Catalyst for air electrode, air electrode including this catalyst for air electrode, and air secondary battery including this air electrode

Abstract

To provide an air electrode catalyst that can contribute to improvement of cycle life characteristics more than conventional, an air electrode containing this air electrode catalyst, and an air secondary battery containing this air electrode.SOLUTION: A battery 2 includes an electrode group 10 including an air electrode 16 and a negative electrode 12 superimposed via a separator 14, and a container 4 containing the electrode group 10 together with an alkaline electrolyte 82, and the air electrode 16 contains an air electrode catalyst, which is an air electrode catalyst made of a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, and Si / Bi, which represents the atomic ratio of Si to Bi, is 0.007 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an air electrode catalyst, an air electrode including this air electrode catalyst, and an air secondary battery including this air electrode. [Background technology]

[0002] In recent years, air batteries, which use atmospheric oxygen as the positive electrode active material, have attracted attention due to their high energy density and ease of miniaturization and lightweight design. Among such air batteries, zinc-air primary batteries have been put to practical use as power sources for hearing aids.

[0003] Additionally, research is being conducted on rechargeable air batteries, which use metals such as Li, Zn, Al, and Mg for the negative electrode. These air secondary batteries are expected to be new secondary batteries with the potential to exceed the energy density of lithium-ion secondary batteries.

[0004] One type of such air secondary battery is known as an air-hydrogen secondary battery, which uses an alkaline aqueous solution (hereinafter also referred to as alkaline electrolyte) as the electrolyte and hydrogen as the negative electrode active material (see, for example, Patent Document 1). Air-hydrogen secondary batteries such as those described in Patent Document 1 use a hydrogen storage alloy as the negative electrode metal. However, the negative electrode active material in air-hydrogen secondary batteries is hydrogen absorbed and released by the hydrogen storage alloy. Therefore, dissolution and precipitation reactions of the hydrogen storage alloy itself do not occur during chemical reactions (hereinafter also referred to as battery reactions) during battery charging and discharging. Therefore, air-hydrogen secondary batteries have the advantage of not being susceptible to problems such as internal short circuits caused by so-called dendrite growth, in which the negative electrode metal precipitates in a dendritic form, or a decrease in battery capacity due to shape change.

[0005] In an air secondary battery that uses an alkaline electrolyte, such as the above-mentioned air hydrogen secondary battery, the following charge and discharge reactions occur at the positive electrode (hereinafter also referred to as the air electrode).

[0006] Charging (oxygen generation reaction): 4OH -→O2+2H2O+4e - (I) Discharge (oxygen reduction reaction): O2 + 2H2O + 4e - →4OH - (II)

[0007] As shown in reaction formula (I), oxygen and water are generated at the air electrode during charging in an air secondary battery. The generated oxygen passes through the voids inside the air electrode and is released into the atmosphere from the part of the air electrode that is open to the atmosphere. On the other hand, during discharge, oxygen taken in from the atmosphere is reduced to generate hydroxide ions as shown in reaction formula (II).

[0008] A catalyst that promotes the above-mentioned charge / discharge reactions is used in the air electrode, which is the positive electrode of the above-mentioned air secondary battery. In air secondary batteries, it is desirable to reduce the overvoltage during the charge / discharge reactions of the air electrode in order to improve energy efficiency and increase power output. Therefore, materials that are effective in reducing overvoltage have been investigated as catalyst materials for use in the air electrode. Various metal oxides are promising materials for reducing such overvoltage. Among these metal oxides, pyrochlore-type bismuth ruthenium composite oxides have the "dual function" of oxygen reduction and oxygen generation, and are therefore considered to be particularly effective as air electrode catalysts because they can reduce overvoltage in both charge and discharge reactions.

[0009] This bismuth ruthenium composite oxide is produced, for example, by a production method in which a precursor is produced by a coprecipitation method using bismuth nitrate and ruthenium chloride as starting materials, and then the precursor is calcined.

[0010] In the above-described method for producing bismuth ruthenium composite oxide, by-products are formed during the process. When an air electrode is fabricated using a catalyst containing these by-products and an air-hydrogen secondary battery equipped with the air electrode undergoes repeated charge-discharge cycles, the bismuth ruthenium oxide itself does not undergo dissolution-precipitation reactions, but the by-products do. Specifically, as the chemical reactions (hereinafter referred to as battery reactions) occur during battery charge and discharge, the metal components (mainly bismuth) in the by-products undergo repeated dissolution-precipitation reactions, resulting in dendritic growth, in which the metal components precipitate in a dendritic pattern on the electrode plate. This dendritic growth of the metal components extends into the separator and ultimately penetrates the separator. This results in the problem of micro-short circuits. When micro-short circuits occur, not only ionic conductivity via the electrolyte but also electronic conductivity exists between the positive and negative electrodes within the battery. If electronic conductivity exists, the battery is self-discharging. The dendrite growth of the metal components increases with the number of charge-discharge cycles, which leads to an increase in the self-discharge rate, resulting in a decrease in the battery's discharge capacity after a relatively short number of cycles and an early end of life for the battery.

[0011] In order to solve the problem of shortening the cycle life, for example, a manufacturing method has been proposed as disclosed in Patent Document 2. According to Patent Document 2, by-products can be removed by treating a bismuth ruthenium composite oxide with an acid, and a decrease in the cycle life caused by the by-products can be suppressed. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Patent No. 6444205 [Patent Document 2] Japanese Patent Application Publication No. 2019-179592 Summary of the Invention [Problem to be solved by the invention]

[0013] By employing the method described in Patent Document 2, the by-products are almost completely removed by the acid treatment, and the decrease in cycle life caused by the by-products is almost completely suppressed, thereby extending the life of the air secondary battery. However, there are other factors that impair the cycle life characteristics besides the by-products. For example, the formation of a three-phase interface, which serves as the discharge reaction site in the air electrode catalyst, may become difficult as the number of cycles increases, resulting in a decrease in the cycle life characteristics. Therefore, it is believed that there is room for further improvement in the cycle life characteristics by further investigating and addressing the factors that impair the cycle life characteristics.

[0014] Air secondary batteries are expected to be used in a variety of applications in the future, so further extending their cycle life is an extremely important issue.

[0015] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide an air electrode catalyst that can contribute to improving cycle life characteristics more than conventional ones, an air electrode including this air electrode catalyst, and an air secondary battery including this air electrode. [Means for solving the problem]

[0016] In order to achieve the above object, the present invention provides an air electrode catalyst comprising a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, in which Si / Bi, which represents the atomic ratio of Si to Bi, is 0.007 or less. [Effects of the Invention]

[0017] The air electrode catalyst according to the present invention is an air electrode catalyst made of a bismuth-ruthenium composite oxide containing at least Bi, Ru, and Si, wherein the Si / Bi ratio, which represents the atomic ratio of Si to Bi, is 0.007 or less. When the Si / Bi ratio is 0.007 or less, a stable three-phase interface can be formed in the air electrode catalyst. Therefore, an air secondary battery using an air electrode containing the air electrode catalyst according to the present invention can discharge stably even with an increased number of cycles, thereby improving cycle life characteristics. Therefore, the present invention can provide an air electrode catalyst that can contribute to improved cycle life characteristics compared to conventional methods, an air electrode containing this air electrode catalyst, and an air secondary battery containing this air electrode. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view schematically illustrating an air hydrogen secondary battery according to one embodiment. [Figure 2] 1 is an X-ray diffraction (XRD) profile of the air electrode catalyst of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0019] An air-hydrogen secondary battery (hereinafter also referred to as battery) 2 including an air electrode catalyst for an air secondary battery according to one embodiment will be described below with reference to the drawings.

[0020] As shown in FIG. 1, the battery 2 includes a container 4 and an electrode group 10 placed in the container 4 together with an alkaline electrolyte 82 .

[0021] The electrode group 10 is formed by stacking a negative electrode 12 and an air electrode (positive electrode) 16 with a separator 14 interposed therebetween.

[0022] The negative electrode 12 includes a conductive negative electrode substrate having a porous structure and a large number of pores, and a negative electrode mixture supported in the pores and on the surface of the negative electrode substrate. For example, foamed nickel can be used as the negative electrode substrate.

[0023] The negative electrode mixture contains a hydrogen storage alloy powder as a negative electrode active material, which is an aggregate of hydrogen storage alloy particles capable of absorbing and releasing hydrogen. Here, the conductive material may be graphite powder, which is an aggregate of graphite particles, or carbon black powder, which is an aggregate of carbon black particles.

[0024] The hydrogen storage alloy constituting the hydrogen storage alloy particles is not particularly limited, but for example, a rare earth-Mg-Ni based hydrogen storage alloy is preferably used. The composition of this rare earth-Mg-Ni based hydrogen storage alloy can be freely selected, but for example, General formula: Ln 1-a Mg a Ni b-c-d Al c M d (III) It is preferable to use one represented by the following formula:

[0025] 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; and the subscripts a, b, c, and d represent numbers that satisfy the relationships 0.01≦a≦0.30, 2.8≦b≦3.9, 0.05≦c≦0.30, and 0≦d≦0.50, respectively.

[0026] Here, the hydrogen storage alloy particles can be obtained, for example, as follows. First, metal raw materials are weighed and mixed to obtain a predetermined composition. This mixture is melted in an inert gas atmosphere, for example, in a high-frequency induction melting furnace, and then cooled to form an ingot. The resulting ingot is heated to 900-1200°C in an inert gas atmosphere and homogenized by heat treatment, maintaining the temperature at that temperature for 5-24 hours. The ingot is then crushed and sieved to obtain a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles of the desired particle size.

[0027] Examples of the binder include sodium polyacrylate, carboxymethyl cellulose, and styrene butadiene rubber.

[0028] Here, the negative electrode 12 can be produced, for example, as follows. First, a negative electrode mixture paste is prepared by kneading a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles, a conductive material, a binder, and water. The resulting negative electrode mixture paste is filled into a negative electrode substrate, which is then dried. After drying, the negative electrode substrate with the hydrogen storage alloy particles and other components attached is rolled to increase the amount of alloy per unit volume, and then cut to obtain the negative electrode 12. This negative electrode 12 has a plate-like shape overall. The negative electrode mixture layer contained in the negative electrode 12 is formed from hydrogen storage alloy particles, conductive material particles, and other components, and therefore has gaps between the particles, resulting in an overall porous structure.

[0029] Next, the air electrode 16 includes a conductive air electrode substrate having a mesh structure and an air electrode mixture layer (cathode mixture layer) formed by the air electrode mixture (cathode mixture) supported on the air electrode substrate. For example, a nickel mesh can be used as the air electrode substrate.

[0030] The air electrode mixture contains an oxidation-reduction catalyst (air electrode catalyst), a conductive material, and a binder. The redox catalyst used has a dual redox function. Such a dual-function catalyst contributes to reducing the overvoltage of the battery during both the charging and discharging processes. For example, a pyrochlore-type bismuth ruthenium composite oxide is used as such a redox catalyst. This bismuth ruthenium composite oxide has the dual functions of oxygen generation and oxygen reduction.

[0031] The bismuth ruthenium composite oxide of this embodiment is an air electrode catalyst made of a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, and the Si / Bi atomic ratio of Si to Bi is 0.007 or less, and preferably 0.002 or more.

[0032] The inventors of the present application have conducted extensive research into the factors that hinder the improvement of the cycle life characteristics of air secondary batteries, and have discovered that a high amount of Si contained in the air electrode catalyst deteriorates the cycle life characteristics, and that the cycle life characteristics can be improved by minimizing the amount of Si. The details are as follows.

[0033] Si is contained as an impurity in the bismuth ruthenium composite oxide and exists mainly in the form of Si or SiO2 (silica). When such a bismuth ruthenium composite oxide containing excess Si or SiO2 comes into contact with an alkaline electrolyte (e.g., a KOH aqueous solution), the Si in the Si or SiO2 forms a silanol bond (Si-OH), which further neutralizes hydrogen ions, forming Si-OK, and producing alkaline silica gel. This alkaline silica gel has the property of absorbing moisture and expands as it absorbs surrounding moisture. The alkaline silica gel formed on the bismuth ruthenium composite oxide mainly absorbs and stores water generated during the charging reaction of the air secondary battery, so the more cycles it uses, the more moisture it can store. As the moisture-rich portion of the bismuth ruthenium composite oxide increases, the formation of a three-phase interface becomes more difficult, and the area of ​​the three-phase interface decreases. The reduced three-phase interface, which serves as a discharge reaction site, makes discharge more difficult and causes the cycle life to expire earlier. In other words, the amount of Si affects the cycle life characteristics, and it was found that the cycle life characteristics of air secondary batteries can be improved by regulating the amount of Si.

[0034] The inventors of the present application believed that limiting the amount of Si would improve the cycle life characteristics of air secondary batteries, and more specifically studied the relationship between the cycle life and Si in air secondary batteries. As a result, they found that there is a relationship between the amount of Si contained in bismuth ruthenium composite oxide and the cycle life, and that a significant improvement in the cycle life characteristics can be achieved when the Si / Bi ratio, which represents the atomic ratio of Si to Bi in the bismuth ruthenium composite oxide, is 0.007 or less. While minimizing the amount of Si is considered effective in improving the cycle life characteristics, in the course of this research, they confirmed that an improvement in the cycle life characteristics can be achieved at least up to a Si / Bi ratio of 0.002.

[0035] The pyrochlore-type bismuth ruthenium composite oxide described above can be produced, for example, as follows.

[0036] Prepare Bi(NO3)3·5H2O and RuCl3·3H2O. Then, measure out the Bi(NO3)3·5H2O and RuCl3·3H2O to the specified amounts. Here, it is preferable to measure out Bi(NO3)3·5H2O and RuCl3·3H2O so that the atomic ratio of Ru to Bi is 1.00 to 1.00 or less. Note that in the bismuth ruthenium composite oxide finally obtained by performing the acid treatment described below, it is preferable that the atomic ratio of Bi to Ru is 1.00 to 0.70 to 0.78.

[0037] Next, the measured Bi(NO3)3·5H2O and RuCl3·3H2O are added to distilled water and stirred to prepare a mixed aqueous solution of Bi(NO3)3·5H2O and RuCl3·3H2O. The temperature of the distilled water is set to 60°C or higher and 90°C or lower. Then, this mixed aqueous solution and a 1 mol / L or higher and 3 mol / L or lower NaOH aqueous solution are simultaneously added dropwise to a glass reaction vessel. The mixed aqueous solution and NaOH aqueous solution are added dropwise until the reaction vessel overflows. At this time, a precursor is formed in the reaction vessel. The mixed aqueous solution and NaOH aqueous solution stored in the reaction vessel are then transferred to a glass stirring vessel and stirred.

[0038] This stirring operation is carried out for 12 to 48 hours with oxygen bubbling. During the stirring operation, the pH of the mixed aqueous solution is maintained at 11, and the temperature is maintained at 60°C or higher and 90°C or lower. After the stirring operation is completed, the mixed aqueous solution is left to stand for 12 to 48 hours. After standing, the resulting precipitate is collected by suction filtration. The collected precipitate is kept at 80°C or higher and 100°C or lower to evaporate some of the water and form a paste. This paste is transferred to an evaporating dish, heated to 100°C or higher and 150°C or lower, and dried at that temperature for 1 hour to 5 hours to obtain a dried paste. The dried paste obtained is placed in a mortar and crushed with a pestle to obtain a powder. This powder is then fired by heating to 400°C or higher and 700°C or lower in an air atmosphere and held for 0.5 hours to 4 hours. The powder after the calcination treatment is washed with distilled water at a temperature of 60°C or higher and 90°C or lower, and then dried. This yields a pyrochlore-type bismuth ruthenium composite oxide. This bismuth ruthenium composite oxide contains Bi 2-x Ru2O 7-z (where x satisfies the relationship 0≦x≦1, and z satisfies the relationship 0≦z≦1.)

[0039] Next, it is preferable to immerse the obtained bismuth ruthenium composite oxide in an aqueous nitric acid solution to perform an acid treatment. Specifically, the procedure is as follows.

[0040] First, an aqueous nitric acid solution is prepared. The concentration of the aqueous nitric acid solution is preferably 5 mol / L or less, and more preferably 2 mol / L or less. The amount of the aqueous nitric acid solution is preferably 20 mL per 1 g of bismuth ruthenium composite oxide. The temperature of the aqueous nitric acid solution is preferably set to 20°C or higher.

[0041] The bismuth ruthenium composite oxide is then immersed in the prepared nitric acid aqueous solution and stirred for at least one hour and not more than six hours. After the predetermined time has passed, the bismuth ruthenium composite oxide is suction filtered from the nitric acid aqueous solution. The filtered bismuth ruthenium composite oxide is washed with distilled water set to at least 60°C and not more than 80°C.

[0042] The washed bismuth ruthenium composite oxide is subjected to a drying treatment by being kept in an environment of 100° C. or higher and 130° C. or lower for 1 hour or longer and 4 hours or shorter.

[0043] In this manner, an acid-treated bismuth ruthenium composite oxide is obtained. By carrying out the acid treatment in this manner, by-products generated in the manufacturing process of the bismuth ruthenium composite oxide can be removed. The acidic aqueous solution used for the acid treatment is not limited to an aqueous nitric acid solution; in addition to an aqueous nitric acid solution, an aqueous hydrochloric acid solution or an aqueous sulfuric acid solution can also be used. These aqueous hydrochloric acid and aqueous sulfuric acid solutions also have the effect of removing by-products, similar to an aqueous nitric acid solution.

[0044] In this manner, a powder of bismuth ruthenium composite oxide, which is an aggregate of particles of bismuth ruthenium composite oxide from which by-products have been removed, is obtained.

[0045] The resulting bismuth ruthenium composite oxide contains Si on its surface or in its bulk, where it is thought to be in a state where it has substituted for a portion of Bi.

[0046] There are various possible routes by which Si may be mixed into the bismuth ruthenium composite oxide, such as dissolution from a glass reaction vessel, contamination during the drying process, dissolution from a glass funnel used for filtration, and dissolution from a crucible used during firing. Because these routes may be combined, it is difficult to precisely identify the route. Among the routes mentioned above, dissolution from a glass reaction vessel is considered to be a typical route by which Si may be mixed. When glass comes into contact with a strongly alkaline aqueous solution such as a sodium hydroxide solution, Si and silica contained in the glass components are eluted. This eluted Si and the Si contained in the eluted silica are incorporated into the bismuth ruthenium composite oxide. Therefore, controlling the dissolution of Si from the glass reaction vessel is considered to be the most effective way to minimize the amount of Si in the bismuth ruthenium composite oxide.

[0047] Next, the conductive material will be described. The conductive material is used to reduce the internal resistance in order to increase the output of the air secondary battery, and as a support for the oxidation-reduction catalyst described above.

[0048] As such a conductive material, for example, nickel powder made of nickel particles is preferably used. The average particle size of the nickel particles is not particularly limited, but it is preferable that the nickel particles have a size that can impart the desired conductivity to the air electrode.

[0049] The nickel powder is preferably contained in the air electrode mixture in an amount of 60 mass % or more, and the upper limit of the nickel powder content is preferably 80 mass % or less in relation to the other constituent materials in the air electrode mixture.

[0050] The binder functions to bind the constituent materials of the air electrode mixture and to impart appropriate water repellency to the air electrode 16. The binder is not particularly limited, and for example, a fluororesin is used. A preferred example of the fluororesin is polytetrafluoroethylene (hereinafter also referred to as PTFE).

[0051] The air electrode 16 can be manufactured, for example, as follows. First, a catalyst powder which is an aggregate of bismuth ruthenium composite oxide particles, a conductive powder which is an aggregate of Ni particles as a conductive material, a binder, and water are prepared.The catalyst powder, conductive powder, binder, and water are then kneaded together to prepare an air electrode mixture paste.

[0052] The obtained air electrode mixture paste is formed into a sheet by, for example, roller pressing to obtain an air electrode mixture sheet. The air electrode mixture sheet is then press-bonded to a nickel mesh (air electrode substrate). This results in an intermediate air electrode product.

[0053] The resulting intermediate product is then placed in a firing furnace and fired. This firing is performed in an inert gas atmosphere. Examples of the inert gas include nitrogen gas and argon gas. The firing is performed by heating the intermediate product to a temperature of 200°C to 400°C and maintaining this temperature for 10 minutes to 40 minutes. The intermediate product is then naturally cooled in the firing furnace and removed into the atmosphere when its temperature reaches 150°C or less. This results in a fired intermediate product. The fired intermediate product is then cut into a predetermined shape to obtain an air electrode 16. The air electrode 16 includes an air electrode mixture layer formed from an air electrode mixture. The air electrode mixture contains particles of bismuth ruthenium composite oxide, and the air electrode mixture layer formed from this air electrode mixture has a porous structure with numerous pores overall, resulting in excellent gas diffusion properties.

[0054] The air electrode 16 and negative electrode 12 obtained as described above are stacked with a separator 14 interposed therebetween, thereby forming an electrode group 10. This separator 14 is disposed to prevent short circuits between the air electrode 16 and the negative electrode 12, and is made of an electrically insulating material. Materials that can be used for this separator 14 include, for example, a polyamide fiber nonwoven fabric to which hydrophilic functional groups have been added, or a polyolefin fiber nonwoven fabric such as polyethylene or polypropylene to which hydrophilic functional groups have been added.

[0055] The formed electrode group 10 is placed in a container 4 together with an alkaline electrolyte. The container 4 is not particularly limited as long as it can accommodate the electrode group 10 and the alkaline electrolyte, and for example, an acrylic box-shaped container 4 is used. For example, as shown in FIG. 1 , the container 4 includes a container body 6 and a lid 8.

[0056] The container body 6 is box-shaped and has a bottom wall 18 and side walls 20 extending upward from the peripheral edge of the bottom wall 18. The portion surrounded by the upper edge 21 of the side wall 20 is open. In other words, an opening 22 is provided on the opposite side of the bottom wall 18. Furthermore, the side wall 20 has through holes at predetermined positions on the right wall 20R and the left wall 20L, respectively, which serve as lead wire outlets 24, 26, which will be described later.

[0057] Furthermore, an electrolyte solution storage unit 80 is attached to the container body 6. This electrolyte solution storage unit 80 is a container that stores an alkaline electrolyte solution 82, and is attached, for example, via a connecting unit 84 that communicates with a through-hole 19 provided in the bottom wall 18. The connecting unit 84 is a flow path for the alkaline electrolyte solution 82 that communicates between the interior of the container 4 and the electrolyte solution storage unit 80. Since the interior of the container 4 and the electrolyte solution storage unit 80 are thus connected, the alkaline electrolyte solution 82 can move between the interior of the container 4 and the electrolyte solution storage unit 80.

[0058] The lid 8 has the same shape as the container body 6 in a plan view, and is placed on top of the container body 6 to close the opening 22. The gap between the lid 8 and the upper edge 21 of the side wall 20 is liquid-tightly sealed.

[0059] An air vent 30 is provided in the inner surface 28 of the lid 8, which faces the inside of the container body 6. The air vent 30 is open at the portion facing the inside of the container body 6, and has a serpentine shape overall. Furthermore, an inlet air hole 32 and an outlet air hole 34 are provided at predetermined positions in the lid 8, penetrating the lid 8 in the thickness direction. The inlet air hole 32 communicates with one end of the air vent 30, and the outlet air hole 34 communicates with the other end of the air vent 30. In other words, the air vent 30 is open to the atmosphere via the inlet air hole 32 and the outlet air hole 34. It is preferable to attach a pressure pump (not shown) to the inlet air hole 32. By driving this pressure pump, air can be sent from the inlet air hole 32 to the air vent 30.

[0060] An adjustment member 36 is placed on the bottom wall 18 of the container body 6 as needed. The adjustment member 36 is used to align the electrode group 10 in the height direction inside the container 4. As the adjustment member 36, for example, a foamed nickel sheet is used.

[0061] The electrode group 10 is disposed on the adjustment member 36. At this time, the negative electrode 12 of the electrode group 10 is disposed so as to be in contact with the adjustment member 36.

[0062] On the other hand, a water-repellent ventilation member 40 is disposed on the air electrode 16 side of the electrode group 10 so as to be in contact with the air electrode 16. This water-repellent ventilation member 40 is a combination of a PTFE porous membrane 42 and a nonwoven diffusion paper 44. The water-repellent ventilation member 40 exhibits a water-repellent effect due to the PTFE and also allows gas to pass through. The water-repellent ventilation member 40 is interposed between the lid 8 and the air electrode 16 and is in close contact with both the lid 8 and the air electrode 16. This water-repellent ventilation member 40 is large enough to cover the entire ventilation channel 30, inlet vent 32, and outlet vent 34 of the lid 8.

[0063] The container body 6, which houses the electrode group 10, the adjusting member 36, and the water-repellent and breathable member 40 as described above, is covered with the lid 8. Then, as schematically depicted in FIG. 1 , peripheral edge portions 46, 48 of the container 4 (container body 6 and lid 8) are sandwiched from above and below by connectors 50, 52. Thereafter, a predetermined amount of alkaline electrolyte 82 is poured from the electrolyte reservoir 80, and the container 4 is filled with the alkaline electrolyte 82. In this manner, the battery 2 is formed.

[0064] As the alkaline electrolyte 82, a general alkaline electrolyte used in alkaline secondary batteries is suitably used, and specifically, an aqueous solution containing at least one of NaOH, KOH, and LiOH as a solute is used.

[0065] In the battery 2, the ventilation channel 30 of the lid 8 faces the water-repellent ventilation member 40. The water-repellent ventilation member 40 allows gas to pass through but blocks moisture, so the air electrode 16 is open to the atmosphere via the water-repellent ventilation member 40, the ventilation channel 30, the inlet ventilation hole 32, and the outlet ventilation hole 34. In other words, the air electrode 16 comes into contact with the atmosphere through the water-repellent ventilation member 40.

[0066] In addition, in this battery 2, an air electrode lead (positive electrode lead) 54 is electrically connected to the air electrode (positive electrode) 16, and an anode lead 56 is electrically connected to the anode 12. Although these air electrode lead 54 and negative electrode lead 56 are depicted schematically in FIG. 1 , they are drawn out of the container 4 from the outlets 24, 26 while maintaining airtightness and liquid tightness. An air electrode terminal (positive electrode terminal) 58 is provided at the tip of the air electrode lead 54, and an anode terminal 60 is provided at the tip of the negative electrode lead 56. Therefore, in the battery 2, current is input and output during charging and discharging using these air electrode terminal 58 and negative electrode terminal 60.

[0067] [Example] 1. Battery manufacturing Example 1 (1) Synthesis of the air electrode catalyst 1) Co-precipitation process Bi(NO3)3·5H2O and RuCl3·3H2O were prepared. Bi(NO3)3·5H2O and RuCl3·3H2O were weighed out so that the atomic ratio of Ru was 1.00 and Bi was 0.75. The Bi / Ru ratio at this time was 0.75. The weighed Bi(NO3)3·5H2O and RuCl3·3H2O were added to distilled water at 70°C and stirred to prepare a mixed solution of Bi(NO3)3·5H2O and RuCl3·3H2O. A 2 mol / L NaOH solution was also prepared. The mixed solution and the NaOH solution were simultaneously added dropwise to a glass reaction vessel. The reaction vessel described above includes a cylindrical main body with an open top and a closed bottom, and a discharge pipe that is connected to the inside of the main body at a side wall near the top of the main body and discharges the solution accumulated in the main body to the outside. The discharge pipe extends diagonally downward from the side wall. Therefore, when a certain amount of the solution injected into the main body accumulates and the liquid level of the solution reaches the discharge pipe, the solution overflows from the discharge pipe and is discharged to the outside. The opening of the discharge pipe on the main body side (hereinafter referred to as the main body side opening) is positioned at a predetermined height so that the solution overflows when the volume of the injected solution reaches 100 mL. In other words, this reaction vessel can store solution until the volume reaches 100 mL. Therefore, the volume of this reaction vessel is 100 mL.

[0068] In Example 1, the dripping time was adjusted so that the mixed aqueous solution and the NaOH aqueous solution would overflow after an average residence time of 50 minutes in the reaction vessel. Here, the average residence time is calculated as average residence time = reaction vessel volume [mL] / dripping rate [mL / min], where dripping rate = drip amount [mL] / dripping time [min]. That is, when a fixed amount of solution is dripped into a reaction vessel of a certain volume, the dripping time of the solution was adjusted so that the liquid level of the solution dripped at a predetermined drip amount reaches the opening of the outlet tube on the main body side and overflows. Specifically, the average residence time is 50 minutes when dripping into a 100 mL reaction vessel at a dripping rate that causes overflow after 50 minutes.

[0069] As described above, the precursor was precipitated by reacting the mixed aqueous solution with the NaOH aqueous solution. The overflowed mixed aqueous solution containing the precursor was then placed in a stirring vessel and stirred. This stirring was continued for 24 hours while oxygen bubbling was performed. During this stirring, the pH of the mixed aqueous solution was maintained at 11, and the temperature was maintained at 70°C. After the stirring was completed, the mixed aqueous solution was allowed to stand for 24 hours. After standing, the resulting precipitate was recovered by suction filtration. The recovered precipitate was kept at 85°C to evaporate a portion of the water, forming a paste. The resulting paste was transferred to an evaporating dish, heated to 120°C, and dried at that temperature for 3 hours to obtain a dried precursor.

[0070] 2) Firing process The dried precursor obtained was placed in a mortar and crushed with a pestle to form a powder. The resulting precursor powder was subjected to a calcination treatment in which it was heated to 500°C in an air atmosphere and held for 3 hours. After the calcination treatment, the precursor was washed with distilled water at 70°C, filtered by suction, and then held at 120°C for 3 hours to be dried. This yielded a bismuth ruthenium composite oxide (air electrode catalyst).

[0071] 3) Acid treatment process The bismuth ruthenium composite oxide was placed in a stirring tank with a stirrer together with an aqueous nitric acid solution, and the nitric acid solution was stirred for 1 hour while maintaining the temperature at 25°C to perform an acid treatment. The amount of the aqueous nitric acid solution was 20 mL per 1 g of bismuth ruthenium composite oxide powder. The concentration of the aqueous nitric acid solution was 2 mol / L.

[0072] After stirring was completed, the bismuth ruthenium composite oxide powder was removed from the nitric acid aqueous solution by suction filtration. The removed bismuth ruthenium composite oxide powder was washed with distilled water heated to 70°C. After washing, the bismuth ruthenium composite oxide powder was dried by being kept in an atmosphere of 120°C for 3 hours.

[0073] In this manner, an acid-treated bismuth ruthenium composite oxide powder, that is, an air electrode catalyst powder, was obtained.

[0074] (2) Manufacturing of the air electrode Ni powder was prepared, which was an aggregate of Ni particles. The Ni particles were filament-shaped and had an average particle size of 10 to 20 μm.

[0075] Furthermore, a polytetrafluoroethylene (PTFE) dispersion and ion-exchanged water were prepared.

[0076] Nickel powder, polytetrafluoroethylene (PTFE) dispersion, and ion-exchanged water were added to and mixed with the bismuth ruthenium composite oxide powder (air electrode catalyst) obtained as described above. At this time, 20 parts by weight of the bismuth ruthenium composite oxide powder, 70 parts by weight of the nickel powder, 10 parts by weight of the PTFE dispersion, and 10 parts by weight of the ion-exchanged water were uniformly mixed to produce an air electrode mixture paste.

[0077] The obtained air electrode mixture paste was formed into a sheet and dried at room temperature of 25°C to obtain an air electrode mixture sheet. The obtained air electrode mixture sheet was press-bonded to a nickel mesh with a mesh count of 60, a wire diameter of 0.08 mm, and an opening ratio of 60%, thereby obtaining an intermediate air electrode product.

[0078] Next, the intermediate air electrode product was subjected to a calcination treatment in a calcination furnace. The calcination treatment conditions were that the intermediate air electrode product was heated to a calcination temperature of 340°C in a nitrogen gas atmosphere and held at this temperature for 13 minutes. The calcined intermediate product was cut into a length of 40 mm and a width of 40 mm, thereby obtaining an air electrode 16. The thickness of this air electrode 16 was 0.23 mm. In the obtained air electrode 16, the amount of bismuth ruthenium composite oxide powder (air electrode catalyst) was 0.23 g.

[0079] (3) Manufacturing of the negative electrode The metal materials Nd, Mg, Ni, and Al were mixed to a predetermined molar ratio, then placed in a high-frequency induction melting furnace and melted under an argon gas atmosphere. The resulting molten metal was poured into a mold and cooled to room temperature of 25°C to produce an ingot.

[0080] The ingot was then heat-treated at 1000°C in an argon gas atmosphere for 10 hours, and then cooled to room temperature of 25°C. After cooling, the ingot was mechanically pulverized in an argon gas atmosphere to obtain a rare earth-Mg-Ni-based hydrogen storage alloy powder. The volume average particle size (MV) of the obtained rare earth-Mg-Ni-based hydrogen storage alloy powder was measured using a laser diffraction / scattering particle size distribution analyzer. The volume average particle size (MV) was found to be 60 μm.

[0081] The composition of this hydrogen storage alloy powder was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the composition was found to be Nd 0.89 Mg 0.11 Ni 3.33 Al 0.17 It was.

[0082] The electrochemical alloy capacity of the obtained hydrogen storage alloy was measured. Specifically, a measurement sample was prepared by setting aside a portion of the hydrogen storage alloy powder obtained as described above, and nickel powder. 0.25 g of the hydrogen storage alloy powder as the measurement sample was mixed with 0.75 g of nickel powder to prepare a mixed powder, which was then molded into a circular pellet electrode with a diameter of 10 mm.

[0083] Next, 100 mL of an 8 mol / L KOH aqueous solution was poured into a cylindrical resin container, and a pellet electrode and a mercury oxide reference electrode were placed in the center of the container and in the KOH aqueous solution. A nickel hydroxide counter electrode with a capacity sufficiently large relative to the negative electrode (pellet electrode) was then placed along the edge of the container. In this manner, a battery with a negative electrode capacity limit was formed. This battery was subjected to a charge / discharge test, in which the battery was charged at 0.5 It for 200 minutes and discharged at 0.5 It until the negative electrode potential reached −0.3 V relative to the mercury oxide reference electrode, to determine the electrochemical alloy capacity. Note that in the charge / discharge test using the pellet electrode described above, the negative electrode capacity was calculated as 1 It, assuming an alloy capacity of 300 mAh / g.

[0084] To 100 parts by weight of the obtained hydrogen storage alloy powder, 0.2 parts by weight of sodium polyacrylate powder, 0.04 parts by weight of carboxymethyl cellulose powder, 1.0 part by weight of a styrene butadiene rubber dispersion, 0.3 parts by weight of carbon black powder, and 22.4 parts by weight of water were added and kneaded in an environment of 25°C to prepare a negative electrode mixture paste.

[0085] This negative electrode mixture paste is applied to a substrate with an areal density (weight per unit area) of approximately 300 g / m 2 The mixture was then filled into a foamed nickel sheet approximately 1.7 mm thick. The negative electrode mixture paste was then dried to obtain a foamed nickel sheet filled with the negative electrode mixture. The obtained sheet was rolled to increase the amount of alloy per unit volume, and then cut into a length of 40 mm and a width of 40 mm. In this way, a negative electrode 12 was obtained. The thickness of the negative electrode 12 was 0.75 mm. The negative electrode capacity calculated from the above electrochemical alloy capacity was 2500 mAh.

[0086] Next, the obtained negative electrode 12 was subjected to activation treatment. The procedure of this activation treatment is as follows. First, a typical sintered nickel hydroxide positive electrode was prepared. The positive electrode capacity of this nickel hydroxide positive electrode was sufficiently greater than the negative electrode capacity of the negative electrode 12. This nickel hydroxide positive electrode and the resulting negative electrode 12 were then stacked together with a separator made of polyethylene nonwoven fabric interposed therebetween to form an electrode group for activation treatment. This electrode group for activation treatment was then placed in an acrylic resin container together with a predetermined amount of alkaline electrolyte. This resulted in the formation of a single-electrode cell for a nickel-metal hydride secondary battery with a negative electrode capacity restriction.

[0087] This single-electrode cell was left standing for 5 hours in an environment at a temperature of 25° C., then charged at 0.5 It for 2.8 hours, and then discharged at 0.5 It until the battery voltage reached 0.70 V. This charge-discharge cycle was repeated five times to activate the negative electrode 12.

[0088] Thereafter, charging was carried out at 0.5 It for 2.8 hours, and then the negative electrode 12 was removed from the single-electrode cell. In this way, the negative electrode 12 that had been activated and charged was obtained.

[0089] (4) Manufacturing of air hydrogen secondary batteries The obtained air electrode 16 and negative electrode 12 were stacked with a separator 14 sandwiched between them to produce an electrode group 10. The separator 14 used in producing this electrode group 10 was formed from a nonwoven fabric made of polypropylene fibers having sulfonic groups, and had a thickness of 0.2 mm (basis weight 100 g / m 2 ) was.

[0090] Next, a container body 6 was prepared, and the above-described electrode group 10 was housed in this container body 6. At this time, a foamed nickel sheet serving as an adjusting member 36 was placed on the bottom wall 18 of the container body 6, and the electrode group 10 was placed on this adjusting member 36. Here, the foamed nickel sheet was 1 mm thick and had a square shape measuring 40 mm long and 40 mm wide.

[0091] Next, a water-repellent ventilation member 40 was disposed on the electrode group 10 (on the air electrode 16). Here, the water-repellent ventilation member 40 was formed by combining a PTFE porous membrane 42 having a length of 45 mm, a width of 45 mm, and a thickness of 0.1 mm with a nonwoven fabric diffusion paper 44 having a length of 40 mm, a width of 40 mm, and a thickness of 0.2 mm.

[0092] Next, the lid 8 was placed on the container body 6 to close the opening 22. At this time, the water-repellent ventilation member 40 was brought into close contact with the inner surface 28 of the lid 8 so that the entire area including the ventilation channel 30, the inlet ventilation hole 32, and the outlet ventilation hole 34 was covered with the water-repellent ventilation member 40. Here, the ventilation channel 30 has a single serpentine shape as a whole. The cross section of the ventilation channel 30 is rectangular, with a vertical dimension of 1 mm and a horizontal dimension of 1 mm. The ventilation channel 30 is open on the water-repellent ventilation member 40 side.

[0093] The container 4 is formed by combining the container body 6 and the lid 8, and its peripheral edges 46, 48 are sandwiched from above and below by connectors 50, 52. A resin packing (not shown) is provided at the contact point between the container body 6 and the lid 8 to prevent leakage of the alkaline electrolyte.

[0094] Next, a 5 mol / L KOH aqueous solution was poured as the alkaline electrolyte 82 into the electrolyte reservoir 80. The amount of the KOH aqueous solution poured at this time was 50 mL. In this manner, a battery 2 as shown in FIG. 1 was produced.

[0095] An air electrode lead 54 is electrically connected to the air electrode 16, and an anode lead 56 is electrically connected to the anode 12, and these air electrode lead 54 and anode lead 56 appropriately extend from the lead wire outlets 24, 26 to the outside of the container 4 while maintaining the airtightness and liquid-tightness of the container 4. An air electrode terminal 58 is attached to the tip of the air electrode lead 54, and an anode terminal 60 is attached to the tip of the anode lead 56.

[0096] Example 2 Except for adjusting the average residence time in the coprecipitation step to 30 minutes, an air-hydrogen secondary battery was produced in the same manner as in Example 1. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Example 2 was 0.24 g.

[0097] Example 3 An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that in the coprecipitation step, Bi(NO3)3·5H2O and RuCl3·3H2O were weighed out so that the atomic ratio of Ru to Bi was 1.00. Here, the Bi / Ru value at the time of preparation in Example 3 was 1.00. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Example 3 was 0.24 g.

[0098] (Comparative Example 1) Except for adjusting the average residence time to 2 minutes in the coprecipitation step, an air-hydrogen secondary battery was produced in the same manner as in Example 1. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.25 g.

[0099] (Comparative Example 2) An air-hydrogen secondary battery was manufactured in the same manner as in Example 1, except that in the coprecipitation step, the average residence time was adjusted to 5 minutes and the production amount was tripled. Here, "triple scale" means that the dropwise addition time was tripled and the quantity per lot was tripled. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.24 g.

[0100] (Comparative Example 3) Except for changing the reaction vessel to one with a volume of 200 mL in the coprecipitation step and adjusting the average residence time to 5 minutes, an air-hydrogen secondary battery was produced in the same manner as in Example 1. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.25 g.

[0101] 2. Analysis of the air electrode catalyst (1) Analysis of composition ratio For analytical samples of the air electrode catalyst powders obtained in Examples 1 to 3 and Comparative Examples 1 to 3, quantitative analysis of Bi, Ru, and Si was performed by X-ray fluorescence analysis using the fundamental parameter (FP) method. An energy-dispersive X-ray fluorescence analyzer was used for the X-ray fluorescence analysis. The analytical conditions were a CuKα X-ray source, a tube voltage of 50 kV, and a tube current of 60 mA for Bi and Ru, and a tube voltage of 30 kV and a tube current of 100 mA for Si. Specifically, the analytical samples were irradiated with X-rays, and the intensity of the characteristic X-rays (fluorescent X-rays) generated by the irradiation was measured, and the content of each element was determined using the FP method. Based on the obtained Bi, Ru, and Si contents, Si / Bi, which represents the atomic ratio of Si to Bi, and Bi / Ru, which represents the atomic ratio of Bi to Ru, were calculated. The results are shown in Table 1.

[0102] The analyzed Bi / Ru value is smaller than the Bi / Ru value at the time of preparation. This is because the Bi / Ru value at the time of preparation is roughly maintained up to the coprecipitation and calcination steps, but the Bi and Bi compounds that are not incorporated into the crystals of the bismuth ruthenium composite oxide are removed by the subsequent acid treatment step and the washing performed in the acid treatment step, so the Bi / Ru value in the finally obtained air electrode catalyst (bismuth ruthenium composite oxide) is smaller than the Bi / Ru value at the time of preparation.

[0103] (2) Measurement of lattice constant The analytical samples of the air electrode catalyst powders obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were analyzed by X-ray diffraction (XRD). A parallel-beam X-ray diffractometer was used for the XRD analysis. The analytical conditions were a CuKα X-ray source, a tube voltage of 15 kV, a tube current of 15 mA, a scan speed of 1 degree / min, and a step width of 0.01 degree. From the diffraction chart patterns of the obtained XRD profiles, the air electrode catalysts were confirmed to be Bi2Ru2O7, which has a pyrochlore-type crystal structure and a similar crystal structure. An example of the XRD profile of Example 1 is shown in Figure 2. Next, the lattice constants were calculated from the peaks of the (111), (222), (400), (331), (440), (622), and (444) planes in the obtained diffraction chart patterns. The results are shown in Table 1.

[0104] 3. Battery characteristic evaluation The air-hydrogen secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were repeatedly charged and discharged in an atmosphere of 25° C. via the air electrode terminal 58 and the negative electrode terminal 60 at 0.1 It for 10 hours, and then discharged at 0.2 It until the battery voltage reached 0.4 V, forming one cycle. In the charge and discharge operations of the air-hydrogen secondary batteries described above, the rated capacity was 2.0 Ah, which corresponds to 80% of the negative electrode capacity.

[0105] In the above-described charge / discharge operations, a rest period of 10 minutes was provided between charge and discharge, and between discharge and charge.

[0106] The discharge capacity was determined for each of the above charge / discharge cycles. The number of cycles was then counted, and the number of cycles at which the percentage of discharge capacity / rated capacity fell below 70% was recorded. This number of cycles is shown in Table 1. A higher number of cycles indicates a longer cycle life.

[0107] During the above-described charge / discharge operation, regardless of whether the battery was being charged or discharged, air was continuously supplied to the ventilation path 30 at a rate of 33 mL / min by introducing air through the inlet ventilation hole 32 and discharging air through the outlet ventilation hole 34. The air supplied to the ventilation path 30 was air (CO2 concentration: approximately 100 ppm) that had been bubbled through a KOH aqueous solution.

[0108] [Table 1]

[0109] 4. Discussion Table 1 shows that the number of cycles for the batteries of Examples 1 to 3 was 49 to 58, while the number of cycles for the batteries of Comparative Examples 1 to 3 was 13 to 18, indicating that the cycle life of the batteries of Examples 1 to 3 was significantly improved compared to the batteries of Comparative Examples 1 to 3. The batteries of Examples 1 to 3 had Si / Bi values ​​of 0.002 to 0.007, while the batteries of Comparative Examples 1 to 3 had Si / Bi values ​​of 0.011 to 0.014. In other words, the amount of Si relative to Bi in the batteries of Examples 1 to 3 was kept lower than that in the batteries of Comparative Examples 1 to 3. This suggests that setting the amount of Si relative to Bi in the air electrode catalyst to 0.007 or less can contribute to improving the cycle life characteristics of air secondary batteries.

[0110] Example 3 has a larger amount of Bi than Example 1. It is believed that in the bulk of the bismuth ruthenium composite oxide, Si substitutes for Bi sites in the crystal, so if there is a large amount of Bi, Si is less likely to penetrate into the bulk. Therefore, in Example 3, the amount of Bi is larger than in Example 1, and therefore the amount of Si taken up into the bulk is correspondingly smaller, resulting in a Si / Bi value of 0.002. As a result, the number of cycles in Example 3 was 58, and it is believed that Example 3 exhibits particularly excellent cycle life characteristics. [Explanation of symbols]

[0111] 2. Battery (air hydrogen secondary battery) 4 containers 6 Container body 8 Lid 10 electrode groups 12 Negative electrode 14 Separator 16 Air electrode (positive electrode) 30 Ventilation channel 40 Water-repellent and breathable material

Claims

1. An air electrode catalyst comprising a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, wherein Si / Bi, which represents the atomic ratio of Si to Bi, is 0.007 or less.

2. 2. The air electrode catalyst according to claim 1, wherein the Si / Bi ratio is 0.002 or more.

3. an air electrode substrate; an air electrode mixture held by the air electrode substrate, The air electrode mixture comprises the air electrode catalyst according to claim 1 or 2.

4. A container and an electrode group disposed in the container; an alkaline electrolyte injected into the container; the electrode group includes an air electrode and a negative electrode stacked together with a separator interposed therebetween, An air secondary battery, wherein the air electrode is the air electrode according to claim 3 .

5. The air secondary battery according to claim 4 , wherein the negative electrode contains a hydrogen storage alloy.

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