JPH0794176A - Hydrogen storage electrode - Google Patents

Abstract

PURPOSE:To provide a metal oxide-hydrogen storage battery having lower cost than a conventional battery and yet having equivalent function by restraining an extent of refining alloy powder for the improvement of a hydrogen storage electrode through the use of a hydrogen storage alloy containing a less cobalt amount in an alloy composition than the conventional case. CONSTITUTION:The predetermined amount of one or more types of powder of iron oxide Fe2O3 and FeOO is added to hydrogen storage alloy powder. As a result, the aforementioned iron oxide powder enters the crack of the allay powder generated due to repeated charging and discharging processes. An extent of refining the alloy power is thereby restrained, and the lifetime of a hydrogen storage electrode can be extended to such level as similar to the conventional case. An amount of the iron oxide to be added is preferably between 0.1 pts.wt, and 10 pts.wt. in consideration of a balance between electrode lifetime characteristics and initial capacity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage electrode used as a negative electrode of a metal oxide-hydrogen storage battery.

[0002]

2. Description of the Related Art A hydrogen storage alloy powder that reversibly stores and releases hydrogen, and a current collector having a two-dimensional porous structure such as punching metal or a three-dimensional porous structure such as a foamed metal porous body. The hydrogen storage electrode according to the present invention is generally manufactured by the following method. That is, various metals are weighed so as to match the alloy composition, a mixture of various metals is melted by using an arc melting furnace or a high frequency melting furnace, an alloy having a desired composition is manufactured, and this alloy is further crushed. To obtain a powder having a particle size of 300 mesh or less. This powder was kneaded with water and a binder / thickener such as carboxymethyl cellulose (CMC) to form a paste, which was filled in an electrode support such as punching metal and then pressed to form a hydrogen storage electrode.

The hydrogen-absorbing alloy powder used in the above-mentioned electrode is refined by stress cracking due to a change in crystal lattice due to changes in crystal lattice due to repeated storage and release of hydrogen (charge / discharge as an electrode reaction), and a reaction site with hydrogen. , And the initial electrode characteristics (discharge capacity, high rate discharge characteristics) are improved. However, depending on the alloy composition, if excessive miniaturization occurs, the contact between alloy powders or between the alloy powder and the current collector will deteriorate due to repeated charging and discharging, and the conductivity of the electrode will decrease, resulting in discharge. Capacity tends to decrease.

Conventionally, as a means for solving this problem, a method has been adopted in which a part of the alloy composition is replaced by, for example, Co of 0.5 to 1.0 atom with respect to Ni to suppress the refinement. . However, since Co is expensive and has a small reserve,
An alloy composition with a small amount of Co substitution has been desired.

However, if the amount of Co substitution is simply reduced from the conventional value, the life characteristics are greatly reduced due to the refinement of the alloy powder. Further, when Co is replaced with another element, it is very difficult to find an alloy composition that satisfies the electrode characteristics such as initial capacity and life characteristics in a well-balanced manner. That is, it is difficult to obtain a hydrogen storage electrode having the same characteristics as conventional ones only by taking an alloy composition approach of reducing the Co substitution amount.

On the other hand, in order to improve the battery characteristics, C
Attempts have also been made to add o, Cu, Bi oxides.
(For example, Japanese Patent Publication No. 1-283767, Japanese Patent Publication No. 2-2395)
66, Japanese Patent Publication No. 306539, etc.). However, it has been impossible to improve the life characteristics of alloys having a smaller amount of Co than in the past even if these additives are added.

[0007]

As described above, the hydrogen storage electrode using an alloy, which is expensive and has a small amount of Co substitution, has a problem that the life characteristics are significantly reduced. The present invention intends to realize a low-cost metal oxide-hydrogen storage battery using an alloy having a small amount of Co substitution and having a hydrogen storage electrode having other electrode characteristics equal to or higher than those of the conventional one.

[0008]

In order to solve the above-mentioned problems, the present invention provides a hydrogen storage alloy powder that electrochemically stores and releases hydrogen, at least iron oxide Fe ₂ O ₃ and FeOOH powder. One is added to form a hydrogen storage electrode. Further, the addition amount of the iron oxide is 0. 0 with respect to the hydrogen storage alloy powder from the balance of life characteristics and initial capacity.
1 to 10% by weight is preferable.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present invention as above, a hydrogen absorbing electrode was added to the iron oxide to the hydrogen absorbing alloy powder, iron oxide Fe ₂ O
The addition of at least one of the powders of ₃ and FeOOH suppresses the refinement of the hydrogen storage alloy powder. The mechanism has not been analyzed in detail, but iron oxide powder enters the cracks generated in the alloy due to the stress caused by repeated charging and discharging, preventing the alloy powder from becoming finer and maintaining the electronic conductivity of the electrode. It is thought that it is doing. This action is not seen in an electrode using an alloy having iron in the alloy composition or an electrode added with another oxide, and is a unique effect of the addition of iron oxide Fe ₂ O ₃ and FeOOH powder.
The metal oxide-hydrogen storage battery provided with such a hydrogen storage electrode has the same battery characteristics as conventional ones, and further, iron oxide Fe ₂ O
By adding ₃ , FeOOH, an inexpensive alloy with a small amount of Co substitution can be used, and the cost can be reduced as compared with the conventional metal oxide-hydrogen storage battery.

[0010]

EXAMPLES Examples of the present invention will be described below.

Example 1 Misch metal (Mm: a mixture of light rare earth elements) containing 20% by weight of lanthanum (La) having a purity of 99.5% or more, nickel (Ni), manganese (Mn), aluminum (Al). , Cobalt (Co) are mixed in a predetermined ratio, and MmNi _3.9 M is prepared by the arc melting method.
An n _0.4 Al _0.3 Co _0.4 alloy was produced. This alloy was crushed in an inert atmosphere to obtain a powder having a particle size of 300 mesh or less. For 3 g of this alloy powder (equivalent to 1 Ah), 0.
15 g (5% by weight) of iron oxide (Fe ₃ O ₄ , Fe ₂ O ₃
And FeOOH, both of which have a particle size of 0.5 to 5 μm), are mixed and kneaded with 0.1% by weight of water-soluble binder CMC and water to form a paste, and the paste is filled into a foamed nickel porous body. Then, it was dried and pressed to produce a hydrogen storage electrode.

One of the above hydrogen storage electrodes was inserted into a bag-shaped separator, and two known nickel positive electrodes (2 Ah
(Corresponding), and using 200 ml of an aqueous potassium hydroxide solution having a specific gravity of 1.25 as an electrolytic solution, a nickel-hydrogen storage battery having a negative electrode capacity regulation was assembled.

For each of these batteries, at a constant current of 0.5 A
It was charged and discharged. Charging time is 2.4 hours and discharging is finished
The voltage was set to 1 V and a 200-cycle life test was performed.
Cycle change of discharge capacity per alloy g
Electrodes prepared by using an alloy with a large amount of substitution without adding iron oxide
Pole 1 (alloy composition: MmNi_3.7Mn_0.4Al_0.3Co
_0.6), And using the same alloy composition but without adding iron oxide
It is shown in FIG. 1 together with the test results of the manufactured electrode 2.

The life characteristics of the electrode 2 are much lower than those of the conventional electrode 1. This is due to the scanning electron beam (SE
M) It is clear from the result of observation. That is, 2 for electrode 1
After the 00 cycle, the alloy powder has a slight crack, but the refinement has hardly progressed, while the electrode 1
1 for the electrode 2 using an alloy having a smaller Co substitution amount than
The width of the alloy powder with a diameter of about 20 μm is 2 by the cycle charge and discharge.
It was found that cracks of about 3 μm were generated, and the alloy powder was refined to about several μm after 50 cycles. It is considered that the decrease in the capacity of the electrode 2 occurred because the refinement of the alloy significantly progressed due to repeated charging and discharging, and the conductivity of the electrode decreased.

On the other hand, in the electrode _{3 in} which Fe ₃ O ₄ is added to the electrode 2, the capacity in the first cycle is 20 mAh compared with the electrode B.
/ Alloy g increased. This is because Fe ₃ O ₄ itself has conductivity and the conductivity of the electrode is improved. However, the life characteristics are similar to those of electrode 2, and the SEM image also shows that of electrode 2.
It was the one that was miniaturized similarly to.

On the other hand, the electrode 4 in which Fe ₂ O ₃ is added to the electrode 2 and the electrode 5 in which FeOOH is added have the same capacity as that of the electrode 2 in the first cycle, but have the life characteristics of the electrode 2.
It was significantly improved as compared with No. 1 and showed a life characteristic almost equal to that of the conventional electrode 1. As a result of observing the SEM image, it was confirmed that although the alloy powder had cracks after the first cycle of charging and discharging, Fe ₂ O ₃ or FeOOH powder penetrated into the cracks and prevented the alloy powder from becoming fine. did it. It was found that this effect was maintained even after 200 cycles, and the electrodes 4 and 5 had the life characteristics improved to the same extent as in the conventional case.

Although iron oxide powder having a particle size of 0.5 to 5 μm was used in this embodiment, Fe ₂ O ₃ and FeOOH are particularly used.
Since is an insulator, it is desirable that the range is such that conductivity can be maintained between the alloy powders, specifically, a range that does not exceed the average particle size of the alloy powders.

Further, in this embodiment, the hydrogen storage electrode formed by filling the alloy paste into the foamed nickel porous body is used as an example, but the alloy paste is used as a punching metal or the like.
Similar results were obtained by using an electrode coated on a three-dimensional porous body.

(Example 2) MmNi _4.3-Y Mn _0.4 Al
Using a hydrogen storage alloy represented by _{0.3 Co Y (Y = 0~0.6)} , the Fe ₂ O ₃ powder was added 5 wt% of the alloy, further addition of Fe ₂ O ₃ powder for comparison Don't
A hydrogen storage electrode similar to that in Example 1 was produced. A battery similar to that of Example 1 was assembled using these electrodes, and each battery was charged and discharged at a constant current of 0.5 A. The charging time was set to 2.4 hours, the discharge end voltage was set to 1 V, and a 200-cycle charge / discharge test was performed. The relationship between the alloy composition, the presence or absence of Fe ₂ O ₃ addition, and the discharge capacity at the 200th cycle in this test is shown in FIG. The curve A in the figure is Fe ₂
O ₃ there is added, the curve B shows the case of Fe ₂ O ₃ without addition.

When Y (Co substitution amount) is 0.4 or less, F
By adding the e ₂ O ₃ powder, it is possible to increase the capacity at the 200th cycle, that is, improve the life characteristics, in accordance with the mechanism shown in Example 1. This tendency is more remarkable as Y is smaller. On the other hand, when Y exceeds 0.4, the capacity at the 200th cycle is slightly decreased by adding the Fe ₂ O ₃ powder. The reason for this is that the alloy composition alone has the effect of suppressing miniaturization, and the demerit that Fe ₂ O _3, which is an insulator, coexists with a hydrogen storage alloy and reduces the conductivity of the electrode affects the characteristics. It is possible that it has become a form. That is, as a range in which the electrode characteristics are improved by the present invention, the Co substitution amount in the alloy is 0 to
It is preferably 0.4 atoms.

In this embodiment, as an example, MmNi
A hydrogen storage alloy having a stoichiometric composition represented by _4.3-Y Mn _0.4 Al _0.3 Co _Y was used, but Mm and Al including the non-stoichiometric composition were also changed, and the Mm
(Ni _5-XY A _X Co _Y ) _Z (A: at least one of Al · Mn, 0.5 ≦ X ≦ 0.9, 0 ≦ Y ≦ 0.4, 0.
The life characteristics of the electrode were improved as in the case of any of the alloys having the composition represented by 9 ≦ Z ≦ 1.1).

(Example 3) 3 g of the same alloy powder as in Example 1
Seven kinds of Fe ₂ O ₃ powders of 0 to 0.36 g (0 to 12% by weight) were added to (corresponding to 1 Ah), and a hydrogen storage electrode similar to that of Example 1 was produced. A battery similar to that of Example 1 was assembled using these electrodes, and 0.5% was obtained for each battery.
Charging / discharging was performed with the constant current of A. The charging time was set to 2.4 hours, the discharge end voltage was set to 1 V, and a 200-cycle charge / discharge test was performed. FIG. 3 shows the relationship between the added amount of Fe ₂ O ₃ powder and the discharge capacities at the first cycle and the 200th cycle in this test. Curve A in the figure shows the case of the first cycle, and curve B shows the case of the 200th cycle.

The amount of Fe ₂ O ₃ powder added is 0.1
Within the range of 10 to 10% by weight, the effect on the life characteristics is observed, but when it is 12% by weight, the initial capacity is remarkably reduced as compared with the case of no addition. This is because Fe ₂ O ₃ itself is an insulator, and this oxide covers the surface of the hydrogen storage alloy, so that the conductivity between the alloy powders decreases as the addition amount increases. Therefore, iron oxide (especially Fe
The addition amount of ₂ O ₃ ) powder is preferably in the range of 0.1 to 10% by weight.

[0024]

As described above, according to the present invention, even if an inexpensive hydrogen storage alloy having a small amount of Co substitution is used, it is possible to provide an inexpensive hydrogen storage electrode having a cycle life equivalent to that of the conventional one. it can.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the effect of iron oxide addition on life characteristics.

FIG. 2 is a characteristic diagram showing the relationship between the alloy composition and the presence or absence of Fe ₂ O ₃ addition, and the discharge capacity at the 200th cycle.

FIG. 3 is a characteristic diagram showing the effect of the amount of Fe ₂ O ₃ powder added on the life characteristics.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Yamamoto 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A hydrogen storage electrode comprising a hydrogen storage alloy powder that electrochemically stores and releases hydrogen, and at least one of iron oxide Fe ₂ O ₃ and FeOOH powder. 2. The hydrogen storage electrode according to claim 1, wherein the amount of the iron oxide powder added is 0.1 to 10% by weight based on the hydrogen storage alloy powder.