JP6201307B2 - Hydrogen storage alloy, electrode, nickel metal hydride storage battery, and method for producing hydrogen storage alloy - Google Patents

Description

  The present invention relates to a hydrogen storage alloy having excellent corrosion resistance, an electrode and a nickel-metal hydride storage battery using the same, and a method for producing the hydrogen storage alloy.

  The hydrogen storage alloy is a material that can be stored safely and easily and is expected as a clean energy source, and has attracted attention as a new energy storage and conversion material.

  Applications of such hydrogen storage alloys include hydrogen storage / transport, heat storage / transport, thermal-mechanical energy conversion, hydrogen separation / purification, hydrogen isotope separation, nickel-metal hydride storage batteries, catalysts in synthetic chemistry However, nickel-metal hydride storage batteries using a hydrogen storage alloy as a negative electrode active material are in increasing demand because they have advantages such as small size, light weight and high output.

As a negative electrode active material of a nickel metal hydride storage battery, an AB ₅ type alloy containing rare earth elements and Ni as main constituent elements has been used conventionally. Such hydrogen storage alloys can store batteries in a high temperature atmosphere. When charging / discharging is repeated, it tends to corrode, and a rare earth hydroxide or the like is generated on the surface due to the corrosion, resulting in a change in its form.

  For this reason, it is important to improve the corrosion resistance of the hydrogen storage alloy in order to extend the life of the nickel metal hydride storage battery. As a method for improving the corrosion resistance of the hydrogen storage alloy, it has been proposed to contain Y (yttrium) in the negative electrode (see Patent Documents 1 and 2). However, these methods have not been able to sufficiently improve the corrosion resistance of the hydrogen storage alloy.

JP-A-6-215765 JP 2001-307721 A

  In view of the above, the present invention has been made to provide a hydrogen storage alloy having excellent corrosion resistance, an electrode and a nickel-metal hydride storage battery using the same, and a method for producing the hydrogen storage alloy.

The present inventor has found that Y is prevented from forming a stable compound with La or Ni by segregating rather than uniformly distributing Y in the hydrogen storage alloy. Then, it became easy to form a film of yttrium hydroxide (Y (OH) ₃ ) in the alkaline solution, and succeeded in dramatically improving the corrosion resistance of the hydrogen storage alloy. In general, segregation in an alloy is considered to lead to a decrease in performance, and is not preferable. The present invention has been completed based on such novel findings.

That is, the hydrogen storage alloy according to the present invention is a hydrogen storage alloy having at least two phases containing La, Ni, and Y or a heavy rare earth element, and the first phase has the general formula R1 _a R2 _b Ni _c Co _d R3 _e (wherein R1 is La and at least one element selected from the group consisting of rare earth elements excluding Y and heavy rare earth elements, Mg, Ca and Zr, and R2 is Y and heavy R3 is at least one element selected from the group consisting of rare earth elements, R3 is at least one element selected from the group consisting of Mn, Al, Zn, Fe, Cu and Si, and a, b, c, d And e are numerical values satisfying 0 <b <0.3, a + b = 1, 5.15 <c + d + e <5.45, 0 ≦ d ≦ 1, and 0 ≦ e ≦ 1. And the second phase is Y or heavy rare earth element The elemental concentration is higher than that of the first phase, the concentration of Ni is 0.02 times or less that of the first phase, and is dispersed in the first phase. Here, heavy rare earth elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) have an ionic radius and an atomic radius close to Y, and the reaction order is almost the same as Y. It is considered that the same effect can be obtained.

  In the hydrogen storage alloy, R1 is La and Ce, R3 is Mn and / or Al, c, d and e are 5.20 <c + d + e <5.45, and 0 ≦ d ≦ 0. .45 is preferred.

  An electrode containing the hydrogen storage alloy according to the present invention and a nickel metal hydride storage battery including the electrode as a negative electrode are also one aspect of the present invention.

  Moreover, the manufacturing method of the hydrogen storage alloy which concerns on this invention is also one of this invention. That is, in the method for producing a hydrogen storage alloy according to the present invention, the raw metal is melted at a temperature lower than the melting point of Y or heavy rare earth element using a high frequency induction melting method and alloyed, and the obtained molten alloy is cooled And a step of heat-treating the cooled alloy at 900 to 1080 ° C.

  Since this invention consists of the structure mentioned above, the corrosion resistance of a hydrogen storage alloy can be improved dramatically and a long-life nickel-metal hydride storage battery can be obtained.

The figure which showed typically the parent phase and the segregation phase in the cut surface of the hydrogen storage alloy which concerns on this invention. The photograph which shows the cut surface of the hydrogen storage alloy ingot of Example 1 imaged with the EPMA apparatus.

  The present invention is described in detail below.

The hydrogen storage alloy according to the present invention is a hydrogen storage alloy having at least two phases containing La, Ni, and Y or heavy rare earth elements, wherein the concentration of Y or heavy rare earth elements is the parent phase (first higher than the phase) segregation phase (secondary phase concentration of Ni is less than 0.02 times the mother phase) is AB ₅ type hydrogen storage alloy is dispersed in the mother phase. The presence or absence of the segregation phase can be analyzed using EPMA (Electron Probe Micro Analyzer). FIG. 1 schematically shows a matrix phase and a segregation phase on the cut surface of the hydrogen storage alloy. In the present invention, having a segregation phase (second phase) means that the ratio of component elements in the alloy is different from the parent phase (first phase) in the analysis of EPMA. The “second phase” means other than AB type ₅ (CaCu type ₅ ).

  When the hydrogen storage alloy corrodes, rare earth hydroxides and the like are formed on the surface and the shape changes. Therefore, the specific surface area of the hydrogen storage alloy powder before and after the cycle test is measured, and this is used to determine the corrosion amount of the hydrogen storage alloy. It can be an index to measure. When the inventors measured the specific surface area of the hydrogen storage alloy powder before and after the cycle test, the hydrogen storage alloy with segregated Y had a smaller specific surface area value and increased corrosion resistance compared to the homogenized hydrogen storage alloy. I found out. This is because when the hydrogen storage alloy in which Y is segregated comes into contact with the electrolyte solution which is an alkaline solution, the segregated Y preferentially elutes, and a passive film of yttrium hydroxide is quickly formed on the alloy surface. it is conceivable that. On the other hand, in a hydrogen storage alloy in which Y is evenly distributed, Y forms a stable alloy with La and Ni, especially Ni, so it is considered that a passive film of yttrium hydroxide is difficult to form. Further, such a behavior of Y is such that the ionic radius and the atomic radius are close to Y, and the heavy rare earth elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, whose reaction order is substantially the same as Y. ).

  The concentration of Y or heavy rare earth element in the segregation phase is preferably at least twice the concentration of Y or heavy rare earth element in the parent phase, more preferably 4 to 100 times.

  The ratio of the segregation phase is preferably 0.05 to 2%, more preferably 0.1 to 1%. If the ratio of the segregation phase is 0.05 to 2%, a passive film can be effectively formed on the alloy surface. If the ratio of the segregation phase is 0.1 to 1%, the alloy is more prominent. A passive film can be formed on the surface, and corrosion of the hydrogen storage alloy can be suppressed. The ratio of the segregation phase means the ratio of the area of the segregation phase to the area of the parent phase in a predetermined region of the cross section including the approximate center of the ingot or particle of the alloy.

  The size of the segregation phase is preferably 1 μm or more, more preferably 5 μm or more. If the size of the segregation phase is 1 μm or more, a passive film can be effectively formed on the alloy surface. If the size of the segregation phase is 5 μm or more, the passive film is more prominently formed on the alloy surface. It can form and can suppress corrosion of a hydrogen storage alloy. In addition, the magnitude | size of a segregation phase measures the long side and short side of a segregation phase, and calculates the average value.

Matrix of the hydrogen storage alloy according to the present invention have the general formula _{R1 a R2 b Ni c Co d} R3 e ( wherein, R1 is La, and rare earth elements except Y and heavy rare earth elements, Mg, Ca and Zr R2 is at least one element selected from the group consisting of Y and heavy rare earth elements, and R3 is composed of Mn, Al, Zn, Fe, Cu and Si. A, b, c, d, and e are 0 <b <0.3, a + b = 1, 5.15 <c + d + e <5.45, 0 ≦ d ≦ 1 And a numerical value satisfying 0 ≦ e ≦ 1). The hydrogen storage alloy is naturally an alloy represented by the above general formula, and may contain an element not defined by the general formula as an inevitable impurity, for example.

  In the above general formula, R1 is La and Ce, R3 is Mn and / or Al, c, d and e are 5.20 <c + d + e <5.45 and 0 ≦ Those satisfying d ≦ 0.45 are preferred. More preferably, 5.25 ≦ c + d + e ≦ 5.35 and 0 ≦ d ≦ 0.2.

  Although it does not specifically limit as a manufacturing method of the hydrogen storage alloy which concerns on this invention, For example, it can be based on the following method. First, a raw material metal weighed so that the obtained alloy has a target composition is put in a crucible, and it is less than the melting point of Y or heavy rare earth element in an atmosphere of an inert gas such as argon gas, for example, 1500 ° C. , The raw metal is completely melted by high frequency induction heating at 1450 ° C. or lower. When the melting temperature exceeds the melting point of Y or heavy rare earth elements, homogenization thereof tends to occur. The lower limit of the melting temperature is not particularly limited as long as it can melt other raw material metals. Further, by arranging a metal having a low melting point in the crucible, an alloy having a homogeneous matrix can be cast. Furthermore, since Mg and Ca having a low melting point are likely to volatilize, it is preferable to put them in the last or alloyed ones.

  Next, the molten raw metal is cooled at a rate of 10,000 ° C./second or less, preferably 5 to 100 ° C./second, more preferably 10 to 100 ° C./second by a die casting method or a melt spinning method to obtain an ingot. If the cooling rate is within this range, segregation of Y or heavy rare earth elements tends to occur.

  Furthermore, the hydrogen storage alloy which concerns on this invention can be obtained by heat-processing the obtained ingot at 900-1080 degreeC, Preferably 940-1050 degreeC. When the heat treatment temperature is within this range, the matrix phase is easily homogenized.

  The use of the hydrogen storage alloy according to the present invention is not particularly limited, and can be applied to various uses including nickel-metal hydride storage batteries, fuel cells, fuel tanks for hydrogen automobiles, among others, It is suitably used as a negative electrode active material for nickel metal hydride storage batteries. Thus, the nickel metal hydride storage battery provided with the negative electrode containing the hydrogen storage alloy according to the present invention is also one aspect of the present invention.

  The nickel metal hydride storage battery according to the present invention includes, for example, a positive electrode (nickel electrode) containing a positive electrode active material mainly composed of nickel hydroxide in addition to a negative electrode containing the hydrogen storage alloy according to the present invention as a negative electrode active material. ), A separator, an alkaline electrolyte, and the like.

  The negative electrode is one in which the hydrogen storage alloy according to the present invention is blended as a negative electrode active material. The hydrogen storage alloy which concerns on this invention is mix | blended in a negative electrode as powdered hydrogen storage alloy powder, for example.

  The average particle size of the hydrogen storage alloy powder is preferably 20 to 100 μm, more preferably 40 to 70 μm. When the average particle size is less than 20 μm, the activation of the alloy becomes insufficient, while when the average particle size exceeds 100 μm, the productivity may be lowered. The hydrogen storage alloy powder is obtained, for example, by pulverizing the hydrogen storage alloy according to the present invention with a machine in the presence of an inert gas.

  In addition to the hydrogen storage alloy powder, the negative electrode may contain a conductive agent, a binder (including a thickener), and the like.

  Examples of the conductive agent include natural graphite (scale-like graphite, scale-like graphite, earth-like graphite, etc.), artificial graphite, carbon black, acetylene black, ketjen black, carbon whisker, carbon fiber, vapor-grown carbon, etc. Examples thereof include metal-based conductive agents; metal-based conductive agents composed of powders or fibers of metals such as nickel, cobalt, and copper; yttrium oxide. These electrically conductive agents may be used independently and 2 or more types may be used together. Moreover, you may contain rare earth oxides, such as an yttrium oxide, as a corrosion inhibitor.

  The blending amount of the conductive agent is preferably 0.1 to 10 parts by mass, more preferably 0.2 to 5 parts by mass with respect to 100 parts by mass of the hydrogen storage alloy powder. When the blending amount of the conductive agent is less than 0.1 parts by mass, it is difficult to obtain sufficient conductivity. On the other hand, when the blending amount of the conductive agent exceeds 10 parts by mass, the effect of improving the discharge capacity is not good. May be sufficient.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyolefin resins such as polyethylene and polypropylene, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, polyvinyl Alcohol, methylcellulose, carboxymethylcellulose, xanthan gum, etc. are mentioned. These binders may be used independently and 2 or more types may be used together.

  The blending amount of the binder is preferably 0.1 to 1.0 part by mass, more preferably 0.5 to 1.0 part by mass with respect to 100 parts by mass of the hydrogen storage alloy powder. . When the blending amount of the binder is less than 0.1 parts by mass, sufficient thickening is difficult to be obtained. On the other hand, when the blending amount of the binder exceeds 1.0 parts by mass, the performance of the electrode is improved. May fall.

  Examples of the positive electrode include an electrode in which a nickel hydroxide composite oxide obtained by mixing zinc hydroxide or cobalt hydroxide with nickel hydroxide as a main component is blended as a positive electrode active material. As the nickel hydroxide composite oxide, those uniformly dispersed by a coprecipitation method are preferably used.

The positive electrode preferably contains an additive for improving electrode performance in addition to the nickel hydroxide composite oxide. Examples of the additive include conductive modifiers such as cobalt hydroxide and cobalt oxide, and the nickel hydroxide composite oxide coated with cobalt hydroxide, and the nickel hydroxide composite oxide. Part of the product may be oxidized with oxygen or oxygen-containing gas, K ₂ S ₂ O ₈ , hypochlorous acid, or the like.

  As the additive, a substance that improves oxygen overvoltage, such as a compound containing a rare earth element such as Y or Yb or a compound containing Ca, can also be used. Since some of rare earth elements such as Y and Yb are dissolved and disposed on the negative electrode surface, an effect of suppressing corrosion of the negative electrode active material is also expected.

  The positive electrode may further contain the above-described conductive agent, binder and the like, similarly to the negative electrode.

  Such a positive electrode and a negative electrode were obtained by kneading these with water or an organic solvent such as alcohol or toluene after adding the above-mentioned conductive agent, binder, or the like to each active material as necessary. The paste can be produced by applying it to a conductive support and drying it, followed by rolling.

  Examples of the conductive support include a steel plate and a plated steel plate obtained by plating a steel plate with a metal material such as nickel. Examples of the shape of the conductive support include a foam, a molded product of a fiber group, a three-dimensional base material subjected to unevenness processing, and a two-dimensional base material such as a punching plate. Of these conductive supports, for the positive electrode, a foam made of nickel having excellent corrosion resistance and oxidation resistance with respect to alkali and having a porous structure having a current collecting property is preferable. On the other hand, for a negative electrode, a perforated steel sheet obtained by applying nickel plating to an iron foil that is inexpensive and excellent in conductivity is preferable.

  The thickness of the conductive support is preferably 30 to 100 μm, more preferably 40 to 70 μm. When the thickness of the conductive support is less than 30 μm, the productivity may be reduced. On the other hand, when the thickness of the conductive support exceeds 100 μm, the discharge capacity may be insufficient. .

  When the conductive support is porous, the inner diameter is preferably 0.8 to 2 μm, more preferably 1 to 1.5 μm. If the inner diameter is less than 0.8 μm, the productivity may be lowered. On the other hand, if the inner diameter exceeds 2 μm, the holding performance of the hydrogen storage alloy may be insufficient.

  Examples of a method for applying each electrode paste to the conductive support include roller coating using an applicator roll and the like, screen coating, blade coating, spin coating, and per coating.

  Examples of the separator include a porous film and a nonwoven fabric made of a polyolefin resin such as polyethylene or polypropylene, acrylic, polyamide, or the like.

The basis weight of the separator is preferably 40 to 100 g / m ² . If the basis weight is less than 40 g / m ² , a short circuit or a decrease in self-discharge performance may occur. On the other hand, if the basis weight exceeds 100 g / m ² , the proportion of the separator per unit volume increases. Tend to go down. The separator preferably has an air permeability of 1 to 50 cm / sec. If the air permeability is less than 1 cm / sec, the internal pressure of the battery may be too high. On the other hand, if the air permeability exceeds 50 cm / sec, a short circuit or a decrease in self-discharge performance may occur. Furthermore, the average fiber diameter of the separator is preferably 1 to 20 μm. When the average fiber diameter is less than 1 μm, the strength of the separator may decrease, and the defect rate in the battery assembling process may increase. On the other hand, when the average fiber diameter exceeds 20 μm, short circuit and self-discharge performance decrease. Sometimes.

The separator is preferably subjected to a hydrophilic treatment on the fiber surface. Examples of the hydrophilization treatment include sulfonation treatment, corona treatment, fluorine gas treatment, and plasma treatment. Among them, the separator whose fiber surface has been sulfonated has a high ability to adsorb impurities such as NO ₃ ⁻ , NO ₂ ⁻ , NH ₃ ^{− and the} like, which cause a shuttle phenomenon, and an elution element from the negative electrode. The suppression effect is high and preferable.

  Examples of the alkaline electrolyte include alkaline aqueous solutions containing potassium hydroxide, sodium hydroxide, lithium hydroxide and the like. The said alkaline electrolyte may be used independently and 2 or more types may be used together.

  The concentration of the alkaline electrolyte is preferably a total ion concentration of 9.0 M or less, more preferably 5.0 to 8.0 M.

  Various additives may be added to the alkaline electrolyte in order to improve oxygen overvoltage at the positive electrode, improve corrosion resistance of the negative electrode, improve self-discharge, and the like. Examples of such additives include oxides and hydroxides such as Y, Yb, Er, Ca, and Zn. These additives may be used independently and 2 or more types may be used together.

  When the nickel-metal hydride storage battery according to the present invention is an open-type nickel-metal hydride storage battery, for example, the battery is sandwiched between a negative electrode and a positive electrode via a separator, and the electrodes are fixed so that a predetermined pressure is applied to these electrodes. It can be produced by injecting an alkaline electrolyte and assembling an open cell.

  On the other hand, when the nickel-metal hydride storage battery according to the present invention is a sealed nickel-metal hydride storage battery, the battery is injected with an alkaline electrolyte before or after the positive electrode, the separator, and the negative electrode are stacked, and sealed with an exterior material. Can be manufactured. Further, in a sealed nickel-metal hydride storage battery in which a power generation element in which a positive electrode and a negative electrode are stacked via a separator is wound, an alkaline electrolyte is injected into the power generation element before or after the power generation element is wound. Is preferred. The method of injecting the alkaline electrolyte is not particularly limited, and may be injected at normal pressure. For example, a vacuum impregnation method, a pressure impregnation method, a centrifugal impregnation method, or the like may be used. Moreover, as an exterior material of a sealed nickel-metal hydride storage battery, for example, a material made of iron, stainless steel, polyolefin resin, or the like plated with a metal material such as iron or nickel can be cited.

  The embodiment of the sealed nickel-metal hydride storage battery is not particularly limited. For example, a battery including a positive electrode such as a coin battery, a button battery, a square battery, and a flat battery, a negative electrode, and a single-layer or multi-layer separator, a roll And a cylindrical battery including a positive electrode, a negative electrode and a separator.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

<Production of nickel metal hydride storage battery>
A nickel-metal hydride storage battery was produced by the method described below.

(1) Production of hydrogen storage alloy (Examples 1-5, Comparative Examples 2-4)
Each raw material ingot was weighed in a predetermined amount and put La, Ce, Pr, Nd, Al, and Mn so that the chemical composition was as shown in Table 1 below, and then Ni, Co, and Y were put in a crucible, The material was melted by heating to 1500 ° C. using a high-frequency melting furnace in a reduced-pressure argon gas atmosphere. After melting, the alloy was solidified by cooling at 50 ° C./second using a water-cooled mold. Next, the obtained alloy was heat-treated at 1000 ° C. for 5 hours in an argon gas atmosphere and then pulverized to obtain a hydrogen storage alloy powder having an average particle size (D50) of 50 μm. The average particle size was measured using an MT3000 apparatus manufactured by Microtrack.

(Comparative Example 1)
A hydrogen storage alloy powder was obtained in the same manner as in Examples 1 to 5 and Comparative Examples 2 to 4 except that the heating temperature in the high frequency melting furnace was changed to 1550 ° C.

(Reference Examples 1 and 2)
Each raw material ingot is weighed in a predetermined amount so that the chemical composition is as shown in Table 1 below, La and Al are added, then Ni and Y are put in a crucible, and a high frequency melting furnace is used in a reduced pressure argon gas atmosphere. The mixture was heated and melted to 1500 ° C., and MgNi ₂ and Ca were added and melted. After melting, the alloy was quenched by applying a melt spinning method to solidify the alloy at 1000 ° C./second. Next, the obtained alloy was heat-treated at 970 ° C. for 5 hours in an argon gas atmosphere and then pulverized to obtain a hydrogen storage alloy powder having an average particle size (D50) of 50 μm.

(2) Measurement of Y concentration distribution A hydrogen storage alloy ingot was solidified with resin, then polished with sandpaper and washed. In addition, you may use a powder or a nickel hydride storage battery electrode group instead of an ingot. Polishing is performed so that a cross section passing through the approximate center of the alloy ingot or particle is exposed, and the cross section is coated with Pt-Pd, and then 0.5 mm using an EPMA apparatus (model number 8705 manufactured by Shimadzu Corporation). Y concentration analysis was performed in the range of 0.5 mm. The results are shown in Table 1 below. The Y concentration and Ni concentration of the segregation phase relative to the parent phase were calculated by comparing the strength ratio. Moreover, as the ratio of the segregation phase of Y, the area ratio of the segregation phase distributed and precipitated in the matrix was calculated. Further, the size of the segregation phase of Y was calculated by measuring the average of the long side and the short side of the segregation phase distributed and precipitated in the matrix phase.

  As shown in Table 1 below, segregation of Y could be confirmed in the raw material ingot melted at 1500 ° C. Since the melting temperature was lower than the melting point temperature of Y, it was presumed that the homogenization was not sufficient and the segregation phase was precipitated. FIG. 2 shows the cut surface of the hydrogen storage alloy ingot of Example 1 imaged by the EPMA apparatus, and the segregation phase is distributed in a circled region.

(3) Production of an open-type nickel-metal hydride storage battery To 100 parts by mass of the hydrogen storage alloy powder obtained as described above, an aqueous solution in which 5 parts by mass of a conductive agent (nickel powder) and a thickener (methyl cellulose) are dissolved is added, Further, a paste obtained by adding 1 part by mass of a binder (styrene butadiene rubber) was applied to both sides of a 35 μm-thick perforated steel sheet (opening ratio 50%), dried, and then 0.33 mm thick. To obtain a negative electrode plate (500 mAh).

  Further, a sinter type nickel hydroxide electrode having a capacity three times as large as the negative electrode capacity was used for the positive electrode plate.

  Further, the negative electrode was sandwiched between the positive electrodes through the separator, these electrodes were fixed so that a force of 1 N was applied to these electrodes, and a 7M potassium hydroxide aqueous solution was injected to assemble an open cell.

<Charge / discharge test>
At 20 ° C., a charging / discharging cycle of charging for 15 hours at 0.1 ItA, resting for 1 hour, and discharging to −0.6 V with respect to the Hg / HgO reference electrode at 0.2 ItA was repeated 10 cycles. Thereafter, a charge / discharge cycle of charging at 20 ° C. for 45 minutes at 1 ItA, resting for 15 minutes, and discharging the Hg / HgO reference electrode to −0.6 V at 0.5 ItA was repeated 40 cycles.

<Battery disassembly and measurement of specific surface area>
The negative electrode after the charge / discharge test was taken out and washed with hot water at 100 ° C. until neutrality. After drying, the specific surface area value was determined using a specific surface area measuring device (Quantochrome monosorb, BET method). The results are shown in Table 1 below.

  In Table 1, the “difference in specific surface area value for each B / A ratio” refers to the specific surface area value of hydrogen storage alloys that do not contain Y between hydrogen storage alloys having substantially the same B / A ratio. When the specific surface area value is smaller than the standard hydrogen storage alloy, it is expressed as a negative value. When the specific surface area value is larger than the standard hydrogen storage alloy, Expressed as a positive value. Further, in Table 1, the case where “difference in specific surface area value for each B / A ratio” is blank indicates that it has not been measured.

  As a result of corrosion, rare earth hydroxide and the like are generated on the surface of the hydrogen storage alloy, and the form changes. Therefore, by measuring the specific surface area value, it can be used as an index of the alloy corrosion amount, and it can be said that the larger the specific surface area value, the larger the corrosion amount. It can be seen that an alloy having a segregation phase of Y has a smaller specific surface area value than an alloy having no segregation phase. This is presumably because when the hydrogen storage alloy comes into contact with the alkaline solution, the segregated Y preferentially elutes and quickly forms a yttrium hydroxide coating on the alloy surface. The effect was particularly great when the B / A ratio was 5.25 to 5.35. Moreover, even when the amount of Co was as small as 0.2 in the Co / A ratio, it was found that the corrosion resistance was improved by adding Y. Note that such an effect of Y is that a heavy rare earth element (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, whose ionic radius and atomic radius are close to Y and whose reaction order is almost the same as Y. ).

Claims (5)

A hydrogen storage alloy having a parent phase and a segregation phase containing La, Ni, and Y or heavy rare earth elements,
Parent phase, in the general formula _{R1 a R2 b Ni c Co d} R3 e ( wherein, R1 is La, and at least one element selected from rare earth elements except Y and heavy rare earth elements, the R2 Y And at least one element selected from the group consisting of heavy rare earth elements, R3 is at least one element of Mn or Al, and a, b, c, d and e are 0 <b <0.3. A + b = 1, 5.15 <c + d + e <5.45, 0 ≦ d ≦ 1, and 0 ≦ e ≦ 1).
The concentration of Y or heavy rare earth elements in the segregation phase is at least twice the concentration of Y or heavy rare earth elements in the matrix phase,
The concentration of Ni in the segregation phase is not more than 0.02 times the concentration of Ni in the matrix phase,
The segregation phase is dispersed in the matrix at a ratio of 0.05 to 2% with respect to the area of the matrix and has a size of 1 μm or more . In the general formula:
R1 is La and Ce;
R3 is Mn and / or Al,
2. The hydrogen storage alloy according to claim 1, wherein c, d, and e are 5.20 <c + d + e <5.45 and 0 ≦ d ≦ 0.45.   An electrode comprising the hydrogen storage alloy according to claim 1.   A nickel-metal hydride storage battery comprising the electrode according to claim 3 as a negative electrode. A method for producing a hydrogen storage alloy according to claim 1 or 2,
Melting the raw material metal below the melting point temperature of Y or heavy rare earth elements using a high frequency induction melting method, and alloying;
Cooling the obtained molten alloy;
A process for heat-treating the cooled alloy at 900 to 1080 ° C., and a method for producing a hydrogen storage alloy.