thus, a complicated process for controlling the Zr amount, etc., is required when forming the mixed phase.
Further, Zr has a relatively large atomic weight, and thereby deteriorates the hydrogen storage capacity per unit weight of the hydrogen storage alloy.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a hydrogen storage material, which can be activated in a short period of time, even under low temperature and low pressure conditions.
A principal object of the present invention is to provide a hydrogen storage material that can be activated without complicated processes.
Another object of the present invention is to provide a hydrogen storage material having a large hydrogen storage capacity per unit weight.
According to an aspect of the present invention, there is provided a hydrogen storage material comprising a poorly activatable hydrogen storage component and an activated hydrogen storage component, wherein 10 hours or more are required to activate the poorly activatable hydrogen storage component at a hydrogen pressure of 10 MPa or less at a temperature of 100° C. or lower.
In the present invention, the activated hydrogen storage component is mixed with the poorly activatable hydrogen storage component. The poorly activatable hydrogen storage component, after being mixed with the activated hydrogen storage component, can be activated in a remarkably short period of time, even under low temperature and low pressure conditions. Thus, compared to a case of activating the poorly activatable hydrogen storage component on its own, the time required for activating the poorly activatable hydrogen storage component is significantly shorter when mixed with the activated hydrogen storage component. In short, according to the present invention, the poorly activatable hydrogen storage component can be activated remarkably easily.
Since the poorly activatable hydrogen storage component can be activated under low temperature and low pressure conditions, the hydrogen storage material can be activated in a short time even though it is contained in a hydrogen storage vessel. Thus, even in cases where a substance contained in the hydrogen storage material is likely to be readily deactivated by air, the hydrogen storage material can be activated in a short time, while preventing deactivation of the substance by using the hydrogen storage vessel.
Further, in the present invention, an additional element is not added during production of the hydrogen storage material, so that a complicated process for controlling the element amount, etc., is not required. In addition, the hydrogen storage capacity per unit weight of the hydrogen storage alloy does not become deteriorated by such an element.
The reason that activation of the poorly activatable hydrogen storage component is accelerated due to mixing thereof with the activated hydrogen storage component is presumed to be because an active hydrogen atom stored in the activated hydrogen storage component is transferred over to an oxide layer formed on the surface of the poorly activatable hydrogen storage component, and thus the oxide layer is reduced by the presence of the hydrogen atom.
The poorly activatable hydrogen storage component is not particularly limited. Preferred examples thereof include hydrogen storage alloys having body-centered cubic (BCC) crystal structures. Thus, in the present invention, a substance considered to be resistant to activation can be activated remarkably easily.
Preferred examples of the activated hydrogen storage component include MgH_x(0.1≦x≦2) doped with a nanoparticle of at least one atom selected from the group of Ni, Fe, Ti, Mn, and V. Such an MgH_xcomponent does not become significantly deactivated in air, and therefore the MgH_xcomponent can be easily handled. For example, the MgH_xcomponent can be mixed with the poorly activatable hydrogen storage component in air.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing elapsed time-stored hydrogen amount relations of hydrogen storage materials according to an example of the present invention, compared with a comparative hydrogen storage material; and

FIG. 2 is a graph showing elapsed time-stored hydrogen amount relations of hydrogen storage materials according to another example of the present invention, compared with another comparative hydrogen storage material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the hydrogen storage material of the present invention will be described in detail below with reference to the accompanying drawings.
The hydrogen storage material according to the present embodiment comprises a mixture of a poorly activatable hydrogen storage component and an activated hydrogen storage component, wherein the mixture (the hydrogen storage material) is contained in a hydrogen storage vessel.
Activation of the poorly activatable hydrogen storage component requires a long time, i.e., 10 hours or more, within allowable hydrogen pressure and temperature ranges of the hydrogen storage vessel. More specifically, the poorly activatable hydrogen storage component is such that 10 hours or more are required to activate the component at a hydrogen pressure of 10 MPa or less and at a temperature of 100° C. or lower. Preferred examples of such poorly activatable hydrogen storage components include BCC hydrogen storage alloys, specifically, V—Cr—Ti, V—Cr—Al, V—Cr—Mo, V—Ti—Mo, V—W, V—Cr—Ti—Al, and V—Cr—Mo—Al hydrogen storage alloys.
Such a BCC hydrogen storage alloy cannot be activated easily. However, once the BCC hydrogen storage alloy has been activated, it can store a large amount of hydrogen therein, so that the hydrogen storage capacity per unit weight of the hydrogen storage material increases.
Preferred examples of the poorly activatable hydrogen storage components further include alloys having a high hydrogen storage equilibrium pressure (a gauge pressure of 1 MPa or more) at room temperature, specifically AB2 alloys (such as Ti—Zr—Fe—Cr—Ni alloys, Ti—Fe—Cr—Mn alloys, and Ti—Fe—Cr—Cu alloys) and AB5 alloys (such as La—Ce—Ni—Mn alloys and La—Ce—Ni—Fe alloys).
The activated hydrogen storage component is not particularly limited, and may be any substance as long as it is activated beforehand. For example, the activated hydrogen storage component may be prepared by activating the above described poorly activatable hydrogen storage component, or by activating LaNi₅, TiCr₂, or the like, all of which can be activated relatively easily.
Alternatively, the activated hydrogen storage component may be MgH_x(0.1≦x≦2) doped with a nanoparticle of at least one atom selected from the group of Ni, Fe, Ti, Mn, and V. The term “nanoparticle” is defined as a fine particle having an average particle diameter of 10 nm or less.
Such an MgH_xcomponent has a hydrogenated surface, and therefore the MgH_xcomponent is not significantly oxidized in air. In other words, the MgH_xcomponent is hardly deactivated, even in air. Thus, the MgH_xcomponent can be mixed together with the poorly activatable hydrogen storage component in air, thus enabling the hydrogen storage material of the present embodiment to be produced easily. Further, the hydrogen storage material can store hydrogen slowly but steadily, even at a relatively low temperature such as room temperature.
The nanoparticle-doped MgH_xcomponent may be prepared by mixing Mg particles and nanoparticles, followed by mechanically grinding the mixture under an increased hydrogen pressure.
Before mixing of the activated hydrogen storage component with the poorly activatable hydrogen storage component is carried out, hydrogen may either be stored or not stored in the activated hydrogen storage component. Of course, a component storing hydrogen and a component that does not store hydrogen therein may be used in combination.
The weight ratio of the activated hydrogen storage component to the hydrogen storage material may be 0.1% to 10% by weight.
The hydrogen storage material of the present embodiment may be produced by mixing and stirring the poorly activatable hydrogen storage component together with the activated hydrogen storage component. If such a production method is used, the content of the activated hydrogen storage component should be about 0.1% to 10% by weight.
If the activated hydrogen storage component is a substance that is likely to be deactivated in air, the activated hydrogen storage component may be mixed with the poorly activatable hydrogen storage component in an inert gas atmosphere such as nitrogen or argon.
The hydrogen storage material produced in the above manner may be activated inside the hydrogen storage vessel. In this case, the hydrogen storage vessel is heated to a predetermined temperature, and is filled with hydrogen at a predetermined pressure. The temperature and the hydrogen pressure may be determined depending on the heat resistance and pressure resistance of the hydrogen storage vessel. Generally, this temperature is 100° C. or lower and the hydrogen pressure is 10 MPa or less. More preferably, the temperature is 80° C., whereas the hydrogen pressure is about 4 to 8 MPa, respectively.
In accordance with the present embodiment, activation of the hydrogen storage material is completed in a remarkably short period of time, as compared to a case of activating a poorly activatable hydrogen storage component on its own. For example, in the case of activating a poorly activatable BCC hydrogen storage alloy of V-13.5Cr-4Ti alone, at 80° C. and 5 Mpa respectively, the activation time for converting the alloy to a hydrogen storable state is long, requiring 75 hours. In the composition formula V-13.5Cr-4Ti, as well as in the composition formulae to be described hereinafter, each numeral represents an atomic percentage unless otherwise noted. In contrast, in the case of adding 1% by weight of an activated V-13.5Cr-4Ti alloy to an unactivated V-13.5Cr-4Ti alloy, the activation time for converting the mixture to a hydrogen storable state is considerably shorter, requiring a time of only one hour. Also, in the case of using a mixture of another poorly activatable hydrogen storage component, the required activation time also is shorter, requiring a time of at most 10 hours.
When the hydrogen storage material becomes deactivated in the hydrogen storage vessel, the material can be activated again within a short period of time. As described above, the hydrogen storage material can be activated in a remarkably short time.
As made clear from the above, in the present embodiment, the hydrogen storage material can be activated without complicated processes. Further, an additional element is not added during production of the hydrogen storage material, so that the hydrogen storage capacity per unit weight of the hydrogen storage alloy does not become deteriorated by such an additional element.
The reasons why the time required for activating the poorly activatable hydrogen storage component can be significantly shortened simply by adding the activated hydrogen storage component thereto are not fully understood at present. However, it is presumed that a phenomenon, which is similar to spillover in catalytic chemistry, is caused in the hydrogen storage material. Thus, it is presumed that an active hydrogen atom stored in the activated hydrogen storage component is transferred over to an oxide layer on the poorly activatable hydrogen storage component, so that the oxide layer is reduced by the active hydrogen atom, whereupon the poorly activatable hydrogen storage component becomes activated into a hydrogen storable state.

EXAMPLE 1

According to the alloy composition formula V-13.5Cr-4Ti, 16.49 kg of V, 2.75 kg of Cr, and 0.75 kg of Ti were melted in a high-frequency melting furnace under an inert atmosphere.
The obtained melt was cast to form an ingot. The ingot was mechanically crushed and classified, in order to obtain 15 kg of V-13.5Cr-4Ti alloy particles, having an average particle diameter of 500 μm.
Then, 3 g of the particles were weighed, and placed inside a sealable vessel. The vessel was evacuated, then maintained at 400° C. at a hydrogen pressure of 8 MPa for one hour, and cooled to room temperature. The steps of evacuating, maintaining, and cooling the vessel were repeated three times in order to obtain an activated V-13.5Cr-4Ti alloy.
0.1 g of the activated V-13.5Cr-4Ti alloy and 1.9 g of an unactivated V-13.5Cr-4Ti alloy were added to a pressure-resistant vessel under a nitrogen gas atmosphere. The pressure-resistant vessel was closed, whereupon the alloys were stirred and mixed to produce a hydrogen storage material. Thus, the content of the activated V-13.5Cr-4Ti alloy was 5% by weight within the hydrogen storage material. In the same manner, hydrogen storage materials having activated V-13.5Cr-4Ti alloy contents of 1% by weight and 3% by weight were produced at a yield of 2 g respectively.
Each of the produced hydrogen storage materials was maintained for one hour in the pressure-resistant vessel at 80° C. and at a hydrogen pressure of 5 MPa. Then, the pressure-resistant vessel was cooled to room temperature.
The hydrogenation rate of each of the hydrogen storage materials was measured using a volumetric hydrogen pressure-composition isotherm (PCT) measurement apparatus. The hydrogen storage materials were introduced into a sample cell of the PCT measurement apparatus, and the sample cell was evacuated at 80° C. for one hour, then maintained at 80° C. and at a hydrogen pressure of 5 MPa for one hour, and cooled to room temperature. Measurements were initiated after completion of the cooling step.
For purposes of comparison, 2 g of an unactivated V-13.5Cr-4Ti alloy was added to a pressure-resistant vessel having the same shape, and the pressure-resistant vessel was maintained at 80° C. and at a hydrogen pressure of 5 MPa for one hour, after which the vessel was cooled to room temperature. The hydrogenation rate of the alloy was measured in the same manner as described above.
The results are shown in the graph of FIG. 1. In FIG. 1, elapsed time is shown on the horizontal axis, whereas the stored hydrogen amount of the hydrogen storage material is shown on the vertical axis. The stored hydrogen amount is represented by a weight ratio (% by weight) of the stored hydrogen to the hydrogen storage material.
As is clear from FIG. 1, the stored hydrogen amounts of the hydrogen storage materials, which were produced by adding the activated V-13.5Cr-4Ti alloy to the unactivated V-13.5Cr-4Ti alloy, at activated alloy contents of 1%, 3% and 5% by weight respectively, increased over time. This implies that the residual unactivated V-13.5Cr-4Ti alloy was activated, while the mixture was maintained at a temperature of 80° C. and a hydrogen pressure of 5 MPa for one hour.
On the other hand, in the Comparative Example using only the unactivated V-13.5Cr-4Ti alloy, the stored hydrogen amount was not increased. Thus, the unactivated V-13.5Cr-4Ti alloy could not be activated by being maintained at a temperature of 80° C. and a hydrogen pressure of 5 MPa for one hour, without the addition of the activated V-13.5Cr-4Ti alloy.
Although not indicated in FIG. 1, the stored hydrogen amount of the unactivated V-13.5Cr-4Ti alloy was increased after maintaining the alloy under a condition wherein the temperature thereof was 80° C. and the hydrogen pressure was 5 Mpa for 75 hours. Thus, the alloy was capable of being activated after maintaining the above condition for 75 hours.
As is clear from the above results, the time required for activation can be significantly shortened, simply by adding the activated V-13.5Cr-4Ti alloy.

EXAMPLE 2

5 g of an Mg powder, 0.036 g of an Ni powder, and 0.023 g of an Fe powder were added, together with 18 stainless-steel balls having a diameter of 10 mm, in a ball mill pot having a volume of 80 ml. Hydrogen was introduced into the ball mill pot at a pressure of 1 MPa, and the pot was closed.
Then, the ball mill pot was placed on a bedplate of a planetary ball mill having a diameter of 300 mm, and ball milling, as one type of mechanical grinding, was carried out for 10 hours at a bedplate revolution of 350 rpm and at a ball mill pot revolution of 800 rpm, in order to obtain 5.059 g of MgH_x(0.1≦x≦2) doped with Ni and Fe nanoparticles. Immediately after the ball milling, the surface of the MgH_xwas in a hydrogenated state, and thus the MgH_xwas in an activated state. Further, the rate of reaction between the hydrogenated surface and oxygen was remarkably low, so that the MgH_xwas not significantly deactivated in air.
0.1 g of MgH_xand 1.9 g of the above prepared unactivated V-13.5Cr-4Ti alloy particles were added to a sealable pressure-resistant vessel, and then stirred and mixed in air to produce a hydrogen storage material. Within the hydrogen storage material, the content of the activated MgH_xwas 5% by weight. In the same manner, hydrogen storage materials, each having an activated MgH_xcontent of 1% by weight and 3% by weight respectively, were produced at a yield of 2 g.
Each of the produced hydrogen storage materials was activated in the same manner as described above, by maintaining the materials at 80° C. and at a hydrogen pressure of 5 Mpa, for one hour inside the pressure-resistant vessel. Then, the pressure-resistant vessel was cooled to room temperature. Further, the hydrogenation rate of each of the hydrogen storage materials was measured using a PCT measurement apparatus.
The results are shown in the graph of FIG. 2, together with the aforementioned results of the Comparative Example. As clearly shown in FIG. 2, the time required for activation can be significantly shortened also in the case of adding the MgH_xactivated hydrogen storage component, which is derived from a substance other than the poorly activatable hydrogen storage component (the V-13.5Cr-4Ti alloy).
As described above, by mixing the activated hydrogen storage component with the poorly activatable hydrogen storage component in order to produce the hydrogen storage material, the poorly activatable hydrogen storage component can be activated in the hydrogen storage material in a remarkably short period of time.
Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims

1. A hydrogen storage material comprising a poorly activatable hydrogen storage component and an activated hydrogen storage component, wherein 10 hours or more are required to activate said poorly activatable hydrogen storage component at a hydrogen pressure of 10 MPa or less and at a temperature of 100° C. or lower.

2. A hydrogen storage material according to claim 1, wherein said poorly activatable hydrogen storage component comprises a body-centered cubic hydrogen storage alloy.

3. A hydrogen storage material according to claim 2, wherein said body-centered cubic hydrogen storage alloy is one of a V—Cr—Ti hydrogen storage alloy, a V—Cr—Al hydrogen storage alloy, a V—Cr—Mo hydrogen storage alloy, a V—Ti—Mo hydrogen storage alloy, a V—W hydrogen storage alloy, a V—Cr—Ti—Al hydrogen storage alloy, and a V—Cr—Mo—Al hydrogen storage alloy.

4. A hydrogen storage material according to claim 1, wherein said poorly activatable hydrogen storage component comprises one of a Ti—Zr—Fe—Cr—Ni alloy, a Ti—Fe—Cr—Mn alloy, and a Ti—Fe—Cr—Cu alloy.

5. A hydrogen storage material according to claim 1, wherein said poorly activatable hydrogen storage component comprises one of an La—Ce—Ni—Mn alloy and an La—Ce—Ni—Fe alloy.

6. A hydrogen storage material according to claim 1, wherein said activated hydrogen storage component comprises MgH_x(0.1≦x≦2) doped with a nanoparticle of at least one atom selected from the group of Ni, Fe, Ti, Mn, and V.

7. A hydrogen storage material according to claim 1, wherein said activated hydrogen storage component comprises an activated product of said poorly activatable hydrogen storage component.

8. A hydrogen storage material according to claim 1, wherein said activated hydrogen storage component comprises activated LaNi₅or TiCr₂.

9. A hydrogen storage material according to claim 1, wherein a weight ratio of said activated hydrogen storage component to said hydrogen storage material is 0.1% to 10% by weight.