JPH097588A - Hydrogen absorbing electrode - Google Patents

Abstract

PURPOSE: To provide a high capacity and a long life and to suppress increasing an internal pressure in the case of a sealed battery, by having a hydrogen absorbing alloy formed with a transition metal rich layer, and containing a simple substance or a compound of rare earth element as an additive agent. CONSTITUTION: A hydrogen absorbing alloy having a composition of MmNi3.9 Al0.3 Co0.7 Mn0.2 is crushed into a suitable size. Crushed alloy powder is immersed in an acetic acid-sodium acetate buffer solution adjusted to pH3.6, stirred and dried after washing by water. This alloy sample and 0.5% powder of Er2 O3 are well mixed and formed into a paste state by adding a thickener to charge a nickel fiber substrate, to be pressed after drying to prepare an electrode A. On the other hand, the hydrogen absorbing alloy manufactures a hydrogen absorbing electrode without mixing Er2 O3 powder, to obtain a reference electrode 1. In the hydrogen absorbing alloy not immersed in an acetic acid-sodium acetate butter solution, an electrode 2 of mixing 0.5% Er2 O3 powder and an electrode 3 of not mixing Er2 O3 powder are manufactured. The electrode A and reference electrodes 1, 2, 3 are used, to perform charge/discharge with a nickel electrode serving as a mate electrode.

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.

[0002]

2. Description of the Related Art Nickel-hydride secondary batteries using a hydrogen storage alloy as a negative electrode material are low in pollution and have high energy density. Therefore, they can be used as portable power sources for nickel-cadmium batteries, portable devices, electric vehicles, etc. , And is being actively researched and developed.

[0003]

The nickel-hydride secondary battery is used as a sealed battery, and in this case,
The rise in the pressure inside the battery is closely related to the problem of safety and the problem of performance deterioration due to electrolyte outflow when the safety valve operates. Therefore, in addition to high capacity and long life of the battery, suppression of increase in internal pressure is an important issue in the development of a high performance sealed battery.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a hydrogen storage electrode which has a high capacity and a long life and which does not cause an increase in internal pressure when a sealed battery is formed.

[0005]

The first aspect of the present invention is characterized by having a hydrogen storage alloy in which a transition metal rich layer is formed and containing a rare earth element simple substance or compound as an additive. It is a hydrogen storage electrode.

A second aspect of the present invention is that the rare earth element is L
a, Ce, PrNd, Sm, Eu, Gd, Tb, Dy,
The hydrogen storage electrode is at least one kind selected from Ho, Y, Er, Tu, Yb, Lu and Sc.

A third aspect of the present invention is a hydrogen storage electrode, wherein the compound of the rare earth element is at least one selected from oxides, hydroxides and halides.

A fourth aspect of the present invention is the hydrogen storage electrode, wherein the transition metal rich layer is formed on the surface of the hydrogen storage alloy by immersing the hydrogen storage alloy in an acidic aqueous solution or an alkaline aqueous solution. .

A fifth aspect of the present invention is the hydrogen storage electrode, wherein the transition metal rich layer is mainly composed of nickel.

[0010]

It has been known that the increase in the internal pressure of the battery due to repeated charging and discharging is caused by hydrogen gas. Therefore, it is possible to suppress the increase in the internal pressure by improving the negative electrode characteristics.

In the sealed battery, the negative electrode capacity is larger than the positive electrode capacity, and hydrogen gas generation from the negative electrode should not occur with some overcharge. Nevertheless, there are mainly two causes of the generation of hydrogen gas from the negative electrode at the end of charging. First, the initial activation of the negative electrode is slow, so the charging efficiency of the negative electrode is poor and hydrogen gas is generated at the end of charging. Secondly, the reserve balance is deviated, the charge reserve is reduced, and hydrogen gas is generated at the end of charge.

Of these, the first solution to the cause is a method of surface-treating the alloy. The initial activation of the hydrogen storage alloy electrode is a step of dissolving the concentrated rare earth element on the alloy surface to form a transition metal rich layer. Since this transition metal layer acts as an electrode reaction field, if it is formed in advance by surface treatment, the initial activation becomes faster. Here, the treatment liquid for performing the surface treatment includes two types, a weakly acidic aqueous solution and an alkaline aqueous solution. As a result of various studies in a weakly acidic aqueous solution, a rare earth element on the surface of the hydrogen storage alloy can be selectively dissolved in a specific pH range, and a transition metal rich layer can be easily formed on the alloy surface layer without generating an insulating substance. I have found that it is possible. When an acetic acid-sodium acetate buffer solution is used, it is easy to control the pH. Here, the weak acid is used because the strong acid may corrode even nickel, which is the main component of the transition metal rich layer. By treating these operations at a high temperature, the treatment time can be shortened. In addition, it is generally known that the treatment with the alkaline aqueous solution produces needle-like insulating rare earth hydroxide precipitates from the alloy surface. However, when a treatment solution having the same concentration and composition as the electrolytic solution used in the battery is used, the production of rare earth hydroxide is considerably suppressed. The rare earth element is easily ionized in an aqueous solution system containing the LiOH aqueous solution, and since the electrolytic solution used is a mixed aqueous solution of KOH and LiOH, the rare earth element is unlikely to precipitate as a hydroxide in the treatment using the electrolytic solution. Therefore, an electrode having an early activation equivalent to that of the weak acid treatment can be obtained.

The second major cause is considered to be alloy corrosion. For example, when the rare earth metal in the hydrogen storage alloy is oxidized, the reduction reaction which is a counter reaction thereof is a hydrogen generation reaction, but in this case, hydrogen is stored in the hydrogen storage alloy. As a result, the discharge reserve increases,
Charge reserve is reduced. Therefore, the solution to this problem is the inhibition of alloy corrosion. For that purpose, it is effective to mix a small amount of an additive composed of a simple substance or a compound of a rare earth element. This additive, once dissolved in the electrolytic solution, re-precipitates as a thin and dense film of several tens of liters to cover the alloy surface and suppress alloy corrosion. The reason why the amount of addition is small is that if the amount of addition is large, the passivation film becomes too thick and the activation of the hydrogen storage alloy is delayed.

The effect of suppressing the increase in internal pressure is effectively exhibited when both of the above two measures are simultaneously satisfied. Even if a rare earth element is added to an alloy which has not been surface-treated, the activation of the alloy having a slower activation is further delayed. In addition, alloys that have undergone only surface treatment proceed rapidly to alloy corrosion, and the internal pressure tends to rise when a sealed battery is formed.

By satisfying both the surface treatment of the hydrogen storage alloy and the mixture of the rare earth element alone or the compound as described above, it is possible to suppress the increase in the internal pressure when the sealed battery has a high capacity and a long life.

[0016]

EXAMPLES The present invention will be described below based on examples. (Example 1) First, MmNi _3.8 Al _0.3 Co _0.7 Mn
A hydrogen storage alloy having a composition of _0.2 is prepared and crushed to an appropriate size. In addition, Mm is misch metal, L
It is a composite of rare earth elements containing at least one of a, Ce, Pr and Nd. Next, this crushed alloy powder was immersed in an acetic acid-sodium acetate buffer solution whose pH was adjusted to 3.6, stirred, washed with water and dried.

This alloy sample and Er ₂ O ₃ powder 0.5% were mixed well in a mortar, and then a thickener was added to form a paste, which was filled in a nickel fiber substrate, dried and pressed to form a book. Invention electrode A was produced. On the other hand, the hydrogen storage alloy is the same, Er
_A hydrogen storage electrode not mixed with ₂ O ₃ powder was produced in the same manner as above. This is referred to as reference electrode 1. Further, a hydrogen storage alloy having the same composition as the above, which was not immersed in an acetic acid-sodium acetate buffer solution, was prepared, and a hydrogen storage electrode obtained by mixing 0.5% of Er ₂ O ₃ powder therein was used in the same manner as above. It was made. This is referred to as a reference electrode 2. Further, a hydrogen storage electrode was prepared in the same manner as described above, in which the hydrogen storage alloy of the same composition as above, which was not immersed in the acetic acid-sodium acetate buffer solution, was not mixed with Er ₂ O ₃ powder. This is referred to as a reference electrode 3.

Using the electrode A of the present invention thus produced and the comparative electrodes 1, 2, and 3, charging / discharging was carried out using a normal nickel electrode as a counter electrode. The result is shown in FIG. As is clear from FIG. 1, the comparison electrodes 2 and 3 which are not surface-treated have a slow initial activation. Only in the comparative electrode 2, since the mixed Er ₂ O ₃ powder is passivated, the activation is slower. The surface-treated electrode A of the present invention and the comparative electrode 1 were activated quickly and showed a high capacity.

Nominal 1000 using these four types of electrodes
A mA size AA sealed battery was fabricated. The invention battery A, the comparative battery 1, the comparative battery 2, and the comparative battery 3 are referred to as the present invention, respectively. Attach a pressure sensor for measuring the internal pressure of each battery,
It was charged and discharged. The result is shown in FIG. As is clear from FIG. 2, the internal pressure of the comparative battery 1 tends to increase as the number of cycles increases, but the internal pressure increase of the battery A of the present invention is significantly suppressed. Comparative battery 2 and comparative battery 3 showed a significant increase in internal pressure. Further, it is understood that the battery A of the present invention and the comparative battery 1 are also excellent in discharge capacity from the beginning.

The above battery was disassembled, the hydrogen storage alloy was taken out from the hydrogen storage electrode, and X-ray diffraction and electron microscope observation were carried out. FIG. 3 shows the X-ray diffraction result. The peak of the portion surrounded by a circle in FIG. 3 indicates the peak of the rare earth hydroxide. FIG. 4 shows an enlarged view of the peak in the center of FIG. Er
_It can be seen that in the electrode A of the present invention in which ₂ O ₃ powder was mixed, the amount of rare earth hydroxide produced was small, and the Er ₂ O ₃ mixture suppressed alloy corrosion. 5 shows an electron micrograph of the electrode A of the present invention, and FIG. 6 shows a battery micrograph of the comparative electrode 1. The acicular precipitation in the figure is the rare earth hydroxide, and it can be seen from FIGS. 5 and 6 that alloy corrosion is suppressed by mixing Er ₂ O ₃ .

Example 2 First, MmNi _3.8 Al _0.3
A hydrogen storage alloy having a composition of Co _0.7 Mn _0.2 is prepared and crushed to an appropriate size. Next, this crushed alloy powder was immersed in a high temperature alkaline aqueous solution in which KOH and LiOH were mixed, stirred, washed with water and dried. The alkaline aqueous solution has the same composition and concentration as that used as the electrolytic solution.

This alloy sample and Yb ₂ O ₃ powder 0.5%
Was thoroughly mixed in a mortar, a thickener was added to form a paste, the nickel fiber substrate was filled, dried and pressed to prepare an electrode B of the present invention. On the other hand, a hydrogen storage electrode having the same hydrogen storage alloy and containing no Yb ₂ O ₃ powder was prepared in the same manner as above. This is referred to as a reference electrode 4. Also,
A hydrogen storage alloy having the same composition as the above, which is not immersed in a KOH / LiOH mixed alkaline aqueous solution, is prepared.
A hydrogen storage electrode mixed with 0.5% of b ₂ O ₃ powder was prepared in the same manner as above. This is referred to as a reference electrode 5. Further, a hydrogen storage electrode in which Yb ₂ O ₃ powder was not mixed with the hydrogen storage alloy having the same composition as above, which was not immersed in the KOH / LiOH mixed alkaline aqueous solution, was prepared in the same manner as above. This will be referred to as a reference electrode 6.

Using the electrode B of the present invention thus produced and the comparative electrodes 4, 5, and 6, a normal nickel electrode was used as a counter electrode for charging and discharging. FIG. 7 shows the result. As is clear from FIG. 7, the initial activation of the comparative electrode 5 and the comparative electrode 6 not subjected to the surface treatment is slow. Only in the comparative electrode 5, since the mixed Yb ₂ O ₃ powder is passivated, the activation is slower. The surface-treated electrode B of the present invention and the comparative electrode 4 were activated quickly and showed a high capacity.

Nominal 1000 using these four types of electrodes
A mA size AA sealed battery was fabricated. The present battery B, the comparative battery 4, the comparative battery 5, and the comparative battery 6, respectively. Attach a pressure sensor for internal pressure measurement to each battery,
It was charged and discharged. FIG. 8 shows the result. As is clear from FIG. 8, the internal pressure of Comparative Battery 4 increased as the number of cycles increased, but the internal pressure of Battery B of the present invention was significantly suppressed. Comparative battery 5 and comparative battery 6 showed a remarkable increase in internal pressure at the initial stage of charging and discharging. Further, it is understood that the battery B of the present invention is also excellent in discharge capacity.

Example 3 First, MmNi _3.8 Al _0.3
A hydrogen storage alloy having a composition of Co _0.7 Mn _0.2 is prepared and crushed to an appropriate size. Next, this crushed alloy powder was immersed in a high temperature alkaline aqueous solution in which KOH and LiOH were mixed, stirred, washed with water and dried. The alkaline aqueous solution is the same as that used as the electrolytic solution.

This alloy sample and Er (OH) ₃ powder 0.
Mix well with 5% in a mortar, add thickener and add
The electrode C of the present invention was produced by forming a strike, filling a nickel fiber substrate, drying and pressing. Further, YbF ₃ was mixed to prepare a hydrogen storage electrode in the same manner as the electrode D of the present invention.
Using the electrode C of the present invention and the electrode D of the present invention thus produced, an AA size sealed battery of nominally 1000 mA was produced. The present invention battery C and the present invention battery D are respectively referred to. A pressure sensor for measuring the internal pressure of each battery was attached and charged and discharged. The result is shown in FIG. As is clear from FIG. 9, any additive has an effect of suppressing an increase in internal pressure.

[0027]

As described above, in the hydrogen storage electrode of the present invention, both the surface treatment of the hydrogen storage alloy and the mixture of the rare earth element or the compound of the rare earth element provide a high capacity and a long life, and at the same time, the sealed type. An extremely excellent effect that an increase in internal pressure can be suppressed when used as a battery is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the number of cycles and discharge capacity.

FIG. 2 is a diagram showing the relationship among the number of cycles, discharge capacity, and battery internal pressure.

FIG. 3 is an X-ray diffraction diagram of an electrode A of the present invention and a comparative electrode 1.

FIG. 4 is an enlarged view of a central portion of FIG.

FIG. 5 is an electron micrograph of electrode A of the present invention.

FIG. 6 is an electron micrograph of comparative electrode 1.

FIG. 7 is a diagram showing the relationship between the number of cycles and the discharge capacity.

FIG. 8 is a diagram showing the relationship among the number of cycles, discharge capacity, and battery internal pressure.

FIG. 9 is a diagram showing the relationship among the number of cycles, discharge capacity, and battery internal pressure.

Claims (5)

[Claims] 1. A hydrogen storage electrode, comprising a hydrogen storage alloy having a transition metal rich layer formed thereon, and containing a rare earth element simple substance or compound as an additive. 2. The method according to claim 1, wherein the rare earth element is La, Ce, Pr,
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, E
The hydrogen storage electrode according to claim 1, which is at least one selected from r, Tu, Yb, Lu, and Sc. 3. The method according to claim 1, wherein the rare earth compound is at least one selected from oxides, hydroxides, and halides.
The hydrogen storage electrode according to claim 1, wherein the hydrogen storage electrode is of a kind. 4. The hydrogen storage electrode according to claim 1, wherein the transition metal rich layer is formed on the surface of the hydrogen storage alloy by immersing the hydrogen storage alloy in an acidic aqueous solution or an alkaline aqueous solution. 5. The hydrogen storage electrode according to claim 1, wherein the transition metal-rich layer is mainly composed of nickel.