JPH04328252A - Hydrogen storage alloy electrode - Google Patents

Abstract

PURPOSE:To reduce a necessary charge/discharge number of times for initial activation, and increase an initial capacity of charge/discharge by applying an immersion process for hydrogen storage alloy in water solution of hypophosphorous acid, and forming a hydrogen storage electrode using obtained hydrogen storage alloy powders. CONSTITUTION:After mixing to form a composition of MnNi4.0CO0.5Al0.5, it is heated and melted, and an obtained ingot of hydrogen storage alloy is crushed to be fine powders of the alloy. A predetermined quantity of carbon nickel powders for conductive agent, and ethylene tetrafluoride powders for binding agent are mixed to the alloy powders, an obtained matter is pressure moulded with a nickel gauze, and it is rolled to be a hydrogen storage alloy electrode plate. This electrode plate is then immersed in water solution of sodium or potassium hypophosphorous acid at a density of 1mol/l, and it is washed and dried to be an electrode. The charge/discharge number of times required for initial activation can thus be small, and a high capacity can be taken even in an initial stage of charge/discharge. The alloy powders arethen arbon nickel powders.

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 used as a negative electrode of a secondary battery or the like.

0002

2. Description of the Related Art Conventionally, nickel-cadmium batteries, lead batteries, and the like are well known as secondary batteries, but these batteries have the drawback of relatively low energy density per unit weight or unit volume. Therefore, the negative electrode is an electrode using a hydrogen storage alloy that can electrochemically absorb and release a large amount of hydrogen, the positive electrode is nickel oxide, and the electrolyte is a nickel-hydrogen with high energy density, using an alkaline aqueous solution. Batteries were proposed. The negative electrode here is La
Hydrogen storage alloys such as Ni5 have been used, but their cycle life for repeated charging and discharging is as short as about 30 cycles, making them impractical. To improve this, LaNi
In addition to replacing a part of Ni in No. 5 with Co, Al, Mn, etc. to diversify the alloy, from an economical point of view, it was proposed to use an alloy in which La was replaced with Mm (misch metal), which is a mixture of rare earth elements. Recently, the use of Laves phase AB2 type alloys such as Zr-V-Ni has also been proposed.

0003

The above-mentioned improved alloy has a relatively long cycle life, and the effects of improvement are recognized. However, these alloys generally require repeated charging and discharging many times before initial activation, that is, a state in which a stable capacity can be obtained. Therefore, when a sealed nickel-metal hydride battery is fabricated using a hydrogen storage electrode made of these alloys, the battery capacity will exceed the rated capacity because almost no capacity can be extracted from the hydrogen storage alloy electrode at the beginning of the charge/discharge cycle. There were some inconveniences such as lower than expected. Therefore, a hydrogen storage electrode that eliminates the disadvantages of conventional hydrogen storage electrodes, reduces the number of charge/discharge cycles required for initial activation, and makes it possible to obtain the rated capacity from the initial stage of charging and discharging. desired.

0004

[Means for Solving the Problems] The present invention relates to a hydrogen storage electrode that solves the above-mentioned problems and satisfies the above-mentioned needs, one of which is formed by immersion treatment in a hypophosphite aqueous solution. The other one is manufactured using hydrogen storage alloy powder immersed in a hypophosphite aqueous solution.

[0005]

[Function] Although the function of the present invention is not clear, the reducing action of the hypophosphite aqueous solution removes the oxide film formed on the surface of the alloy particles, and the catalytic action of the alloy itself reduces the hypophosphite to the alloy. It is thought that hydrogen from sodium phosphate is taken in, distortion remaining in the crystal lattice is removed, and this results in a highly active state in which it is easy to absorb and release hydrogen. As a result, the initial activation of this electrode can be done quickly, and a high capacity can be obtained from the beginning as a battery.

[0006]

EXAMPLES Next, examples of the present invention will be described in detail. Commercially available misch metal, nickel, cobalt, and aluminum powders were mixed at a predetermined composition ratio, for example, MmNi4.0CO0.5A.
They were weighed and mixed so that the weight was 10.5, and then heated and melted using an arc melting method to obtain a hydrogen storage alloy. This alloy ingot was crushed using a crusher, and then made into fine powder of 250 mesh or less using a ball mill or the like. For example, 15 wt. %, for example, 5 wt.% of tetrafluoroethylene powder as a binder. % and mixed. This mixed powder was filled into a mold, pressed together with a nickel wire gauze, and then removed from the mold, and the molded plate was rolled to a predetermined thickness to produce a hydrogen storage electrode plate. According to the invention, this electrode may be
After being immersed in an aqueous solution of sodium hypophosphite (NaPH2O2) or potassium hypophosphite (KPH2O2) at room temperature for 1 hour at a concentration of , it was taken out, washed with water, and dried. This is referred to as the electrode of the present invention. Although this drying was carried out at 60° C. for several hours, no heat generation or ignition occurred. This is because a nickel-phosphoric acid film is formed on the surface of the alloy powder particles. On the other hand, for comparison, the hydrogen storage electrode plate manufactured above, which was not subjected to the immersion treatment in the sodium hypophosphite aqueous solution, was used as a comparison electrode. The weight of the hydrogen storage alloy powder in each electrode is approximately 1 g. The electrode of the present invention and the reference electrode prepared in this way were combined as working electrodes, and a nickel plate was used as a counter electrode, and 30 wt. % potassium hydroxide aqueous solution was used to prepare an open type test cell. Using each of these test cells, the capacity that could be taken out during the first charge and discharge was confirmed. Furthermore, charging and discharging were repeated to determine the number of charging and discharging cycles required for the capacity to stabilize, that is, for initial activation. Charge/discharge each test cell at 6 mA/
After charging the hydrogen storage electrode to 130% of its electrochemical hydrogen storage capacity at a current density of cm2, the voltage of the hydrogen storage electrode was -0.75V vs. at a current density of 10mA/cm2.
This was done by discharging until Hg/Hg0. The above test results are as shown in Table 1.

[0007]

[Table 1]

[0008] As can be seen from Table 1, the effect of increasing the initial discharge capacity and decreasing the number of initial activation cycles was observed by immersing the electrode in the hypophosphite aqueous solution. In the above example, the immersion treatment in the hypophosphite aqueous solution was performed at room temperature, but increasing the treatment temperature is effective in shortening the treatment time. Further, when the oxide on the alloy surface is easily reduced, the hypophosphite aqueous solution can be further diluted with water, and vice versa, the concentration of the aqueous solution can be increased.

Incorporating the above electrode of the present invention as a negative electrode,
When a AA size 1000 mAh hydrogen-nickel cylindrical sealed alkaline storage battery was manufactured, it was possible to obtain a capacity that satisfied the rating from the first charge/discharge.

[0010] In the above example, the hydrogen storage alloy powder was immersed in a hypophosphite aqueous solution at the stage of forming the electrode plate. The same effect as above was also obtained by immersing the hydrogen-absorbing alloy powder in a hypophosphite aqueous solution and using it to make a hydrogen-absorbing electrode in the same manner as above.

[0011] In addition to the above-mentioned Mm-Ni alloy, the hydrogen storage alloys to be subjected to the immersion treatment in the hypophosphite aqueous solution of the present invention include La-Ni, Zr-Ni, Ti-Ni, and Ti.
-Zr-Ni, Zr-V-Ni, Ti-Fe, etc.

0012

Effects of the Invention As described above, according to the present invention, a hydrogen storage electrode manufactured using a hydrogen storage electrode immersed in a hypophosphite aqueous solution or a hydrogen storage alloy powder immersed in a hypophosphite aqueous solution is provided. All of the electrodes require fewer charging and discharging cycles for initial activation, and have the effect of allowing high capacity to be obtained from the initial stages of battery charging and discharging.

Claims (2)

[Claims] Claim: 1. A hydrogen storage electrode formed by immersion in a hypophosphite aqueous solution. 2. A hydrogen storage electrode manufactured using hydrogen storage alloy powder immersed in a hypophosphite aqueous solution.