JPH1150263A - Production of stabilized hydrogen storage alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove oxide film, to maintain hydrogen occluding ability over a long period of time and to improve hydrogenation characteristics by melting a very small amt. of a rare earth metal in an alloy consisting essentially of Ti and Ni and treating the alloy with an aq. soln. contg. F ions. SOLUTION: A rare earth metal is melted and incorporated by <=10 at.% into a hydrogen storage alloy having a compsn. consisting essentially of Ti and Ni or a compsn. obtd. by substituting Zr for part of the Ti and the alloy is treated with an aq. soln. contg. F ions to form fluoride or hydroxide of the rare earth metal on the surface and grain boundary faces of the alloy. It is preferable that the aq. soln. contg. F ions is adjusted to pH 4.5-8.0 to accelerate hydrogenation reaction on the surface of the alloy and the specific surface area is increased by about 50-500 times by forming a fine complex surface structure by lattice expansion and shrinkage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for forming a fluoride on a surface or a grain boundary, whereby hydrogenation reaction characteristics and electrochemical characteristics are remarkably improved, and characteristics are deteriorated over a long period of time. The present invention relates to a method for producing a hydrogen storage alloy that can be used without any problem.

[0002]

2. Description of the Related Art In recent years, attention has been paid to hydrogen storage alloys as one of new metallic materials, and applied research has been actively conducted.
This hydrogen-absorbing alloy absorbs hydrogen when pressure is applied or lowers the temperature to become a metal hydride, and generates heat.On the contrary, when the pressure is lowered or the temperature is increased, the absorbed hydrogen is absorbed. Has the property of releasing heat again to take away heat. Therefore, utilizing such properties, the hydrogen storage alloy is used to store hydrogen itself and thermal energy,
Alternatively, it can be used for conversion between chemical energy and heat energy, and for example, it has been attempted to use it for a hydrogen storage device, a heat pump utilizing waste heat, a chemical engine, an electrode material of a nickel-hydrogen battery, and the like. .

[0003] In this hydrogen storage alloy, AB
Labase phase-based hydrogen storage alloy having a C-14 structure or a C-15 structure containing titanium or zirconium and nickel as main components classified as type _2, for example, Ti _0.1 Z
r _0.9 V _0.2 Mn _0.6 Co _0.1 Ni _1.1 , or a hydrogen storage alloy classified as AB type containing titanium or zirconium and nickel as main components, for example, Ti _0.7 Zr _0.3 Ni _0.5 M
Since such n _0.5 is larger theoretical hydrogen storage capacity many electrochemical capacity, it has been regarded as the hydrogen storage material and the electrode material, the hydrogenation reaction properties by these that the surface is oxidized in a dense state And the charge-discharge characteristics, particularly the initial activation characteristics, are poor, and the function is rapidly deteriorated by the repetition of the hydrogenation-dehydrogenation reaction and the charge-discharge. The layer formed by the above-mentioned oxidation can be temporarily removed by an alkali treatment, but when used for a long period of time, it loses its oxidation resistance and poisoning resistance. It was difficult.

The present inventor has previously found that a hydrogen storage alloy containing nickel and a rare earth metal as main components can be stably used over a long period of time by subjecting it to fluorination treatment. JP-A-7-207493), AB ₂ type, AB type and BCC type alloys,
When a similar fluorination treatment was performed, surface oxides could be easily removed, but no fluoride was formed on the surface, and the stabilization was unsuccessful.

[0005]

SUMMARY OF THE INVENTION The present invention relates to hydrogen having a composition containing titanium and nickel as main components or a composition in which titanium is partially substituted with zirconium, which has been difficult to stabilize by fluorination treatment. The purpose of the present invention is to provide a method for stabilizing a storage alloy by fluorination treatment.

[0006]

SUMMARY OF THE INVENTION The inventor of the present invention has made a keen effort to stabilize AB ₂ type, AB type and BCC type alloys containing titanium and nickel or both of them and zirconium as components by fluorination treatment. As a result of repeated studies, a small amount of rare earth metal was dissolved in these alloys, and a new phase of a rare earth nickel alloy, for example, LaNi ₃ was formed in a dispersed state between the master alloys, and then fluorinated. As a result, it has been found that the object can be achieved, and the present invention has been made based on this finding.

That is, the present invention relates to a hydrogen storage alloy having a composition containing titanium and nickel as main components or a composition in which titanium is partially substituted with zirconium.
After the rare earth metal is melted and contained in an amount of not more than atomic%, a fluoride or hydroxide of the rare earth metal is formed on the surface and the grain interface of the alloy by treating with an aqueous solution containing fluorine ions. It is intended to provide a method for producing a stabilized hydrogen storage alloy. Also, at this time, the hydrogen ion concentration of the aqueous solution containing fluorine ions is adjusted within the range of pH 4.5 to 8.0 to promote the hydrogenation reaction on the alloy surface, thereby causing fine and complicated lattice expansion and contraction caused by lattice expansion and contraction. The present invention provides a method for forming a specific surface structure and increasing the specific surface area by about 50 to 500 times.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The hydrogen storage alloy used as a raw material in the present invention is of AB ₂ type, AB type and BCC type having a composition containing titanium and nickel as main components or a composition in which titanium is partially substituted with zirconium. Alloy. This alloy can contain manganese, vanadium, cobalt, aluminum, calcium, etc. in addition to titanium, nickel and zirconium. Among these alloys, examples of the AB ₂ type include Ti _0.1 Zr
_0.9 V _0.2 Mn _0.6 Co _0.1 Ni _1.1 , examples of the AB type include Ti _0.7 Zr _0.3 Ni _0.5 Mn _0.5 , and examples of the BCC type include TiV ₃ Ni _0.5 .

Next, scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, etc. Although there are ytterbium and lutetium, in the method of the present invention, these may be used alone, or a mixture of two or more kinds, for example, misch metal may be used. A particularly preferred rare earth metal is lanthanum.

In the method of the present invention, it is necessary to add the above rare earth metal to the hydrogen storage alloy as a raw material in an amount of 10 atomic% or less, preferably in the range of 0.5 to 5 atomic%, and to melt the metal. . If this content is too small, sufficient fluoride will not be produced during the subsequent fluorination treatment, and if it exceeds 10 atomic%, the hydrogen storage capacity will be reduced.

In the method of the present invention, in order to include a rare earth metal in a predetermined hydrogen storage alloy, a previously prepared hydrogen storage alloy may be melted, and a predetermined amount of the rare earth metal may be added thereto to be uniformly dispersed. Alternatively, a predetermined amount of a rare earth metal may be mixed with each component corresponding to the composition of the hydrogen storage alloy as a raw material, and this may be melted to form an alloy. The melting temperature at this time depends on the composition of the hydrogen storage alloy, but is usually in the range of 900 to 1800 ° C. This melting can be performed by any method generally used for producing an alloy, for example, an electric furnace method or an arc melting method. At this time, it is desirable to carry out in an atmosphere of an inert gas such as argon. By including the rare earth metal in this way, the hydrogen storage capacity of the raw material hydrogen storage alloy is not impaired at all.

Next, the hydrogen storage alloy containing a rare earth metal thus obtained is mechanically less than 500 μm, preferably 5 μm or less.
It is pulverized into particles having a particle size of 0 to 300 μm and treated with an aqueous solution containing fluorine ions. At this time, if necessary, the alloy particles may be subjected to repeated hydrogenation / dehydrogenation treatment so as to be further finely pulverized to reduce the particle size.
Further, the alloy particles thus obtained can be formed into pellets by press molding according to a conventional method, and then supplied to a subsequent step.

In the method of the present invention, the aqueous solution containing fluorine ions used for fluorinating the rare earth metal-containing hydrogen storage alloy is disclosed in JP-A-7-207493 and Japanese Patent Application No. 8-1.
No. 32264, for example, an aqueous solution of a metal hexafluoride compound or an alkali fluoride of 0.2 to 1
An aqueous solution containing 0.0% by weight is adjusted to pH with hydrogen fluoride.
A processing solution adjusted to 2.0 to 6.5 can be mentioned. As the alkali fluoride at this time, sodium fluoride, potassium fluoride, ammonium fluoride or the like is used, and potassium fluoride is particularly preferable. These alkali fluorides are dissolved in water at a concentration ranging from 0.2 to 10.0% by weight. Particularly preferred concentrations are 0.3 to 3.0% by weight for sodium fluoride, 0.5 to 5.0% by weight for potassium fluoride, and 0.1% for ammonium fluoride.
It is in the range of 5 to 8.0% by weight.

[0014] obtained by a hexafluoride metal compound such as K ₃ AlF ₆ was dissolved in water F ^- ions, HF ₂ ^- with ion, hydrogen fluoride aqueous solution containing H ⁺ ions, K ⁺ ions, NH A fluoride aqueous solution in which a cation such as ₄ ⁺ ion coexists can be used. As the solvent at this time, water is usually used alone, but in some cases, a water-soluble solvent such as alcohol and acetone may be used in combination. Then, a nickel compound such as nickel fluoride is previously contained in the fluorine ion-containing aqueous solution to suppress the elution of nickel from the hydrogen storage alloy or to supplement the nickel component reduced by the elution. it can.

In order to suitably perform the treatment with the fluorine ion-containing aqueous solution, for example, first, M ₃ AlF ₆ , M ₂ TiF ₆ ,
Metal fluoride compounds such as M ₂ ZrF ₆ and M ₂ SiF ₆ (where M is an alkali metal such as potassium or sodium) are used in a weight / volume ratio (W / V) of about 0.01 to 0.5. The resulting solution is dissolved in water to obtain a supersaturated aqueous solution. Next, the alloy particles are immersed in the supersaturated aqueous solution, and usually at about 0 to 60 ° C. under normal pressure,
The treatment is preferably performed at a temperature of 15 to 40 ° C for 0.5 to 5 hours.

By performing the treatment as described above, a rare earth nickel alloy such as LaNi _{3 is} added to a hydrogen storage alloy having a composition containing titanium and nickel as main components or a composition in which titanium is partially substituted with zirconium.
A new phase is formed, which forms fluoride in a state dispersed between the master alloy phases. And this fluorinated L
The strong selectivity and permeability to hydrogen of the a-Ni alloy phase leads to a significant improvement in the initial properties of the hydrogenation reaction and electrochemical properties. At this time, the hydrogen ion concentration was adjusted to pH
When the relationship between the treatment time and the hydrogen ion concentration is appropriately controlled while maintaining the value in the range of 4.5 to 8.0, a rare earth metal hydroxide such as hydroxide is used instead of the rare earth metal fluoride such as lanthanum fluoride. Lanthanum forms at the alloy surface and grain interface. In addition, if nickel ions are previously dissolved in a fluorine ion-containing aqueous solution, this treatment causes reduction of metallic nickel at the same time as generation of fluoride, improving hydrogen permeability, and having high conductivity together with a surface protection function. A surface layer is formed.

On the other hand, when fine cracks occur in the hydrogen storage alloy due to expansion / contraction of the lattice during repetition of hydrogenation / dehydrogenation or charging / discharging, a new fluoride or nickel protective layer is not provided. Surfaces can occur, which can lead to poisoning resistance and reduced cycle life. For this reason, it is desired that a new surface due to fine cracks is formed in advance and a protective layer is formed on the surface to avoid the above-described trouble.

In the method of the present invention, the hydrogen ion concentration is reduced by replenishing the aqueous solution of hydrogen fluoride or adding a buffer such as an aqueous solution of acetic acid or sodium acetate during the treatment with the aqueous solution containing fluorine ions. p
By adjusting the H within the range of 4.5 to 8.0, particles having a large specific surface area and a small particle diameter can be produced. This is because hydrogen ions in the aqueous solution form hydrides by a hydrogenation reaction on the alloy surface, and the lattice expands at that time, causing fine cracks. In addition to forming alloyed particles, cracks are simultaneously generated to increase the specific surface area.

[0019]

According to the method of the present invention, the following effects can be obtained. (A) The oxide film is removed, and the hydrogen storage capacity is maintained for a long time. (B) The hydrogenation characteristics are improved by the formation of the rare earth metal nickel-based alloy phase and the fluoride at the surface and at the grain interface. (C) Metal nickel and hydroxide are formed on the surface and at the grain interface, and the electrical conductivity and the thermal conductivity are improved. (D) A composite layer is formed by reduction of nickel metal, so that surface protection functions such as corrosion resistance and poisoning resistance, and electrical conductivity and thermal conductivity are improved. (E) The hydrogenation reaction that proceeds in the process of fluoride formation causes fine cracks, thereby increasing the specific surface area, and increasing the surface and grain interface between the rare earth metal fluoride or hydroxide and metal nickel. The composite phase improves surface protection functions such as corrosion resistance and poisoning resistance, and electrical and thermal conductivity.

[0020]

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

Example 1 Hydrogen storage alloy Zr _0.9 Ti _0.1 Ni _1.1 V _0.2 Co _0.1 Mn
5 atomic% of lanthanum was added to _0.625 g, and after melting by an arc melting method in an argon atmosphere, the operation of cooling was repeated five times to obtain a uniform composition. The maximum voltage at this time was 250V. Next, this was mechanically pulverized to prepare a lanthanum-containing hydrogen storage alloy powder having a particle diameter of 26 μm or less. Separately, JP-A-7-207493
In accordance with the method described in JP-A No. H10-240, 40 g of K ₃ AlF ₆ was dissolved in 100 g of water, and 0.5 g of NiF ₂ was further added and dissolved, and then adjusted to pH 5.3 with HF to prepare an aqueous solution containing fluorine ions. . 5 g of the above-mentioned lanthanum-containing hydrogen-absorbing alloy powder was put into 100 ml of the thus-obtained fluorine-ion-containing aqueous solution, stirred at 30 ° C. for 30 minutes, filtered, and filtered until no fluorine ions were found in the washing water. By washing with water and drying under reduced pressure,
A stabilized hydrogen storage alloy was manufactured.

Example 2 3 g of potassium fluoride was dissolved in 100 ml of a 0.3% by weight aqueous solution of hydrogen fluoride.
g and a solution in which 7.4 g of trisodium citrate hydrate was dissolved, and a solution in which 4.0 g of sodium hypophosphite was dissolved in 100 ml of a 0.3% by weight aqueous hydrogen fluoride solution,
An aqueous solution containing fluorine ions for stabilization treatment was prepared. Next, 10 g of the same lanthanum-containing hydrogen storage alloy powder as in Example 1 was put into 200 ml of the fluorine ion-containing aqueous solution, and the mixture was stirred at 30 ° C. for 30 minutes.
A stabilized hydrogen storage alloy was produced by washing with water and drying in the same manner as described above.

Example 3 A median diameter of 73.9 μm and a specific surface area of 0.068 m ² / g
5 g of a lanthanum-containing hydrogen storage alloy powder having
Using the same fluoride ion-containing aqueous solution 200 ml as used in the above, using an acetic acid aqueous solution containing sodium acetate, pH
Was fluorinated at 30 ° C. for 30 minutes while controlling within the range of 6.0 to 8.0. FIG. 1 shows the change over time in pH when this treatment was repeated in a 30-minute cycle. The specific surface area of the lanthanum-containing hydrogen storage alloy powder after the treatment was measured by the BET method and found to be 5.72 m ² / g, which is about 84 times larger than that before the treatment. I understand.

Reference Example 1 Hydrogen storage alloy Zr _0.9 Ti _0.1 Ni _1.1 V _0.2 Co _0.1 Mn
_0.6 powder (A), an alloy powder containing 5 atomic% of lanthanum in the same manner as in Example 1 (B), the above (A)
Was tested as follows using (C) which was fluorinated in the same manner as in Example 1 and (D) which was fluorinated from (B) in the same manner as in Example 1. That is,
To each powder sample, 5% by weight of polytetrafluoroethylene was added and compression-molded using a pressure of 4 ton / cm ² to obtain pellets having a diameter of 13 mm and a thickness of 1.5 mm. Next, this pellet was used as a test electrode,
The impedance was measured in the frequency range of １００100 kHz, and the results are shown in FIGS. As is clear from these figures, regardless of the presence or absence of lanthanum, the impedance of the fluorinated samples (C) and (D) is significantly lower than that of the untreated samples (A) and (D). doing. In addition, the addition of lanthanum lowers the impedance, which means that there is no oxide film with low electrical and ionic conductivity and a surface morphology covered with highly conductive metallic nickel is produced. Suggests. The most remarkable decrease in impedance is the sample (D) in which the lanthanum-added alloy is fluorinated, and FIG. 2 shows that the remarkable decrease in the impedance is closely related to the improvement in the electrical and ionic conductivity. You.

Reference Example 2 Hydrogen storage alloy Zr _0.9 Ti _0.1 Ni _1.1 V _0.2 Co _0.1 Mn
_0.6 powder (A), and alloy powders (B) and (A) containing 5 atomic% of lanthanum, which were fluorinated in the same manner as in Example 2 (E) and (B). The following tests were performed using the fluorinated (F) in the same manner as in Example 2 as each sample powder. That is, 300 parts by weight of nickel powder and 20 parts by weight of polytetrafluoroethylene powder were added to 100 parts by weight of each powder sample, mixed well, and then compression-molded under a pressure of 4 ton / cm ² to form a 13 mm diameter, A pellet having a thickness of 1.5 mm was produced. Next, this pellet was used as a test electrode and 6N-
In a KOH electrolyte, a positive electrode Ni (OH) ₂ , a reference electrode Hg
/ HgO was used to perform a charge / discharge test. The test conditions at this time were as follows: ambient temperature 20 ° C., charging 200 mA / g.
Time, rest time 10 minutes, discharge 150 mA / g, discharge depth -0.6 V. FIG. 4 shows the test results as a graph showing the relationship between the number of cycles and the discharge capacity. As can be seen from this figure, in the samples (A) and (B) which were not subjected to the fluorination treatment, the discharge capacity was higher than that of the sample (A) to which lanthanum was not added (B). Discharge capacity is low. In addition, the samples (E) and (F) subjected to the fluoridation treatment
It is clear that the case (F) to which lanthanum is added is clearly superior, and that a stable high discharge capacity can be obtained by adding lanthanum and performing fluorination treatment.

[Brief description of the drawings]

FIG. 1 is a graph showing a time-dependent change in pH in Example 3.

FIG. 2 is a graph showing the results of impedance measurement of a sample to which lanthanum is not added in Reference Example 1, in which the imaginary part is the vertical axis and the real part is the horizontal axis.

FIG. 3 is a graph showing the results of impedance measurement of a sample to which lanthanum is added in Reference Example 1, with the imaginary part as the ordinate and the real part as the abscissa.

FIG. 4 is a graph showing the relationship between the number of cycles and the discharge capacity of each sample in Reference Example 2.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI // H01M 4/38 H01M 4/38 A

Claims (3)

[Claims] 1. A hydrogen storage alloy having a composition containing titanium and nickel as main components or a composition in which a part of titanium is replaced by zirconium contains 10 atomic% or less of a rare earth metal in a molten state and then contains fluorine. A method for producing a stabilized hydrogen storage alloy, comprising forming a fluoride or hydroxide of a rare earth metal on the surface and grain interface of the alloy by treating with an ion-containing aqueous solution. 2. The method according to claim 1, wherein the rare earth metal is lanthanum. 3. The production method according to claim 1, wherein the hydrogen ion concentration of the fluorine ion-containing aqueous solution is controlled within a range of pH 4.5 to 8.0.