Patent Description:
Air secondary batteries, which use oxygen in air as the positive electrode active material, have attracted attention recently as high-efficient and clean energy conversion devices.

Among these, air hydrogen secondary batteries, in which an alkaline aqueous solution (alkali electrolyte solution) is used as the electrolyte solution and which includes, as the negative electrode, a hydrogen storage alloy capable of absorbing and releasing hydrogen as the negative electrode active material, have advantages described below, and thus are expected to be next-generation secondary batteries.

First, an air hydrogen secondary battery, in which oxygen in air is used as the positive electrode active material, does not require to reserve space for storing the positive electrode active material therein. Eliminating this space has an advantage of saving the space for the battery correspondingly. Alternatively, in the case where this space is not eliminated but utilized for storing a hydrogen storage alloy, the battery capacity of the air hydrogen secondary battery depends only on the capacity of the negative electrode, and thus, there is an advantage of enhancing the capacity in accordance with the increase in the amount of the hydrogen storage alloy. In other words, such an air hydrogen secondary battery may achieve a higher energy density than that of a nickel hydrogen secondary battery, which also employs a hydrogen storage alloy.

In an air secondary battery in which an alkali electrolyte solution is used as in the air hydrogen secondary battery described above, a charge and discharge reaction shown below occurs in the air electrode.

Discharge: O<NUM> + <NUM><NUM>O + 4e- → 4OH-     (I).

Charge: 4OH- → O<NUM> + <NUM><NUM>O + 4e-     (II).

The air electrode in the air hydrogen secondary battery reduces oxygen to produce hydroxide ions on discharging, as represented by the reaction formula (I) and produces oxygen and water on charging, as represented by the reaction formula (II). Oxygen generated in the air electrode is released into the atmosphere from the portion open to the atmosphere in the air electrode.

In the air electrode, which is the positive electrode of the air hydrogen secondary battery as described above, a pyrochlore-type oxide is used as a catalyst. Examples of this pyrochlore-type oxide include transition element oxides. For example, a bismuth-ruthenium oxide is known, as disclosed in Patent Document <NUM>. This bismuth-ruthenium oxide, which has catalytic activity on oxygen generation and oxygen reduction, is used in the positive electrode of an air hydrogen secondary battery.

This bismuth-ruthenium oxide is produced by a production method in which a precursor is produced by a coprecipitation method by use of bismuth nitrate and ruthenium chloride as the starting material and thereafter the precursor is calcined, for example.

Patent Document <NUM>: <CIT>
<NPL> discloses a new series of mixed metal oxides with the pyrochlore structure according to the general formula A<NUM>[B<NUM>-xAx]O<NUM>-y where A = Pb or Bi, B = Ru or Ir, <NUM> ≤ x ≤ <NUM> and <NUM> ≤ y ≤ <NUM>. Values of x appreciably greater than zero may only be obtained at relatively low temperatures. It has been observed that when A = Pb and B : Ru, y = <NUM> for all values of x. Linear correlations between lattice parameter and x are presented for both Pb<NUM>[Ru<NUM>-xPbx]O<NUM> and Bi<NUM>[Ru<NUM>-xBix]O<NUM>-y. The mixed metal oxides with the pyrochlore structure were obtained by carrying out solid state syntheses of a pyrochlore compound at temperatures between <NUM> and <NUM>. These syntheses involved hand grinding, with an agate mortar and pestle, the metal-containing reactants and then firing in air or oxygen at the specified temperatures. These firings were usually interrupted at frequent intervals for additional grinding in order to facilitate the reaction. The reactants employed were reagent grade oxides and/or nitrates of the appropriate metals. The reactants were generally combined in a post transition metal to noble metal molar ratio between <NUM>:<NUM> and <NUM>:<NUM>, depending upon the composition of the product desired. The reactant mixture often contained a post transition metal to noble metal ratio appreciably higher than the final reacted product. After reaction, the excess post transition metal oxide phase could be removed by selective leaching. For example, second phases of lead oxide were often removed by leaching in dilute acetic acid or concentrated caustic. <CIT> discloses a liquid solution method of preparing electrically conductive pyrochlore compounds of high surface area, having the formula A<NUM>[B<NUM>-xAx]O<NUM>-y is disclosed wherein A is selected from lead, bismuth and mixtures thereof, B is selected from ruthenium, iridium and mixtures thereof, <NUM> < x ≦ <NUM> and <NUM> ≦ y ≦ <NUM>. The method involves reacting A and B cations to yield a pyrochlore oxide by precipitation of A and B cations from an aqueous solution source of these cations in a liquid alkaline medium having a pH of at least about <NUM> in the presence of an oxygen source at a temperature below about <NUM>° C for a sufficient time for reaction to occur. <NPL> discloses the synthesis of a bismuth ruthenium pyrochlore by the dissolution of a metal chloride and a metal oxide in a mixture of a weak hydroxy carboxylic acid, such as citric acid, and a polyol. A clear solution was obtained by heating a mixture of citric acid and ethylene glycol (<NUM>:<NUM> molar ratio) at <NUM>, RuCI<NUM>. <NUM><NUM>O and Bi<NUM>O<NUM> were added in appropriate amount. The solution was heated at <NUM> and Bi<NUM>Ru<NUM>O<NUM> was obtained. Several samples were prepared by heating the Bi<NUM>Ru<NUM>O<NUM> at temperatures ranging from <NUM> to <NUM>. <CIT> discloses a liquid solution method of preparing electrically conductive pyrochlore compounds having the formula A<NUM>[B<NUM>-xAx]O<NUM>-y, wherein A is selected from lead, bismuth and mixtures thereof, B is selected from ruthenium, iridium and mixtures thereof, <NUM>≦×≦<NUM> and <NUM>≦y≦<NUM>. The method involves reacting A and B cations to yield a pyrochlore oxide by precipitation of A and B cations in a liquid medium having a pH which is equal to or greater than <NUM>, but less than <NUM>, in the presence of an oxygen source at a temperature below about <NUM>° C. for a time sufficient for reaction to occur. In those instances in which amorphous reaction products are obtained, these amorphous reaction products are subsequently heat treated at a temperature of about <NUM>° C. to about <NUM>° C. for a time sufficient to convert amorphous reaction products to crystalline pyrochlore(s).

In the method for producing the bismuth-ruthenium oxide described above, by-products are formed in the process thereof. When an air electrode is produced with a catalyst including by-products mixed therein, and charge and discharge cycles are performed on an air hydrogen secondary battery including the air electrode, the bismuth-ruthenium oxide itself does not cause a dissolution and precipitation reaction, but the by-products described above cause the dissolution and precipitation reaction. Particularly, the dissolution and precipitation reaction of the metal components (principally bismuth) in the by-products is repeated due to a chemical reaction on charging and discharging in the battery (hereinbelow, referred to as a battery reaction), and socalled dendritic growth occurs, in which the metal components are dendritically precipitated on the electrode plates. When the metal components thus dendritically grow, the metal components extend into the separator and penetrate the separator in the end. As a result, micro short-circuiting problematically occurs. When the micro short-circuiting thus occurs, there is not only ion conductivity via the electrolyte, but also electron conductivity, between the positive electrode and the negative electrode inside the battery. In the case where electron conductivity exists, the battery is self-discharging. The dendritic growth of the metal components increases in accordance with the charge and discharge cycle. Thus, the amount self-discharged increases as the charge and discharge cycle proceeds. As a result, the discharge capacity of the battery decreases with a relatively small number of cycles, and the battery life is expired in the early stage.

For this reason, it is demanded to develop an air secondary battery that is unlikely to be subjected to decrease in the discharge capacity than before and has a stable discharge capacity, even when charging and discharging are repeated.

The present invention has been achieved in view of the above-described circumstances, and it is an object thereof to provide a method for producing a catalyst for an air secondary battery, a method for producing an air secondary battery, a catalyst for an air secondary battery, and an air secondary battery that are capable of preventing occurrence of micro short-circuiting.

In order to achieve the object described above, according to the present invention, there is provided a method for producing a catalyst for an air secondary battery for use in the air electrode of the air secondary battery according to claim <NUM>, a method for producing an air secondary battery according to claim <NUM>, a catalyst for use in an air electrode of the air secondary battery according to claim <NUM>, and an air secondary battery according to claim <NUM>. Preferred embodiments are set forth in the subclaims.

The method for producing a catalyst for an air secondary battery according to the present invention includes an acid treatment step of immersing the pyrochlore-type oxide in an acid aqueous solution to apply an acid treatment. Subjecting the oxide to this acid treatment step enables removal of by-products of the pyrochlore-type oxide to thereby prevent dendrites from occurring due to a dissolution and precipitation reaction of the metal components in the by-products. Thus, according to the present invention, it is possible to provide a method for producing a catalyst for an air secondary battery capable of preventing occurrence of micro short-circuiting.

Hereinbelow, an air hydrogen secondary battery (hereinbelow, simply referred to as the battery) <NUM> incorporating an air electrode including a catalyst for an air secondary battery according to the present invention will be described with reference to the drawings.

As shown in <FIG>, the battery <NUM> is formed by sandwiching an electrode group <NUM> accommodated in a container <NUM> between a top plate <NUM> and a bottom plate <NUM>.

The electrode group <NUM> is formed by stacking an air electrode (positive electrode) <NUM> on a negative electrode <NUM> with a separator <NUM> therebetween.

The negative electrode <NUM> includes a conductive negative electrode substrate that forms a porous structure and has a large number of pores and a negative electrode mixture supported inside the pores and the surface of the negative electrode substrate.

As such a negative electrode substrate, foam nickel can be used, for example.

A negative electrode mixture includes a hydrogen storage alloy powder, which is an assembly of hydrogen storage alloy particles that can absorb and release hydrogen, as a negative electrode active agent, a conductive material, and a binder. Here, as the conductive agent, graphite, carbon black, or the like can be used.

As the hydrogen storage alloy constituting the hydrogen storage alloy particles, which is not particularly limited, a rare earth-Mg-Ni-based hydrogen storage alloy is used. The composition of this rare earth-Mg-Ni-based hydrogen storage alloy can be optionally selected. For example, one represented by the general formula:.

In general formula (III), Ln represents at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, Zr, and Ti, M represents at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B, subscripts a, b, x, and y respectively represent a number satisfying <NUM> ≤ a ≤ <NUM>, <NUM> ≤ b ≤ <NUM>, <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y ≤ <NUM>.

Hydrogen storage alloy particles are obtained as follows, for example.

First, the metal raw materials are weighed and mixed to achieve a predetermined composition. This mixture is dissolved under an inert gas atmosphere in a high-frequency induction melting furnace, for example, to form an ingot. The ingot obtained is heated to <NUM> to <NUM> under an inert gas atmosphere. The ingot is subjected to a thermal treatment at this temperature for <NUM> to <NUM> hours and thus homogenized. Thereafter, this ingot is pulverized and sieved to thereby obtain a hydrogen storage alloy powder, which is an assembly of hydrogen storage alloy particles having a desired particle size.

Examples of a binder to be used include sodium polyacrylate, carboxymethyl cellulose, and styrene butadiene rubber.

The negative electrode <NUM> here can be produced as follows, for example.

First, a hydrogen storage alloy powder, which is an assembly of hydrogen storage alloy particles, a conductive agent, a binder, and water are kneaded to prepare a negative electrode mixture paste. The negative electrode mixture paste obtained is packed into a negative electrode substrate and then dried. After drying, the negative electrode substrate having the hydrogen storage alloy particles and the like attached thereto is rolled to increase the alloy content per volume. Thereafter, the rolled substrate is cut to thereby produce the negative electrode <NUM>. This negative electrode <NUM> is in a plate form as a whole.

Next, the air electrode <NUM> includes a conductive air electrode substrate that forms a porous structure and has a large number of pores and an air electrode mixture (positive electrode mixture) carried inside the pores and the surface of the air electrode substrate.

As such an air electrode substrate, foam nickel or nickel mesh can be used, for example.

The air electrode mixture includes a catalyst for an air secondary battery, a conductive agent, and a binder.

As the catalyst for an air secondary battery, used is a pyrochlore-type oxide subjected to an acid treatment of immersion in an acidic aqueous solution. According to the present invention, as the pyrochlore-type oxide to be subjected to the acid treatment, a pyrochlore-type transition element oxide having a composition represented by the general formula, as defined in claim <NUM>, is used.

The catalyst for an air secondary battery is produced as follows.

First, a pyrochlore-type bismuth-ruthenium oxide is prepared. The preparation is specifically as follows.

Bi(NO<NUM>)<NUM>·<NUM><NUM>O and RuCl<NUM>·<NUM><NUM>O were introduced at the same concentration into distilled water and stirred to prepare a mixed aqueous solution of Bi(NO<NUM>)<NUM>·<NUM><NUM>O and RuCl<NUM>·<NUM><NUM>O. The temperature of the distilled water at this time is set at <NUM> or more and <NUM> or less. Then, a <NUM> mol%/l or more and <NUM> mol%/l or less NaOH aqueous solution is added to this mixed aqueous solution. While the bath temperature at this time is maintained at <NUM> or more and <NUM> or less, the solution is stirred under oxygen bubbling. While the solution containing a precipitate generated by this operation is maintained at <NUM> or more and <NUM> or less, a portion of the moisture is evaporated to form a paste. This paste is transferred to an evaporating dish and heated to <NUM> or more and <NUM> or less. The paste is dried while maintained in the state for <NUM> hours or more and <NUM> hours or less to obtain a dried product of the paste. After pulverized in a mortar, this dried product is heated under an air atmosphere to <NUM> or more and <NUM> or less. The dried product is calcined while maintained for <NUM> hours or more and <NUM> hours or less to obtain a calcined product. The calcined product obtained is washed with distilled water at <NUM> or more and <NUM> or less and then dried. This results in a pyrochlore-type bismuth-ruthenium oxide.

Subsequently, the bismuth-ruthenium oxide prepared is subjected to a nitric acid treatment as the acid treatment, wherein the oxide is immersed in a nitric acid aqueous solution. The treatment is specifically as follows.

First, a nitric acid aqueous solution is provided. Here, the concentration of the nitric acid aqueous solution is preferably <NUM> mol%/l or more and <NUM> mol%/l or less. The amount of the nitric acid aqueous solution to be provided is preferably <NUM> per <NUM> of a bismuth-ruthenium oxide. The temperature of the nitric acid aqueous solution is preferably set at <NUM> or more and <NUM> or less.

Then, in the nitric acid aqueous solution provided, the bismuth-ruthenium oxide is immersed and stirred for <NUM> minutes or more and <NUM> hours or less. After a predetermined time period elapses, the bismuth-ruthenium oxide is filtered with suction from the nitric acid aqueous solution. The bismuth-ruthenium oxide filtered is introduced in and washed with ion exchanged water set at <NUM> or more and <NUM> or less.

The washed bismuth-ruthenium oxide is maintained under a reduced pressure environment of a room temperature (<NUM>) for <NUM> hours or more and <NUM> hours or less and dried. Note that, for drying the washed bismuth-ruthenium oxide, a drying condition for drying the oxide while the oxide is maintained in air under a temperature environment of <NUM> to <NUM> for one hour or more and <NUM> hours or less may be employed.

In the manner as mentioned above, the bismuth-ruthenium oxide subjected to the nitric acid treatment is obtained. Applying the nitric acid treatment as mentioned above enables removal of by-products generated during the production process of the pyrochlore-type oxide. Note that the acidic aqueous solution used in the acid treatment is not limited to nitric acid aqueous solutions and hydrochloric acid aqueous solution and sulfuric acid aqueous solution can be used in addition to nitric acid aqueous solutions. In these hydrochloric acid aqueous solutions and sulfuric acid aqueous solutions, an effect of enabling by-products to be removed can be provided as in the case of nitric acid aqueous solutions.

Subsequently, as the conductive agent, which is not particularly limited, for example, a nickel powder, which is an assembly of nickel particles, is preferably used.

The binder binds a redox catalyst and additionally serves to impart suitable water repellency to the air electrode <NUM>. Here, examples of the binder include, but are not particularly limited to, fluorine resins. Note that, as a preferable fluorine resin, polytetrafluoroethylene (PTFE) is used, for example.

The air electrode <NUM> can be produced as follows, for example.

First, prepared is an air electrode mixture paste including a bismuth-ruthenium oxide, a binder, and water.

The air electrode mixture paste obtained is shaped into a sheet form and then is pressure-bonded by a press onto nickel mesh (an air electrode substrate). Thereby, an intermediate product for an air electrode is obtained.

The intermediate product obtained is then introduced in a calciner and subjected to a calcining treatment. This calcining treatment is carried out under an inert gas atmosphere. As this inert gas, nitrogen gas or argon gas is used, for example. As the calcining treatment conditions, the intermediate product is heated to a temperature of <NUM> or more and <NUM> or less and maintained in this state for <NUM> minutes or more and <NUM> minutes or less. Thereafter, the intermediate product is naturally cooled in the calciner and taken out in the atmosphere when the temperature of the intermediate product is lowered to <NUM> or less. Thereby, obtained is an intermediate product subjected to the calcining treatment. The air electrode <NUM> is obtained by cutting this intermediate product into a predetermined shape.

The air electrode <NUM> and the negative electrode <NUM> obtained as described above are stacked with the separator <NUM> therebetween to thereby form the electrode group <NUM>. This separator <NUM> is provided to prevent short-circuiting between the air electrode <NUM> and the negative electrode <NUM>, and an electrically insulating material is employed therefor. As the material employed for this separator <NUM>, non-woven fabric of polyamide fibers to which hydrophilic functional groups are imparted, and non-woven fabric of polyolefin fibers such as polyethylene and polypropylene to which hydrophilic functional groups are imparted can be used.

The electrode group <NUM> formed is accommodated along with an alkali electrolyte solution into the container <NUM>. The container <NUM> is not particularly limited as long as the container <NUM> can accommodate the electrode group <NUM> and the alkali electrolyte solution, and as an example thereof, a bagshaped container made of polyethylene (hereinbelow, referred to as the accommodating bag <NUM>) is used. In this accommodating bag <NUM>, for example, an insertion/ejection port having a fastener (not shown) is provided in a portion thereof, and an opening <NUM> is provided in another portion thereof. The electrode group <NUM> is accommodated through the insertion/ejection port described above in the accommodating bag <NUM>.

When the electrode group <NUM> is accommodated into the accommodating bag <NUM>, carbon non-woven fabric <NUM> is disposed on the air electrode <NUM> side of the electrode group <NUM> so as to be in contact with the air electrode <NUM>. This carbon non-woven fabric <NUM> has been subjected to a water repellent treatment with PTFE. Additionally, a separator <NUM> is disposed on the negative electrode <NUM> side of the electrode group <NUM> so as to be in contact with the negative electrode <NUM>. As this separator <NUM>, one made of the same material and having the same shape as that of separator <NUM> described above is used, for example.

Here, the electrode group <NUM> accommodated into the accommodating bag <NUM> is mounted on the separator <NUM> disposed on the negative electrode side, as shown in <FIG>. Then, the carbon non-woven fabric <NUM> is disposed on the air electrode <NUM> of the electrode group <NUM>. Then, only the carbon non-woven fabric <NUM> is exposed from the opening <NUM> of the accommodating bag <NUM>.

Subsequently, the electrode group <NUM> accommodated into the accommodating bag <NUM> as described above is sandwiched between the top plate <NUM> and the bottom plate <NUM>.

The top plate <NUM> is a plate material made of an acryl resin, having a venting passage <NUM> at a position opposite to the opening <NUM> of the accommodating bag <NUM>, as shown in <FIG>. This venting passage <NUM> entirely takes one serpentine shape, and the ends of the passage <NUM> are open to the atmosphere.

The bottom plate <NUM> is a plate material made of an acryl resin, having the same size as the top plate <NUM>. Note that the bottom plate <NUM> has no venting passage.

The accommodating bag <NUM> accommodating the electrode group <NUM> therein is mounted on the bottom plate <NUM> with a flat negative electrode side buffer plate <NUM> formed of a resin interposed therebetween. Then, the top plate <NUM> is mounted on the accommodating bag <NUM> accommodating the electrode group <NUM>. The electrode group <NUM> accommodated in the accommodating bag <NUM> is thus sandwiched vertically by the top plate <NUM> and the bottom plate <NUM>. At this time, the venting passage <NUM> of the top plate <NUM> is opposite to the carbon non-woven fabric <NUM>. The carbon non-woven fabric <NUM> allows gas to permeate therethrough but blocks moisture, and thus the air electrode <NUM> is open to the atmosphere via the carbon non-woven fabric <NUM> and the venting passage <NUM>. That is, the air electrode <NUM> is brought into contact with the atmosphere through the carbon non-woven fabric <NUM>.

As for the top plate <NUM> and the bottom plate <NUM> vertically sandwiching the electrode group <NUM> accommodated in the accommodating bag <NUM>, the peripheral edge <NUM> of the top plate <NUM> and the peripheral edge <NUM> of the bottom plate <NUM> are clamped vertically by connectors <NUM> and <NUM>, as schematically depicted in <FIG>. The battery <NUM> is thus formed.

Here, in this battery <NUM>, an air electrode lead (positive electrode lead) <NUM> is electrically connected to the air electrode (positive electrode) <NUM>, and a negative electrode lead <NUM> is electrically connected to the negative electrode <NUM>. The air electrode lead <NUM> and negative electrode lead <NUM>, which are schematically depicted in <FIG>, are drawn outside the accommodating bag <NUM> while air-tightness and water-tightness are maintained. Then, an air electrode terminal (positive electrode terminal) <NUM> is provided on the tip of air electrode lead <NUM>, and a negative electrode terminal <NUM> is provided on the tip of the negative electrode lead <NUM>. Accordingly, in the battery <NUM>, these air electrode terminal <NUM> and negative electrode terminal <NUM> are used to input and output electric currents on charging and discharging.

As a first step, a predetermined amount of Bi(NO<NUM>)<NUM>·<NUM><NUM>O and RuCl<NUM>·<NUM><NUM>O was provided. These Bi(NO<NUM>)<NUM>·<NUM><NUM>O and RuCl<NUM>·<NUM><NUM>O were introduced at the same concentration into distilled water at <NUM> and stirred to prepare a mixed aqueous solution of Bi(NO<NUM>)<NUM>·<NUM><NUM>O and RuCl<NUM>·<NUM><NUM>O. Then, a <NUM> mol%/l NaOH aqueous solution was added to this mixed aqueous solution. The bath temperature at this time was set at <NUM>, and the solution was stirred under oxygen bubbling. While the solution containing a precipitate generated by this operation was maintained at <NUM>, a portion of the moisture was evaporated to form a paste. This paste was transferred to an evaporating dish and heated to <NUM>. This paste was dried while maintained in the state for <NUM> hours to obtain a dried product of the paste (precursor). Then, as a second step, this dried product was pulverized in a mortar then heated under an air atmosphere to <NUM>, calcined while maintained for <NUM> hour to obtain a calcined product. The calcined product obtained was washed with distilled water at <NUM>, then filtered with suction, and dried. This resulted a pyrochlore-type bismuth-ruthenium oxide.

The bismuth-ruthenium oxide obtained was pulverized using a mortar to obtain a bismuth-ruthenium oxide powder, which was an assembly of particles having a predetermined particle size. As a result of observation on a secondary electron image of this bismuth-ruthenium oxide powder obtained using a scanning electron microscope, the bismuth-ruthenium oxide had a particle size of <NUM> or less.

Subsequently, as a third step, <NUM> of the bismuth-ruthenium oxide powder, along with <NUM> of nitric acid aqueous solution, was placed in the stirring tank of a stirrer and stirred for <NUM> hours while the temperature of the nitric acid aqueous solution was maintained at <NUM>. Here, the concentration of the nitric acid aqueous solution was set at <NUM> mol%/l.

After the stirring was completed, the bismuth-ruthenium oxide powder was taken out from the nitric acid aqueous solution by filtering with suction. The bismuth-ruthenium oxide powder taken out was washed with <NUM> liter of ion exchanged water heated to <NUM>. After washing, the bismuth-ruthenium oxide powder was placed in a reduced pressure vessel at room temperature of <NUM> and dried while maintained under a reduced pressure environment for <NUM> hours.

In the manner as mentioned above, obtained was a bismuth-ruthenium oxide powder subjected to a nitric acid-treatment, that is, a catalyst for an air secondary battery.

The bismuth-ruthenium oxide powder subjected to the nitric acid treatment, a nickel powder, a polytetrafluoroethylene (PTFE) dispersion, and ion exchanged water were uniformly mixed at a mass ratio of <NUM>:<NUM>:<NUM>:<NUM> to produce an air electrode mixture paste.

The air electrode mixture paste obtained was shaped into a sheet form. The air electrode mixture paste in a sheet form was pressure-bonded by a press onto nickel mesh having a mesh number of <NUM>, a wire diameter of <NUM>, and an opening ratio of <NUM>%.

The air electrode mixture pressure-bonded on the nickel mesh was heated to <NUM> under a nitrogen gas atmosphere and calcined while maintained at this temperature for <NUM> minutes. After the calcination, the calcined product was cut to a size of <NUM> in length and <NUM> in width to thereby obtain an air electrode <NUM>. The air electrode <NUM> had a thickness of <NUM>.

Metal materials of Nd, Mg, Ni, and Al were mixed at a predetermined molar ratio, then introduced in a high-frequency induction melting furnace, and melted under an argon gas atmosphere. The melted metal obtained was poured into a mold and cooled to room temperature of <NUM> to produce an ingot.

Subsequently, this ingot was subjected to a thermal treatment by being maintained under an argon gas atmosphere at a temperature of <NUM> for <NUM> hours. Then, the ingot was pulverized mechanically under an argon gas atmosphere to obtain a rare earth-Mg-Ni-based hydrogen storage alloy powder. The volume average particle size (MV) of the rare earth-Mg-Ni-based hydrogen storage alloy powder obtained was measured by a laser diffraction-scattering particle size distribution analyzer. As a result, the volume average particle size (MV) was <NUM>.

The composition of this hydrogen storage alloy powder was analyzed by inductively coupled high-frequency plasma spectroscopy (ICP) to find that the composition was Nd<NUM>Mg<NUM>Ni<NUM>Al<NUM>.

To <NUM> parts by mass of the hydrogen storage alloy powder obtained were added <NUM> parts by mass of a sodium polyacrylate powder, <NUM> parts by mass of a carboxymethyl cellulose powder, <NUM> parts by mass of a dispersion of styrene butadiene rubber, <NUM> parts by mass of a carbon black powder, and <NUM> parts by mass of water, and the mixture was kneaded under an environment of <NUM> to prepare a negative electrode mixture paste.

This negative electrode mixture paste was packed into a foam nickel sheet having an areal density (basis weight) of about <NUM>/m<NUM> and a thickness of about <NUM> and dried to obtain a foam nickel sheet packed with the negative electrode mixture. The sheet obtained was rolled to increase the alloy content per volume, and cut to a size of <NUM> in length and <NUM> in width to thereby obtain a negative electrode <NUM>. The negative electrode <NUM> had a thickness of <NUM>.

Next, the negative electrode <NUM> obtained was subjected to an activation treatment. The procedure of this activation treatment is shown below.

First, a common sintered nickel hydroxide positive electrode was provided. Note that, as this nickel hydroxide positive electrode, provided was one having a positive electrode capacity sufficiently larger than the negative electrode capacity of the negative electrode <NUM>. Then, this nickel hydroxide positive electrode and the negative electrode <NUM> obtained were stacked with a separator formed of polyethylene non-woven fabric interposed therebetween to form an electrode group for an activation treatment. This electrode group for an activation treatment, along with a predetermined amount of an alkali electrolyte solution, was accommodated in a container made of an acrylic resin. Thereby, a single electrode cell for a nickel hydride secondary battery was formed.

As an initial charge and discharge operation, this single electrode cell was left to stand under an environment at a temperature of <NUM> for <NUM> hours, then charged at <NUM> It for <NUM> hours, and then discharged at <NUM> It until the battery voltage reached <NUM> V. Next, as a second charge and discharge operation, under an environment at a temperature of <NUM>, the single electrode cell, after left to stand for <NUM> hours, was charged at <NUM> It for <NUM> hours and then discharged at <NUM> It until the battery voltage reached <NUM> V. The second charge and discharge operation described above was taken as one cycle. In the second and later operations, the negative electrode <NUM> was subjected to an activation treatment by performing this charge and discharge cycle in a plurality of times. In each charge and discharge cycle, the capacity of the single electrode cell was determined. Then, the maximum value of the capacities obtained was taken as the capacity of the negative electrode. The negative electrode had a capacity of <NUM> mAh.

Thereafter, the single electrode cell was charged at <NUM> It for <NUM> hours and then, the negative electrode <NUM> was removed from the single electrode cell. In this manner, obtained was a negative electrode <NUM> subjected to the activation treatment and charging.

The air electrode <NUM> and the negative electrode <NUM> obtained were stacked with a separator <NUM> sandwiched therebetween to produce an electrode group <NUM>. The separator <NUM> used for the production of this electrode group <NUM> was formed of non-woven fabric made of polypropylene fiber having a sulfone group and had a thickness of <NUM> (basis weight <NUM>/m<NUM>).

Subsequently, an accommodating bag for evaluation <NUM> was provided, and the electrode group <NUM> described above was accommodated into this accommodating bag <NUM>. In this accommodating bag <NUM>, which is a bag made of polyethylene, for example, an insertion/ejection port having a fastener (not shown) is provided in a portion thereof, and an opening <NUM> of <NUM> in length and <NUM> in width is provided in another portion thereof.

The electrode group <NUM> was accommodated through the insertion/ejection port described above in the accommodating bag <NUM>. In the accommodating bag <NUM>, a separator <NUM>, different from the separator <NUM>, was disposed below the electrode group <NUM> (below the negative electrode <NUM>). Additionally, carbon non-woven fabric (<NUM> in length, <NUM> in width, and <NUM> in thickness) <NUM> subjected to a water repellent treatment with PTFE was disposed above the electrode group <NUM> (above the air electrode <NUM>). Then, the peripheral portion of the opening <NUM> of the accommodating bag <NUM> was brought into a close contact with the carbon non-woven fabric <NUM>, and only the carbon non-woven fabric <NUM> was exposed through the opening <NUM> from the accommodating bag <NUM>. Then, <NUM> of an alkali electrolyte solution (<NUM> mol%/l KOH aqueous solution) was poured through the insertion/ejection port. Thereafter, the fastener of the insertion/ejection port was closed, and the inside of the accommodating bag <NUM> was defoamed under reduced pressure.

The electrode group <NUM> accommodated in the accommodating bag <NUM> in the state described above was sandwiched along with the accommodating bag <NUM> between the top plate <NUM> and the bottom plate <NUM>. At this time, a negative electrode side buffer plate <NUM> was interposed between the accommodating bag <NUM> and the bottom plate <NUM>. Then, the top plate <NUM> and the bottom plate <NUM> were connected and fixed with connectors <NUM> and <NUM>. Here, the top plate <NUM> is a plate material made of an acryl resin, having a venting passage <NUM>, the ends of which are open to the atmosphere. This venting passage <NUM> entirely takes one serpentine shape having a width of <NUM>, an end width of <NUM>, a depth of <NUM>, and a peak width of <NUM>. This venting passage <NUM> faces the carbon non-woven fabric <NUM> via the opening <NUM>. The bottom plate <NUM> is a plate material made of an acryl resin, having the same size as the top plate <NUM> and having no venting passage.

In the manner described above, a battery <NUM> was produced, as shown in <FIG>. The battery <NUM> obtained was left to stand under an environment at a temperature of <NUM> for <NUM> hours to allow the alkali electrolyte solution to permeate the electrode group <NUM>.

Note that an air electrode lead <NUM> is electrically connected to the air electrode <NUM> and a negative electrode lead <NUM> is electrically connected to the negative electrode <NUM>. The air electrode lead <NUM> and negative electrode lead <NUM> are appropriately extend from the inner side to the outside of the accommodating bag <NUM> while air-tightness and water-tightness of the accommodating bag <NUM> are maintained. Additionally, an air electrode terminal <NUM> is attached to the tip of the air electrode lead <NUM>, and a negative electrode terminal <NUM> is attached to the tip of the negative electrode lead <NUM>.

The battery <NUM> obtained was discharged via the air electrode terminal <NUM> and the negative electrode terminal <NUM> under a condition where the current value per unit area of the air electrode <NUM> reached <NUM> mA/cm<NUM> to provide a battery <NUM> before property evaluation.

An air hydrogen secondary battery was produced in the same manner as in Example <NUM> except that no nitric acid treatment was conducted on the bismuth-ruthenium oxide and the bismuth-ruthenium oxide not subjected to a nitric acid treatment was used.

A portion of the bismuth-ruthenium oxide powder subjected to the nitric acid treatment in Example <NUM> and a portion of the bismuth-ruthenium oxide powder not subjected to the nitric acid treatment in Comparative Example <NUM> were reserved in advance as analysis samples. The analysis samples were subjected to an X-ray diffraction (XRD) analysis. A parallel beam X-ray diffraction apparatus was used for the analysis. The analysis conditions here included an X-ray source of CuKα, a tube voltage of <NUM> kV, a tube current of <NUM> mA, a scan speed of <NUM> degrees/min, and a step width of <NUM> degrees. The profiles of the analysis results are shown in <FIG>.

In the profiles of the analysis results, the peak of portions marked with a triangle are the peaks of by-products. From the profiles of the analysis results, it can be seen that, from the catalyst for an air secondary battery according to Comparative Example <NUM>, which was not subjected to the nitric acid treatment, by-products were generated.

In contrast, in Example <NUM>, the peaks of by-products disappeared. The catalyst for an air secondary battery of Example <NUM> was subjected to the nitric acid treatment, and it is conceived that this nitric acid treatment removed by-products having high crystallinity. Also in Example <NUM>, the background intensity has been lowered entirely compared with Comparative Example <NUM>, and it is conceived that by-products seeming to be amorphous have been removed simultaneously.

A portion of the bismuth-ruthenium oxide powder subjected to the nitric acid treatment in Example <NUM> and a portion of the bismuth-ruthenium oxide powder not subjected to the nitric acid treatment in Comparative Example <NUM> were reserved in advance as analysis samples. The analysis samples were observed with a scanning electron microscope (SEM), and simultaneously, the composition of the samples was analyzed using an energy dispersive X-ray spectrometer (EDS).

A SEM image (magnification: <NUM> times) photograph of the analysis result of Example <NUM> is shown in <FIG>, and a SEM image (magnification: <NUM> times) photograph of the analysis result of Comparative Example <NUM> is shown in <FIG>. From the SEM images of the analysis results, a composition analysis was conducted based on area mapping. The composition of elements detected with the composition analysis is shown in Table <NUM>. The ratio of the amount of bismuth to the amount of ruthenium (Bi/Ru) was also shown.

The ratio of the amount of bismuth to the amount of ruthenium (Bi/Ru) in the catalyst for an air secondary battery of Example <NUM> was <NUM>. Meanwhile, the amount of bismuth to the amount of ruthenium (Bi/Ru) in the catalyst for an air secondary battery of Comparative Example <NUM> was <NUM>. That is, the ratio of the amount of bismuth to the amount of ruthenium in Example <NUM> is lower than that in Comparative Example <NUM>. It is conceived that this resulted from removal of the by-products by the nitric acid treatment.

From those described above, it can be seen that the Bi/Ru value is <NUM> when the bismuth-ruthenium oxide is not subjected to the nitric acid treatment, and when the bismuth-ruthenium oxide is subjected to the nitric acid treatment, the by-products are removed, and the Bi/Ru value falls below <NUM>.

Each of the batteries before property evaluation according to Example <NUM> and Comparative Example <NUM>, under an environment at a temperature of <NUM>, after left to stand for three hours, was charged for <NUM> hours at a charging current at <NUM> mA, and thereafter, left to stand for <NUM> minutes.

Then, the battery after left to stand for <NUM> minutes, under the same environment, was discharged at a discharging current at <NUM> mA until the battery voltage reached <NUM> V, and left to stand for <NUM> minutes.

The charging and discharging cycle described above was taken as one cycle, and <NUM> cycles were repeated.

Note that <NUM>/minute of air was continuously allowed to flow through the venting passage <NUM> irrespective of charging and discharging.

The charge capacity on charging and the discharge capacity on discharging were determined in each cycle. From the charge capacity and the discharge capacity obtained, the percentage of the discharge capacity to the charge capacity in each cycle was determined as the discharge capacity ratio. Then, the variation in the discharge capacity was determined from the relationship between the discharge capacity ratio and the number of cycles. The results were shown in <FIG>.

Additionally, the relationship between the battery voltage and the elapsed time was determined in the pause state after the charge of the 20th cycle was conducted. From this relationship between the battery voltage and the elapsed time, the variation in the voltage was determined. The results were shown in <FIG>.

In the graph of the variation in the discharge capacity ratio according to the battery of Example <NUM>, the discharge capacity ratio exhibits substantially constant values of the order of <NUM> to <NUM>% and stable, even when the charge and discharge cycle proceeds. In other words, the amount discharged of the battery of Example <NUM> is substantially equal to the amount charged, and additionally, the state is maintained even when the charge and discharge cycle proceeds. In other words, self-discharging in the battery of Example <NUM> is kept low. Conceivably, this is because micro short-circuiting has not occurred in the battery of Example <NUM>.

In contrast, in the graph of the variation in the discharge capacity ratio according to the battery of Comparative Example <NUM>, the discharge capacity ratio decreases as the charge and discharge cycle proceeds. In other words, the amount discharged is lower than the amount charged in the battery of Comparative Example <NUM>, and the battery is self-discharging. Conceivably, this is because micro short-circuiting has occurred in the battery of Comparative Example <NUM>.

In the graph of the variation in the voltage according to the battery of Example <NUM>, the value of the battery voltage lingers at of the order of <NUM> V and is stable with no sharp drop. Conceivably, this is because micro short-circuiting has not occurred in the battery of Example <NUM>.

In contrast, in the graph of the variation in the voltage according to the battery of Comparative Example <NUM>, the value of the battery voltage sharply drops immediately after the pause state is started. Conceivably, this is because micro short-circuiting has occurred in the battery of Comparative Example <NUM>.

The batteries of Example <NUM> and Comparative Example <NUM>, after <NUM> cycles of charging and discharging were repeated in the property analysis of the air hydrogen secondary batteries described above, were disassembled, and the separators were removed. A portion was cut off from each of the separator according to Example <NUM> and the separator according to Comparative Example <NUM> to take analysis samples. The analysis samples obtained were subjected to an X-ray diffraction (XRD) analysis. A parallel beam X-ray diffraction apparatus was used for the analysis. The analysis conditions here included an X-ray source of CuKα, a tube voltage of <NUM> kV, a tube current of <NUM> mA, a scan speed of <NUM> degrees/min, and a step width of <NUM> degrees. The profiles of the analysis results is shown in <FIG>.

Additionally, an analysis sample of an unused separator was prepared and subjected to an X-ray diffraction (XRD) analysis under the same conditions as described above. The profile of the analysis results obtained is also shown in <FIG>.

The profile of the separator according to Comparative Example <NUM> has peaks at portions marked with a black solid triangle. These peaks correspond to the peaks of bismuth. In other words, it can be seen that bismuth is deposited on the separator according to Comparative Example <NUM>. Thus, it is conceived that by-products remained in Comparative Example <NUM> not subjected to the nitric acid treatment, bismuth included in the by-products caused a dissolution and precipitation reaction due to the battery reaction, and the bismuth dendritically grew to extend into the separator. It is conceived that this bismuth extended into the separator is responsible for the micro short-circuiting.

The profile of the separator according to Example <NUM> has no peak at the portions marked with a black solid triangle. The profile of the unused separator substantially coincides with the profile of the separator of Example <NUM>. In the separator of Example <NUM>, the initial state is maintained even when charging and discharging are repeated. From those described above, it can be said that no bismuth is deposited on the separator according to Example <NUM>. It is conceived that this is caused by removal of the by-products by the nitric acid treatment.

From those described above, it can be said that deposition of bismuth does not occur to thereby prevent micro short-circuiting from occurring as long as the by-products are removed by the nitric acid treatment. When the bismuth-ruthenium oxide is subjected to the nitric acid treatment and the Bi/Ru value reaches <NUM> or less, it is conceived that the by-products are removed from the oxide such that an effect of preventing deposition of bismuth is achieved. For this reason, it is conceived that the bismuth-ruthenium oxide is subjected to the nitric acid treatment and the value of Bi/Ru is <NUM> or less and as low as possible, measured by using an energy dispersive X-ray spectrometer (EDS). However, when the value of Bi/Ru is less than <NUM>, the crystalline structure of the bismuth-ruthenium oxide may change. Thus, it is conceived that the value of Bi/Ru is more preferably <NUM> or less and <NUM> or more, measured by using an energy dispersive X-ray spectrometer (EDS).

Claim 1:
A method for producing a catalyst for an air secondary battery for use in an air electrode of the air secondary battery, the method comprising:
a precursor preparation step of preparing a pyrochlore-type oxide precursor,
a calcination step of calcining the precursor to form a pyrochlore-type oxide, and
an acid treatment step of immersing the pyrochlore-type oxide obtained from the calcination step in an acidic aqueous solution to apply an acid treatment,
wherein the pyrochlore-type oxide is a pyrochlore-type transition element oxide having a composition represented by the general formula: Bi<NUM>-XRu<NUM>-YO<NUM>-Z, wherein x, y, and z each represent a numerical value of <NUM> or more and <NUM> or less, and
wherein the acid treatment is applied such that, when X represents the amount of bismuth in terms of atom% contained in the bismuth-ruthenium oxide and Y represents the amount of ruthenium in terms of atom% contained in the bismuth-ruthenium oxide, the value of X/Y, which is the ratio of the amount of bismuth to the amount of ruthenium, is <NUM> or less, measured by using an energy dispersive X-ray spectrometer (EDS).