JP2014165056A - Positive electrode for metal air secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air electrode (positive electrode) for a safe battery pack whose charging voltage is stable even during charging and that has no electrolyte leakage and such.SOLUTION: The positive electrode for a metal air secondary battery includes: a gas diffusion layer 1; an oxygen generation catalyst layer 2 laminated on the gas diffusion layer 1; and a hydrophilic porous layer 3 laminated on the oxygen generation catalyst layer 2, wherein the hydrophilic porous layer 3 is disposed in a state where the hydrophilic porous layer 3 is in contact with an alkaline electrolyte.

Description

  The present invention relates to a positive electrode for a metal-air secondary battery such as a lithium-air battery, a zinc-air battery, and an aluminum-air battery.

  An air battery that uses oxygen in the air as the positive electrode active material of the battery is advantageous for increasing the energy density of the battery. As the types of air batteries, metal-air batteries (primary batteries and secondary batteries) and fuel cells using zinc, aluminum, lithium or the like as a negative electrode active material are known and some of them are put into practical use. The basic structure of these air batteries is such that the positive electrode side is open to the atmosphere as shown in FIG. When these air batteries are discharged, an oxygen reduction reaction occurs at the positive electrode. In this case, it is known that the oxygen reduction reaction occurs rapidly in an aqueous electrolyte, and further in an alkaline solution, and an alkaline aqueous solution is usually used as the battery electrolyte.

  The discharge reaction of the positive electrode in the alkaline electrolyte is based on the following reaction formula (1), and the discharge reaction at the negative electrode is based on the following reaction formula (2).

Positive electrode reaction: 1/4 O ₂ + 1/2 H ₂ O + e- → OH ^- Reaction formula (1)
Negative electrode reaction: M → M ^{n +} + ne- Reaction formula (2)
Here, when M is Li, n = 1, when Zn is n = 2, when Al is n = 3, and when H is n = 1.

  The positive electrode reaction layer 12 in FIG. 1 is an oxygen reduction catalyst layer in which Pt is supported on carbon black, for example, and has a function of promoting the reaction of the reaction formula (1). Moreover, the gas diffusion layer 11 of FIG. 1 is a carbon fiber sheet, for example.

  When the air battery is used as a rechargeable secondary battery, the electrode reaction between the positive electrode and the negative electrode during charging is as follows.

Positive ^{reaction: OH - → 1/4 O} 2 + 1/2 H 2 O + e- reaction formula (3)
Negative electrode reaction: M ^{n +} + n e- → M Reaction formula (4)
In general, the discharge product (usually generated in the form of M (OH) n) due to the discharge reaction is very stable, and as shown in the reaction formula (4), the negative electrode metal ion is converted into the original metal. It is very difficult to return to electrochemical. Therefore, until now, it has not been easy to use a metal-air battery as a secondary battery that can be electrochemically regenerated (charged).

However, in Li-air batteries using metallic Li as the negative electrode, the electrochemical reduction of discharge products is relatively easy, and recently it has been shown that repeated discharge / charge operations can actually be performed. -Research and development of air secondary batteries is active. The Li-air battery secondary battery is composed of a non-aqueous Li-air secondary battery using an organic electrolyte or ionic liquid as a solvent for the electrolyte as shown in FIG. 1, and water as shown in FIG. It can be classified into two types: water-based Li-air secondary batteries used as a solvent for the electrolyte. The water-based Li-air secondary battery includes a lithium metal negative electrode 20 and Li ₁ + x (M, Al, Ga) x (Ge _{1 -yTiy} ) _{2 -x} (PO ₄ ) ₃ (where X ≦ 0.8 and 0 ≦ Y ≦ 1.0 and M is an element selected from the group consisting of Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) and / or Li _{1 + x + y} QxTi _{2 −} Water-impermeable lithium ion conductive glass ceramic (hereinafter referred to as _x Siy P _{3 -y} O ₁₂ (where 0 <X ≤ 0.4 and 0 <Y ≤ 0.6, and Q is Al or Ga)) , Omitted as LATP), and by stacking and protecting 50, it has become possible to use an aqueous electrolyte solution 30 (see, for example, Patent Document 1).

The positive electrode reaction during discharge of the water-based Li-air secondary battery is shown by the reaction formula (1), and the conventional Zn-air primary battery, Al-air primary battery, alkaline fuel cell, etc. used as an aqueous electrolyte solvent Is the same. The functions necessary for the positive electrode of these primary batteries and fuel cells include high supply ability (diffusibility) of oxygen and water as electrode active materials, high conductivity of OH ^- ions and electrons, and high oxygen reduction ability (high catalytic ability) ) And a stable holding ability of the liquid electrolyte.

As a method for realizing them, a great number of techniques (see Non-Patent Document 1) are known. A conventional method for producing a general discharge positive electrode is as follows. Using hydrophilic carbon black (CB) and water-repellent polytetrafluoroethylene (PTFE), hydrophilic holes and water-repellent holes are formed in the positive electrode. As a result, the electrolyte solution is infiltrated into the hydrophilic hole to form an OH ^- ion conductive path, while preventing the electrolyte solution from entering the water-repellent hole, and simultaneously supplying the oxygen supply path. It is to secure. In order to smoothly perform oxygen reduction during the discharge reaction, an oxygen reduction catalyst such as platinum, silver, a metal oxide, or a cyclic metal complex is added and applied to the inside or surface of the positive electrode as a positive electrode reaction layer.

  On the other hand, when an air battery is used as a secondary battery, the charge reaction (oxygen generation reaction) is different from the discharge reaction (oxygen reduction reaction). There is a problem that even if the generation reaction is performed, it does not function sufficiently.

In the case of a charging reaction (oxygen generation reaction), the reaction requires OH 2 ⁻ ions according to the reaction formula (3), and this is supplied from the electrolytic solution. Is advantageous. However, since oxygen gas is generated by the reaction, a function of discharging it to the outside of the battery is necessary. The catalyst used as the reaction layer requires an oxygen generation catalyst. As shown in Non-Patent Document 2, as a conventional technology, a hydrophilic positive electrode for charging specifications installed on the electrolyte side and a water-repellent positive electrode for discharge specifications installed on the atmosphere side are separately manufactured, and in the thickness direction Bi-functional positive electrode structures are known that are stacked and bonded. However, in this case, oxygen generated during the charging reaction needs to pass through the positive electrode of the discharge specification and be discharged into the atmosphere, and OH ⁻ ions generated during the discharge reaction pass through the positive electrode of the charging specification through the electrolyte solution. It needs to be discharged inside. Therefore, there is a possibility that efficient charge / discharge cannot be performed. Further, as a catalyst, a pyrochlore oxide containing iridium having catalytic ability for both oxygen reduction / oxygen generation (see Patent Document 2) and a polynuclear metal complex (see Patent Document 3) are reacted with a bifunctional catalyst. An example for use with layers is shown. However, although materials that catalyze both oxygen reduction / oxygen generation are known, in general, the catalytic ability for oxygen reduction and the catalytic ability for oxygen generation are contradictory, and both functionalities are There is a problem that the use of a catalyst does not lead to an improvement in performance during discharging and charging. Furthermore, in Patent Document 4 and Patent Document 5, a charging electrode (referred to as an auxiliary electrode or a third electrode) is separately installed in the battery in addition to the discharging electrode, and an electric circuit installed outside the battery at the time of charging is installed. A technique for switching from a discharging electrode to a charging electrode is known. In this case, there is a problem that an external electric circuit for switching between the discharge / charge electrodes is required. In addition, it is necessary to devise a place where the auxiliary electrode is installed in the battery or to evacuate oxygen generated from the auxiliary electrode, and there are also indications that a short circuit easily occurs in the battery by installing the auxiliary electrode. .

Special table 2007-7513464 Japanese Patent No. 4568124 JP 2012-238591 A Japanese Patent No. 2655810 Japanese Patent No. 3523506

Kim Kinoshita, “Electrochemical Oxygen Technology”, JOHN WILEY & SONS, INC. (1992) M. Klein and S. Viswanathan, "Zinc / Air Battery R & D, Research and Development of BifunctionalOxygen Electrode, Tasks I & II Final Report, Lawrence Berkeley National Laboratory, Berkeley, CA (Dec, 1986)

In the positive electrode reaction at the time of discharge, according to the reaction formula (1), it is necessary to form a path in the positive electrode reaction layer for supplying oxygen and water and discharging generated OH ⁻ ions in the direction of the electrolyte. As a measure for realizing this, a technique in which a hydrophilic material such as carbon black (CB) and a water repellent material such as PTFE are mixed to form a hydrophilic hole and a water repellent hole in the positive electrode reaction layer is usually used. In this case, as shown in FIG. 3, the electrolyte solution 30A is impregnated into the positive electrode reaction layer 12 through the hydrophilic holes, thereby forming a path for the electrolyte solution for water supply and movement of OH ⁻ ions. On the other hand, oxygen in the atmosphere passes through the gas diffusion layer 11, enters the positive electrode reaction layer 12 through the water repellent holes in the positive electrode reaction layer 12, and meets the electrolyte (reaction region R, usually three-phase). The discharge reaction occurs at the interface). In order to cause this reaction more and generate more oxygen reduction current, the positive electrode reaction layer 12 has a certain thickness (usually about 300 to 1,000 μm) and causes a discharge reaction also in the thickness direction. Designed to be able to. Therefore, in the positive electrode structure as shown in FIG. 3, the reaction region of the discharge reaction extends in the thickness direction.

On the other hand, when the charge reaction is performed using the positive electrode structure shown in FIG. 3, the positive electrode reaction requires the supply of OH ^- ions according to the reaction formula (3). Since a strong alkaline electrolyte is usually used as the electrolyte of the water-based air battery, OH ^- ions are supplied in a large amount and stably from the electrolyte. Therefore, the reaction at the time of discharge occurs in a relatively narrow region R at the interface between the positive electrode reaction layer 12 and the electrolytic solution 30A as shown in FIG. Oxygen generated in the reaction region R due to the charging reaction should pass through the gas diffusion layer 11 and be released into the atmosphere. However, oxygen generated in the vicinity of the electrolyte 30A has an oxygen diffusion distance L. For this reason, it is difficult to escape to the gas diffusion layer 11 side, and the electrolyte impregnated in the positive electrode reaction layer 12 may be moved and discarded to cut the OH ^- ion conductive path and increase the charging voltage. Further, there is a possibility that the oxygen diffuses into the electrolyte solution 30A on the side opposite to the gas diffusion layer 11, and the oxygen that has moved into the electrolyte solution 30A reduces the contact area between the positive electrode reaction layer 12 and the electrolyte solution 30A, or the battery pack. May cause problems such as swelling and electrolyte leakage.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an air electrode (positive electrode) for a safe battery pack that has a stable charging voltage even during charging and does not leak electrolyte. .

  The positive electrode for a metal-air secondary battery of the present invention made to solve the problem includes a gas diffusion layer, an oxygen generation catalyst layer stacked on the gas diffusion layer, and a hydrophilic layer stacked on the oxygen generation catalyst layer. A porous porous layer, wherein the hydrophilic porous layer is exposed to an alkaline electrolyte.

  Since the oxygen generation catalyst layer is disposed in the gas diffusion layer, oxygen is generated during the charging reaction in the vicinity of the gas diffusion layer, and the generated oxygen passes through the gas diffusion layer and is released to the atmosphere. Furthermore, since there is a hydrophilic porous layer between the reaction region where oxygen is generated and the electrolytic solution, the impregnation property of the electrolytic solution is enhanced and the mobility of ions is enhanced. Moreover, the movement of the generated oxygen to the electrolyte side is suppressed. As a result, an increase in charging voltage is suppressed, and electrolyte leakage and the like are suppressed.

In the metal-air secondary battery positive electrode, the oxygen generation catalyst layer may be porous. As a result, the generated oxygen can be more efficiently discharged. Further, it has a hydrophilic / water-repellent oxygen reduction catalyst layer laminated on the gas diffusion layer, and the hydrophilic / water-repellent oxygen reduction catalyst layer is also added to the alkaline electrolyte. It should be exposed.

Since it has a hydrophilic / water-repellent oxygen reduction catalyst layer laminated on the gas diffusion layer, the electrolyte solution is impregnated in the hydrophilic / water-repellent oxygen reduction catalyst layer through the hydrophilic pores during the discharge reaction, and water is supplied. and OH ^- ions path of the electrolyte to move is formed. On the other hand, oxygen in the atmosphere passes through the gas diffusion layer, enters the hydrophilic / water-repellent oxygen reduction catalyst layer through the water-repellent holes in the hydrophilic / water-repellent oxygen reduction catalyst layer, and meets the electrolyte. A discharge reaction occurs. As a result, the discharge reaction occurs efficiently.

  Moreover, the thickness of the oxygen generating catalyst layer is preferably in the range of 10 to 500 μm. If the thickness of the oxygen generating catalyst layer is large, a charging reaction is likely to occur, but oxygen is hardly discharged. When the thickness is in the range of 10 to 500 μm, oxygen can be easily discharged and the charging reaction can be effectively performed.

  Since the oxygen generation catalyst layer is disposed on the gas diffusion layer, oxygen is generated during the charging reaction in the vicinity of the gas diffusion layer, and the generated oxygen passes through the gas diffusion layer and is released to the atmosphere. Furthermore, since there is a hydrophilic porous layer between the reaction region where oxygen is generated and the electrolytic solution, the impregnation property of the electrolytic solution is enhanced and the mobility of ions is enhanced. Moreover, the movement of the generated oxygen to the electrolyte side is suppressed. As a result, an increase in charging voltage is suppressed, and electrolyte leakage and the like are suppressed.

It is sectional drawing which shows a concept of a metal-air battery concerning a prior art typically. FIG. 3 is a cross-sectional view schematically showing a concept of a water-based lithium-air battery according to a conventional technique. It is a schematic diagram of the positive electrode reaction at the time of discharge of the conventional metal-air battery. It is a schematic diagram of the positive electrode reaction at the time of charge of the conventional metal-air battery. It is sectional drawing which shows typically the structure of the positive electrode for metal-air secondary batteries which concerns on Embodiment 1 of this invention. It is sectional drawing which shows typically the structure of the water-system Li-air battery with the positive electrode for metal-air secondary batteries which concerns on Embodiment 2 of this invention. It is the front view (a) and AA sectional view (b) of the positive electrode of Embodiment 2 (FIG. 6). It is a manufacturing process figure of the positive electrode of Embodiment 2 (FIG. 6, 7). It is the front view (a) and BB sectional view (b) of the positive electrode of a deformation | transformation aspect. It is the front view (a) and CC sectional view (b) of the positive electrode of another deformation | transformation aspect. It is the schematic of the cell for battery reaction measurement. It is a graph which shows the relationship between an overvoltage and a current density. It is a graph which shows the change over time of the voltage at the time of discharge.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the invention will be described in detail with reference to the drawings.

(Embodiment 1)
In the positive electrode of the present embodiment, as shown in FIG. 5, an oxygen generating catalyst layer 2 having oxygen generation catalytic ability is disposed in the vicinity of the gas diffusion layer 1, and a hydrophilic porous layer 3 is provided on the oxygen generating catalyst layer 2. It has an arranged structure.

  As the gas diffusion layer 1, for example, a fluorine resin (PTFE, FEP, PVF, etc.) filter, a hydrocarbon resin (PE, PP, etc.) filter, or the like can be used. Further, a metal mesh, a metal foam, or a carbon fiber filter that has been subjected to a water repellent treatment with a fluororesin can be used.

Examples of the material for the oxygen generating catalyst layer 2 include iridium, ruthenium and oxides thereof, La _1-x St _x MO ₃ (M = Co, Fe, Mn, etc.), etc. You may carry | support and use on support | carriers, such as black (CB) and a zeolite. Further, in order to facilitate the discharge of oxygen generated in the reaction region R during charging, it is preferable to mix powder such as PTFE.

  The thickness of the oxygen generation catalyst layer 2 is determined in consideration of the magnitude of the charging current and the diffusion distance of the generated oxygen. When the oxygen generating catalyst layer 2 is thick, a charging reaction is likely to occur, but on the other hand, oxygen is hardly discharged. The thickness of the oxygen generating catalyst layer 2 is preferably 10 to 500 μm, and more preferably 20 to 100 μm.

  Examples of the material of the hydrophilic porous layer 3 include carbon fiber paper (CP), a metal mesh such as titanium, a metal foam, and a nonwoven fabric such as nylon. When CP or a metal mesh is selected as the material of the hydrophilic porous layer 3, it has electronic conductivity and can be used as a current collector. The thickness of the hydrophilic porous layer 3 is preferably 20 to 2,000 μm, and more preferably 50 to 1,000 μm.

  In the positive electrode of the present embodiment, since the oxygen generation catalyst layer 2 is disposed in the vicinity of the gas diffusion layer 1, oxygen generation during charging occurs in the reaction region R in the vicinity of the gas diffusion layer 1. Accordingly, the generated oxygen is released to the atmosphere through the gas diffusion layer 1. That is, the oxygen diffusion distance L is substantially equal to the thickness of the gas diffusion layer 1, and the generated oxygen is quickly released into the atmosphere.

Further, since the hydrophilic porous layer 3 is disposed between the oxygen generating catalyst layer 2 and the electrolytic solution 30A, the impregnating property of the electrolytic solution 30A is high, and the movement of OH ⁻ ions during charging is facilitated. Furthermore, among H ₂ O and O ₂ generated in the reaction region R, H ₂ O easily moves to the electrolyte solution 30A side, but O ₂ is suppressed from moving.

(Embodiment 2)
The positive electrode of Embodiment 1 is designed so that the charging reaction is satisfactorily performed. Therefore, the positive electrode of the present embodiment is a positive electrode in which a part of the positive electrode is the positive electrode of the first embodiment and the remaining part is excellent in the discharge reaction.

  That is, the positive electrode of the present embodiment has a half of the area as the positive electrode of the first embodiment, and the hydrophilic / water-repellent oxygen reduction catalyst layer 4 is disposed in the remaining half of the gas diffusion layer.

  FIG. 6 schematically shows the structure of an aqueous Li-air secondary battery equipped with the positive electrode of the present embodiment.

  In the water-based Li-air secondary battery, since the metal lithium 20 used for the negative electrode reacts with water, the negative electrode reaction needs to occur in a non-aqueous atmosphere. The battery reaction of the negative electrode in a non-aqueous atmosphere is characterized by being very slow compared to the battery reaction of the positive electrode that occurs in a water atmosphere. Therefore, the current that can be extracted from the negative electrode per unit area (negative electrode current density) is one digit or more smaller than the positive electrode current density, and the current value that can be extracted from the water-based Li-air secondary battery is regulated by the size of the negative electrode area. Is in a situation. Therefore, even if the area of the positive electrode is reduced with respect to the area of the negative electrode, the battery performance does not deteriorate so much.

  Utilizing the characteristics of this water-based Li-air secondary battery, the function is separated into the charge reaction part and the discharge reaction part within the surface of the positive electrode, and a bifunctional positive electrode having two functions of oxygen reduction / oxygen generation at the same time. be able to.

  As shown in FIGS. 6 and 7, the positive electrode of the present embodiment includes a region A1 in which an oxygen generation catalyst layer 2 and a hydrophilic porous layer 3 are sequentially laminated on a half of the area of the gas diffusion layer 1, and a gas diffusion layer. And a region A2 in which a hydrophilic / water-repellent oxygen reducing layer 4 is laminated on half of the area of 1.

  Since area | region A1 is the same as Embodiment 1, it is a charge area | region which has an oxygen generation function and a charge reaction is performed favorably. On the other hand, since the region A2 has the same structure as that of the conventional positive electrode, the region A2 is a discharge region having an oxygen reduction function and in which a discharge reaction is favorably performed. Therefore, the positive electrode of the present embodiment is a so-called bifunctional positive electrode having two functions.

  What is necessary is just to produce the structure and production method of area | region A2 according to the technique of the conventional air primary battery. Examples of the catalyst material for the hydrophilic / water-repellent oxygen reduction catalyst layer 4 include oxygen reduction catalysts such as noble metals such as platinum, ruthenium, iridium, palladium, silver, alloys thereof, metal oxides, and cyclic metal complexes. They may be used alone or supported on a carrier such as CB or zeolite. The hydrophilic / water-repellent oxygen reduction catalyst layer 4 has three-dimensional distribution of hydrophilic holes and water-repellent holes.

  The material and manufacturing method of the structure of the region A1 are the same as the material and manufacturing method of the air electrode of the first embodiment.

The oxygen generation catalyst layer 2 (reaction layer during charging) in the region A1 has a structure that is easy to charge, and the hydrophilic / water-repellent oxygen reduction catalyst layer 4 (reaction layer during discharge) in the region A2 has a structure that is easy to discharge. However, the catalyst specifications used for them are as follows: the oxygen reduction catalytic ability of the catalyst in the region A2> the oxygen reducing catalytic ability of the catalyst in the region A1, the oxygen generating catalytic ability of the catalyst in the region A1> the oxygen of the catalyst in the region A2 It is preferable to use a combination of catalysts having generation catalytic ability. As such a combination, platinum (Pt) is used as the oxygen reduction catalyst of the hydrophilic / water-repellent oxygen reduction catalyst layer 4 in the region A2, and iridium oxide (IrO) is used as the oxygen generation catalyst of the oxygen generation catalyst layer 2 in the region A1. ₂ ) can be exemplified.

  By using such a combination of catalysts, it becomes possible to more clearly regulate the reaction area at the time of discharge / charge to the discharge electrode / charge electrode part, and the battery outside the system at the time of discharge / charge There is no need to switch the electrical circuit installed in Further, the charging electrode (region A1) and the discharging electrode (region A2) are integrated, and it is possible to unify the air supply port during discharge and the oxygen discharge port during charging. The electrode area of the region A1 (charge reaction portion) and the region A2 (discharge reaction portion) formed in the positive electrode surface is smaller than the negative electrode area, but in the case of a water-based Li-air secondary battery, the performance is hardly affected. Does not affect.

  Next, an example of the manufacturing method of the positive electrode of this embodiment is demonstrated. FIG. 8 is a manufacturing process diagram of the positive electrode shown in FIGS.

  First, the hydrophilic / water-repellent oxygen reduction catalyst layer 4 in the region A2 and the hydrophilic / water-repellent porous structure 3 and the hydrophilic porous layer 3 installed between the oxygen-generating catalyst layer 2 in the region A1 and the electrolyte are formed. For this purpose, the lower half of the hydrophilic carbon fiber paper (CP) is immersed in a PTFE dispersion solution, dried and fired. Then, CP with the lower half water-repellent is obtained.

Next, IrO ₂ powder as an oxygen generation catalyst, PTFE powder as a pore-forming material, a conductive auxiliary material such as carbon black, and a resin binder such as polyvinylidene fluoride are dissolved and dispersed in a suitable solvent, and a paste-like solution is obtained. Is coated with an applicator or the like on one surface of the hydrophilic upper CP and dried to form an oxygen generation catalyst layer (charging reaction layer) 2. In this case, it is preferable to adjust the type of solvent and the viscosity of the paste so that the oxygen generating catalyst layer (charging reaction layer) 2 does not enter the inside of the hydrophilic upper CP (hydrophilic porous layer).

  Next, a hydrophilic / water-repellent oxygen reduction catalyst layer (discharge reaction layer) 4 is combined with a PTFE dispersion solution as a binder with a catalyst (PtC catalyst) in which platinum fine powder as an oxygen reduction catalyst is supported on CB. In order to improve the wettability with aqueous CP, prepare a solution in which a solvent such as ethanol is dissolved and dispersed, and paste it, and then dip the CP part in which the lower half is water-repellent and dry Thus, a reaction layer having two functions of discharge / charge can be formed.

  Finally, a gas diffusion layer 1 such as a PTFE filter is joined to the oxygen generation catalyst layer 2 side by hot pressing, whereby a positive electrode having two functions of discharge / charge can be obtained.

  In the positive electrode of this embodiment, as shown in FIGS. 6 and 7, the region facing the negative electrode 20 is divided into two equal parts, and the upper side is the region A1 and the lower side is the region A2. is not. For example, as shown in FIG. 9, the region facing the negative electrode 20 may be divided into eight equal parts, and the regions A1 and A2 may be alternately arranged.

  Moreover, as shown in FIG. 10, you may divide | segment into 4 in the shape of a rice field and arrange | position the area | region A1 and area | region A2 in a staggered pattern.

  In the case of FIGS. 9 and 10, the oxygen generation catalyst layer paste and the oxygen reduction catalyst layer paste can be easily prepared by screen printing or the like. Moreover, the area of area | region A2 (discharge reaction area | region) and area | region A1 (charge area | region) does not necessarily need to be equal. If the area ratio is larger, a larger charge / discharge current can be obtained. However, the ratio between the discharge area and the charge area may be adjusted according to the demand for the battery.

  Next, a comparative evaluation test of charge / discharge characteristics of the positive electrode of the present invention (Examples 1 and 2) and a conventional positive electrode (Comparative Example) will be described.

(Comparative example)
As a positive electrode of a comparative example, a conventional electrode was produced as follows. A hydrophilic CP (TGP H120; manufactured by Toray, 350 μm) is dip-coated with a solution obtained by diluting PTFE dispersion (D-210C; manufactured by Daikin, 60% by mass) three times with pure water. Baking was performed at 350 ° C. for 1 hour to prepare a water-repellent CP. To form a reaction layer for the discharge reaction, platinum-supported carbon (TEC10EA50E made by Tanaka Kikinzoku), pure water, ethanol, and PTFE dispersion (D-210C; made by Daikin, 60% by mass) in a mass ratio of 1: 7 The solution was mixed at a ratio of 20: 1 and uniformly dispersed and pasted with an ultrasonic homogenizer (hereinafter abbreviated as “discharge paste for paste”). The water-repellent CP prepared earlier was dip-coated with this discharge reaction layer paste, and was naturally dried at room temperature all day and night to form a discharge reaction layer. In this case, the oxygen reduction reaction during the discharge reaction occurs in the entire reaction layer impregnated and held with platinum-supporting carbon as a catalyst. Cut out the produced reaction layer for discharge into a rectangular size of 40mm in length and width, PTFE filter (PF060; manufactured by Advantech, φ70mm) is used as a gas diffusion layer, and an expanded metal made of titanium is sandwiched between the reaction layer, They were integrated by hot pressing (150 ° C., 10 MPa, 1 min.) To obtain a positive electrode for a comparative example.

Example 1
In order to form a reaction layer for charging reaction, iridium oxide powder (IrOx · (H ₂ O) y (SA = 50m ² / g); made by Tanaka Kikinzoku), pure water, ethanol, PTFE dispersion (D- 210C (made by Daikin, 60% by mass) mixed in a mass ratio of 1: 7: 20: 1, uniformly dispersed and pasted with an ultrasonic homogenizer (hereinafter abbreviated as “charge reaction layer paste”) ) Was produced. This charging reaction paste was applied to one side of an untreated hydrophilic CP using an applicator (gap 300 μm), and then naturally dried at room temperature all day and night to form an oxygen-generating catalyst layer. In this case, the oxygen generation reaction during the charging reaction occurs on the side of the surface on which iridium oxide as a catalyst is coated and held. Cut the produced oxygen generation catalyst (discharge reaction) layer into a rectangular size of 40 mm in length and width, use a PTFE filter as the gas diffusion layer, and install it so that the reaction layer surface is on the gas diffusion layer side, with titanium as the current collector The expanded metal made by sandwiching the two was integrated and integrated by hot pressing (150 ° C., 10 MPa, 1 min.) To obtain a positive electrode of Example 1.

(Example 2)
Half of the area is dipped in a solution of hydrophilic CP, diluted PTFE dispersion 3 times with pure water, dip-coated with PTFE solution, dried, and then baked at 350 ° C in the atmosphere for 1 hour. A half water-repellent CP was produced. The charging reaction layer paste was applied with an applicator (gap: 300 μm) to the upper half CP area which was not water-repellent and allowed to dry naturally all day and night. The water-repellent lower half CP region was dipped in the above-mentioned discharge reaction layer paste, dip-coated, and air-dried all day and night. The fabricated bifunctional reaction layer is cut into a rectangular size of 40 mm in length and width so that the area of the discharge reaction layer is equal to the area of the charge reaction layer, and a PTFE filter is used as the gas diffusion layer. It was installed so as to be on the layer side, and an expanded metal made of titanium was sandwiched as a current collector between them and integrated by hot pressing (150 ° C., 10 MPa, 1 min.) To obtain a positive electrode of Example 2.

<Evaluation test>
The positive electrode of the comparative example and Examples 1 and 2 was installed in a battery reaction evaluation apparatus as shown in FIG. 11, and the electrochemical performance was examined.

  In the battery reaction evaluation apparatus, both ends of a cylindrical acrylic container are closed with a PTFE sheet and PTFE filter paper, and an electrolytic solution is placed therein, and humidified air is supplied from the PTFE filter paper side.

10mol / L LiCl + 4.3mol / L LiOH is used as the electrolyte, 200cc / min. Of atmospheric air is passed through 1M LiOH as the supply gas, and de-CO ₂ air is allowed to flow to the back of the positive electrode. Evaluation was performed.

  First, as an electrochemical characteristic, the overvoltage during charging / discharging was evaluated. As the method, from the open circuit state, a current having a set constant current density is passed, and the charge / discharge voltage (E) of the positive electrode (WE) with respect to the reference electrode (RE) after 15 minutes is recorded. Thereafter, the circuit is returned to the open circuit state, and the open circuit voltage (E0) of the positive electrode (WE) with respect to the reference electrode (RE) after 10 minutes is recorded. The overvoltage was defined as the difference between the charge / discharge voltage (E) and the open circuit voltage (E0).

Next, in order to evaluate the voltage stability during charging of the positive electrode of Example 1, a constant discharge current density of ² mA / cm ² is maintained, and the charging voltage of the positive electrode (WE) with respect to the reference electrode (RE) Time changes were recorded.

<Evaluation test results>
FIG. 12 shows a plot of the overvoltage during charging / discharging of the positive electrode of various specifications measured according to the above procedure against the logarithm of the current density.

  Moreover, the time change of the charging voltage of the positive electrode of Example 1 and the positive electrode of a comparative example was shown in FIG.

  In FIG. 12, a curve Ac is an overvoltage characteristic during charging of the positive electrode of the comparative example, a curve Bc is an overvoltage characteristic during charging of the positive electrode of Example 1, and a curve Cc is an overvoltage characteristic during charging of Example 2. Further, the curve Ad is the overvoltage characteristic during discharge of the positive electrode of the comparative example, the curve Bd is the overvoltage characteristic during discharge of the positive electrode of Example 1, and the curve Cc is the overvoltage characteristic during discharge of Example 2.

  The smaller the overvoltage during charging / discharging, the more preferable as the characteristics of the battery positive electrode. From FIG. 12, the discharge overvoltage is the comparative positive electrode <the positive electrode in Example 2 <the positive electrode in Example 1; 1 positive electrode <positive electrode of Example 2 <comparative positive electrode.

  Therefore, the positive electrode of the comparative example has a problem that the overvoltage at the time of charging is high instead of the low overvoltage at the time of discharging. Moreover, the positive electrode of Example 1 has the problem that the overvoltage at the time of discharge is high instead of the low overvoltage at the time of charge. On the other hand, the positive electrode of Example 2 shows that the overvoltage at the time of charging / discharging is moderately low.

In FIG. 13, curve A is the positive electrode of the comparative example, and curve B is the positive electrode of Example 1. In the case of the positive electrode of the comparative example, a phenomenon in which the charging voltage increases in a relatively short time is observed. This is because the oxygen gas generated by the charging reaction is generated on the surface and inside of the positive electrode of the comparative example, and the electrolyte solution existing on the surface and inside of the positive electrode is removed, so that the OH ^- ion conductive path is cut. It is thought that this occurred. On the other hand, in the case of the positive electrode of Example 1, it can be seen that the charging voltage is low, the charging voltage is stable for a long time compared to the positive electrode of the comparative example, and the charging reaction proceeds stably.

DESCRIPTION OF SYMBOLS 1 ... Gas diffusion layer 2 ... Oxygen generation catalyst layer 3 ... Hydrophilic porous layer 4 ... Hydrophilic / water-repellent oxygen reduction catalyst layer

Claims (4)

A gas diffusion layer;
An oxygen generation catalyst layer laminated on the gas diffusion layer;
A hydrophilic porous layer laminated on the oxygen generating catalyst layer;
A positive electrode for a metal-air secondary battery, characterized in that the hydrophilic porous layer is exposed to an alkaline electrolyte.   The positive electrode for a metal-air secondary battery according to claim 1, wherein the oxygen generation catalyst layer is porous.   Furthermore, it has a hydrophilic / water-repellent oxygen reduction catalyst layer laminated | stacked on the said gas diffusion layer, The said hydrophilic / water-repellent oxygen reduction catalyst layer is arrange | positioned in the state exposed to the said alkaline electrolyte. Or the positive electrode for metal-air secondary batteries of 2.   4. The positive electrode for a metal-air secondary battery according to claim 1, wherein the thickness of the oxygen generation catalyst layer is in a range of 10 to 500 μm.

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