JPH04121958A - Metal hydride electrode - Google Patents

Abstract

PURPOSE:To facilitate the control of the surplus discharge amount of the negative electrode and reduce work danger and remove the generation of impurities that cause self discharge and the non-activation of battery constituent elements, by retaining at a conductive support body metal bismuth powder together with hydrogen storage alloy powder. CONSTITUTION:A pastelike mixture is prepared by adding water to 100 wt parts of hydrogen storage alloy powder, 3 wt parts of metal bismuth powder, 2 wt parts of furnace black that is a conduction promoting agent and 2 wt parts (solid portion) of synthetic latex consisting of an acrylic-styrene copolymer that is a bonding agent. This pastelike mixture is painted on both sides of a nickel plated iron punching metal that is a conductive support body, and after drying, pressing and cutting off a metal hydride electrode is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a hydrogen storage electrode used as a negative electrode of a sealed alkaline storage battery and comprising an electrode comprising a hydrogen storage alloy. is equipped with a hydrogen storage alloy. This hydrogen storage alloy contains LaN
There are intermetallic compounds such as i5 and TiMn2, and the hydrogen storage capacity of these alloys can be increased by replacing some of the constituent elements of these alloys with other elements or by changing the stoichiometric number. They are used in electrodes by changing the equilibrium hydrogen pressure of these metal hydrides, and by improving the corrosion resistance of the alloy in alkaline electrolytes.

The conventional manufacturing method for this electrode involves holding the above-mentioned hydrogen storage alloy powder on a conductive support such as punched metal or foamed nickel, and then using a highly alkali-resistant material such as polyvinyl alcohol, fluororesin, or acrylic-styrene resin. Some are made by bonding molecules together, while others are made by sintering a hydrogen-absorbing alloy.

An alkaline storage battery is constructed using this negative electrode, a positive electrode such as a nickel hydroxide electrode, and an alkaline electrolyte such as potassium hydroxide.

In an alkaline storage battery using a hydrogen storage electrode, the discharge capacity of the hydrogen storage electrode is approximately twice that of a cadmium electrode of the same volume. Therefore, for example, just as the capacity of a hydrogen storage electrode is about 1.5 times that of a cadmium electrode,
In the case of a nickel metal hydride battery, by reducing the volume of the hydrogen storage electrode and increasing the volume of the positive electrode by the amount of the volume reduction, increasing the capacity of the positive electrode by about 1.5 times. , its capacity can be increased to about 1.5 times that of a nickel-cadmium battery of the same volume.

When an alkaline storage battery is constructed using a hydrogen storage electrode as a negative electrode, the battery is constructed as follows to achieve hermetic sealing.

First, regarding overcharging, it is as follows. That is,
The chargeable capacity of the positive electrode is made smaller than the chargeable capacity of the negative electrode so that charging of the positive electrode ends before that of the negative electrode.

When this sealed battery is overcharged, the water is electrolyzed and the oxygen scum generated from the positive electrode is electrochemically reduced on the surface of the negative electrode to produce water, so no accumulation of oxygen scum occurs inside the battery and the electrolyte The amount does not change either. At the negative electrode, a reduction reaction of oxygen gas occurs during overcharging, and charging of the negative electrode is stopped. This prevents hydrogen gas from being generated when the negative electrode finishes charging, and prevents hydrogen gas from accumulating inside the battery. do not have. Therefore, even if the battery is overcharged, gas will not accumulate within the battery and the internal pressure will not significantly increase, and the battery's sealed system will be maintained. Conversely, if the battery is configured so that the negative electrode finishes charging before the positive electrode, hydrogen scum will be generated from the negative electrode during overcharging, but the rate at which the hydrogen scum is electrolytically oxidized at the positive electrode is extremely slow. It is difficult to effectively prevent the accumulation of hydrogen gas within the battery. Therefore, in practical sealed storage batteries, charging of the positive electrode ends before charging of the negative electrode.

The principle of sealing during overcharging is the same as that of sealed nickel-cadmium batteries. This battery configuration can be realized by making the chargeable capacity of the positive electrode smaller than the chargeable capacity of the negative electrode.

Next, the case of overdischarge is as follows. That is, the dischargeable capacity of the positive electrode active material is made smaller than the dischargeable capacity of the negative electrode so that the discharge of the positive electrode ends before that of the negative electrode. When this sealed battery is over-discharged, water is electrolyzed and the hydrogen gas generated from the positive electrode is electrochemically oxidized on the surface of the negative electrode to produce water, so hydrogen gas does not accumulate inside the battery and the electrolyte The amount does not change either. And at the negative electrode,
During overdischarge, an oxidation reaction of hydrogen gas occurs and the discharge of the negative electrode is stopped. This prevents the generation of oxygen gas when the discharge of the negative electrode ends, and no accumulation of oxygen gas occurs within the battery. Therefore, even if the battery is over-discharged, gas will not accumulate within the battery and the internal pressure will not significantly increase, and the battery's sealed system will be maintained. Conversely, if a battery is constructed so that the discharge of the negative electrode ends before that of the positive electrode, oxygen gas will be generated from the negative electrode during overdischarge, but the rate at which oxygen gas is electrolytically reduced at the positive electrode is extremely slow. It is difficult to effectively prevent the accumulation of oxygen gas within the battery. Therefore, a practical sealed storage battery is configured such that the discharge of the positive electrode ends before the discharge of the negative electrode.

If the gas absorption reaction does not occur at a sufficiently high rate during overcharging or overdischarging, the gas generated by the electrolysis of water in the alkaline electrolyte will accumulate inside the battery, causing a significant increase in the internal pressure of the battery. Then, the safety valve is activated and the gas inside the battery is released.

If this happens repeatedly, the gas generated by the electrolysis of water will leave the battery system, and the amount of electrolyte in the battery will decrease significantly, making it difficult to charge and discharge the battery.
The charge/discharge cycle life of the battery will be significantly shortened. In particular, the principle of sealing during overdischarge is unique to sealed alkaline storage batteries that use a hydrogen storage electrode or a hydrogen gas diffusion electrode as a negative electrode.

Note that in the case of a hydrogen storage electrode, the amount of hydrogen that is difficult to discharge increases during high rate discharge or discharge at low temperatures, resulting in a decrease in discharge capacity. In such a case, if the discharge of the negative electrode ends before that of the positive electrode and the hydrogen storage electrode of the negative electrode is over-discharged and oxygen gas is generated, the surface of the hydrogen storage alloy is anodically oxidized and becomes inactive. This causes the inconvenience that subsequent charging and discharging becomes difficult. Therefore, it is useful to prevent this inconvenience by sealing a sealed alkaline storage battery that has a hydrogen storage electrode at the negative electrode, limiting battery discharge at the positive electrode, and preventing the negative electrode from being over-discharged. .

Problems to be Solved by the Invention In a sealed alkaline storage battery using a hydrogen storage electrode as the negative electrode, as mentioned above, the following is a means of configuring the battery so that the discharge of the positive electrode ends before the discharge of the negative electrode. Several methods have been proposed, but each has its own problems.

The first method is to charge the hydrogen storage electrode in advance before configuring the battery. Specifically, a certain amount of gaseous hydrogen gas is charged to the hydrogen storage alloy of the hydrogen storage electrode. This method involves storing hydrogen or partially charging the hydrogen storage electrode in an electrolyte. These methods have the following problems. In other words, when storing hydrogen scum in the gas phase, it is difficult to store the hydrogen scum uniformly in the hydrogen storage electrode, and it is necessary to pressurize and heat the hydrogen scum during storage, which is dangerous for work. accompanied by.

In addition, when partially charging a hydrogen storage electrode, it is necessary to wash and dry the electrode before assembling the battery, but at this time the hydrogen in the electrode is released and the amount of storage is controlled. difficult to do.

The second method is to add a reducing agent to the alkaline electrolyte and take advantage of the phenomenon in which the negative electrode is charged as the reducing agent oxidizes. Specifically, hydrazine or alcohol is added. There are things. These methods are advantageous in that the amount of the reducing agent added can be controlled to easily control the amount of excessive discharge of the negative electrode, but the oxidation products of the reducing agent have various adverse effects on the battery. That is, nitrogen compounds, which are oxidation products of hydrazine, promote self-discharge by a "shuttle mechanism" similar to that of nickel-cadmium batteries. In addition, alcohol is oxidized and produces a large amount of carbonate radicals, which accelerate the corrosion of the metallic nickel, which is the conductive skeleton of the positive nickel hydroxide electrode, and cause the inactivation of the nickel hydroxide. .

The third method is to electrochemically support a metal that is more base than the hydrogen electrode potential in an alkaline aqueous solution in a hydrogen storage electrode, and when the hydrogen storage electrode comes into contact with an alkaline electrolyte, this base metal is oxidized. As the reaction progresses, the hydrogen storage electrode is partially charged by a local battery mechanism. Specifically, it has been proposed to support a base metal such as zinc. This method has the following disadvantages. In other words, the hydrogen storage alloy has a low hydrogen overvoltage, and the overvoltage of the charging reaction of a hydrogen storage alloy that has not been charged or discharged is high, so when the hydrogen storage electrode supporting this base metal comes into contact with an alkaline electrolyte, the hydrogen storage alloy The charging reaction is slow and hydrogen scum is generated from the hydrogen storage alloy, making it impossible to accurately control the amount of electricity that is partially charged.

The fourth method is to use a non-sintered nickel hydroxide electrode containing metal cobalt or cobalt hydroxide as the positive electrode, and when charging the battery, the metal cobalt or cobalt hydroxide in the positive electrode becomes trivalent cobalt oxide. It takes advantage of the phenomenon in which substances are irreversibly oxidized, and in this case, the negative electrode is partially charged without charging the active material of the positive electrode. This method is excellent in that it is easy to control the partial charge amount of the negative electrode.

However, metallic cobalt and cobalt hydroxide are easily oxidized during the active material impregnation process of sintered nickel hydroxide electrodes, so this method cannot be applied when using sintered nickel hydroxide electrodes as positive electrodes. There are drawbacks.

A fifth method is to make the capacity of the negative electrode larger than the capacity of the positive electrode, charge the battery in an open state until oxygen gas is generated from the positive electrode, and then seal the battery. In this case, the positive electrode is overcharged when the battery is open, and at least a portion of the oxygen scum generated from the positive electrode is released outside the battery system without being electrolytically reduced at the negative electrode. The positive electrode is charged in excess by the amount of electricity required to reduce the released oxygen. Therefore, it is possible to make the discharge capacity of the positive electrode smaller than that of the negative electrode. This method is advantageous in that it does not require additives and can be applied regardless of the type of electrode, but it is important to control the efficiency of the oxygen gas reduction reaction at the negative electrode when overcharging the battery in an open state. Therefore, it is difficult to control the oxygen gas released outside the battery system, and it is difficult to control the amount of excessive discharge of the negative electrode.

As described above, in a sealed alkaline storage battery using a negative electrode as a hydrogen storage electrode, when the discharge of the positive electrode ends before the discharge of the negative electrode, it is easy to control the excessive discharge amount of the negative electrode. There was a desire for a method that would be less dangerous during work, would not generate impurities that would cause self-discharge or inactivation of battery components, and would also be applicable when using a sintered nickel hydroxide electrode as the positive electrode. .

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a hydrogen storage electrode formed by holding metal bismuth powder together with hydrogen storage alloy powder on a conductive support.

An alkaline storage battery in which the working negative electrode is composed of a hydrogen storage electrode comprising a hydrogen storage alloy and metal bismuth powder has the following effects.

In other words, the potential at which bismuth metal is oxidized in an alkaline electrolyte is higher than the equilibrium potential of hydrogen, so even if this hydrogen storage electrode comes into contact with an alkaline electrolyte, hydrogen gas will not spontaneously be generated. There is no.

The potential at which the metal bismuth provided in this hydrogen storage electrode is electrolytically oxidized in an alkaline electrolyte according to equation (1) is about 0.37 V nobler than the equilibrium potential of hydrogen, and the oxygen gas It is about 0.86 V less than the equilibrium potential of the generated reaction.

2Bi + 6QH--+ Bi2O3+ 3820
+6e-(1) Therefore, after the discharge of the hydrogen storage alloy is almost completed and before the oxygen gas generation reaction occurs, the electrolytic oxidation reaction of the metal hismuth, that is, the discharge reaction occurs. Furthermore, even without charging the electrode, the hydrogen storage electrode containing metal bismuth already has a discharge capacity equivalent to the electrolytic oxidation reaction of metal bismuth by the time the oxygen scum generation reaction occurs. .

In other words, compared to a hydrogen storage electrode that does not include metal bismuth, a hydrogen storage electrode that includes metal bismuth is in a state equivalent to being overcharged by the discharge capacity of metal bismuth, and the negative electrode has a higher discharge capacity than the positive electrode. It becomes possible to increase the discharge capacity of.

When over-discharging until Bi2O3 is generated and then charging, the equilibrium potential of the reaction in equation (1) is higher than the potential at which the charging reaction of the hydrogen storage electrode occurs, so first After the reaction occurs, charging of the hydrogen storage alloy progresses.

Furthermore, even when metal copper or metal lead is used as a hydrogen storage electrode, when this electrode is anodically polarized in an alkaline electrolyte,
An anodic oxidation reaction of metallic steel and metallic lead occurs before the oxygen scum generation reaction occurs. However, divalent copper and divalent lead, which are dissolved species of these metals, have high solubility in alkaline electrolytes, and when these metals are anodized, these dissolved species dissolve in a supersaturated state. The concentration becomes even higher. Therefore, if a sealed alkaline storage battery is over-discharged and the positive electrode is over-discharged to a base potential where a hydrogen gas generation reaction occurs, high-concentration dissolved species of copper and lead will form a dendritic charge on the positive electrode. As a result, an electrically conductive internal short path may form between the positive and negative electrodes. Additionally, even if by chance an internal short circuit path does not form during overdischarge, high concentrations of dissolved species of these metals in the electrolyte can cause dendritic formation at the negative electrode when the battery is overdischarged and then charged. may be electrolytically deposited to form internal short-circuit paths. In this way, it is not preferable to include metal copper or metal lead in the hydrogen storage electrode because there is a risk of internal short circuit.

However, since the solubility of bismuth oxide in an alkaline electrolyte is extremely low, the risk of an internal short circuit occurring is extremely small compared to when metal copper or metal lead is provided.

Since the potential at which bismuth metal is oxidized in an alkaline electrolyte is higher than the equilibrium potential of hydrogen, even if this hydrogen storage electrode comes into contact with an alkaline electrolyte, hydrogen gas will not be spontaneously generated. There is no.

Therefore, the excess discharge capacity of the negative electrode can be easily controlled by simply changing the amount of metal bismuth powder added. Furthermore, since high temperature and high pressure hydrogen gas is not used during this control, there is little danger in the work.

Furthermore, even when bismuth is oxidized, it does not produce nitrogen compounds or carbonate radicals. In addition, in the battery of the present invention, in order to make the discharge capacity of the negative electrode more excessive than that of the positive electrode, metal bismuth is provided in the negative electrode to obtain this effect, so the positive electrode is a non-sintered nickel hydroxide electrode. In addition, the same effect can be obtained when using a sintered nickel hydroxide electrode.

EXAMPLES The present invention will be explained in more detail by the following examples.

[Hydrogen storage electrode A] (Embodiment of the present invention) The hydrogen storage alloy is made by replacing La in LaN i 5 with Mitshu metal (raw material is bastnaesite) phase, and replacing N1 with a mixture of nickel, cobalt, aluminum and mankan. So, is its component element chemical formula 111+1Ni3.55c00.7
Melt it in a high frequency induction furnace with an argon atmosphere to give an sAI of 0.41'fn 0.3, pour it into an iron mold and cast it, coarsely crush this casting with a show crusher, sieve it, and determine the particle size. A coarse powder having a diameter of 1 mm or less was obtained.

Next, this coarse powder was moistened with ethanol and ground in a ball mill using an alumina pot and balls. This powder is then vacuum dried and classified.
A fine powder of a hydrogen storage alloy that passed through a 330 mesh sieve was obtained.

Then, 100 parts by weight of this hydrogen storage alloy powder, 3 parts by weight of metal hismuth powder of a chemical reagent manufactured by Nacalai Tesque,
A paste-like mixture is prepared by adding water to 2 parts by weight of a synthetic latex (solid content) consisting of 2 parts by weight of furnace black as a conductive agent and an acrylic-styrene copolymer as a binder, and this paste-like mixture is Coat it on both sides of a nickel-plated iron Bantenku metal with a thickness of 0.09 mm and an aperture ratio of about 0.5 as a conductive support, dry it, press it, and cut it to produce hydrogen storage electrode A. did.

The size of one hydrogen storage electrode A is such that the hydrogen storage alloy attached portion is 55 mm in height and 15 mm in width, and the thickness of the electrode is 0.5 mm.
It's 4 fights. The weight of the hydrogen storage alloy contained in one electrode was about 1.25 grams.

[Hydrogen storage electrode B (conventional example) Hydrogen storage electrode B was manufactured in the same manner as hydrogen storage electrode A without using the metal bismuth powder in hydrogen storage electrode A. The dimensions of the electrode and the amount of hydrogen storage alloy supported were almost the same as those of hydrogen storage electrode A.

[Hydrogen Storage Electrode C (Conventional Example) Hydrogen storage electrode C was manufactured using metal zinc powder instead of the metal bismuth powder in hydrogen storage electrode A, and using the same method as hydrogen storage electrode A in other respects.

The dimensions of the electrode and the amount of hydrogen storage alloy supported were almost the same as those of hydrogen storage electrode A.

When the above three types of hydrogen storage electrodes were immersed in a 5.8M potassium hydroxide electrolyte and observed, only the hydrogen storage electrode C containing zinc powder generated a large amount of hydrogen gas bubbles. From this, it can be seen that in a hydrogen storage electrode including zinc powder, hydrogen gas leaves the battery system, making it difficult to control the excessive discharge capacity of the negative electrode. Therefore, hydrogen storage electrode C was not used in the following tests.

Next, the sealed alkaline storage battery was continuously over-discharged, and the internal pressure of the battery was measured, as well as the remaining capacity of the negative electrode. We have confirmed through experiments that the increase in battery internal pressure is suppressed and the increase in battery internal pressure is prevented.The results will be explained below.

[Sealed alkaline storage battery A (Battery according to the present invention) Sealed alkaline storage battery A was constructed as follows.

Four hydrogen storage electrodes A were used as negative electrodes. In the positive electrode, the dimensions of the active material holding part are 54 mm in height and 14-5 mm in width.
The thickness is 0.82 mm, and the discharge capacity is 34 mm assuming that the charge/discharge reaction of nickel hydroxide follows a one-electron reaction.
Three known sintered nickel hydroxide electrodes of 0rnAh were used. In addition, since this positive electrode plate is chemically formed by charging and discharging in an alkaline electrolyte using a conventional method, the positive electrode plate already contains nickel higher oxide, which is difficult to charge and discharge. A polysulfone non-woven fabric with a thickness of 0-12 mm is used for the separator, and each positive electrode plate is wrapped in one separator, and the positive electrode plate and negative electrode plate are laminated alternately. This electrode group was inserted into a battery case, and an electrolyte prepared by dissolving 15g of lithium hydroxide in 1β potassium hydroxide solution with a concentration of 6M was injected into the battery case and then sealed. , produced a sealed alkaline storage battery.

This battery has a height of 66.2 mm and a width of 6.4 mm, including a safety valve that operates at an internal pressure of approximately 5 to 8/cIT12.
, and has a prismatic shape with an external dimension of 5.6 mm in thickness. The nominal capacity of this battery is 900mAh.

[Sealed alkaline storage battery A] (Conventional battery) Sealed alkaline storage battery A uses hydrogen storage electrode B instead of negative electrode hydrogen storage electrode A in sealed alkaline storage battery A, and the other configuration is that of a sealed alkaline storage battery. It has the same configuration as A.

The above two sealed alkaline storage batteries are charged at a current of 90 mA for 15 hours in the first cycle, and then charged at a current of 180 mA.
Each battery was discharged at a current of 0.8V until the terminal voltage of each battery reached 0.8V. Next, the battery was charged with a current of 180 mA for 6 hours, discharged with a current of 180 mA until the terminal voltage reached 0■, and then over-discharged with the same current for another 1 hour. A charge/discharge cycle was performed 10 times. At the final overdischarge, the internal pressure of the battery was measured. These charging and discharging were performed at 25°C.

FIG. 1 shows the change over time in the internal pressure of the battery during this overdischarge. From Figure 1, we can see the following.

In other words, the sealed alkaline storage battery according to the present invention, which holds metallic hismuth powder, suppresses the internal pressure at the time of overdischarge to a small value, and does not reach the operating pressure of the safety valve of 5 kg/cm2, making it possible to seal the battery. or has been achieved. On the other hand, in a conventional sealed alkaline storage battery that does not hold metal hismuth powder, the internal pressure increases as overdischarge progresses, and the internal pressure reaches the operating pressure of the safety valve.

Further, these two batteries were disassembled after overdischarging, the negative electrode plate was taken out, this was used as a test electrode, and the battery was discharged under the following conditions to examine the discharge capacity up to the generation of oxygen scum.

That is, each of the hydrogen storage alloys described above was used as a test electrode, and one nickel plate was placed on each side of the hydrogen storage alloy so that the distance between the electrodes and the test electrode was 2 + 5 cm, and these were used as counter electrodes. Using a potassium hydroxide electrolyte with a concentration of .8M,
A flooded open test cell was constructed.

For measuring the potential of the test electrode, a mercuric oxide electrode was used as a reference electrode.

These test electrodes were then heated to 25℃ without charging.
In this case, we want to discharge +0.25V with a current of 45mA based on the mercuric oxide electrode.The balanced type 1)7 of the oxygen gas generation reaction is +〇, 303V, so at +0.25V, the oxygen force is Does not occur. ).

The results are shown in Table 1.

Table 1 From Table 1, the closed alkaline storage battery A according to the present invention is as follows:
Since the residual capacity of the negative electrode is obtained, it can be seen that the discharge is limited by the capacity of the positive electrode.

On the other hand, in the conventional sealed alkaline storage battery, it is difficult to limit the discharge by the capacity of the positive electrode because the remaining capacity of the negative electrode cannot be obtained.

In the above embodiments, a specific composition was used as the hydrogen storage alloy, but not only this alloy, but also alloys with different constituent elements, hydrogen storage alloys belonging to the Laves phase, etc. may be used. The present invention is similarly applicable to these cases.

Furthermore, in the above example, a synthetic latex consisting of an acrylic-styrene copolymer was used as the alkali-resistant polymer that acts as a binder, but it is not limited to just this alkali-resistant polymer. The present invention is also applicable to cases where resins, fluororesins, methyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, etc. are used.

In addition, in the above embodiments, punched metal is used as the conductive support, but conductive porous materials such as metal mesh, expanded metal, metal foam, and metal fiber sintered body may also be used. The present invention can also be applied when using the following.

Effects of the Invention The present invention provides a sealed alkaline storage battery that uses a hydrogen storage electrode as the negative electrode, and is configured so that the discharge of the positive electrode ends before the discharge of the negative electrode, thereby effectively suppressing the rise in internal pressure of the battery during overdischarge. It has the effect of Moreover, there is no need to work in dangerous conditions such as high-temperature, high-pressure hydrogen scum atmospheres, and since solid metal powder that does not generate hydrogen gas is added to the alkaline electrolyte, it is possible to control the excessive discharge of the negative electrode. It is easy to use, does not generate nitrogen compounds or carbonate radicals that are harmful to the self-discharge performance of the battery, and has the advantage that it can be applied even when a sintered nickel hydroxide electrode is used as the positive electrode.

[Brief explanation of the drawing]

Figure 1: A diagram showing the change over time in the internal pressure of a sealed alkaline storage battery as overdischarge progresses. A...Battery according to the present invention B...Battery according to the conventional example (JE Lj)

Claims (1)

[Claims] A hydrogen storage electrode comprising bismuth metal powder held on a conductive support together with hydrogen storage alloy powder.