JPH03289042A - Hydrogen storage electrode - Google Patents

Abstract

PURPOSE:To improve the charge/discharge life of an alloy and improve the discharge characteristic at a high rate and a low temperature by constituting an electrode with a specific hydrogen storage alloy. CONSTITUTION:A hydrogen storage electrode is constituted of a hydrogen storage alloy indicated by a formula A1-alphaNi5-y-2CoyMz, where A is rare earth elements, their mixture or a mixture of rare earth elements and Zr, Hf, M is Al, Mn or their mixture, 0<alpha<0.06, y=0.3-1.2, z=0.2-1.2. The high-rata discharge characteristic and low-temperature discharge characteristic can be improved while the cycle life of the alloy is maintained.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a hydrogen storage electrode.

[Conventional technology]

In order to improve energy storage capacity, a secondary battery using a hydrogen storage alloy that reversibly stores and releases hydrogen as a negative electrode has been proposed.

As a hydrogen storage alloy for electrodes, generally LaNi5
or MmNis (Mm stands for mitshu metal). In order to keep the hydrogen equilibrium dissociation pressure of the hydrogen storage alloy below 1 atm within the battery operating temperature range of -20 to 60°C, some of the Ni is replaced with ^l or Mn, and the It is common practice to replace Ni with Co to prevent oxidative decomposition of the alloy.

[Problem to be solved by the invention]

LaNi5 type has a large charge/discharge capacity but is expensive.

On the other hand, the MmN i s type has a small charge/discharge capacity but is inexpensive. In addition, so far, both alloy systems have been used to charge and discharge 3
Although alloys have been developed that exhibit almost no capacity loss even after 00 cycles, they have the drawback of poor discharge characteristics at high rates and low temperatures.

The present invention aims to eliminate such conventional drawbacks.

[Means to solve the problem]

The hydrogen storage alloy of the present invention that achieves the above object is general
+-aJJis-y-zcoyM, made of a hydrogen storage alloy indicated by z.

However, in the above general formula, A is a rare earth element, a mixture thereof or a mixture of a rare earth element and Zr, If, H
is Al, Mn or a hybrid thereof, and 0<α<
0.06, y=0.3-1.2, z=0.2-1.
It is 2.

The mixture of rare earth elements (Mm: misch metal) may be prepared artificially, or naturally occurring elements may be used as they are.

As a mixture of naturally occurring rare earth metals, the following
As shown in the table, various chemical compositions (wt%) are obtained depending on the type of ore and the extent of the separation process.

Table 1 Composition A h is generally the most readily available and inexpensive, but has a low charge/discharge capacity due to its low La content. Composition and h in E have a high charge/discharge capacity due to a high content of La, but are expensive because Ce etc. are separated.

On the other hand, h of composition C has a high La content and is low in price, but there are concerns about mass supply. Composition 1 can be obtained at a lower price than A, but has the disadvantage of requiring preliminary refining before producing the alloy due to its high Mg and CI content.

In this way, since each 1'bw has its own characteristics, it can be used depending on the purpose.

In Mm, as the content of Ce and Nd in the rare earth metal increases, a sufficiently long charge/discharge cycle life (less than 10% decrease in charge/discharge capacity after 300 cycles) can be obtained even if the content of expensive Co is reduced. However, since the dissociation pressure increases, it is necessary to increase the content of old and Mn to lower it.

In addition, in alloy systems with a high La content, it is necessary to use a large amount of Co (y > 1.5 in equation -a above) in order to obtain a sufficiently long cycle life, but in this case, the discharge capacity decreases. and cause deterioration of high rate discharge characteristics. Therefore, if 3 to 8 atomic percent of La is replaced with Zr or Hf, C
Even if the content of o is set to y<1, O, a sufficiently long life can be obtained. Therefore, the Co content is y = 0.3~1.2,
Preferably y-0,5 to 0,9, more preferably y=
It is 0.6 to 0.8. In addition, in order to adjust the hydrogen dissociation pressure of the alloy, part of the Ni is replaced with old or Mn, but the amount of replacement is determined by the battery operating temperature range, and z = 0.
．． 2 to 1.2, and preferably z = 0.4 to 0.8 in the temperature range of -20 to 60°C.

Note that 0<α<0.06, preferably 0.1≦α≦0.
The range of 0.04, more preferably 0.1<α≦0.3 will be explained in the following examples.

Examples of the present invention will be described below.

〔Example〕

Example 1 Chemical composition Lao, 5-azNdo, + 5Zr(H
f) o, osNi*, 8COO,? A1. ．． ．． Manufacture of electrodes.

A hydrogen storage alloy with the above composition was produced in an argon arc melting furnace using commercially available La, Nd, Zr, or Hf, Ni, Co, and ^1 metals with a purity of 99.5% or higher. This alloy was pulverized to 100 mesh or less, and a copper coating layer of about 20% by weight was formed on the surface of the alloy powder by electroless copper plating. Next, a fluororesin (tetrafluoroethylene/fluoropropylene copolymer, resin addition amount = equivalent to 10 weights and 4%) was added as an adhesive to the resulting copper-coated hydrogen storage alloy, and cold pressed to a diameter of 13 mm. A molded article weighing 300 cm was obtained.

This was sandwiched between nickel meshes as current collectors from both sides, and hot press molded at a temperature of 300°C to produce a hydrogen storage electrode.

These hydrogen storage electrodes were used as negative electrodes, sintered nickel oxide electrodes were used as positive electrodes, and mercury oxide electrodes were used as reference electrodes.
A test battery was assembled using a 6N potassium hydroxide solution as an electrolyte. Note that all test batteries were negative electrode regulated types in which the battery capacity depends on the capacity of the negative electrode. These test batteries were placed in a constant temperature room at a temperature of 200°C, and a charging current of 4
A long-term charging/discharging test was conducted by charging at 0 mA for 2.5 hours, resting for 0.5 hours, and then discharging at a discharge current of 20 mA until the voltage dropped to 10.6 V with respect to the reference electrode. . Further, the high rate discharge test was conducted up to a discharge current of 200 mA. The results for various alloys are shown in Table 2 and Figure 1.

Table 2 Note: 200n+A=926mA/g, capacity 280a+
When expressed as Ah/g, this corresponds to a discharge at a rate of 3.3C.

As is clear from Table 2 and Figure 1, when La shifts to the insufficient side (α>0), high rate discharge performance improves, but when the shift is too large (α = 0.06), the cycle life decreases. do. On the other hand, when La shifts to the rich side (α<0), high rate discharge performance is impaired and the life span is also shortened.

Therefore, 0<α<0.06 is used for this alloy, and 0
．． 01≦α≦0.04 is preferably used, and 0.01<
α≦0.03 is more preferably used.

This is because on the La-rich side, La precipitates at grain boundaries and is oxidized by contact with the electrolyte, so this oxide layer acts as a resistance to the electrode reaction and causes a decrease in capacity during high-rate discharge. Conceivable. On the other hand, on the La-deficient side, a Ni-rich phase precipitates at the grain boundaries, which becomes a catalyst for the electrode reaction, and is thought to facilitate the high-rate discharge reaction.

Example 2 The chemical composition was determined in the same manner as in Example 1. Iwl-(Jlil
, 5Coo, ,A1. An electrode was manufactured using an alloy of , , and tested in the same manner as in Example 1. Note that Mm for the composition shown in Table 1 above was used for h.

The test results are shown in Table 3 below.

Table 3: The discharge characteristics are significantly degraded. Therefore 0〈α≦0.06
The range of 0.01≦α≦0.04 is preferably used, and the range of 0.01<α≦0.03 is more preferably used.

Example 3 Chemical composition Mm+-acNi3.5Coo, sMno,
An electrode of aAlo.3 was manufactured in the same manner as in Example 1, and the same tests were conducted. In addition, the Mm of the composition shown in Table 1 above was used for Mm. The results are shown in Table 4 below.

(Leaving space below next summer) As is clear from Table 3, if Mm shifts to the shortage side (
α>0), the high rate discharge characteristics are improved, but when α=0.06, the life is shortened.

On the other hand, when Mm shifts to the rich side (α<0), it becomes high. characteristics and low-temperature discharge characteristics can be significantly improved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the discharge capacity of the hydrogen storage electrode of the present invention and the deviation from the stoichiometric amount of La. As is clear from Table 4, when M- shifts to the shortage side, (
α>0), the high rate discharge characteristics are improved, but when α=0.06, the life is short. On the other hand, when h shifts to the rich side (α<0), the high rate discharge characteristics deteriorate significantly. Therefore, the range of 0.01≦α≦0.04 is preferably used.

Claims (1)

[Claims] General formula A_1_-_αNi_3_-_y_-_zCo_
A hydrogen storage electrode comprising a hydrogen storage alloy represented by yM_z. However, in the above general formula, A is a rare earth element, a mixture thereof, or a mixture of a rare earth element and Zr or Hf, M is Al, Mn, or a hybrid thereof, and 0<α<0.
06, y=0.3-1.2, z=0.2-1.2.