JPH11222601A - Surface treating method for hydrogen storage alloy powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treating method for a hydrogen storage alloy capable obtaining an optimum amt. of nickel-enriched layer in spite of the variation of process conditions in a nickel-enriched layer forming treatment. SOLUTION: This surface treating method for hydrogen storage alloy powder is the one executing nickel-enriched layer forming treatment in such a manner that hydrogen storage alloy powder contg. nickel is treated to form a nickel- enriched layer on the surface of the hydrogen storage powder, in which the magnetic properties of the hydrogen storage alloy powder varying in accordance with the change of the surface properties of the hydrogen storage alloy powder in the process of the nickel-enriched layer forming treatment are detected, and the treatment is finished at the point of time when the magnetic properties reach the prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for surface treating a hydrogen storage alloy powder.

[0002]

2. Description of the Related Art LaNi ₅ -based, Ti-Fe-based, and hydrogen-absorbing alloys capable of absorbing and releasing hydrogen at around normal temperature and pressure.
Include Zr alloy Laves phase system, in particular, LaNi ₅ or MmNi ₅ (Mm: misch metal over lanthanum, mixtures of rare earth elements such as cerium) representative to AB ₅ type, Z
rV _0.4 Ni _1.6 TiZrVNi based Laves phase alloy representatives to AB ₂ type hydrogen storage alloy, etc. is equilibrium pressure 1 atm around the room temperature, as well as a possible reversibly absorbing and desorbing hydrogen, an alkaline aqueous solution Because of its relatively long contact resistance, it is used as a negative electrode active material for secondary batteries, and is also used as a chemical heat pump or hydrogen tank.

However, there has been a problem that such a hydrogen storage alloy surface easily forms an oxide layer when exposed to air, and this oxide layer hinders the storage and release of hydrogen.
In addition, in the nickel hydride battery using the negative electrode composed of the hydrogen storage alloy powder, the battery characteristics in the early stage of use in which no Ni layer as a hydrogen dissociation catalyst was formed on the surface of the hydrogen storage alloy powder, that is, the initial activation characteristics were poor.

In order to solve this problem, Japanese Patent Laid-Open No. 5-1 has been proposed.
3077 and JP-A-4-137361, an alloy powder is immersed in a high-temperature alkaline aqueous solution to remove an oxide layer, a misch metal, Co, Al, and Mn from the surface of a hydrogen storage alloy powder and leave a Ni catalyst layer while leaving only Ni. It is proposed to form In addition, proposals have been made to immerse the hydrogen storage alloy powder in an acidic aqueous solution or to treat it with hydrogen gas or water vapor to pursue a similar effect.

[0005]

However, when performing the above-described various processes (hereinafter, also referred to as a nickel-rich layer forming process), it is not easy to control the processes. That is, the degree of oxidation and the composition of the hydrogen storage alloy powder, which is the raw material before the treatment, naturally have variations due to the history up to that point, and the conditions of the nickel-rich layer forming process such as the temperature, the solution concentration, and their temporal changes. This is also due to various variations. For example, a decrease in the solution concentration or an increase in the concentration of a component harmful to the reaction tends to delay the nickel-rich layer forming process, and an increase in the temperature tends to accelerate it. It is very difficult to control completely constant.

[0006] These variations in the manufacturing history vary the degree of removal of oxides from the surface of the hydrogen-absorbing alloy powder and the degree of formation of the nickel-rich layer. This causes a change in the electrode resistance of the alloy negative electrode. More specifically, these nickel-rich layer forming treatments do not have to be performed for a long time to increase the ratio of the nickel-rich layer without limit, but if these treatments are performed excessively, the hydrogen in the hydrogen-absorbing alloy powder may be reduced. The proportion of the portion effective for occlusion and release is reduced, and the nickel-rich layer formed too thick conversely hinders hydrogen transfer of the hydrogen storage alloy powder between the outside and the effective portion.

The present invention has been made in view of the above-mentioned problems, and a method of surface treating a hydrogen-absorbing alloy powder capable of obtaining an optimal amount of a nickel-rich layer despite fluctuations in process conditions in the nickel-rich layer forming treatment. Is the issue to be solved. Conventionally,
It has been known that the nickel-rich layer forming treatment improves the initial activation characteristics of the hydrogen storage alloy negative electrode. However, as described above, the formation of the nickel-rich layer varies depending on various manufacturing conditions. The decision was difficult.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is another object of the present invention to provide a method for surface treating a hydrogen storage alloy powder capable of obtaining good properties despite various production conditions. It should be an issue to be done.

[0009]

According to the method for treating the surface of a hydrogen storage alloy powder according to the present invention, the hydrogen storage alloy powder containing nickel is treated with water vapor, hydrogen gas, acid, alkali or the like to store the hydrogen. A nickel-rich layer forming process for forming a nickel-rich layer on the surface of the alloy powder is performed.

Since nickel alone formed by such a nickel-rich layer forming process has ferromagnetism, the magnetic characteristics of the hydrogen storage alloy powder tend to gradually approach the magnetic characteristics of the ferromagnetic material. Therefore, in this configuration, while monitoring the magnetic characteristics of the hydrogen storage alloy powder, the nickel-rich layer forming process ends when the magnetic characteristics reach a predetermined level.
In this way, the hydrogen having an optimal amount of the nickel-rich layer regardless of the variation in the processing conditions, and as a result, having an excellent reaction activity with good hydrogen without unnecessarily reducing the hydrogen storage capacity. An occlusion alloy powder can be obtained.

According to a second aspect of the present invention, in the method for treating a surface of a hydrogen storage alloy powder according to the first aspect, the magnetic susceptibility of the hydrogen storage alloy powder is further employed as a magnetic property to be monitored. This is effective in improving the detection accuracy. According to the surface treatment method for a hydrogen storage alloy powder according to the third aspect, the nickel-rich layer forming treatment for treating the hydrogen storage alloy powder containing nickel using steam, hydrogen gas, acid, alkali, or the like is performed by the hydrogen storage alloy powder. Dry weight of (g)
This is performed until the magnetic susceptibility (emu) per hit becomes 0.3 to 0.8.

In this way, it is possible to remarkably improve the hydrogen reaction activity of the hydrogen storage alloy powder while suppressing the decrease in the hydrogen storage and release capabilities of the hydrogen storage alloy powder due to the excessive formation of the nickel-rich layer.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a hydrogen storage alloy powder, a powder containing nickel is used. The nickel-rich layer forming process may be any process that forms a nickel-rich layer as a hydrogen dissociation catalyst on the surface. An increase in the amount of ferromagnetic material causes a change in various magnetic properties and, consequently, a change in electrical properties. Can be adopted. More specifically, the change in the amount of ferromagnetic material in the hydrogen storage alloy powder can be detected in a non-contact manner as a change in the voltage, current, and frequency of a magnetic circuit that forms a part of the magnetic circuit. The change in the magnetic flux density can be detected by a magnetic sensor.

As an example of the magnetic properties to be detected, the magnetic properties of the hydrogen-absorbing alloy powder may be such that they surround the hydrogen-absorbing alloy powder being processed, are inserted into the hydrogen-absorbing alloy powder, or A predetermined DC or AC current is applied to the coil disposed near the storage alloy powder, or a magnetic field H is applied by a permanent magnet or the like, and the magnetic flux density B near the hydrogen storage alloy powder at this time is detected by a magnetic sensor. The magnetic characteristics can be detected based on the relationship between B and H. For example, as the ferromagnetic nickel-rich layer increases, the magnetic resistance decreases and the magnetic density increases. By detecting this, the amount of the nickel-rich layer formed can be determined.

Further, if an oscillator is formed using the above coil, the inductance of the coil increases with the increase of the nickel-rich layer and the oscillation frequency of the oscillator decreases. The amount can be detected. When a hydrogen storage alloy negative electrode is prepared using this hydrogen storage alloy powder, methyl cellulose, carboxymethyl cellulose, or the like can be used as a thickener, and polytetrafluoroethylene ( PTFE), a styrene-butadiene copolymer, or the like can be employed. As an electrolyte for a battery using a hydrogen storage alloy electrode, a KOH aqueous solution,
A mixed aqueous solution of KOH and LiOH can be used, and as the current collector inside the hydrogen storage alloy negative electrode, foamed nickel, punching metal, or the like can be used.

[0016]

Embodiment 1 In this embodiment, a surface treatment method (nickel-rich layer forming treatment) using an acidic aqueous solution was performed while detecting the saturation magnetic susceptibility. As the hydrogen storage alloy powder before the treatment, MmNi _3.6 Co was mechanically pulverized to 100 mesh or less.
_0.7 Mn _0.3 Al _0.3 (La / Mm = 0.6) powder was used.

The hydrogen storage alloy powder is placed in a glass tube which is a non-magnetic material, immersed in an acetic acid aqueous solution having a pH of 3.5 to 4.5, and a vibrating sample magnetometer is adhered to the outer surface of the glass tube. Then, the magnetic susceptibility of the solution (more precisely, the hydrogen storage alloy powder) was measured. When the magnetic susceptibility became 0.5 emu / g, the process was terminated. Thereafter, the powder was taken out from the glass tube, washed with distilled water, and dried under vacuum to obtain a surface-treated (activated) hydrogen storage alloy powder.

For reference, FIG. 1 shows how the magnetic susceptibility measured by the vibrating sample magnetometer changes with the lapse of time in the above processing.
FIG. 2 shows the MH alloy utilization rate (discharge capacity / theoretical discharge capacity) of the hydrogen storage alloy negative electrode using the hydrogen storage alloy powder when the treatment time was variously changed. However, the manufacturing conditions of the hydrogen storage alloy negative electrode and the charge / discharge conditions thereof are described below.

After the hydrogen storage alloy powder having the above composition was activated, the paste was prepared by mixing a 2 wt% aqueous solution of methylcellulose with the hydrogen storage alloy powder at 30 wt%. This was applied to a 30 × 40 mm foamed nickel current collector, dried, and pressed to obtain a paste negative electrode having a thickness of about 0.6 mm. Using a paste nickel electrode of 45 × 60 mm as a nickel positive electrode, a nickel-metal hydride secondary battery regulated by a negative electrode was produced as follows. A battery stack was prepared by superimposing two positive electrodes on one negative electrode via a separator. A mixed aqueous solution of 6.8N KOH + 0.8N LiOH, which is an electrolytic solution, was injected into this to make a battery.

For charging and discharging, a charge of 0.2 C is reduced to 1 of the theoretical capacity.
Discharge to a final voltage of 0.8 V
C. From FIG. 1, it can be seen that excellent initial activation characteristics are selected when the magnetic susceptibility (emu) per dry weight (g) of the hydrogen storage alloy powder is in the range of 0.3 to 0.8. That is, if it is less than 0.3, the initial activity characteristics are poor,
If it exceeds 0.8, the ratio of the nickel-rich layer in the hydrogen storage alloy powder increases, so that the absolute value of the capacity decreases.

After all, according to the present embodiment, it is possible to form an optimal amount of the nickel-rich layer irrespective of the variation of the composition of the hydrogen-absorbing alloy powder, the manufacturing conditions, and the processing conditions for forming the nickel-rich layer. And it can be realized without complicating the apparatus or the processing configuration.

[0022]

Embodiment 2 Another nickel rich layer formation amount detection method will be described with reference to FIG. The detection system of this embodiment is more suitable for mass production than the susceptibility detection system of the first embodiment. 1 is a magnetic head, 2 is an oscillation circuit, 3 is a power supply circuit, and 4 is a signal including a microcomputer. The processing circuit 5 is a processing tank. The processing tank 5 is charged with the hydrogen storage alloy powder and the acetic acid aqueous solution as in the first embodiment.

The magnetic head 1 is made of a soft magnetic ferrite rod wound with a coil, covered with an acid-resistant resin, and put into a processing tank 5. The magnetic head may be constituted by an air-core coil, but in such a case, it is preferable from the viewpoint of improving the sensitivity that the liquid also flows through the through-hole inside the air-core coil. The oscillating circuit 2 is an oscillating circuit that is oscillated by being supplied with power from the power supply circuit 3, and its oscillating frequency is determined mainly by the inductance L of the magnetic head 1 and the capacitance C of an internal capacitor. Therefore, as the processing time elapses, the inductance L of the magnetic head 1 increases, and the oscillation frequency decreases.

The oscillating voltage of the oscillating circuit 2 is shaped into a binary signal by a comparator in the signal processing circuit 4, and its pulse period is counted and converted into a digital frequency signal. (Not shown). Thus, when the oscillation frequency decreases to a predetermined value, the operator may take out the hydrogen storage alloy powder from the processing tank, clean it, and terminate the processing reaction.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in magnetic susceptibility of a hydrogen storage alloy powder by acetic acid treatment in Example 1.

FIG. 2 is a diagram showing a change in discharge capacity at the beginning of a cycle of a nickel-metal hydride battery using a hydrogen storage alloy powder obtained by treating the acetic acid treatment of Example 1 for various treatment times as a negative electrode.

FIG. 3 is a block circuit diagram showing a circuit for estimating a surface treatment state of a hydrogen storage alloy powder based on a frequency change in Example 2.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yutaka Oya 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. 41 at Yokomichi, Toyota Central Research Laboratory, Inc.

Claims (3)

[Claims] 1. A method for treating a surface of a hydrogen-absorbing alloy powder, comprising: treating a hydrogen-absorbing alloy powder containing nickel to form a nickel-rich layer on a surface of the hydrogen-absorbing alloy powder. Detecting a magnetic property of the hydrogen storage alloy powder that changes according to a change in the surface state of the hydrogen storage alloy powder during the layer formation processing, and ending the processing when the magnetic property becomes a predetermined value. Characteristic surface treatment method for hydrogen storage alloy powder. 2. The surface treatment method for a hydrogen storage alloy powder according to claim 1, wherein a magnetic susceptibility of the hydrogen storage alloy powder is adopted as the magnetic characteristic. 3. A method for treating a surface of a hydrogen-absorbing alloy powder, comprising: treating a hydrogen-absorbing alloy powder containing nickel to form a nickel-rich layer on a surface of the hydrogen-absorbing alloy powder. A surface treatment method for hydrogen-absorbing alloy powder, wherein the nickel-rich layer forming treatment is performed until the magnetic susceptibility (emu) per dry weight (g) of the alloy powder becomes 0.3 to 0.8.