JPH01267955A - Hydrogen absorption alloy electrode and its manufacture - Google Patents

Abstract

PURPOSE:To assure excellent discharging characteristics even though discharge with large current takes place in a low-temp. atmosphere by fixing transfer metal particles to the surface of hydrogen absorption alloy powder, which occludes and emits hydrogen as active substance electrochemically. CONSTITUTION:Transfer metal particles 2 are fixed onto the surface of hydrogen absorption alloy powder 1 to constitute an electrode of hydrogen absorption alloy. This suppresses overvoltage at the time of discharging. That is, in discharging reaction in which hydrogen atoms occluded in the powder 1 are ionized, the overvoltage is dropped by electrochemical catalyzer effect of the transfer metal 2 fixed to the surface of the alloy, and thus the discharging characteristics are enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improvements in hydrogen storage alloy electrodes used as negative electrodes of alkaline storage batteries. More specifically, the present invention relates to a high energy density electrode made of a hydrogen storage alloy that can electrochemically absorb and release hydrogen as an active material.

BACKGROUND OF THE INVENTION Currently, most sealed storage batteries in practical use are lead-acid storage batteries and nickel-cadmium storage batteries. Although the former is inexpensive, it is disadvantageous in terms of weight efficiency wh*Ky-', cycle life, etc. for electrophoresis in portable devices that are used for a long period of time. On the other hand, although the latter type of battery is relatively expensive, it is possible to improve the shortcomings of the former type, and the demand for it has increased significantly in recent years, and it has become widely used in applications that require particularly high reliability. Ta.

While taking advantage of these characteristics, there is an urgent need for higher energy density as a power source for portable devices and the like. Recently, instead of a cadmium negative electrode, an electrode using a hydrogen storage alloy (hereinafter referred to as a J hydrogen storage alloy electrode) as an electrode material that can electrochemically absorb and release hydrogen, which is an active material, has been attracting attention. The energy density per unit volume of this negative electrode can far exceed that of a cadmium negative electrode. Therefore, by using this negative electrode, a battery can be constructed with a smaller volume than a cadmium negative electrode.

In other words, since more positive electrode active material can be used in the remaining battery internal volume, higher energy density can be expected.

However, hydrogen-absorbing alloy electrodes have significantly lower activity in the initial electrochemical reaction than cadmium electrodes, so their discharge capacity is small for several cycles after battery construction, and sufficient discharge capacity is not achieved after a dozen or more cycles of charging and discharging. It becomes possible to obtain. In particular, this tendency is remarkable when high rate discharge is performed at a low temperature (0° C.). This is due to the fact that the initial discharge overvoltage of the hydrogen storage alloy electrode is very large.

Therefore, conventionally, this type of electrode generally increases the activity of the initial electrochemical reaction by the following method.

(1) A method of activating a hydrogen storage alloy electrode by chemically storing and releasing hydrogen in a high-pressure hydrogen atmosphere before sealing the battery.

(2) A method in which positive and negative electrodes are assembled into a group with a separator interposed between them, and after being inserted into a case, hydrogen is chemically absorbed and released in a high-pressure hydrogen atmosphere to activate the hydrogen-absorbing alloy electrode.

(3) A method of activating a hydrogen storage alloy electrode by charging and discharging it in an electrolytic solution.

However, the methods (1) and (2) above require chemical hydrogen storage and release operations in a high-pressure hydrogen atmosphere. Furthermore, when an activated hydrogen storage alloy electrode comes into contact with oxygen in the atmosphere, the alloy surface is oxidized and activity is lost. Therefore, it is necessary to carry out storage and desorption operations in a high-pressure hydrogen gas atmosphere, and to maintain the steps from constructing the battery to sealing it under an inert gas atmosphere, making the manufacturing process extremely complicated.

In the case of (3), in addition to the same problems as (1) and (2),
Complicated steps are required, such as charging and discharging in an electrolytic solution, and washing and drying the charged and discharged electrodes.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a hydrogen storage alloy electrode with a simple configuration and manufacturing method, and excellent discharge characteristics, and a manufacturing method thereof.

Means for Solving the Problem In order to solve this problem, the present invention constitutes a hydrogen storage alloy electrode using a powder in which transition metal particles are immobilized on the surface of hydrogen storage alloy powder.

Further, in order to obtain the powder, a mixture of hydrogen storage alloy powder and transition metal particles is fired in an inert gas atmosphere.

Effect: With this configuration, the present invention reduces overvoltage during discharge by immobilizing transition metal particles on the surface of the hydrogen storage alloy powder. In other words, during the discharge reaction, the hydrogen atoms stored in the hydrogen storage alloy powder are ionized, and the electrochemical catalytic action of the transition metal fixed on the alloy surface reduces the overvoltage and improves the discharge characteristics. .

EXAMPLES Hereinafter, the effects of the present invention will be explained using examples with reference to FIGS. 1 to 3.

<Example 1> A hydrogen storage alloy that electrochemically absorbs and releases hydrogen as an active material and its electrode were prepared in the following manner.

Approximately 40 wt% cerium, approximately sowt% lanthanum.

Mitshu metal (hereinafter referred to as Mm) containing about 13 wt% neodymium, nickel, and cobalt.

Aluminum and manganese each in an atomic ratio of 1:
The basis weight is adjusted to 3.66:0.75:0.3:0.4, melted in a high-frequency melting furnace, and caau
MmNi, 55Mno, 4Ad with type 5 crystal structure
, ,Goo, , hydrogen storage alloys were created. Next, this alloy was heat-treated at a temperature of 1000° C. in a murky gas atmosphere to improve the homogeneity of the sample. Thereafter, this alloy was mechanically pulverized to have an average particle size of 16 μm and 20 μm.

Various alloy powders 1 of 30 μm and 36 μm were obtained.

Next, 6 parts by weight of nickel particles having an average particle diameter of 6 μm were mixed with 100 parts by weight of these powders, and the mixture was heated at 80 parts by weight in vacuum.
By firing at a temperature of 0° C. for 1 hour, various powders in which nickel particles 2 were immobilized on the surface of hydrogen storage alloy powder 1 were obtained. 1st
The figure shows a schematic cross-sectional view of this powder. 3 in FIG. 1 indicates a portion where the alloy and nickel particles are partially melted together and fixed.

Powders with various average particle diameters in which nickel particles are immobilized on the surface of a hydrogen storage alloy are made into a paste form with a 1.5 wt% aqueous solution of polyvinyl alcohol to a thickness of 0.9 wn.
The material was filled into a sponge-like porous nickel support having a porosity of about 96%. This was dried at 100° C. and then pressed to form an electrode plate with an average thickness of 0.5yLrln. Then, it was cut into a width of 39 rran and a length of BOrran to obtain hydrogen storage alloy electrodes having various average particle diameters and a charge/discharge capacity of 1600!0 AIIk.

The hydrogen storage alloy electrode obtained in this way was used as a negative electrode,
A well-known foamed metal Ninocle positive electrode with a capacity of 100,100 O was wound spirally through a general-purpose polyamide nonwoven fabric separator and inserted into a Mumu-sized case, and then a 7.1 N KOH aqueous solution was poured into a 2.2-liter case. After injecting the liquid into the tube, the tube was sealed. Table 1 shows the numbers of batteries using hydrogen storage alloy electrodes having various average particle diameters and the average particle diameters of the alloys. As a comparative example, an electrode E using a hydrogen storage alloy electrode having an average particle diameter of 20 μm and having powder on the surface of which the nickel was not immobilized was prepared using a process similar to the above.

(Leaving space below) Table 1 These batteries were charged for the first time at 100°C in an atmosphere of 20°C.
After 15 hours at nA, discharge was performed at 200 m to 1 OY. Thereafter, these batteries were charged under the same conditions as above, left in an atmosphere at 0° C. for 2 hours, and discharged at a constant current of 3000 m in this temperature atmosphere. Figure 2 shows the discharge curve when constant current discharge was performed for 3000m people.

As a result, the batteries for humans, B, and C according to the present invention are
．． Since nickel powder is fixed on the surface of 55"o, 4"#, 5C00, 75 alloy powder by firing, the overvoltage of the negative electrode will not increase even if discharge is performed with a large current of 3000m in an atmosphere of 0℃. First, the discharge capacity was 700 mAh or more when the terminal voltage reached 1.0 mAh, demonstrating excellent discharge characteristics. On the other hand, the battery of Comparative Example E has a discharge capacity of about 100 nA up to a terminal voltage of 1.0 mA. The cause of this is
When a large current discharge of 3000 Ilm is performed in an atmosphere of 0°C, the negative electrode MmNi, 5Mno, 4m'0.5c
This is because hydrogen ionization on the surface of the o0.75 alloy powder becomes rate-determining, increasing the overvoltage during discharge. still,
A battery in which the hydrogen storage alloy has a particle size of 36 μm,
The discharge capacity when the terminal voltage reaches 1.0 mAh is about 170 mAh. Despite the fact that nickel particles are immobilized on the surface of the alloy, the discharge overvoltage of the negative electrode increases,
The reason why the discharge capacity decreases is that the reaction in which hydrogen atoms are ionized on the surface of the alloy is not rate-determining, but the rate-determining process is the diffusion process in the alloy powder. Therefore, even if nickel particles are immobilized on the surface of the alloy, there is no effect of reducing the overvoltage of the discharge reaction. From the above, it is appropriate that the average particle diameter of the hydrogen storage alloy powder is 30 μm or less.

Although nickel was used as the transition metal particles in this example, similar results were obtained when transition metals such as cobalt, silver, and platinum were used.

Further, in this example, the firing temperature was 800° C., but if the firing temperature exceeds 900° C., nickel melts and covers the surface of the hydrogen storage alloy, resulting in an increase in overvoltage during discharge. Furthermore, at temperatures below 70° C., nickel particles are not immobilized. Therefore, the firing temperature is 700-900℃
A range of is preferred.

<Example 2> The average particle diameter of the hydrogen-absorbing metal-containing powder in Example 1 was set to 20
μm, and the mixing ratio of nickel particles was kept constant at 6 parts by weight, and the average particle size of these particles was varied in the range of 3 to 15 μm, and the surface of the hydrogen storage alloy was coated in the same manner as in Example 1. The nickel particles were immobilized. Negative electrodes and batteries were produced in the same manner as in Example 1 using these powders in which nickel particles having various average particle diameters were immobilized.

Table 2 shows the average particle diameter of the nickel particles used and the trial battery numbers.

(The following is a blank space) Table 2 The discharge curves of these batteries were examined under the same charging and discharging conditions as the pattern shown in Example 1. The results are shown in FIG.

As a result, each of the F, G, and H batteries according to the present invention contained nickel particles of 10 μm or less in size such as MmNi, MmNi,
55 Mno, 4 M'0. Since it is immobilized on the surface of the S''0.75 alloy powder in the same manner as in Example 1, the overvoltage of the negative electrode does not increase even when discharging at a large current of 3,000,111 people in an atmosphere at 0°C, and the terminal The discharge capacity at voltages up to 1.0 mA was over 700 mAh, showing excellent discharge characteristics. On the other hand, battery J had a discharge capacity of about 170 m1 IAh at terminal voltages up to 1.0 mA. The average particle size of the immobilized nickel particles is 16 μm, which is inferior to the electrochemical catalytic action that ionizes hydrogen atoms during discharge, and because the particle size is large, they are not uniformly dispersed in the hydrogen storage alloy powder. Therefore, in order to obtain excellent discharge characteristics, it is appropriate that the average particle diameter of the nickel particles to be immobilized is 10 μm or less.

In this example, the content of nickel particles was kept constant at 6 parts by weight, but similar results were obtained if the content was in the range of 2 to 10 parts by weight. When the nickel content exceeds 10 parts by weight, the energy density of the hydrogen storage alloy electrode decreases,
It has little industrial value.

Moreover, if it is less than 2 parts by weight, the effect of improving discharge characteristics cannot be obtained.

Effects of the Invention As described above, according to the present invention, a hydrogen storage alloy electrode is provided with a powder on which a transition metal is immobilized on the surface of a hydrogen storage alloy powder that electrochemically absorbs and releases hydrogen as an active material. By using this material, it is possible to provide a negative electrode with excellent discharge characteristics even when discharging at a large current in a low-temperature atmosphere.

Moreover, the transition metal particles can be reliably immobilized on the surface of the hydrogen storage alloy powder by a simple process of firing in vacuum or in an inert gas atmosphere.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a state in which nickel particles are immobilized on the surface of a hydrogen storage alloy powder in an embodiment of the present invention;
Figure 2 is a diagram showing the discharge curve when the average particle size of the hydrogen storage alloy powder is changed while the average particle size of the nickel particles is constant, and Figure 3 is a diagram showing the discharge curve when the average particle size of the hydrogen storage alloy powder is constant. It is a figure which shows the discharge curve when the average particle diameter of the nickel particle to fix|immobilize is changed. 1...Hydrogen storage alloy powder, 2...Nickel particles, 3...Part where the hydrogen storage alloy powder and nickel particles are partially melted together and fixed. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure f--Ichizu Awari and the combination request r-<, Kuguru 1 (tsu: Bow "- 3--Identified as a teacher) 8 Expansion/Station゛Hmi

Claims (6)

[Claims] (1) A hydrogen storage alloy electrode characterized in that transition metal particles are immobilized on the surface of a hydrogen storage alloy powder that electrochemically stores and releases hydrogen as an active material. (2) The hydrogen storage alloy electrode according to claim 1, wherein the hydrogen storage alloy powder has a CaCu_5 type crystal structure. (3) The hydrogen storage alloy electrode according to claim 1, wherein the average particle size of the hydrogen storage alloy powder is 30 μm or less, and the average particle size of the transition metal particles is 10 μm or less. (4) The content of the transition metal particles fixed on the surface of the hydrogen-absorbing alloy powder is 2 to 10 parts by weight based on 100 parts by weight of the alloy powder. The hydrogen storage alloy electrode described. (5) A process of firing a mixture of hydrogen storage alloy powder and transition metal particles that electrochemically absorbs and releases hydrogen, which is an active material, in an inert gas atmosphere or in a vacuum, and supporting the fired powder. 1. A method for producing a hydrogen storage alloy electrode, comprising the steps of applying, press-fitting or filling the electrode onto a body, and then applying pressure to a desired thickness. (6) The method for manufacturing a hydrogen storage alloy electrode according to claim 5, wherein the firing temperature is 700 to 900°C.