JP3185151B2 - Hydrogen storage alloy for hydrogen storage electrode and method for producing hydrogen storage electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an electrode made of a hydrogen storage alloy used for a negative electrode of an alkaline storage battery or the like.

[0002]

2. Description of the Related Art A hydrogen storage electrode uses a hydrogen storage alloy capable of reversibly storing and releasing hydrogen as an electrode. The electrochemical oxidation-reduction reaction of hydrogen is carried out by the electromotive force of a negative electrode of an alkaline storage battery. Use for reaction. Hydrogen storage alloys used for hydrogen storage electrodes include those in which the constituent elements of intermetallic compounds such as LaNi ₅ or Laves phase ZrNi ₂ or TiNi are replaced with other metals to improve the performance as hydrogen storage electrodes. There is.

[0003] Conventional methods of manufacturing a hydrogen storage electrode using these hydrogen storage alloys include the following.

[0004] One is a method in which a powder of a hydrogen storage alloy is held on an alkali-resistant conductive support such as punched metal or foamed nickel by a binder such as a fluororesin. The electrode manufactured by this method is called a plastic bonded type electrode. The other is a method of sintering a powder of a hydrogen storage alloy. The electrode manufactured by this method is called a sintered electrode.

[0005] The plastic bonded electrode can be produced by a method in which a powder of a hydrogen storage alloy is made into a paste, and the paste is applied or filled on a conductive support, and then dried and pressed. It is excellent in that electrodes can be manufactured at high speed by a simple manufacturing apparatus.

On the other hand, the sintered electrode has a high mechanical strength because the hydrogen storage alloy is sintered and bonded thereto, and has a small electrode polarization because the electron conductivity between the hydrogen storage alloys is high. Is excellent.

[0007]

However, since all of these electrodes use a powdered hydrogen storage alloy, the component metals of the hydrogen storage alloy are first dissolved in a vacuum or in an inert atmosphere, and then dissolved. It was necessary to cast and mechanically grind or spray the melt to produce a powder. In the melting step and the pulverizing step, a large-scale equipment and a large amount of energy consumption as a heat source are required. Furthermore, impurities are mixed in from the furnace material in the melting step, and the volatile component elements are evaporated in the melting step. Of alloy composition due to
There were problems such as contamination of the alloy powder by the material of the mill in the pulverizing step.

As a method of manufacturing a hydrogen storage electrode without using such a melting step, a green compact of a powder mixture of a hydride of titanium and nickel disclosed in Japanese Patent Application Laid-Open No. 49-15933 is sintered for a long time at a high temperature. There is a method in which the sintered body is pulverized, mixed with copper powder and hot-pressed. However, although this method does not require a large-scale melting apparatus, it requires a long time of about one day for sintering a green compact of a powder mixture of hydride and nickel of titanium, and is inferior in mass productivity. There is a disadvantage that. Also, this method has low industrial utility in that the hydride of titanium used as a material for the alloy is extremely expensive.

Therefore, an inexpensive alloy material can be used as it is, and furthermore, there is no need for a melting step or a pulverizing step of an alloy which consumes a lot of energy and is liable to mix impurities and shift the alloy composition. There has been a demand for a method of manufacturing a hydrogen storage electrode made of a sintered body of a hydrogen storage alloy having a small polarization.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention relates to a method for producing a hydrogen storage alloy, which comprises mixing a metal powder which generates heat during a reaction for synthesizing a hydrogen storage alloy with a green compact or a non-vacuum compact. A method for producing a hydrogen storage electrode that ignites in an oxidizing atmosphere and performs self-propagating reaction sintering.

[0011]

[Action] Self-Propagating Reaction
Sintering is a phenomenon or process also called combustion synthesis, and its contents are as follows.

That is, when a part of the mixed green compact of the raw material powders involved in the synthesis reaction of the inorganic substance is ignited and an energy higher than the activation energy required for starting the reaction is given, the part of the inorganic substance is decomposed at that part. The synthesis reaction starts. If the reaction is of a type with spontaneous exotherm,
That is, when the enthalpy of formation of the inorganic compound is negative, more heat is generated in the portion where the reaction has occurred than the heat input by ignition. Then, the heat generation heats the green compact in a portion adjacent to the ignition portion, and the reaction propagates to the adjacent portion. Since heat is generated by the reaction also in this adjacent portion, the synthesis reaction propagates and proceeds one after another. In many cases, the temperature at which the exothermic reaction occurs becomes so high that strong light emission is observed.

In the self-propagating reaction sintering, the reaction zone is heated to a high temperature, and a solid phase reaction occurs in this part,
A part of the zone is in a molten state, and this zone sequentially moves to an unreacted portion of the solid phase at a low temperature in front thereof while forming a reacted portion of the solid phase at a low temperature behind the zone.

Therefore, the self-propagating reaction sintering has the same characteristics as those of the ordinary sintering process, and furthermore, since it uses the heat generated by the reaction, it does not need to be heated by an external heat source. And sintering is completed in a short time because the reaction spontaneously propagates.

This phenomenon is also observed in intermetallic compounds. For example, there is an attempt to apply it to the production of a TiNi alloy which is a shape memory alloy.

The present invention intends to apply this process to the production of a hydrogen storage electrode.

For example, the enthalpies of formation ΔH of the intermetallic compounds LaNi ₅ and TiNi used for the hydrogen storage electrode are -40 kcal / mol and -16 kcal / mol, respectively, and ΔH is a negative value. The reaction for synthesizing a compound from its component metals is an exothermic reaction. Therefore, a green compact in which La and Ni powders are mixed in an atomic ratio of 1: 5, and a green compact in which Ti and Ni powders are mixed in an equimolar ratio by atomic ratio, in vacuum, argon, etc. When ignited in an inert atmosphere, the reaction spontaneously propagates due to the reaction heat, and a sintered body mainly containing the intermetallic compounds LaNi ₅ and TiNi is obtained.

The hydrogen storage alloy used for the hydrogen storage electrode has, for example, the composition of LaNi ₅ having a slightly higher equilibrium pressure at the plateau portion in its PCT characteristics (equilibrium hydrogen pressure-hydrogen storage amount-isothermal characteristic). In addition, since the charge-discharge cycle life is short, in order to solve these problems, it is necessary to replace part of La with other rare earth elements or a small amount of Zr, and replace part of Ni with Co or Al. Done. And even in such a case, for example, the enthalpy of formation of ZrNi ₅ and LaCo ₅ are -57 kcal / mol and -17.5 kcal / mo, respectively.
As in l, the reaction for synthesizing a hydrogen storage alloy from these constituent elements is an exothermic reaction in many cases even if these substitution elements are used. Therefore, the method of the present invention can be applied to a practical multi-component hydrogen storage alloy for batteries.

Further, in the present invention, since the heat generated in the synthesis reaction of the hydrogen storage alloy is used, the material powder of the green compact which causes self-propagation reaction sintering is not necessarily a single metal, but is made of an alloy material. There may be. In short, regarding the material metal of the green compact, whether the material metal is a simple metal or an alloy is not an essential constituent requirement in the present invention, but a material that causes a reaction for synthesizing a hydrogen storage alloy to be an exothermic reaction. The use of a metal is an essential component of the present invention.

In the self-propagating reaction sintering of the present invention, since the reaction zone in which the synthesis of the hydrogen storage alloy is occurring is at a high temperature, the oxidation reaction of the alloy is prevented to obtain the desired hydrogen storage alloy. It is necessary to perform reaction sintering in vacuum or in a non-oxidizing atmosphere such as argon or hydrogen.

According to this method, the following points are advantageous as compared with the conventional method for manufacturing a hydrogen storage electrode. (1) Since a compact obtained by molding a compact made of an inexpensive metal powder material by reaction sintering becomes a hydrogen storage electrode as it is, a large-scale alloy melting furnace and a process using a powder manufacturing apparatus are not required. . (2) The synthesis reaction of the hydrogen storage alloy proceeds in a short time while supporting a part of the metal compact, and the hydrogen storage electrode can be obtained as it is. Therefore, the contamination of the alloy from the furnace material in the melting process of the alloy. In addition, the composition shift due to the evaporation of the low melting point element in the melting step and the contamination of the mill material in the pulverizing step do not occur. Accordingly, an electrode made of a hydrogen storage alloy having a small amount of impurities and a small composition shift can be obtained. (3) Since the synthesis of the hydrogen storage alloy occurs only by the reaction heat without the step of heating and melting the alloy material, the energy required for manufacturing the electrode can be significantly reduced. (4) Since the obtained electrode is a sintered body of a hydrogen storage alloy, the electric resistance of the electrode is reduced, the internal resistance of the battery is reduced, and the polarization when discharging the battery with a large current is reduced.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described by way of preferred embodiments. [Example] La _0.8 Ce _0.2 Ni _3.8 Co _0.7 Al _0.3 Mn in molar ratio
La, C with a particle size of 100 μm or less so that the composition becomes _0.2
e, Ni, Co, Al and Mn powders are dry-mixed, and this mixed powder is pressed under a pressure of 1 ton / cm ² to a thickness of about 0.4 m.
m, a compact having a width of about 16 mm and a height of about 57 mm were molded. Since the raw material powder contained a large amount of nickel powder having high plasticity, the density of the green compact could be increased.

Next, the compact is placed on a calcia table having a large number of irregularities on the surface so that the contact area with the compact is reduced, and a nichrome wire is wound around one end of the compact. Put this in a vacuum vessel and evacuate the inside of the vacuum vessel to about 10 ^-5
Atmospheric pressure was applied. When the nichrome wire was energized and glowed red, the green compact ignited from the portion in contact with the nichrome wire and began to emit light intensely. Then, the reaction part emitting light sequentially propagated to the unreacted green compact.
Since the reaction involving light emission was completed within a few minutes, the sintered body was allowed to cool and then dry air was introduced into the vacuum vessel.

FIG. 1 is a perspective view of a green compact in the process of producing a hydrogen storage electrode by the progress of self-propagating reaction sintering.
In FIG. 1, reference numeral 1 denotes an unreacted green compact, 2 denotes a sintered body of a hydrogen storage alloy after the reaction, and 3 denotes a synthesis reaction of the hydrogen storage alloy, which propagates toward the unreacted green compact. The reacting sintering zone 4 is a nichrome wire attached for ignition.

The obtained sintered body retains the shape of a plate-like compact, and its dimensions shrink slightly to a thickness of about 0.4 mm.
It was about 15mm wide and 55mm long. The sintered body is the weight of one sheet porous material about 1.5 g, the sintered body was ground in an agate mortar, the results was carried out X-ray diffraction analysis of the crystal structure, LaNi ₅ in crystalline form The main thing was the structure. And the composition of the alloy was almost equal to the component of the input raw material metal. Then, 100 mg of the alloy powder classified to have a particle size of 45 μm or less and 400 mg of electrolytic copper powder were mixed, pressed and molded to produce an electrode for evaluating the capacity of the alloy, and a current of 15 mA was applied in a 7 M KOH electrolyte. The battery was charged for 2.5 hours and then discharged to 0.7 V with reference to the mercury oxide electrode potential. The discharge capacity was determined as a result. As a result, the discharge capacity per 1 g of the alloy was about 0.32 Ah.

The sintered body was brought into contact with a nickel net to collect electricity, charged in a 7M KOH electrolyte at a current of 0.24 A for 2.5 hours, and 0.7 V based on the potential of the mercuric oxide electrode. The discharge capacity in the case of repeating charge and discharge of discharging up to
It was about 0.48 Ah.

Next, a nickel alloy using this sintered body as a negative electrode
A metal hydride storage battery was manufactured.

As the negative electrode of this battery, four sintered bodies were used, and a nickel net was in contact with the surface for current collection. As the positive electrode, three known sintered nickel hydroxide electrodes were used. The size of one sintered nickel electrode is approximately
0.84 mm, width about 14 mm, length about 54 mm, the total amount of nickel hydroxide and the additive cobalt hydroxide contained in the three positive plates is about 3.5 g, and the reaction is one electron. The theoretical capacity, assuming to follow the process, is about 1.01 Ah.
Nylon nonwoven fabric was used for the separator. Then, the electrode plate group in which the positive electrode plate and the negative electrode plate are alternately laminated via the separator is housed in a rectangular sealed container, and the outer dimensions are height.
A sealed battery of 66.7 mm, width of 16.4 mm and thickness of 5.6 mm was constructed.
The electrolyte used was 7 M KOH to which 20 g / l of LiOH had been added. This battery is called battery A. [Comparative Example] For comparison, a plastic bonded type hydrogen storage electrode was manufactured with the same alloy composition as that of the above-mentioned embodiment by using the following conventional manufacturing method.

First, in terms of molar ratio, La _0.8 Ce _0.2 Ni _3.8 Co _0.7
Al _0.3 so that the composition of the _{Mn 0.2, La, Ce, Ni} , Co, Al
And Mn were charged into a high-frequency melting furnace lined with calcia and melted for 2 hours. Next, this melt was poured into a water-cooled copper mold and solidified. The ingot was roughly pulverized with a jaw crusher and then finely pulverized with an alumina ball mill. Then, classification was performed to obtain a hydrogen storage alloy powder having a particle size of 45 μm or less.

The crystal structure of this alloy powder was mainly of LaNi _{5 type} , but the volatile component Mn volatilized during melting of the alloy, and the stoichiometric number of the alloy powder changed from 0.2 at the time of injection to 0.12. Had declined. Further, about 0.1% by weight of a fine powder of alumina as a material of the mill when pulverized by a ball mill was mixed.

The hydrogen storage alloy powder 100 mg and the electrolytic copper powder
400 mg, and press-molded to produce an electrode for evaluating the capacity of the alloy, charged in a 7 M KOH electrolyte at a current of 15 mA for 2.5 hours, and 0.7 V based on the potential of the mercuric oxide electrode. The discharge capacity was determined by repeating charge / discharge of discharging to about 0.32 Ah per 1 g of the alloy.

Next, 1.5 parts by weight of furnace black as a conductive additive was mixed with 100 parts by weight of the hydrogen storage alloy powder.
An aqueous solution in which 1.5 parts by weight of polyvinyl alcohol as a binder was dissolved was added and kneaded to produce a paste-like mixture. Then, this paste-like mixture is 0.8 mm thick
In and coated on both sides of the open porosity is about 50% nickel plated iron punched metal, after drying, about 1 ton / cm ²
Pressing and cutting at a pressure of 1. The hydrogen absorbing electrode was manufactured by a conventional manufacturing method. The size of this electrode is approximately
It was 0.4 mm, about 15 mm wide and about 55 mm long. The weight of the hydrogen storage alloy contained in one hydrogen storage electrode was about 1.2 g. Therefore, this one hydrogen storage alloy electrode has about 0.38
A hydrogen storage alloy having a discharge capacity of Ah is included.

As described above, the reason that the conventional plastic-bonded electrode has a smaller amount of the hydrogen-absorbing alloy than the electrode according to the method of the present invention is that the plastic-bonded electrode has a mechanically practically applicable degree. In the press,
This is because the porosity of the compact made of the hydrogen storage alloy powder having low plasticity cannot be reduced to a certain degree or more.

Next, a battery B was manufactured in the same manner as the battery A except that four plastic bonded type hydrogen storage electrodes were used instead of the hydrogen storage electrode manufactured by self-propagating reaction sintering.

The batteries A and B described above were heated at 25 ° C.
, A charge / discharge cycle was repeated in which the battery was charged at a current of 1 A for 1.2 hours and discharged at a current of 1 A until the terminal voltage reached 1.0 V. Then, the number of charge / discharge cycles (charge / discharge cycle life) until the discharge capacity was reduced to 60% of the discharge capacity at the 10th cycle was examined. Table 1 shows the results. Also,
When charging with 1A current for 1.2 hours and then discharging with 3A current, 1/2 of the discharge time until the terminal voltage reaches 0.8V
The terminal voltage (discharge intermediate voltage) during the discharge time was examined. The results are also shown in Table 1.

[0036]

[Table 1] Table 1 shows that the battery A using the hydrogen storage electrode manufactured by the method of the present invention has a longer charge / discharge cycle life than the battery B using the hydrogen storage electrode manufactured by the conventional method.

The reason is presumed to be as follows.

That is, in the hydrogen storage alloy manufactured by the conventional method, Mn volatilizes and the stoichiometric number of the hydrogen storage alloy shifts, so that the deterioration rate of the alloy with the progress of the charge / discharge cycle increases. Further, in the case of the plastic bonded electrode, since the amount of the alloy was small, the influence of the deterioration of the alloy was remarkable. Furthermore, since alumina was mixed in the hydrogen storage alloy produced by the conventional method, the alumina was dissolved in the alkaline electrolyte, reached the positive electrode, and its discharge capacity was reduced.

Further, from Table 1, it can be seen that the battery A using the hydrogen storage electrode manufactured by the method of the present invention has a larger discharge current than the battery B using the hydrogen storage electrode manufactured by the conventional method. It can be seen that the discharge intermediate voltage is high.

The cause is presumed as follows.

That is, in the hydrogen storage electrode manufactured by the method of the present invention, since the hydrogen storage alloy is firmly sintered,
As a result of the excellent electrical conductivity in the electrodes, the polarization of the battery when discharging with a large current is reduced.

In the above embodiment, the description has been given of the case of the hydrogen storage alloy in which a part of the component elements of the LaNi ₅ alloy is replaced with another specific element. However, the effects of the present invention are not limited to the TiNi alloy and the Laves alloy. In the case of a phase alloy and an alloy in which some of the constituent elements are replaced with other elements, the same can be obtained if the synthesis reaction of the hydrogen storage alloy is an exothermic reaction.

[0043]

According to the present invention, a compact obtained by molding an inexpensive metal powder material by reaction sintering is used as it is as a hydrogen storage electrode. The step of using the device becomes unnecessary.

Therefore, the contamination of the alloy from the furnace material in the melting step of the alloy, the shift of the composition due to the evaporation of the low melting point component element in the melting step, and the contamination of the mill material in the grinding step do not occur. Accordingly, an electrode made of a hydrogen storage alloy having a small amount of impurities and a small composition shift can be obtained. Furthermore, since the synthesis of the hydrogen storage alloy occurs only by the reaction heat without the step of heating and melting the alloy material, the energy required for manufacturing the electrode can be significantly reduced. Since the obtained electrode is a sintered body of a hydrogen storage alloy, the electric resistance of the electrode is reduced, the internal resistance of the battery is reduced, and the polarization when the battery is discharged with a large current is reduced.

[Brief description of the drawings]

FIG. 1 is a view showing a state of a green compact during the production of a hydrogen storage electrode by progress of self-propagating reaction sintering.

[Explanation of symbols]

 1 Unreacted green compact 2 Sintered body of hydrogen storage alloy after reaction 3 Reaction sintering zone 4 Nichrome wire

Claims (1)

(57) [Claims] 1. A green compact is prepared by mixing a metal powder which generates heat during a reaction for synthesizing a hydrogen storage alloy, and then ignites the green compact in a vacuum or a non-oxidizing atmosphere to cause self-propagation. A method for producing a hydrogen storage electrode, comprising performing reaction sintering.