JPH07105231B2 - Hydrogen storage electrode - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hydrogen storage electrode.

[Conventional technology]

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

In addition, hydrogen storage alloys for electrodes are generally LaNi ₅ and MmN.
It is based on i ₅ (Mm stands for misch metal). Then, in order to keep the hydrogen equilibrium dissociation pressure of the hydrogen storage alloy at 1 atm or less in the battery operating temperature range of −20 to 60 ° C., a part of Ni is replaced with Al or Mn, or the alloy accompanying the charge / discharge cycle Ni is often replaced with Co in order to prevent the oxidative decomposition of.

[Problems to be Solved by the Invention]

The LaNi ₅ series has a large charge / discharge capacity but is expensive. On the other hand, MmN
The i ₅ series has a small charge and discharge capacity but is low price. Also, up to now, both alloy systems have been developed which show almost no capacity loss even after 300 cycles of charge and discharge, but they have the drawback of being inferior in discharge characteristics at high rates and low temperatures.

The present invention aims to eliminate the above-mentioned conventional drawbacks.

[Means for Solving the Problems]

The hydrogen storage alloy of the present invention which achieves the above object has the general formula A
A hydrogen storage alloy represented by _1-α Ni _5-yz Co _y M _z ,
It is composed of an alloy in which a nickel-rich phase corresponding to the deviation α from the stoichiometric composition is precipitated at the grain boundaries.

However, in the above general formula, A represents at least one selected from rare earth elements, Zr and Hf, and M represents at least one of Al and Mn. Also, 0 <α <0.06,
y = 0.3-1.2, z = 0.2-1.2 When two or more rare earth elements are used, the mixture (Mm: misch metal) may be artificially prepared, or naturally. It is also possible to use what is produced as it is. As a mixture of rare earth metals naturally produced,
As shown in Table 1 below, various chemical compositions (% by weight) are obtained depending on the type of ore and the degree of the separation process.

And, the Mm of the composition A is generally most easily available and inexpensive, but since the content of La is low, the charge / discharge capacity is low.
Mm of compositions B and E has a high content of La and thus has a high charge / discharge capacity, but is expensive because it separates and removes Ce and the like.

On the other hand, Mm of composition C has a high La content and a low price, but there is concern about mass supply. Further, although Mm of composition D can be obtained at a lower cost than A, it has the inconvenience of requiring preliminary refining before the alloy production because it contains a large amount of Mg and Cl.

Since each Mm has its own characteristics, it can be used properly according to the purpose.

In Mm, the higher the content of Ce or Nd in the rare earth metal,
Even if the expensive Co content is reduced, a sufficiently long charge / discharge cycle life (10% or less charge / discharge capacity reduction at 300 cycles) can be obtained, but since the dissociation pressure increases, the Al and Mn contents should be reduced. It is necessary to increase and decrease.

In addition, in an alloy system having a large amount of La, in order to obtain a sufficiently long cycle life, a large amount of Co (in the above general formula, y>
It is necessary to use 1.5), but this causes a decrease in discharge capacity and deterioration of high rate discharge characteristics. Therefore, if 3 to 8 atomic% of La is replaced with Zr or Hf, the Co content becomes y <
Even with 1.0, a sufficiently long life can be obtained. Therefore, the Co content is y = 0.3 to 1.2, preferably y = 0.5 to 0.9, and more preferably y = 0.6 to 0.8. Further, in order to adjust the hydrogen dissociation pressure of the alloy, a part of Ni is replaced with Al or Mn, and the replacement amount is determined by the battery operating temperature range and z = 0.2 to 1.2.
And z = 0.4 to 0.8 is preferable in the temperature range of −20 to 60 ° C.

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

Examples of the present invention will be described below.

〔Example〕

Example 1 Chemical composition La _0.8−α Nd _0.15 Zr (Hf) _0.05 Ni _3.8 Co _0.7 Al _0.5
Manufacture of electrodes.

A hydrogen storage alloy having the above composition was produced in an argon arc melting furnace using commercially available La, Nd, Zr or Hf, Nr, Co and Al metals having a purity of 99.5% or more. This alloy is crushed to 100 mesh or less, and the surface of the alloy powder is approximately coated by electroless copper plating.
A 20 wt% copper clad layer was formed. Fluorine resin (tetrafluoroethylene / fluorinated propylene copolymer, resin addition amount: 10% by weight) as an adhesive to the obtained copper-coated hydrogen storage alloy
Equivalent) and cold press 13 mm in diameter, 300 mg in weight
The molded body of This was sandwiched from both sides with a nickel mesh as a current collector, and hot pressed at a temperature of 300 ° C. to fabricate a hydrogen storage electrode.

These hydrogen storage electrodes are used as negative electrodes, positive electrodes are sintered nickel oxide electrodes, and reference electrodes are mercury oxide electrodes.
A test battery using a 6N potassium hydroxide solution as an electrolyte was assembled. All test batteries were of the negative electrode regulation type in which the battery capacity depended on the negative electrode capacity. These test batteries were placed in a temperature-controlled room at a temperature of 200 ° C and charged at 2.5 mA at a charging current of 40 mA.
After a long-time charge and a 0.5-hour rest, a long-term charge / discharge repeated test was performed in a cycle of discharging at a discharge current of 20 mA until the voltage dropped to −0.6 V with respect to the reference electrode. The high rate discharge test was performed up to a discharge current of 200 mA. The results for various alloys are shown in Table 2 and FIG.

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

Therefore, in this alloy, 0 <α <0.06 is used 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 is deposited at the crystal grain boundaries and is oxidized by contact with the electrolytic solution, so that this oxide layer becomes a resistance of the electrode reaction and causes a decrease in capacity during high rate discharge. Conceivable. On the other hand, in the alloy of the present invention, La is deficient in α relative to the stoichiometric composition, so that the Ni-rich phase corresponding to the deviation α from the stoichiometric composition is precipitated in the grain boundaries. , This becomes the catalyst of the electrode reaction,
It is considered that the high rate discharge reaction becomes easy.

Example 2 As in Example 1, the chemical composition Mm _1-α Ni _3.5 Co _0.7 Al
_An electrode was manufactured from the _0.8 alloy and tested in the same manner as in Example 1. The Mm having the composition A shown in Table 1 was used as the Mm.

The test results are shown in Table 3 below.

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

On the other hand, when Mm shifts to the rich side (α <0), the high rate discharge characteristic deteriorates significantly. Therefore, the range of 0 <α ≦ 0.06 is used, 0.01 ≦ α ≦ 0.04 is preferable, and 0.01 <α ≦ 0.03 is more preferably used.

Example 3 An electrode having a chemical composition of Mm _1-α Ni _3.5 Co _0.8 Mn _0.4 Al _0.3 was manufactured by the same method as in Example 1, and the same test was performed. As the Mm, the Mm having the composition D shown in Table 1 above was used. The results are shown in Table 4 below.

As is clear from Table 4, when Mm shifts to the shortage side (α
> 0), the high rate discharge characteristics are improved, but the life is short when α = 0.06.

On the other hand, when Mm shifts to the rich side (α <0), the high rate discharge characteristic is significantly deteriorated. Therefore, the range of 0.01 ≦ α ≦ 0.04 is preferably used.

〔The invention's effect〕

As described above, according to the hydrogen storage electrode of the present invention, the high rate discharge characteristic and the low temperature discharge characteristic can be remarkably improved while maintaining the cycle life of the alloy.

[Brief description 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.

Claims (1)

[Claims] 1. A hydrogen storage alloy represented by the general formula A _1-α Ni _5-yz Co _y M _z , in which a nickel-rich phase corresponding to a deviation α from the stoichiometric composition is precipitated at a grain boundary. A hydrogen storage electrode characterized by comprising an alloy of: wherein A represents at least one selected from rare earth elements, Zr and Hf, and M represents Al and
At least one of Mn is shown; and 0 <α <0.06, y = 0.3 to 1.2, and z = 0.1 to 1.2.