JPH06215762A - Hydrogen absorbing electrode - Google Patents

Abstract

PURPOSE:To provide a hydrogen absorbing electrode which has a long cycle life and can keep high capacity in a low temperature range. CONSTITUTION:In a rare earth metal-based hydrogen absorbing alloy; MmNi5 (Mu is misch metal, a mixture of rare earth elements); having a CaCu5-type crystal structure, is used a hydrogen absorbing alloy wherein the rare earth elements consist of Ce and/or Pr and a part of Ni of the alloy is replaced with two or more elements selected from Co, Mn, Al, and Fe.

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 using a hydrogen storage alloy capable of reversibly storing and releasing hydrogen.

[0002]

2. Description of the Related Art Currently used hydrogen storage alloys for hydrogen storage electrodes are commercially available Mm (Mish metal, a mixture of rare earth elements, such as Ce 45, La 30, Nd 5, Pr and other rare earth elements 20% by weight each). ) And Ni, Co, Mn, Al, etc. are used because the hydrogen dissociation pressure is about 1 atm at the battery operating temperature because the amount of Ce in the rare earth metal is large in the MmNi ₅ alloy using commercially available Mm. To do so, it is necessary to replace part of Ni with elements such as Co, Mn, and Al. However, when a part of Ni is replaced by Co, the effective hydrogen storage amount decreases as the replacement amount increases, and the discharge capacity when used as an electrode also decreases. In addition, although the effective hydrogen storage amount of Al increases as the substitution amount increases, Al has a larger atomic radius than Ni and tends to precipitate at grain boundaries, and excessive substitution is not possible.
This results in the elution of Al into the alkaline electrolyte, resulting in a decrease in cycle life when used as an electrode. In addition, as the substitution amount of Mn increases, the effective hydrogen storage amount increases, but Mn also causes elution into the alkaline electrolyte like Al, and becomes manganese oxide in the electrolyte and adheres to the separator and electrode surface in the battery. As a result, the conductivity of the electrode is lowered, and the battery life is shortened.

[0003]

[SUMMARY OF THE INVENTION Therefore, it is necessary to determine the respective element substitution amount required to obtain an appropriate hydrogen dissociation pressure and cycle life characteristics, MmNi _3.55 Co these were considered alloy
_0.75 Mn _0.4 Al _0.3 , MmNi _3.5 Co _0.7 Al _0.8, etc., but MmNi
_3.55 Co _0.75 Mn _0.4 Al _0.3 has a discharge capacity of 250mAh / g to 270mAah /
It has a sufficient discharge capacity to form a high capacity storage battery with g, but its cycle life is short. MmNi _3.5 Co _0.7 Al _0.8 has a long cycle life, but its discharge capacity is 200 mAh / g to 220 mAh / g, which is insufficient for constructing a substantially high capacity storage battery. The present invention is intended to provide a hydrogen storage electrode for a high-performance storage battery having a high discharge capacity and a long cycle life, which solves the above problems.

[0004]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have deeply studied a rare earth-based hydrogen storage alloy composition having a CaCu ₅ type crystal structure, particularly a rare earth element, and completed the present invention. but, the gist, rare earth-based hydrogen storage alloy having a CaCu ₅ type crystal structure MmNi ₅ (here Mm
Is a misch metal and a mixture of rare earth elements), where the rare earth elements consist of Ce and / or Pr, and some of Ni is C
A hydrogen storage electrode is characterized by using a hydrogen storage alloy substituted with two or more elements selected from o, Mn, Al and Fe.

The present invention will be described in detail below. The greatest feature of the present invention is that in the rare earth-based hydrogen storage alloy MmNi ₅ having a CaCu ₅ type crystal structure (where Mm is a misch metal and a mixture of rare earth elements), the rare earth element is Ce and / or
By limiting to two types of Pr, Ce 100, Pr 100, Ce / N
d = 99.9 to 0.1 / 0.1 to 99.9 Each weight% of each element is set alone or both are set. If elements other than Ce and Pr are mixed, the discharge capacity will decrease and the cycle life will decrease. FIG. 1 shows the relationship between the cycle life and the discharge capacity of a storage battery using the hydrogen storage alloy electrode of the present invention. As a preferable composition from which the discharge capacity can be secured to a certain value or more, Pr75 to 100% by weight, Most preferred is Ce25
% By weight Pr 75% by weight. The initial stable capacity also showed the same tendency. Regarding Ni, a part of Ni is Co, Mn,
It is preferable that two or more elements of Al and Fe are substituted, and the range of substitution amount is 0.5 ≦ Co ≦ 1.0 (hereinafter atomic ratio), 0.1 ≦ Mn
≦ 0.6, 0.2 ≦ Al ≦ 0.8, 0.1 ≦ Fe ≦ 0.5. Co is
If it is less than 0.5, the cycle life will be shortened, and if it exceeds 1.0, the elution into the electrolytic solution will be severe, so the above range is preferable. If the Mn is less than 0.1, the capacity will be insufficient, and if it exceeds 0.6, the elution into the electrolytic solution will be severe, so the above range is preferable.
When Al is less than 0.2, the cycle life is shortened, and when Al exceeds 0.8, it becomes inactive, so the above range is preferable. Also Fe
If it is less than 0.1, the capacity will decrease, and if it exceeds 0.5, the cycle life will decrease, so the above range is preferable. In particular,
CeNi _3.5 Co _0.7 Al _0.8 , PrNi _3.5 Co _0.7 Al _0.8 , (Ce ₅₀ Pr ₅₀ ) Ni
_3.5 Co _0.7 Al _0.8 , (Ce ₂₅ Pr ₇₅ ) Ni _3.5 Co _0.7 Al _0.8 , (Ce ₇₅ Pr ₂₅ )
Examples _include Ni _3.5 Co _0.7 Al _0.8 .

In addition, since the PCT diagram (pressure-composition isotherm diagram, not shown) of these alloys of the invention has a smaller change in dissociation pressure with temperature than that of the conventional product, the hydrogen storage electrode using the alloy has a capacity due to temperature change. It was found that the decrease was small, and the temperature characteristics were also examined. As a result, it was found that the discharge capacity was extremely stable against changes in temperature, especially at high temperatures, compared with Mm-based alloys.

The most effective application of this hydrogen storage alloy is a negative electrode for a nickel-hydrogen storage battery, and its manufacturing process is as follows. First, the rare earth metal Mm with a specified content of Ce and Pr
Is mixed with each rare earth metal and heated and melted in a high-frequency melting furnace to produce an R alloy. Separately, a predetermined amount of each Ni-based metal is mixed and heated and melted in a high-frequency melting furnace to produce an N alloy. And N alloys are weighed so that they have a predetermined composition ratio, heated and melted in a high-frequency melting furnace to form alloys, which are mechanically pulverized to a size of 75 μm or less and hydrogen storage alloy powder And As an example, this alloy powder is made into a paste with a 1.5% by weight aqueous solution of polyvinyl alcohol, and a 3 × 4 cm ² foamed Ni porous body is filled with 2 g of the paste of the alloy powder, dried and pressed to form a plate negative electrode having a thickness of 0.5 to 1.0 mm. Use as an electrode. The positive electrode is nickel hydroxide, and a non-woven fabric is used for the diaphragm.
Assemble the storage battery with 6N-KOH as the electrolyte.

[0008]

EXAMPLES The embodiments of the present invention will be specifically described below with reference to examples, but the present invention is not limited thereto. (Examples 1 to 5, Comparative Examples 1 to 4) The content of Ce: Pr is 10
0: 0, 75:25, 50:50, 25:75, 0: 100 A rare earth metal R (R is a rare earth element Ce, Pr) in a weight ratio is prepared,
The alloys were weighed so that each had a composition ratio of RNi _3.5 Co _0.7 Al _0.8 , and melted by heating in a high-frequency melting furnace to obtain a hydrogen storage alloy, which was mechanically pulverized to a size of 75 μm or less and A hydrogen storage alloy powder was obtained. These alloy powders were kneaded with a 1.5% by weight aqueous solution of polyvinyl alcohol to form a paste, and 2g of the alloy powder was filled in a foamed Ni porous body of 3 x 4 cm ² , dried and pressed to prepare a negative electrode having a thickness of 0.5 to 1.0 mm. . Sintered nickel hydroxide is used for the positive electrode and 6N for the electrolyte.
-Used KOH. Charge at 180mA for 5 hours discharge at 120mA
Up to 1.0V / cell. Also, the cycle life of the initial capacity
The point at which it decreased to 60% was defined as the life. The above-mentioned five types of hydrogen storage alloys are referred to as Examples 1 to 5, and Table 1 shows the test results.
Further, Table 2 shows low temperature discharge capacities at 20 ° C and 0 ° C. As Comparative Examples 1 to 4, La having a conventional alloy composition
Compositions and test results (test conditions are the same as those of the examples) of the H-type and Mm-type hydrogen storage alloys are also shown. As is clear from Table 1, the hydrogen storage electrode using only Ce and Nd has a remarkably long cycle life compared to the conventional product and a life of more than 2,000 cycles can be obtained. FIG. 1 illustrates the relationship between the discharge capacity and the cycle life of Examples 1 to 5. In addition, the initial stable capacity was measured in Example 4 (Pr 7
5) In Example 5 (Pr 100), the discharge capacity was 260 mAh / g, which was a sufficient discharge capacity to form a substantially high capacity storage battery. From Table 2, all of the examples have better low-temperature characteristics than the comparative example of the conventional product.

[0009]

[Table 1]

[0010]

[Table 2]

[0011]

According to the present invention, a hydrogen storage electrode having a long cycle life and capable of maintaining a high capacity in a low temperature range can be provided, and its industrial utility value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between cycle life and discharge capacity in Examples 1 to 5 of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Rui Nakano 2-1, 5 Kitafu, Takefu City, Fukui Prefecture Shinetsu Kagaku Kogyo Co., Ltd. Magnetic Materials Research Center (72) Inventor Toshio Kobayashi 2-chome, Kitafu, Fukui Prefecture No. 5 Shinetsu Kagaku Kogyo Co., Ltd. Magnetic Materials Research Center (72) Inventor Koji Nakamura 2-5-5 Kitafu, Takefu City, Fukui Prefecture Shin-Etsu Kagaku Kogyo Co., Ltd. Magnetic Materials Research Center

Claims (1)

[Claims] 1. In a rare earth-based hydrogen storage alloy MmNi ₅ (where Mm is a misch metal and a mixture of rare earth elements) having a CaCu ₅ type crystal structure, the rare earth elements are Ce and / or Pr.
A hydrogen storage electrode, comprising a hydrogen storage alloy in which a part of Ni is replaced by two or more elements selected from Co, Mn, Al and Fe.