GB2059141A

GB2059141A - Metal-air electric cell

Info

Publication number: GB2059141A
Application number: GB8022619A
Authority: GB
Original assignee: Gould Inc
Current assignee: Gould Inc
Priority date: 1979-07-27
Filing date: 1980-07-10
Publication date: 1981-04-15
Also published as: JPS5622055A; FR2462784A1; DE3025476A1

Abstract

A metal air electric cell uses cobalt phthalocyanine as a catalyst in its cathode assembly to promote electrochemical reduction of the positive active material, oxygen, and obtains increased output with extended service life. A button cell (10) comprises a cathode cup (11) secured to an anode cup (12) with an insulating seal (14) in between. The anode cup (12) contains a metallic material (13) mixed with electrolyte and this is separated from a cathode assembly (15) by a separator (33) which prevents electronic conductance but does not inhibit ionic transport. The cathode assembly (15) includes cathode material (32) incorporating cobalt phthalocyanine catalyst and encompassing a current collector grid (31) electrically connected to the cathode cup (11). A permeable hydrophobic member (30) is provided between the cathode assembly (15) so that gas entering and leaving the cell via openings (20,21) in the cathode cup (11) must pass through the hydrophobic member (30), and a diffuser (16) provides for even distribution of gas entering the openings (20,21) to the cathode assembly (15). <IMAGE>

Description

SPECIFICATION Metal air electric cell BACKGROUND OF THE INVENTION 1. Field of the invention The present invention concerns generally the use of a catalyst in a metal air electric cell, and more particularly, the use of cobalt phthalocyanine (CoPc) for effecting electrochemical reduction of atmospheric oxygen in the air cathode assembly.

2. Description of the priorazt In the prior art, the use of treated or "activated" cobalt phthalocyanine in the electrodes of a fuel cell is known. The term "fuel cell" is used herein and in the art to denote a device, system or apparatus wherein the chemical energy of the fluid combustible fuel is electrochemically converted to electric energy at a non-sacrificial or inert electrode. The true fuel cell is adapted for continuous operation and is supplied with both fuel and oxygen from sources outside the cell proper. Such cells include at least two non-sacrificial or inert electrodes functioning as an anode and the cathode is an electrolyte which provides ionic conduction between the electrodes. Electronically conductive means are provided for electrode connection between the anode and the cathode external to the electrolyte.

Furthermore, it is necessary to treat or "activate" the cobalt phthalocyanine in order to improve its catalytic activity in fuel cells. Examples which detail the procedures necessitated when using cobalt phthalocyanine in a fuel cell environment are Richter, U.S. Patent No.3,585,079 and Jasinski, "Journal of the Electrochemical Society" 112(1965), pages 526-528.

The term "metal air electric cell" is used herein and in the art to denote a device or apparatus which unlike the aforementioned fuel cell does not use a non-sacrificial or inert negative electrode, nor is it continuously supplied with fuel from a source outside the cell proper. The metal air electric cell has an anode made of metallic material which is electrochemically oxidized as the cell produces electrical energy. The reduction reaction takes place in the air cathode of the metal air electric cell, which is supplied with oxygen by the diffusion of air into the cell.

It is well-known that catalysts are employed to promote the electrochemical conversion taking place at the air electrode in the cell. Additionally, the material used as catalysts must also exhibit certain other properties.

For example, the material must be resistant to attack from the electrolyte in the cell.

Good peroxide decomposing properties are also desirable in a catalyst in order to increase cell voltage and to prevent possible capacity loss. The electrical potential of the air cathode is associated specifically with the formation of peroxides and is reversible with respect to the latter The reaction involving the air cathode in an acidic electrolyte can be written as: 2 + 2H* + 2e = H202 and in an alkaline electrolyte as: O2 + H2O + 2e HO2 - + OH# From which, the standard reduction potential 250C in an acid electrolyte is: Poz [ Htl2 EO =E - 0.0295 log [ H2O2 ] and in an alkaline electrolyte is:: POp E =E - 0.0295 log [OH-] [ HO2- ] An excess peroxide concentration will significantly depress the cell voltage. Additionally, since peroxide reacts with the anodic material, an excess peroxide concentration could result in a capacity loss. In order to minimize these effects, peroxide reduction or decomposition catalysts are incorporated in the air cathode.

Furthermore, the oxygen produced by the decomposition of the peroxides is used electrochemically in lieu of equivalent atmospheric oxygen in the cathode assembly.

In order for a material to be a suitable catalyst, many other practical criteria must also be met. For example, commercial acceptability demands the material be relatively inexpensive in the quantities requires as well as convenient to use.

An example of a metal air electric cell is the commercially available zinc air button cell. This button cell is made of an anode cup containing amalgamated zinc, mated with a cathode cup containing the air cathode assembly. A catalyst commonly used in the cathode assembly in manganese dioxide (MnO2). There are one or more small air holes in the cathode cup which restrict passage of air into the cell, thereby limiting the atmospheric oxygen reaching the air cathode assembly. In general, the limiting current of the cell increases as the surface area of the air holes is increased. However, as the surface area of these air holes is increased, the permeation rates of water vapor and carbon dioxide also increase. Excessive transfer of these gases has an adverse effect on the cell capacity and subsequently, its service life.Therefore, enlarging the surface area of the air holes is undesirable.

Because of these limitations, zinc air button cells do not have the ability to provide adequate power to support certain devices, like hearing aids with push-pull type amplification which draw about ten milliamperes, without substantially shortening their service life. Once the surface area of the air holes is increased in order to provide the higher limiting current required by such devices, the environmental tolerance of the cell is drastically lowered and its service life unacceptably shortened.

Brief summary of the invention The invention comprises a metal air electric cell which uses cobalt phthalocyanine as a catalyst in a cathode assembly to promote electrochemical reduction of the positive active material, namely oxygen. The cell includes an anode containing the metallic material in contact with an electrolyte and electrically connected with a first external contact. A separator keeps the anode and the cathode assembly apart, preventing electronic conductance but not inhibiting ionic transport. The cathode assembly is electrically connected with a second external contact. These elements are contained in a housing generally impervious to gas and liquid, but having gas diffusion means for allowing atmospheric oxygen to enter and leave the cathode assembly by diffusion.

An object of the present invention is to provide metal air electric cells with higher voltages at reasonable current drain.

Another object is to increase service life and prevent loss of capacity in metal air electric cells.

Still another object is to provide metal air button cells with higher limiting currents without an increase in surface area of the air holes.

A still further object is to provide an inexpensive, stable catalyst which is simple and convenient to use in the metal air electric cells.

Another object is to provide a catalyst with superior peroxide decomposing properties in metal air electric cells.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

Brief description of the drawings Figure 1 is a cross-sectonal view of a metal air button cell; Figure 2 is a top view of a metal air button cell; Figure 3 is a bottom view of a metal air button cell; Figure 4 displays the data from a half-cell polarization of an air cathode; Figure 5 displays the polarization curves for metal air button cells prepared with cobalt phthalocyanine as the catalyst, compared to cells prepared with manganese dioxide as the catalyst; and Figure 6 displays the cell discharge curves for the metal air button cells prepared with cobalt phthalocyanine as the catalyst, compared to cells prepared with manganese dioxide as the catalyst.

Description of the preferred embodiment Referring to Figures 1,2, and 3, reference numeral 10 generally designates a metal air button cell containing a cathode cup 11, an anode cup 12, anode material 13, a separator 33 and a cathode assembly 15.

Althoughaparticular configuration for these elements is shown in the figures as a "button" type cell, the housing of our metal air electric cell is not limited to any particular design or size.

The anode cup 12 contains an electrolyte mixed with the anode material 13 so that they are in contact.

Means for electrically connecting the anode material 13 with a first external contact is provided in the preferred embodiment by the anode cup 12.

The anode material is separated from the cathode assembly 15 by a separator 33 which is disposed between said anode material and said cathode assembly. The separator 33 prevents electronic conductance, but does not inhibit ionic transport. The anode material 13 in the preferred embodiment includes zinc or amalgamated zinc, which is mixed with the electrolyte, potassium hydroxide.

Although the preferred embodiment of the invention includes zinc or amalgamated zinc as the metallic material in the anode, no limitation is intended. For the sake of example and not for limitation, other metallic materials which can be used include iron, cadmium, magnesium, lead, aluminum, calcium, gallium, indium and their suitable compounds or mixtures. Metallic material includes any material comprising a metal, whether the metal be in pure form, or in a compound or in a mixture with other constituents.

It is particularly significant, that the cathode assembly 15 includes at least catalytic amounts of cobalt phthalocyanine. Additionally, the cathode material 32 typically contains, but is not limited to, carbon and a hydrophobic binder which is dispersed throughout the cathode.

Means for electrically connecting the cathode assembly 15 with a second external contact is preferably; but not limited to, an electrically conductive current collecting member 31, forming a a low resistance electrical contact with the cathode cup 11. The current collecting member 31 may be of any well-known type but there is shown, by way of example, a nickel grid or screen.

A cathode cup 11 is substantially circular in cross-section and has an annular flat portion 24 which slants radially inward from vertical side 26 to meet a crown 23. It is particularly important that a gas diffusion means allows gas to enter and leave the cathode assembly 15 by diffusion. The preferred embodiment of the gas diffusion means are openings 20 and 21 in the crown 23. It should be understood that while two openings are shown, this is not to be interpreted as a restriction on the gas diffusion means. Additionally, the number and surface area of these openings can be varied within a wide range to obtain desired cell performance.

Of significance is the gas limiting means limiting the rate at which gas enters and leaves the cathode assembly 15. Decreasing the surface area of the openings 20 and 21 in the crown 23 to limit the diffusion of gas into the cells is the preferrred embodiment. But, it should be understood that while two small openings can be used as the gas limiting means, this is not to be interpreted as a restriction. Additionally, the number and surface area of these openings can be varied within a wide range to obtain the desired cell performance.

In some applications it may be desired to include a hydrophobic member 30 between the cathode assembly 15 and the gas diffusion means shown in the preferred embodiment as openings 20 and 21,so that gas entering and leaving the cell must pass through the hydrophobic member. Typically, a polymer such as porous polytetrafluoroethylene is used. However, other hydrophobic materials are also suitable for use with these types of metal air cells.

In Figure 1, the bottom portion 27 of the cathode cup 11 has been deformed radially inward to interlock with insulator 14 and the anode cup 12. The insulator 14 includes an annular member which not only prevents electrical contact between cathode cup 11 and anode cup 12, but also forms an electrolyte seal.

Thus, the cell is impervious to gas and liquid except for the gas diffusion means.

The embodiment of Figure 1 is also shown with an additional feature which is not necessary to the invention but may be desirable in the preferred embodiment. This added feature comprises a porous absorbent material such as a blotter 16 which is placed on the gas access side of the cathode assembly 15 to promote even air distribution as the air passes into the cell.

Having described the invention in general terms, the following examples are set forth to more fully illustrate the preferred embodiments of the invention. These examples, however, are not meant to be limiting. It is possible to produce still other embodiments without departing from the inventive concept herein disclosed. Such embodiments are within the ability of one skilled in the art.

Example 1 Figure 4 displays the data from a half cell polarization of an air cathode. The half cells tested were identical except for the catalyst used in the cathode. The construction of the half cells included the use of zinc reference electrodes, a nickel counter electrode, and 30% potassium hydroxide as the electrolyte. The electrode area totalled 2.0 sq. cm. The current collector was 12% Nickel Ply Wire Cloth 40 x 40 mesh, .006 wire from National Standard Corporation. In one group of cathodes, the cobalt phthalocyanine catalyst (5-10% by weight of the final catalyst mix) was deposited on Cabot Vulcan XC-72 Carbon Black (70-75% by weight) supplied by Cabot Corporation, from a concentrated sulfuric acid solution by diluting with ice water.

Sufficient DuPont Teflon 30-B dispersion was added until it reached 20% by weight of the final catalyst mix.

The mix was dried and cathodes were fabricated with two layers of DuPont Teflon backing (Dodge E-125 unsintered Teflon tape, .004 inch thick) and two layers of separator material (one layer each of Pellon 2563 from Pellon Corporation and Celgard K72-2 from Celanese Company). In another group of cathodes, the manganese dioxide catalyst (10% by weight of the final catalyst mix) was deposited on Cabot Vulcan XC-72 Carbon (70% by weight) by decomposing aqueous manganese nitrate. The same procedure previously explained was then followed.

As the data demonstrate, the use of cobalt phthalocyanine as a catalyst in metal air cells results in unexpectedly higher voltages for a given current density when compared to the prior art cells.

Example 2 Metal air button cells were made using amalgamated zinc (approximately 3.5% Hg) as the metallic material. The dimensions of these cells were .455" in diameter and .210" in height. 135 microliters of 30% potassium hydroxide was used as the electrolyte. There were two air holes in the cathode cup, each .015" in diameter. Two groups of cells were tested under identical conditions, except the cathode assembly of one group contained 10% cobalt phthalocyanine, 20% polytetrafluoroethylene, and 70% carbon black, while the cathode assembly of the other group contained 11% manganese dioxide, 20% polytetrafluoroethylene, and 70% carbon black. The polarization curves for these two groups of cells are displayed in Figure 5.

As the data demonstrate, the use of cobalt phthalocyanine as a catalyst in metal air button cells results in unexpectedly higher limiting current for the same surface area of air openings when compared to the prior art catalyst. The amount of cobalt phthalocyanine was varied between 5 and 15% with identical results.

Example 3 The rates of peroxide decomposition in 30% potassium hydroxide on carbon, 11% manganese dioxide on carbon, and 10% cobalt phthalocyanine on carbon have been determined and set forth in the table below. All mixes contained 20% by weight polytetrafluoroethylene. The experimental technique used to measure these rate constants utilized a standard addition of peroxide to a mixture of catalyst mix and 30% potassium hydroxide electrolyte. While stirring the solution, the peroxide concentration was subsequently determined by the indirect iodine method. In this method, the iodine produced by the reaction of the peroxide with 1- was treated with a standard thiosulfate solution. A plot of 1 n (H202) vs. time yields a straight line. The rate constants are calculated from the slope of this line.

The results show that the rate of peroxide decomposition on cobalt phthalocyanine is three times that of 11% manganese dioxide on carbon. It is noteworthy that when the higher molecular weight of cobalt phthalocyanine compared to manganese dioxide is taken into account, the peroxide decomposing ability of cobalt phthalocyanine is 18 times that of manganese dioxide per molecule of catalyst.

TABLE 1 Rate constants for peroxide decomposition in 30% KOH Catalyst mix k,sec#1 B.E.T. Surface Area (m2lgm) none 5.4x105 - 1 gram carbon 6.4x 10-5 128 lgraml0%MnO2oncarbon 1.7 x 10-3 60 1 gram 10% CoPe on carbon 5.7 x10 66 The superior peroxide decomposing property of cobalt phthalocyanine is clearly evident from our data.

These results may in part explain the superior performance of cobalt phthalocyanine as a catalyst in metal air cells.

Example 4 Two groups of metal air button cells were prepared as described in Example 2. The composition of the cathode assembly of one group, however, was changed to 5% cobalt phthalocyanine, 20% polytetrafluoroethylene, and 75% carbon black. The cell discharge curves as 10 milliamperes are displayed in Figure 6.

As evidenced by these data, the present invention significantly overcomes the limitations of the prior art cells. Metal air cells using cobalt phthalocyanine as a catalyst can now be used in devices which demand high voltages and high limiting currents without sacrificing service life and cell capacity.

Although the test cells of the preceding examples are primary cells, this is by no means a limitation.

Secondary metal air cells using cobalt phthalocyanine as a catalyst achieve the same unexpected superior performance.

In the preparation of metal air cells using cobalt phthalocyanine, either the monomeric or polymeric form may be used. In the preferred embodiment the monomeric cobalt phthalocyanine was used. Cobalt phthalocyanine is a relatively inexpensive catalyst. It requires no treatment or "activation" prior to its use in the cell. Since cobalt phthalocyanine is stable in acidic or alkaline electrolyte, it will not deteriorate in a metal air cell. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

Claims

1. A metal air electric cell comprising a housing generally impervious to gas and liquid; an electrolyte located in said housing; an anode located in said housing in contact with said electrolyte, said anode comprising a metallic material; means for electrically connecting said anode with a first external contact; a cathode assembly located in said housing comprising at least catalytic amounts of cobalt phthalocyanine; a separator disposed between said anode and said cathode assembly, preventing electronic conductance but not inhibiting ionic transport between said anode and said cathode assembly; means for electrically connecting said cathode assembly with a second external contact; and said housing further comprising gas diffusion means allowing gas to enter and leave said cathode assembly by diffusion.

2. A cell as recited in Claim 1, wherein said metallic material comprises zinc.

3. A cell as recited in Claim 1, wherein said electrolyte comprises an alkaline material.

4. A cell as recited in Claim 1, wherein said cathode assembly further comprises carbon.

5. A cell as recited in Claim 1, wherein said cathode assembly further comprises a hydrophobic member placed between said cathode assembly and said gas diffusion means, so that the gas entering and leaving said cathode assembly must pass through said hydrophobic member.

6. A cell as recited in Claim 1, wherein said cathode assembly further comprises an electrically conductive current collecting member.

7. A cell as recited in Claim 1, wherein said gas diffusion means comprises an opening in said housing.

8. A cell as recited in Claim 1, wherein said gas diffusion means comprises gas limiting means for limiting the rate at which gas enters and leaves said cathode assembly.

R. A cell as recited in Claim 8, wherein said gas limiting means comprises an opening in said housing.

10. A metal air button cell comprising an anode cup and a cathode cup; said anode cup comprising a surface for engaging said cathode cup and forming an external anode contact; metallic material located in said anode cup; an electrolyte in contact with said metallic material; said cathode cup comprising a surface for engaging said anode cup and forming an external cathode contact; a cathode assembly located in said cathode cup and comprising at least catalytic amounts of cobalt phthalocyanine; a separator disposed between and separating said metallic material and said cathode assembly, preventing electronic conductance but not inhibiting ionic transport between said metallic material and said cathode assembly; insulating means to insulate said external anode contact and said external cathode contact;; gas diffusion means allowing gas to enter and leave said cathode assembly by diffusion; sealing means for combining said anode cup and said cathode cup to produce an electric button cell generally impervious to gas and liquid.

11. A button cell as recited in Claim 10, wherein said metallic material comprises zinc.

12. A button cell as recited in Claim 10, wherein said electrolyte comprises an alkaline material.

13. A button cell as recited in claim 10, wherein said cathode assembly comprises carbon.

14. A button cell as recited in Claim 11, wherein said cathode assembly further comprises a hydrophobic member disposed between said cathode assembly and said gas diffusion means so that the gas entering and leaving said cathode assembly must pass through said hydrophobic member.

15. A button cell as recited in Claim 11, wherein said cathode assembly further comprises an electrically conductive current collecting member.

16. A button cell as recited in Claim 10, wherein said gas diffusion means comprises a gas limiting means for limiting the rate at which gas enters and leaves said cathode assembly.

17. A button cell as recited in Claim 16, wherein said gas limiting means comprises an opening in said cathode cup.

18. Our invention as substantially shown and described.

19. A metal air electric cell wherein the cathode assembly includes cobalt phthalocyanine arranged to serve as a catalyst to promote electrochemical reduction of the positive active material, oxygen.