JPH04324246A - Hydrogen storage electrode - Google Patents

Abstract

PURPOSE:To form an electrode of hydrogen storage alloy very excellent in performance of cycle life, capacity value, etc., by adhesively sticking silver plating or a silver alloy to the hydrogen storage alloy. CONSTITUTION:Silver is plated, for instance, by a method of electroless plating to a hydrogen storage alloy crushed, for instance, under 100mesh. In this silver plating, the silver is plated by mixing silver liquid, consisting of silver nitrate, aqueous ammonia and sodium hydroxide, with reducing liquid, consisting of a methanol-base water solution mainly composed of glucose, by a predetermined proportion while stirred. Hydrogen storage alloy powder, thus obtained by applying silver plating, is fixed to a conductive supporter to form an electrode. As a fixing method, the powder is fixed by using an electrolyte resisting binder and press-attached to this supporter. In this way, electronic conductivity of a negative pole itself is improved to increase a cycle life and a capacity value by improving catalytic assistance at the time of storing/releasing hydrogen gas in a surface and reducing power of the hydrogen gas to water, and so on.

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 cathode of a secondary battery, which has a large electric capacity and a long cycle life.

0002

[Prior Art] Conventionally used secondary batteries include alkaline batteries and lead-acid batteries, but recently hydrogen batteries have been developed which have higher capacity, higher energy density, and may be lighter than these batteries. It has been proposed to use an electrode made of an absorbing alloy as a negative electrode of a secondary battery (Japanese Patent Application Laid-open Nos. 62-271348 and 62-2713).
Publication No. 49). Hydrogen storage alloys used for such purposes include, for example, rare earth hydrogen storage alloys represented by LaNi5, Ti-based alloys, and the like.

[0003] When used as an electrode, these hydrogen storage alloy powders are mixed with conductive material powder, for example, nickel powder of 5 to 20 wt%, filled in a nickel foam metal, and heated at 1000°C in an argon stream. A method of sintering for about 2 hours (Japanese Unexamined Patent Publication No. 58-46827), and a method of mixing conductive material powder and then fixing it with a binder such as fluororesin (Japanese Unexamined Patent Publication No. 163157-1983) have been proposed. Ta.

However, since the above method had many drawbacks, a method was proposed in which the surface of the hydrogen storage alloy powder was coated with a metal, which is a conductive material. The metal used for the surface coating of the hydrogen storage alloy is, for example, copper (Japanese Unexamined Patent Publication No. 61-10
No. 1957), nickel (Japanese Unexamined Patent Publication No. 61-64069)
(Japanese Unexamined Patent Publication No. 185862/1983), and palladium (Japanese Patent Application Laid-open No. 185862/1983).

[0005]

[Problems to be Solved by the Invention] The conventional method of mixing hydrogen storage alloy powder and conductive material powder has a short cycle life. In other words, in terms of cycle life, the alloy becomes pulverized when it absorbs and releases hydrogen through repeated charging and discharging, so the electrode itself may peel off,
It has the disadvantage that it may fall off and deteriorate. In addition, since the conventional surface coating method almost completely covers the surface of the hydrogen storage alloy powder, it is impossible to extract 100% electric capacity from the alloy, and it is impossible to extract electric capacity close to the theoretical electric capacity. However, there are disadvantages in that the discharge capacity rises slowly at the initial stage of charging and discharging, and furthermore, a large amount of coating material such as copper, nickel, etc. is required, at 5 to 10% of the hydrogen storage alloy.

SUMMARY OF THE INVENTION The object of the present invention is to improve the above-mentioned conventional drawbacks and to provide a hydrogen storage electrode having a large electric capacity and a long cycle life.

[0007]

[Means for Solving the Problems] The present invention is a hydrogen storage electrode that uses a powder of a hydrogen storage alloy whose surface is coated with silver or a silver compound.

[0008] Here, the hydrogen storage alloy made of rare earth metals is Mm1-xNiAlaCobMnc (however, a,
b, c are integers), MmAxNiaCobMc (where A is any element of Mg, Ca, Ti, Zr, Th or Hf, M is V, Nb, Ta, Mn, Fe, Cu, Z
n, Mo, Ga, Sn, W, Ag, In, Ge, Tl,
An alloy made of a rare earth metal such as Pb, Sb, or Bi, or a Ti-based hydrogen storage alloy. Ti-based hydrogen storage alloys are based on FeTi with Zr,
It refers to an alloy with a trace amount of Nb, etc. added thereto, or an alloy based on TiMn with trace amounts of V, Ni, Cr, Fe, Zr, Nb, and Cu added thereto.

[0009]

[Function] To illustrate the manufacturing method of the present invention, a hydrogen storage alloy pulverized to a size of 100 mesh, preferably 200 mesh, is plated with silver, for example, by electroless plating. For silver plating, a silver solution consisting of silver nitrate, aqueous ammonia, and sodium hydroxide is mixed with a reducing solution consisting of a methanolic aqueous solution mainly composed of glucose in a ratio of 1:1 to 1:4, and the silver is plated while stirring. do.

[0010] In the case of the silver plating, it is not necessarily necessary to cover the entire surface of the hydrogen storage alloy, and even if silver is scattered on the surface, that is, if it is partially coated, it will be sufficient as an excellent negative electrode material.

That is, when silver is used as a coating material as shown in FIG. 4, it is best if the amount of silver is about 2% with respect to the hydrogen storage alloy powder, and even if silver is partially plated with less than 2%. good.

The silver-plated hydrogen storage alloy powder produced as described above is fixed to a conductive support by a conventional method to form an electrode. Examples of fixing methods include a method of fixing using an electrolyte-resistant binder, a method of press-bonding to a conductive support, and a method of fixing by sintering at a temperature below which the performance of the hydrogen storage alloy does not deteriorate.

By silver plating the hydrogen storage alloy powder in this way, 1. Improving the electronic conductivity of the negative electrode itself 2. Improvement of hydrogen gas storage on the surface, catalyst assistance during release, and ability to reduce oxygen gas to water 3. Increased chemical stability of the surface. Effects and effects such as these can be obtained.

[0014]

[Examples] Next, the present invention will be specifically explained using examples. 1. Alloy plating: MmNi3.5 as hydrogen storage alloy
Silver plating was performed on Al0.3Co0.8Mn0.4 using the following three methods. Silver liquid silver nitrate 3.5 g 25% ammonia water 20 ml Water 60 ml Sodium hydroxide 2.5 g Reducing liquid glucose 45 g Tartaric acid
4.0g methanol
100ml water
1000ml combination 1 silver solution: reducing solution = 20m
l: 20ml combination 2 silver solution: reducing solution = 10ml: 20
ml combination 3 Silver solution: reducing solution = 5 ml: 20 ml

00
15] Silver plating was performed on each of the above formulations 1 to 3 while stirring with a stirrer at room temperature for 20 minutes. The amount of silver deposited is as follows. 2.0g of each alloy used for plating
It is. Silver deposited weight (silver plating weight g/alloy g) formulation 1 0.
1g Mixture 2 0.02g Mixture 3 0.05g

2. Preparation of test electrode: 5, 7, and 9 wt% polytetrafluoroethylene (PTFE) was mixed with 300 mg of the silver-plated hydrogen storage alloy, and a fine nickel mesh was sandwiched between them as a conductive support. An electrode was prepared by pressing for 1 minute at a pressure of about 1.5 t/cm 2 and subjected to a charge/discharge cycle test. In this charge/discharge cycle test, NiOOH was used as the positive electrode, the current was 10 mA,
The current density is 3.3 mA/cm2, and the electrolyte contains 8 mol KO.
The experiment was carried out using H in a constant temperature bath kept at 30°C.

3. X-ray diffraction of alloy: The results of X-ray diffraction of the hydrogen storage alloy before and after plating and after charge/discharge cycles are shown in FIG.

4. Charge/discharge cycle test: An electrode prepared using a hydrogen storage alloy was used as a negative electrode, and a NiOOH electrode was used as a positive electrode. The current is 10mA and the current density is 3.3mA/cm
2. The experiment was conducted in a constant temperature bath kept at 30° C. using 8 mol KOH as the electrolyte. The results are shown in FIGS. 2 and 3.

As shown in FIG. 2, it seems that a binder content of 5% provides good performance. Also, as shown in Figure 3,
The capacitance of the silver-plated capacitor after 10 cycles is approximately 30% larger than that of the non-plated capacitor. Further, as shown in FIG. 3, it is clear that the optimum amount of silver plating is 2%.

Although the case of silver plating has been described above, it is not necessarily necessary to coat the surface of the hydrogen storage alloy with metallic silver, and compounds containing silver such as silver chloride, silver oxide, etc. may be used. Although the details of the reason are not clear, it is known that silver compounds have an excellent catalytic effect on the activation of hydrogen molecules (H2→2H), and [A. Kozaw
a, K. V. Kordesch, Electroch
imica Acta, 26, 1489 (1979)]
, seems to be related to this.

That is, activation of hydrogen molecules is essential for charging and discharging hydrogen storage alloys, and if the activation speed increases due to catalytic action, larger current charging and discharging becomes possible, and as a result, during large current discharging This will lead to an increase in capacity.

[0022]

Effects of the Invention As detailed above, according to the present invention, by attaching silver plating or a silver compound to a hydrogen storage alloy, performance such as cycle life and capacity value can be improved compared to conventional hydrogen storage alloys. It can be made into an extremely excellent hydrogen storage alloy electrode.

[Brief explanation of the drawing]

FIG. 1 is an X-ray diffraction diagram of a hydrogen storage alloy.

FIG. 2 is a diagram showing the relationship between silver plating and discharge capacity.

FIG. 3 is a diagram showing the relationship between silver plating amount and discharge capacity.

Claims (1)

[Claims] 1. A hydrogen storage electrode characterized by using a powder of a hydrogen storage alloy whose surface is coated with silver or a silver compound.