Air cathode for air depolarized cells

Metal/oxygen cells such as zinc/oxygen cells using air as the source of oxygen are provided with a cathode comprising a unitary member which, in a preferred embodiment, satisfies all of the diverse requirements for a cathode in this type of cell, the unitary member consisting of a hydrophobic portion and a hydrophilic portion comprising an ion exchange material which carries the desired amounts of an oxygen reduction catalyst. In a preferred embodiment, the oxygen reduction catalyst likewise serves as the current collector.

RELATED APPLICATION 
Cretzmeyer and Espig, Ser. No. 902,151, filed: May 5, 1978, for: 
"Metal/Oxygen Cells and Method for Optimizing the Active Life Properties 
Thereof", now U.S. Pat. No. 4,189,526. 
This invention relates to gas depolarized electrochemical cells and, more 
particularly, to an improved air cathode for use in an air depolarized 
cell. 
The technology underlying metal/oxygen cells is well known, and much effort 
has been directed to using this technology in a variety of applications. 
In some applications, such as batteries for hearing aids and the like, 
there are used small cells which are commonly referred to as button cells 
due to their button-like appearance. A typical button cell includes a 
casing having an aperture on one side of the cell, often termed the air 
side of the cell, through which air can enter the cell. A porous absorbent 
layer such as filter paper is typically positioned adjacent the aperture 
to promote even air distribution as the air passes into the cell. Adjacent 
to the absorbent layer is a cathode assembly, and many constructions are 
known. 
In general, the cathode assembly will include a thin hydrophobic layer such 
as unsintered polytetrafluoroethylene with a current collecting screen 
positioned within. The cathode assembly likewise often includes carbon 
black, a catalyst for the oxygen reduction desired for operation of the 
cell, and a hydrophobic binder. 
A separator is positioned adjacent to the cathode and separates it from the 
anode, typically zinc or amalgamated zinc powder onto which the requisite 
amount of electrolyte is placed prior to the final assembly of the cell, 
potassium hydroxide solutions commonly being used as electrolyte. Sealing 
of the cell to prevent electrolyte leakage through the cell and out the 
aperture is provided in some fashion; U.S. Pat. No. 3,897,265 is one 
example of such a button cell assembly. 
To optimize the active life characteristics of button cells, an oxygen 
diffusion limiting member such as sintered polytetrafluorethylene can be 
introduced on the air side between the absorbent layer and the cathode. 
The specific details are set forth in the copending Cretzmeyer et al. 
patent, previously identified. 
While the button cells are relatively thin, more recent potential 
applications are more demanding. Thus, such low current applications as 
digital and electric analog watches require even thinner cells than can be 
readily made by available technology. 
Moreover, the manufacture of the conventional type of cathode assembly is a 
somewhat tedious operation, requiring careful quality control to provide 
satisfactory assembly. The present method of making conventional cathode 
assemblies thus involves, in general, pressing a mixture of high surface 
area carbon, a polytetrafluoroethylene emulsion and a catalyst such as 
manganese oxide onto a nickel plated steel screen which is to serve as the 
current collector. A layer of unsintered polytetrafluoroethylene is then 
generally laminated onto what will become the air side of the resulting 
material. The overall thickness of the final cathode assembly may become a 
limiting factor in attempts to make thinner button cells which have 
satisfactory performance characteristics. 
Yet, despite the readily apparent impetus for developing improved methods 
of manufacture of cathodes which allow a thinner configuration, it must be 
appreciated that a variety of functions need to be satisfied by the 
cathode assembly. Thus, according to conventional theory, oxygen diffuses 
into a metal/oxygen cell and into the hydrophobic portion of the cathode. 
Then, under the influence of a concentration gradient caused by operation 
of the cell, oxygen diffuses further into the cathode structure where it 
reaches a hydrophilic portion containing an aqueous electrolyte, an oxygen 
reduction catalyst and an electronically conducting medium. In this 
region, oxygen is reduced by the transfer of electrons from the electronic 
conductor in response to a flow of electric current in a circuit external 
to the cell. To provide an efficient operation, it should be accordingly 
apparent that the cell cathode must incorporate (1) a means allowing an 
adequate supply of oxygen to reach the area where oxygen reduction takes 
place and, as well, a means of preventing flooding of the cell, i.e., - a 
layer of water or electrolyte developing between the incoming air and the 
area at which oxygen reduction takes place thereby limiting the supply of 
oxygen to a value less than that required for the desired current flow, 
(2) a means for providing the catalyst in a form capable of carrying out 
an efficient oxygen reduction and (3) a means for including a satisfactory 
current collector. Still further, the cathode assembly in cells used in 
applications requiring long useful lives must provide satisfactorily low 
water transport as is known. 
It has been proposed to incorporate ion exchange materials in metal/oxygen 
cells for a variety of purposes. As one example, U.S. Pat. No. 4,137,371 
utilizes an ion-exchaning membrane as a zincate restricting membrane to 
prevent poisoning of the electrochemically active material in a 
zinc/oxygen cell. The ion-exchanging membrane is joined directly to the 
oxygen electrode and is positioned between the porous layer of this 
electrode and the zinc electrode. 
U.S. Pat. No. 3,514,336 describes an electrochemical cell which utilizes an 
ion exchange resin matrix having macroporous channels containing a free 
electrolyte in the channels that is disposed between the anode and 
cathode. When used in a fuel cell, such matrix is said to act as mixed 
current carriers so that the resins cannot dehydrate as a result of 
endosmatic transport, thus providing an electrolyte which remains 
homogenous. 
A further use of ion-exchange materials is described in U.S. Pat. No. 
3,097,115 wherein natural and synthetic zeolites are utilized to form 
electrodes for fuel cells. The electrodes are formed, by, in general, 
shaping the electrode as desired, ion exchanging the naturally occuring 
ions from the zeolite with the desired activating metallic exchange 
properties. U.S. Pat. No. 3,097,116 discloses forming an electrode 
structure by bonding the heat stabilized, ion exchange zeolite to a gas 
diffusion membrane, which membrane may be either hydrophilic or 
hydrophobic. 
It is a primary aim of the present invention to provide a gas depolarized 
electrochemical cell which is characterized by an improved cathode 
construction. 
A further object provides a cathode for a metal/oxygen button cell which is 
thinner than available cathode assemblies. A related and more specific 
object provides a cathode having a thickness in the range of from about 
0.0005 to 0.003 inch. 
Yet another object of this invention lies in the provision of a facile 
method for constructing an oxygen cathode for a metal/oxygen cell. A more 
specific object is to provide a method for making a cathode which is 
amenable to continuous production. 
A still further object provides a cathode configuration having sufficient 
versatility to accommodate the particular performance requirements of a 
specific application. A more specific object provides a cathode member 
capable of achieving in a metal/oxygen cell a limiting current density in 
the range of from about 5 to 20 microamperes/sq.cm. of cathode area. 
An additional object of this invention lies in the provision of a cathode 
member capable of achieving the desired gas diffusion limitation. A more 
specific object is to provide a cathode member which effectively prevents 
the flooding of a metal/oxygen cell.

While the invention is susceptible to various modifications and alternative 
forms, there is shown in the drawings and will herein be described in 
detail, the preferred embodiments. It is to be understood, however, that 
it is not intended to limit the invention to the specific forms disclosed. 
On the contrary, it is intended to cover all modifications and alternative 
forms falling within the spirit and scope of the invention as expressed in 
the the appended claims. For example, while the present invention will be 
described in connection with zinc/air cells, and more specifically with 
zinc/air button cells, it should be appreciated that the invention is 
equally applicable to other metal/oxygen or air depolarized cells. 
Moreover, while the present invention will be primarily described in 
connection with the use of silver as the oxygen reduction catalyst, this 
is merely exemplary; and any other type of oxygen reduction catalyst could 
certainly be employed in conjunction with the cathode of this invention. 
In general, the present invention is predicated on the discovery that an 
improved air cathode configuration for a metal/oxygen cell can be provided 
by utilizing a unitary membrane having a hydrophobic portion and a 
hydrophilic portion comprising an ion-exchange material containing therein 
the desired oxygen reduction catalyst. In this fashion, all of the diverse 
requirements needed for a cathode member in this type of cell can be 
provided in a single member, yet allowing an overall configuration which 
is thinner than present cathode assemblies. 
Considering the present invention in greater detail, FIG. 1 illustrates a 
somewhat stylized view of the configuration of a cathode according to this 
invention. Thus, there is shown a cathode 10 having a hydrophobic portion 
12 and a hydrophilic portion 14 comprising an ion-exchange material having 
dispersed therein oxygen reduction catalyst particles 16. The cathode is 
formed from a single material and is thereafter treated to provide the 
hydrophobic and hydrophilic portions as will be hereinafter described. 
In accordance with one embodiment of this invention, the cathode is 
prepared by utilizing any of the materials used for the hydrophobic 
material in conventional cathode assemblies. In general, suitable 
hydrophobic materials should have relatively low carbon dioxide and water 
vapor transmission properties and sufficient oxygen transmission 
properties to support the necessary current requirements for the 
particular intended application. In this connection, many button cell 
applications involve discharge rates of from about 1 to about 35 
microamperes/square centimeter of cathode area. Electronic watch 
applications typically require from about 5 to about 7 microamperes/square 
centimeter of cathode area. As illustrative examples of suitable materials 
for these applications, there can be listed sintered 
polytetrafluorethylene, fluoroethylenepropylene polymers, and the like. 
Materials such as polyethylene, unsintered polytetrafluoroethylene and 
polypropylene may perhaps be useful. 
The thickness of the hydrophobic material used for the cathode may suitably 
range from about 0.0005 inch to about 0.003 inch. Of course, if thicker 
cathodes are needed or desired for any reason, thicknesses in excess of 
0.003 inch may certainly be utilized. Likewise, films of thicknesses less 
than 0.0005 inch may be found useful for some applications. In any event, 
the film utilized should be sufficiently pore-free to prevent electrolyte 
leakage to the oxygen side of the cell in use. 
When a hydrophobic material is used, at least one surface must be converted 
to provide the necessary ion-exchange portion 14. This may be accomplished 
by any known means. As one example, when utilizing sintered 
polytetrafluoroethylene, styrene may be radiation grafted onto the polymer 
backbone; and the grafted portion of the resulting material can be then 
reacted with chlorosulfonic or sulfuric acid to convert such grafted 
portion to the H.sup.+ form of a cation exchange material. As may be 
apparent, the amount of styrene grafted onto the membrane should be 
sufficient to provide the number of ion-exchange sites needed for the 
desired amount of catalyst. To accomplish this objective, the amount of 
styrene should be in the range of 20 to 50%, based upon the dry weight of 
the untreated membrane; the concentration of acid used should be in 
sufficient excess to sulfonate at least a majority of the aromatic rings 
of the styrene molecules present with at least one sulfonic acid group. 
Likewise, the hydrophobic material may be converted to an anion-exchange 
material, as can be achieved by known techniques. This, of course, 
requires that the silver or other oxidation reduction catalyst used, be 
introduced in anionic form. 
The ion-exchange portion may thus be either cationic or anionic in form, 
depending on the particular method utilized. It may be theorized that the 
anionic form would provide better permeability properties. On the other 
hand, the cationic form may well involve a more facile preparation 
technique and result in a somewhat more stable cathode assembly over a 
longer time period than would result when using an anionic-exchange 
material. 
As general functional requirements, the ion-exchange portion should be 
stable in the presence of the type of electrolyte being utilized, 
typically a strong caustic such as KOH. In addition, the ion-exchange 
portion should be sufficiently permeable to water and hydroxyl ions so 
that an efficient oxygen reduction reaction can take place. 
The particular depth of the ion-exchange portion can be varied as desired; 
but, in any event, there should be left a hydrophobic portion thickness 
sufficient to prevent leakage of the cell as well as satisfying the other 
characteristics as described herein. As is known, process parameters in 
forming the exchange portion, such as temperature, strength of reagent, 
and residence time will affect the depth of the ion-exchange portion which 
results. 
As a processing technique, to insure that a satisfactory hydrophilic 
portion is formed, it may be desirable to first convert a hydrophobic 
membrane to a hydrophilic material throughout its thickness. The desired 
hydrophobic portion can then be provided by reconverting part of the 
hydrophilic layer to a hydrophobic character. Suitable reconversion 
techniques are well known, an example being treatment of a sulfonated 
material with steam or immersion in boiling water. 
The hydrophilic, ion-exchange portion may then be converted to the form 
carrying the oxygen reduction catalyst, or its precursor. Silver, platinum 
and oxides of manganese are illustrative examples of suitable catalysts, 
silver being preferred for alkaline and platinum for acidic cells. Many 
techniques for such conversion are well known and allow the conversion to 
be carried out in a straightforward manner with continuous production. 
However, in accordance with a preferred embodiment of this invention, the 
conversion is achieved by a novel technique which disperses the catalyst 
in an extremely uniform manner throughout the hydrophilic portion and 
allows as much catalyst to be incorporated as is desired. To this end, the 
initial step comprises converting the cation-exchange resin to a form 
containing counter ions capable of reducing the oxidation catalyst 
precursor to its active form. The catalyst precursor ions diffuse into the 
membrane and are there reduced to the catalytically active form. After 
washing to remove the anions present, the next step involves treating with 
a salt of the catalyst precursor. The salt used should be soluble in water 
or other solvent employed so that the reaction will proceed at a 
satisfactory rate and should be employed in sufficient excess to insure 
that at least the majority of particles of oxidation catalyst precipitate 
within the hydrophilic portion rather than on the surface thereof. After 
washing to remove the anions present in the oxidation catalyst precursor 
salt, the membrane may be placed in service in a cell or, if desired, the 
membrane may be further treated by contacting with the electrolyte to be 
used. Oxides of the oxidation catalyst, if present within the membrane, 
may be chemically or electrochemically reduced, if desired. As an 
illustrative example, formalin may be used to chemically reduce such 
oxides. 
Considering an illustrative species, when silver is the desired oxidation 
catalyst, the conversion of the hydrophilic portion of the membrane can be 
carried out as follows. The conversion of the ion-exchange membrane from 
its cation form (typically--H.sup.+) to the form containing counter ions 
capable of reducing Ag.sup.+ ions may be accomplished by treating the 
membrane with an aqueous solution or SnCl.sub.2, a 0.5 of 1 molar solution 
being satisfactory. Thorough washing with distilled water will readily 
remove the Cl.sup.- anions present so as to obviate precipitation of AgCl 
in later processing. 
Incorporation of particles of silver dispersed within the hydrophilic 
portion of the membrane is then accomplished by contacting with an aqueous 
solution of a soluble silver salt such as AgF. A 0.5 or 1 molar solution 
may be usefully employed; but to insure that the silver particles are 
dispersed within the membrane, the soluble silver salt should be of 
sufficient concentration to insure that the Ag.sup.+ ions are present in 
an excess in relation to the Sn.sup.++ counter ions. 
After washing to remove the fluoride anions present, as with distilled 
water, the membrane is ready for use. However, the membrane may contain 
some silver present as Ag.sub.2 O; and, if desired for a particular 
application, the membrane may be treated prior to use eliminate the 
presence of such Ag.sub.2 O as has been described herein. 
The resulting membrane will contain silver particles uniformly dispersed 
within the membrane in a highly active catalytic state, perhaps 
approaching an atomic state. If desired, the procedure may be repeated to 
increase the content of silver particles. 
Alternatively, the conversion may be carried out by immersing the 
ion-exchange portion of the cathode membrane in a silver nitrate solution. 
After several rinsings with deionized water, the Ag.sup.+ ions are 
converted to solid, dark brown Ag.sub.2 O by immersing the ion-exchange 
portion in a KOH solution. The resulting cathode material may be rinsed in 
deionized water, dried in air at room temperature and stored until needed 
for use. 
As may be appreciated, cells including Ag.sub.2 O in the cathode member 
initially function as zinc-silver oxide cells. However, after a period of 
operation, the cell will be converted to a zinc/oxygen cell. 
Alternatively, if desired, the silver oxide can be reduced. The result is 
that the silver catalyst particles are dispersed throughout the ionic 
exchange portion of the cathode member so as to permit an efficient 
catalytic reduction. This procedure is not, however, preferred since it is 
more difficult to insure that the catalyst particles are optimally 
dispersed within the membrane. 
In accordance with a preferred embodiment of the present invention, 
sufficient silver is ultimately provided so that the particles are 
sufficiently close together so that the catalyst can likewise serve as the 
current collector. Satisfactory closeness of the particles is readily 
apparent from the current density performance. Of course, if desired, a 
separate current collector may be employed, positioned on the hydrophilic 
side of the cathode. 
FIG. 2 shows the use of the cathode 10 of this invention in a zinc/air cell 
of conventional construction. The cell, shown generally at 18, includes 
the cathode 10 positioned in a casing comprising an anode can 20 and a 
cathode can 22, the hydrophobic portion 12 of the cathode being adjacent 
the exterior or the air side of the cell. Air ingress into the cell is 
achieved through apertures 24. The separator 26 is positioned adjacent the 
hydrophilic side 14 of the cathode 10 and separates the zinc anode 28 from 
the cathode. Suitable sealing against leakage is provided by gasket 30. If 
desired, an air diffusing membrane 32, e.g.--filter paper, may be 
interposed between the hydrophobic portion 12 of the cathode 10 and the 
cathode can 22, as is shown. 
The following Examples are illustrative, but not in limitation of the 
present invention. Unless otherwise indicated, all percentages are by 
weight and all solutions are aqueous. 
EXAMPLE 1 
This Example illustrates the current density-voltage characteristics which 
are capable of being achieved by utilizing the cathode configuration of 
the present invention. 
Two 0.001 inch thick sintered polytetrafluoroethylene films were modified 
by gamma radiation grafting with styrene to provide a level of styrene in 
the range of 30% based upon the dry weight of the untreated films. One 
side was then masked with masking tape, and the films were submerged into 
a dilute solution of chlorosulfonic acid in methylene chloride 
(e.g.--about 1 to 5% chlorosulfonic acid) for about 15 to 30 minutes. 
After this sulfonation, with the masked side having retained its 
hydrophobic characteristics, the films were washed in methanol and boiled 
in water for one hour. 
Cathodes were then formed from each film by boiling for one hour in a 5% 
KOH solution, rinsing in distilled water, immersing at room temperature in 
0.5 M SnCl.sub.2 for one hour, immersing in 0.5 M AgNO.sub.3 for three 
hours, immersing in 40% KOH solution for one hour, and soaking in formalin 
for six days. 
The resulting films were then dried in air at room temperature and then 
placed in conventional, commercial button cells as illustrated in FIG. 2. 
Specifically, the cathode was positioned adjacent the air aperture of the 
cell casing with its hydrophobic side positioned towards the air aperture. 
A separator was positioned between the zinc anode and the hydrophilic 
surface of the cathode. 
FIG. 3 shows the current density-voltage characteristics achieved for the 
two cells, the performance of one cell being indicated on the graph by 
triangles and the other by circles. As can be seen, the cell 
characteristics are adequate for such low current drain applications as 
electronic watch applications. 
EXAMPLE 2 
This further illustrates the preparation of cathodes in accordance with 
this invention and the performance in conventional button cells. 
Cathodes were prepared from 0.001 inch thick polytetrafluorethylene 
modified as in Example 1 by the same technique as set forth in Example 1. 
Specifically, the films were boiled in a 5% KOH solution for one hour, 
rinsed in distilled water, immersed at room temperature for about seven 
hours in a 0.5 M SnCl.sub.2 solution, rinsing in distilled water, 
immersing in a 0.5 M AgF solution for 12 hours, immersing in a 45% KOH 
solution for 11/2 hours, and soaking in a 40% formalin solution for six 
days. 
After drying as in Example 1, the cathodes were positioned in cells as 
likewise described in Example 1. FIG. 4 sets forth the current 
density-voltage characteristics for the two cells tested, the symbols used 
to differentiate the cells being as described in Example 1. Again, the 
cells exhibited characteristics suitable for low current drain 
applications such as electronic watches. 
Thus, as has been seen, the present invention provides a facile procedure 
which can be easily adapted to continuous processing for making an air 
cathode thinner than previous configurations. Because of the ability to 
use various polymeric materials as the hydrophobic base as well as the 
variety of ionic exchange members which may be utilized, the variations in 
the properties which can be achieved are relatively unlimited. Moreover, 
if desired, the cathode configuration can combine in one member not only 
the function of water transfer limitation (i.e.--obviating flooding) and 
catalytic oxygen reduction but can also incorporate the necessary current 
collection. Moreover, by the method of preparation, a highly and uniformly 
dispersed catalytic surface is obtained.