JPH04167365A - Hydrogen storage electrode - Google Patents

Abstract

PURPOSE:To enhance the oxidation resistance of a hydrogen storage alloy by providing the hydrogen storage allay with an amorphous layer, and also providing the layer on the surface of the alloy. CONSTITUTION:The progress of oxidation of a hydrogen storage alloy is caused by the enlargement of the surface area of the allay due to pulverization of crystal as the crystal is expanded and contracted by repetition of the storage and release of hydrogen. When the surface layer of the hydrogen storage alloy is provided with an amorphous layer pulverization of the alloy is restrained and the progress of oxidation of alloy powder is prevented and also the active material is prevented from falling off. When an amorphous layer is provided to an alloy of the same composition the amount of hydrogen stored in the alloy is smaller than that stored in the inner hydrogen storage alloy but the alloy has an ability to store a large amount of hydrogen so the energy density of the hydrogen storage electrode is not lowered so much as when the electrode is covered at its surface with a metal which does not contain hydrogen at all, and a new phase is generated in a boundary area between the surface layer and the internal layer and the ability of the alloy to store hydrogen would not be lost.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an improvement in a hydrogen storage electrode capable of reversibly absorbing and desorbing hydrogen.

Problems with the Prior Art In a nickel-metal hydride battery using a hydrogen storage alloy, deterioration of the hydrogen storage alloy is an important factor in determining the battery life. There are various reasons why alloys deteriorate.
For example, deterioration due to oxidation or corrosion of the alloy may be considered. In particular, in sealed batteries, like nickel-cadmium batteries, airtightness is achieved by reducing oxygen gas generated from the positive electrode at the end of charging on the hydrogen storage electrode of the negative electrode. Therefore, the hydrogen storage alloy is constantly in contact with active oxygen gas, which causes the alloy to be oxidized and gradually deteriorate due to elution of alloy constituent elements, generation of oxides or hydroxides, etc. In addition, when incorporated into electronic equipment, multiple batteries are often connected and used.
In such a case, if a battery with a low capacity is included, the battery will be over-discharged, leading to a situation in which oxygen gas is generated on the surface of the hydrogen storage alloy, making it extremely susceptible to oxidation. In such a state, the electrode becomes covered with an oxide layer, or phenomena such as elution as described above occur, and the hydrogen storage capacity decreases, leading to electrode deterioration. Particularly when covered with an oxide layer, it is thought that the hydrogen ionization reaction during discharge is inhibited.

For the purpose of preventing oxidation of hydrogen storage alloys, as disclosed in Japanese Unexamined Patent Publication No. 61-64069, it has been proposed to cover the alloy surface with an alkali-resistant metal.

However, if such a method is used, a large amount of metal that does not substantially absorb or release hydrogen will be added, and the gravimetric energy density will decrease by as much as 20% when coated with copper. Therefore, there was a problem that the high energy density property of this electrode was lost.

OBJECTS OF THE INVENTION An object of the present invention is to provide a hydrogen storage electrode that can minimize the decrease in energy density, improve the oxidation resistance of a hydrogen storage alloy, and withstand long-term use.

Structure of the Invention The present invention is a hydrogen storage electrode characterized in that a hydrogen storage alloy is provided with an amorphous layer, and the amorphous layer is provided on the surface of the hydrogen storage alloy.

Function: Taking advantage of the high corrosion resistance of rare earth alloys, it is possible to provide a thin film of rare earth alloys on the surface layer of the alloy.
As proposed in Publication No. 175343, in rare earth alloys, the oxide layer formed on the alloy surface is destroyed as a result of absorption and release reactions, and a new alloy surface is constantly exposed.
Even if the surface is oxidized, the storage capacity is prevented from decreasing.

However, when this alloy is provided as a thin film, the rare earth alloy thin layer is quickly oxidized, and even if the internal alloy can be prevented from oxidizing, the surface reaction is inhibited, so the alloy as a whole has no occlusion/release capacity. or decrease.

Unlike crystalline hydrogen storage alloys, amorphous hydrogen storage alloys do not have a clear plateau pressure, so the actual amount of M absorbed is small. It is known to have characteristics such as small size and resistance to oxidation.

In the present invention, oxidation of the internal alloy is prevented by providing the surface layer of the crystalline hydrogen storage alloy with an amorphous hydrogen storage alloy that is less susceptible to oxidation.

Oxidation of hydrogen storage alloys progresses because the crystals expand and contract repeatedly due to repeated storage and desorption, and are pulverized, resulting in an increase in surface area. By providing an amorphous layer in the alloy, pulverization of the alloy can be suppressed and progress of oxidation of the alloy powder can also be prevented. Moreover, by suppressing pulverization, falling off of the active material is also prevented.

Furthermore, by providing an amorphous layer made of an alloy with the same composition, it has the ability to absorb a large amount of hydrogen, although the amount of hydrogen absorption is smaller than that of an internal hydrogen storage alloy, so the surface is coated with a metal that does not absorb hydrogen at all. Compared to the case where the energy density of the hydrogen storage electrode is reduced significantly, it is possible to improve the energy density of the hydrogen storage electrode. Furthermore, there is no problem such as deactivation of storage capacity due to the formation of a new phase in the boundary region between the surface layer and the internal layer.

Example Hereinafter, the present invention will be explained in detail with reference to Examples.

Mm < Mitsushi metal, mixture of rare earth elements), Ni,
A predetermined amount of AN was weighed and melted several times in an Ar atmosphere in an arc melting furnace to obtain MmNi: +, s Affio,
A homogeneous alloy ingot represented by s Coo, 7 was prepared. This was mechanically crushed and passed through a sieve to obtain an alloy powder that passed 300 mesh. On the other hand, sputter coating was performed on the alloy powder using a similarly prepared alloy ingot as a target. A 2vt% PVA aqueous solution was added to this powder to form a paste, which was then filled into a nickel fiber sintered substrate, dried and pressed, and the hydrogen storage electrode according to the present invention (A
). In addition, it was confirmed through various instrumental analyzes that no specific crystal structure was detected in the coating layer. For comparison, an electrode produced using hydrogen storage alloy powder produced in the same manner except that sputter coating was not performed was designated as a conventional electrode (B).

These electrodes were used as negative electrodes, and Co was placed on the nickel fiber sintered substrate.
A sealed battery was constructed using a known fiber-type nickel electrode filled with a nickel hydroxide active material paste containing O as the positive electrode, a polyamide nonwoven fabric as the separator, and an aqueous potassium hydroxide solution with a specific gravity of 128 as the electrolyte. Battery size is A
It is A size (AA size) and has a nominal capacity of 1100IIAH.

Batteries using electrodes (A) and (B) are referred to as battery A and battery B, respectively.

Figure 1 shows the results of examining the cycle characteristics of these batteries. Charging is 0.25CmA for 5 hours, discharging is 0.5CrA
A final voltage was set to 0.8V. As is clear from FIG. 1, the cycle characteristics of the battery of the present invention are very good.
It can be seen that this electrode is highly durable. After this test, the battery was disassembled and the negative electrode alloy was investigated. In conventional battery B, a large amount of acicular crystals were observed on the alloy surface, and TEM analysis confirmed that it was a rare earth metal hydroxide. It was done. On the other hand, in the battery A of the present invention, almost no needle-like crystals were observed, and very little alloy composition elements were eluted into the electrolyte. In other words, it is thought that in the conventional battery, oxidation of the alloy progressed as the battery was charged and discharged, and the electrode characteristics deteriorated. However, in the present invention, it is thought that by making the surface layer amorphous, internal oxidation is prevented and durability is improved. In addition, because there are fewer corrosion products and a metal layer on the surface, the conductivity between alloy particles does not deteriorate, and the efficiency of oxygen gas absorption during charging does not decrease, improving cycle life. it is conceivable that.

In this example, the amorphous layer was formed using an alloy with the same composition, but an alloy with a different composition is also effective in improving the durability of the electrode. However, since there is a possibility that a new alloy layer will be generated or layer separation will occur in the boundary region between the amorphous layer and the crystalline layer, it is preferable that they have the same composition. Further, the alloy composition is not limited to rare earth-based alloys, and any composition that can form a passive film on the alloy surface may be used.

In addition, in the above embodiment, an example using a sintered nickel fiber substrate was shown, but the substrate is not limited to this, and expanded metal, metal mesh, nickel-plated punching metal, etc. may be used as the substrate.

Effects of the Invention As described above, according to the present invention, it is possible to improve the oxidation resistance of the hydrogen storage electrode without reducing the energy density and extend the life of the electrode. It's huge.

[Brief explanation of the drawing]

FIG. 1 is a cycle characteristic diagram of a battery of the present invention and a conventional battery. Applicant Yuasa Battery Co., Ltd. Figure 1 Cycle number (S)

Claims (3)

[Claims] (1) A hydrogen storage electrode capable of reversibly absorbing and releasing hydrogen, characterized in that an amorphous layer is provided on a hydrogen storage alloy. (2) The hydrogen storage electrode according to claim 1, wherein the amorphous layer is a surface layer of the alloy. (3) The hydrogen storage electrode according to claim 2, wherein the alloy composition of the amorphous layer is substantially the same as the alloy composition of the internal crystalline layer.