JPH11130401A - High activation treatment of hydrogen storage alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To activate a Zr-Ni Laves phase hydrogen storage alloy so as to enhance the alloy capacity of a hydrogen storage alloy electrode. SOLUTION: A hydrogen storage alloy is hydrogenated by heating in a hot atmosphere of hydrogen and rapidly cooling immediately after the heating and then degassing is carried out by evacuation to highly activate the hydrogen storage alloy. It is preferable that the activated hydrogen storage alloy is heated in a KOH soln. after dipping in a soln. of HF and NH4 F.HF to which an Ni salt is optionally added. Activation frequency in initial activation is considerably reduced and the alloy capacity of a hydrogen storage alloy electrode can be enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for activating hydrogen storage alloy to promote the reaction of storing and releasing hydrogen.

[0002]

2. Description of the Related Art A hydrogen storage alloy is an alloy that stores a large amount of hydrogen, and its storage density is higher than that of liquid hydrogen (it is possible to store hydrogen in a volume of 1000 times or more). At present, it is a material to be considered for use in many fields such as hydrogen storage, batteries, and heat pumps. In actual use of the hydrogen storage alloy, it is necessary to activate the hydrogen storage alloy so that the hydrogen storage alloy can store and release hydrogen.

Conventionally, a hydrogen storage alloy powder has been melted in an inert gas atmosphere or a vacuum atmosphere, cast into a mold to form a cast material, and then subjected to a homogenization treatment at 900 to 1100 ° C. for a long time. After that, it is manufactured by a method of repeating mechanical crushing and / or occluding and releasing hydrogen in an inert gas atmosphere such as argon gas until a predetermined particle size is obtained.

[0004] Recently, it has been attempted to easily produce powder by employing a gas atomization method using an inert gas and dissolving a raw material to gas atomize a molten metal. However, since the hydrogen storage alloy contains a large amount of active metal elements, its surface is easily oxidized, and the hydrogenation reaction of the hydrogen storage alloy is hindered by the formed oxide layer. In particular, the powder produced by the gas atomization method has a thick oxide layer, which makes it difficult to initially store hydrogen when used as a hydrogen storage material.

Therefore, in order to cause these hydrogen storage alloys to store hydrogen, it is necessary to perform an initial activation process in which vacuum degassing is performed at a high temperature and high pressure hydrogen is introduced and activated many times. There is a problem that processing is complicated and costly. So, in order to solve these problems,
A method of immersing in a supersaturated aqueous solution of a metal fluoride compound, an acid, or an alkali solution has been proposed.

For example, Japanese Patent Application Laid-Open No. Hei 5-213601 discloses that when a hydrogen storage alloy is treated with a supersaturated aqueous solution of a metal fluoride compound such as K ₃ AlF ₆ as a chemical solution, the surface or the surface layer is highly activated. ing. This allows
It is stated that the hydrogen storage alloy can be easily activated, and the conventional high-temperature, high-vacuum degassing and high-temperature, high-pressure activation processes can be greatly eased. It also states that the activated alloy can be stabilized in the atmosphere and can provide a treatment method that is easy to handle.

JP-A-5-225975 discloses that a hydrogen storage alloy electrode from which an oxide film formed on the surface has been removed can be obtained by treating the hydrogen storage alloy with hydrochloric acid. At the time of initial activation of a battery using this hydrogen storage alloy electrode as a negative electrode, the initial discharge amount increases, and the charge / discharge cycle required for the initial activation decreases. In Japanese Patent Application Laid-Open No. 5-135765, a Zr-Ni-based general formula is represented by ABα (α = 1.5 to 2.5), and the alloy layer substantially belongs to the Laves phase of the intermetallic compound. The crystal structure of which is hexagonal C14 type or (and / or) cubic C15
A method is disclosed in which a hydrogen storage alloy powder, which is a mold, is used as an electrode together with a binder, and then immersed in an alkaline solution at 100 to 120 ° C. According to the document, the initial activity of the hydrogen storage alloy electrode can be improved, and the utilization factor and life can be improved.

However, the acid aqueous solution excluding the these publications techniques be applied to the Zr-Ni-based AB ₂ type hydrogen storage alloy, zirconium oxide formed on the Zr-Ni-based hydrogen storage alloy surface hydrofluoric acid , Alkalis, salts and insoluble in all organic solvents, the treatment with a metal fluoride compound described in JP-A-5-213601 and the treatment with hydrochloric acid described in JP-A-5-225975 disclose oxides on the surface of the hydrogen storage alloy. The layer cannot be removed. In addition, since the zirconium oxide film, which is an oxide layer, has poor hydrogen permeability, there is a problem that it is necessary to repeatedly store and release hydrogen many times before the initial activation is completed.

In Japanese Patent Application Laid-Open No. 5-135765, a processing temperature as high as 100 to 120 ° C.
A long processing time of 5 to 5 hours is required, and there is a problem that the processing takes time and costs. For this reason,
JP-A-8-291391 discloses a Zr-Ni Lav.
In the es-phase hydrogen storage alloy, a surface treatment inhibitor such as an oxide formed on the surface of the hydrogen storage alloy by a special chemical treatment is removed, and the surface of the hydrogen storage alloy has a predetermined appropriate surface state (structure). It is disclosed that by doing so, a highly active alloy surface is obtained and the initial activation is improved.

[0010]

However, the discharge capacity per unit alloy amount when the activated hydrogen storage alloy described in JP-A-8-291391 is used as a negative electrode of a hydrogen storage alloy battery is disclosed. The indicated alloy capacity was low. Therefore, it is an object of the present invention to provide a method of activating the Zr—Ni Laves phase hydrogen storage alloy to improve the alloy capacity of the hydrogen storage alloy electrode.

[0011]

In order to solve the above-mentioned problems, if hydrogen is easily diffused on the surface and inside of the electrode, the absorption and release of hydrogen are facilitated, and the charge and discharge characteristics of the electrode are improved. Was found. That is, in the present invention, the hydrogen storage alloy is heated in a high-temperature hydrogen atmosphere, followed by a hydrogen treatment for rapidly cooling immediately after the completion of the heating, and then a high-activation treatment for degassing by reducing the pressure.

After the activation treatment, HF and NH ₄ F · HF solution or HF and N
A process of immersing the hydrogen storage alloy in an H ₄ F · HF solution;
And / or a high activation treatment in which a heating treatment is further performed in a KOH solution.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a hydrogen storage alloy used in the present invention, a general formula based on Zr-Ni is represented by ABα (α
= 1.5-2.5)
An s-phase hydrogen storage alloy is preferred. This alloy is a C14 type hexagonal hydrogen storage alloy or a cubic C15 type hydrogen storage alloy. This alloy has a feature that the electric capacity is much larger than that of the rare earth AB ₅ type hydrogen storage alloy used for conventional electrodes and the like, but it is considered that activation is not easy due to the surface condition. The effect of the application of the present invention is extremely large. ABα (α = 1.5-2.5)
Type hydrogen storage alloy is (Zr-Ti) (V-Ni-M
_{n-Fe) 2.1, Zr (} V-Ni-Mn-Cr) 2 .1,
(Zr-Ti) (V-Ni-Mn-Cr) _{2.1 and the} like are preferable.

In the high activation treatment of the present invention, the hydrogen storage alloy is heated in a high-temperature hydrogen atmosphere, and then rapidly cooled to perform a hydrogen treatment for high activation. Is performed. The high-temperature hydrogen atmosphere for the hydrogen treatment is a hydrogen gas atmosphere of 0.05 to 2 MPa.
After heating at a heating temperature of 50 to 1000 ° C. for 30 minutes or more,
It is preferable to rapidly cool to 100 ° C. or less. The rapid cooling rate is preferably set to 10 to 50 ° C./sec.

The hydrogen gas pressure in the high-temperature hydrogen atmosphere for the hydrogen treatment is 0.05 to 2 MPa, preferably 0.1 to 2 MPa.
The range of １1 MPa is preferable. This is because the treatment can be easily performed by using a heating furnace capable of maintaining a hydrogen gas atmosphere without requiring a special high-pressure vessel. The hydrogen storage alloy powder was placed in a reaction vessel, filled with hydrogen gas, and the reaction vessel was heated. As a result, it was found that the reaction between the hydrogen gas and the hydrogen storage alloy powder occurred at 250 ° C. or higher. From this result, the heating temperature of the hydrogen treatment can be set in the range of 250 to 1000 ° C. When the heating temperature is lower than 250 ° C., hydrogen is easily released from the hydrogen storage alloy, so that the hydrogen treatment becomes insufficient.
If the height is higher, phase separation easily occurs in the hydrogen storage alloy, so that the alloy becomes easy to be heterogeneous, and an alloy phase having a low hydrogen storage amount is formed. Further, if the heating time is less than 30 minutes, the activation is insufficient. Therefore, the heating time is preferably 30 minutes or more to absorb hydrogen. The optimum heating time differs depending on the alloy composition.

[0016] Immediately after the completion of the heating, 100 ° C
Rapid cooling to below. As a result of heating the reaction vessel while depressurizing the rapidly cooled hydrogen storage alloy powder to a vacuum, when the heating temperature exceeds 300 ° C., hydrogen released from the hydrogen storage alloy is absorbed by the hydrogen storage alloy. I understand. It was also found that when the reaction temperature was lower than 60 ° C., the rate of release of hydrogen from the hydrogen storage alloy was reduced. For this reason, the degassing is performed by reducing the pressure to a vacuum at 60 to 300 ° C. A more preferred temperature range is 60-2.
50 ° C.

[0017] Subsequent to the activation treatment, a treatment for removing an oxide layer that inhibits the surface activity of the hydrogen storage alloy, and / or a surface treatment for imparting surface activity by coating a catalytic substance (Ni, Pd, etc.). Can also be performed. The oxide layer removal treatment is performed by using HF and NH ₄ F.HF solution or Ni
The treatment is performed by immersing the hydrogen storage alloy in an HF and NH ₄ F · HF solution to which a salt has been added, or by further performing a heat treatment in a KOH solution.

The treatment liquid for the oxide layer removal treatment includes Zr
Zirconium oxide formed on -Ni-based hydrogen storage alloy surface acid solution except hydrofluoric acid, alkali, salt, since it is insoluble in all organic solvents, HF and NH ₄ F · HF solution is used. In addition, when HF and NH ₄ F.HF solutions are used, Ni in the alloying element is much less eluted into the solution than other elements, so that Ni serving as a catalyst in the hydrogenation reaction is reduced. It is preferable because it is concentrated in the surface layer and a Ni-rich layer can be formed in a short time.

Concentrations of the HF and NH ₄ F.HF solutions, treatment time, and other suitable constitutions for the treatment are adjusted according to the surface state (structure) of the surface active portion obtained by contact with the hydrogen storage alloy. Is appropriately determined. The concentration of the solution is preferably 0.5 wt% or more, more preferably 1 to 5 wt%, but can be appropriately changed depending on the composition of the alloy to be processed, the amount and particle size of the powder to be processed, and the processing time. The treatment time varies depending on the concentration of the solution and the amount of the alloy powder to be treated, but is preferably 5 to 30 minutes. The processing time is preferably in the temperature range of room temperature to 120 ° C,
More preferably, it is in the range of 25 to 35 ° C. The ratio of the processing solution to the alloy powder is preferably about 5 to 10 g of the alloy powder per 20 ml of a 1 wt% solution.

If a Ni salt is added to the HF and NH ₄ F.HF solution, the Ni salt is N
It exists as i-ion. Since Ni ions, which are ions having a smaller ion tendency than the dissolved alloy component ions, are present, the Ni components are dissolved and, at the same time, the Ni ions precipitate on the surface of the hydrogen storage alloy, thereby forming a Ni-rich layer.

The Ni salt can be introduced as NiCl ₂ . The addition amount is 0.1 to 0.5 mol / in terms of Ni ion.
l is preferred. If the addition amount is less than 0.1 mol / l, the effect of precipitating Ni ions on the alloy surface is almost negligible. If the addition amount exceeds 0.5 mol / l, it does not contribute much to the activation effect of the hydrogen storage alloy, The weight of the occlusion alloy is increased, and the properties of the alloy are relatively deteriorated.

The heat treatment with the KOH solution has a concentration of 3
It is preferable that the processing temperature is 80 to 120 ° C. and the processing time is 1 to 30 hours.

[0023]

【Example】

(Example 1) As a hydrogen storage alloy, a Laves phase hydrogen storage alloy (Zr-Ti) (V
-Ni-Mn-Cr) _2.1 , the alloy was vacuum melted,
Alternatively, it was produced by a plasma button melting method, and then pulverized to produce a hydrogen storage alloy powder.

Samples 1 to 3 were prepared by activating the above hydrogen storage alloy powder under heating and degassing conditions shown in Table 1. Also,
Comparative Example 1 in which heating and degassing were not performed was also manufactured. In the initial activation treatment of the hydrogen storage alloy powder, a hydrogen storage alloy powder (particle size: 75 μm or less) is charged into a reaction vessel, and while being heated at 100 ° C. under vacuum for 30 minutes, 1.5 MPa of hydrogen is introduced. The operation of exposing to the atmosphere for 30 to 60 minutes was defined as one cycle, and this operation was repeated until the hydrogen storage amount became constant. In addition, in the above-mentioned repetition operation, when it was confirmed that hydrogen had begun to be absorbed, after the next cycle, the temperature of the reaction vessel was lowered to 0 ° C. after vacuum deaeration, and then hydrogen was introduced.

The number of times of initial activation until reaching 90% of the maximum hydrogen storage amount when initial activation in the gas-solid reaction system was performed on the prepared Samples 1 to 3 and Comparative Example 1 was measured. The results are shown in Table 1. Samples 1 to 3 and Comparative Example 1 are samples that have not been subjected to an oxide layer removal treatment on the alloy surface.

[0026]

[Table 1]

From Table 1, it can be seen that Samples 1 to 3 which had been subjected to the activation treatment of the present invention were subjected to an initial activation operation until reaching 90% of the maximum hydrogen storage amount as compared with Comparative Example 1 which had not been subjected to the treatment. Sample 2 under the heating condition of 800 ° C. for 1 hour is half of Comparative Example 1. (Example 2) Samples 4 to 6 in which a hydrogen storage alloy powder produced by melting and pulverizing in the same manner as in Example 1 was activated under heating and degassing conditions shown in Table 2 and then subjected to an oxide layer removal treatment. Was prepared. The oxide layer removal treatment is a treatment in which 5 g of the alloy powder is placed in 20 ml of a 1% NH ₄ F · HF solution containing 2.7% NiCl ₂ , treated at room temperature for 5 minutes, washed with water, and vacuum-dried. Further, Comparative Example 2 of a hydrogen storage alloy in which heating and degassing were not performed was also manufactured.

Using the prepared samples 4 to 6 and the sample of Comparative Example 2, a hydrogen storage alloy battery described later was constructed.
After the activation described later, the discharge capacity of the battery was measured, and the alloy capacity, which is the discharge capacity per unit alloy amount divided by the amount of the hydrogen storage alloy of the negative electrode, was measured. Was. The discharge conditions were 0.044C and 0.
The alloy capacity when discharged at 2C is also shown.

The hydrogen storage alloy battery was prepared by kneading a sample powder obtained by subjecting a 30 × 40 mm foamed nickel current collector to an oxide removing treatment and a 2 wt% methyl cellulose solution in a ratio of 77:23, filling a paste, and drying the paste. A negative electrode, a positive electrode filled with nickel hydroxide in a foamed Ni current collector, a separator, and an electrolytic solution composed of 5N KOH + 1N LiOH, sandwiching the negative electrode with the separator between the two positive electrodes,
This is an open battery that is fastened from both sides with an acrylic plate.

The method for activating the hydrogen storage alloy battery is as follows.
1204 charge at 044 C (27 mAh at 17 mAh / g current).
After charging for 3 hours), pause for 30 minutes, then 0.044C
And the operation of discharging to 0.8 V was defined as one cycle, and this operation was repeated three times. The alloy capacity was measured by charging the hydrogen storage alloy battery with activated electrodes at 120% at 0.2C (charging for 6h at a current of 78mAh / g), resting for 30 minutes, and then discharging at 0.2C to 0.8V. The discharge capacity was measured under the following cycle conditions, and the measured discharge capacity was divided by the amount of the hydrogen storage alloy of the negative electrode to determine the alloy capacity.

The unit (C) of the electrode evaluation in the above activation is a unit that becomes 1.0 C when the alloy battery is fully charged in one hour, and the unit of 0.2 C is that the alloy battery is charged for 5 hours. Unit when charged.

[0032]

[Table 2]

Sample 4 of Table 2 having undergone the activation treatment of the present invention
In Comparative Examples 2 to 6, Sample 5 having a heating temperature of 800 ° C. was Comparative Example 2.
However, Sample 4 and Sample 6 have lower alloy capacities than Comparative Example 2. Therefore, under the activation conditions of the present invention, when the heating time is 1 hour, the heating temperature is
800 ° C. is preferred. (Example 3) Samples 7 to 9 and Comparative Example 3 were prepared by activating the hydrogen-absorbing alloy powder produced by melting and pulverizing in the same manner as in Example 1 under heating and degassing conditions shown in Table 3. The oxide layer removal treatment was performed, and the alloy capacity was measured using the same hydrogen storage alloy battery as in Example 2.

[0034]

[Table 3] By examining the alloy capacities of the samples 5, 7 to 9 and the sample of Comparative Example 2, the influence of the heating time in hydrogen of the activation treatment of the present invention on the alloy capacities was examined. FIG. 1 shows a change in the alloy capacity with a change in the heating time in hydrogen. The sample 5, used,
7 to 9 are, as can be seen from Tables 2 and 3, other heating temperatures (800 ° C.) and degassing conditions (250 ° C., 30 mi) except for the activation time among the activation treatment conditions.
n) is a sample activated under the same conditions. The sample of Comparative Example 2 is a sample that has not been activated.

As shown in the diagram of FIG. 1, when the heating temperature in the activation treatment of the present invention is 800 ° C., the activation treatment sample of the present invention has a heating time of about 2 hours regardless of the discharge conditions. The capacity is at its maximum. In addition, 0.2C
Under the discharge conditions described above, Sample 7 has a lower alloy capacity than Comparative Example 2 for a heating time of 30 minutes. In addition, the influence of the degassing method in the activation treatment of the present invention on the alloy capacity was examined by determining the relationship between the number of times of charge and discharge and the alloy capacity of Sample 8 and Comparative Example 3. The measurement results of the alloy capacity are shown in FIG. Was. In addition,
Table 8 shows that Sample 8 and Comparative Example 3 used were different from the activation treatment conditions only in the degassing temperature except for other heating conditions (800 ° C., 2 h) and degassing time (30 min).
Is a sample activated under the same conditions.

From FIG. 2, comparing the alloy capacities under the same heating conditions and with the discharge conditions of 0.2 C in the samples in which the degassing conditions have been changed, a comparative example in which the degassing temperature is 800 ° C. and the same temperature as the heating temperature is shown. The alloy capacity of No. 3 is about 100 m irrespective of the number of times of charging and discharging as compared with Sample 8 degassed at 250 ° C.
Ah / g is also low. Further, the effect of the surface treatment for removing the oxide layer on the activation was determined by comparing the alloy capacities of Sample 2, Sample 5, Comparative Example 1 and Comparative Example 2. FIG. 3 shows the measurement results of the alloy capacity at this time. In addition, the measured alloy capacity was an alloy capacity when charge and discharge at 0.044C were repeated.

As shown in FIG. 3, the sample 5 having been subjected to the surface treatment was activated as the alloy utilization exceeded 90% in one charge / discharge, whereas the sample 2 without the surface treatment was subjected to the charge / discharge. Even if it is repeated nine times, the alloy utilization reaches only about 80%. Further, the alloy utilization ratio of Comparative Example 2 in which only the surface treatment was performed without performing the heating / degassing treatment reached only 55% when charge and discharge were repeated nine times. Further, the alloy utilization rate of Comparative Example 1, which was not subjected to the heating / degassing treatment and was not subjected to the surface treatment, reached about 20% only after repeating charging and discharging 9 times.

[0038]

According to the method for highly activating the hydrogen storage alloy of the present invention, the number of times of activation in the initial activation can be greatly reduced and the alloy capacity can be improved. Further, when the surface treatment is performed on the hydrogen storage alloy that has been subjected to the high activation treatment, not only the alloy capacity is further improved, but also the charge / discharge characteristics are greatly improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between heating time and alloy capacity in a high activation treatment of the present invention.

FIG. 2 is a diagram showing the relationship between the degassing temperature and the alloy capacity in the high activation treatment of the present invention.

FIG. 3 is a diagram showing the effect of surface treatment on the alloy capacity of a hydrogen storage alloy.

Claims (1)

[Claims] Claims: 1. A hydrogen storage alloy is heated in a high-temperature hydrogen atmosphere, and after a heating treatment, is rapidly cooled immediately after the hydrogen treatment.
A method for highly activating hydrogen storage alloys in which degassing is performed by reducing the pressure.