Manufacture of Air Oxygen Electrodes

The invention relates to a method of manufacturing an air oxygen electrode which consists of a porous polytetrafluorethylene foil and a catalyst layer attached thereto and essentially containing activated carbon.

The positive electrodes provided for galvanic air oxygen elements are customarily composed of two layers, one of which contains the catalyst material and constitutes the actual air electrode while the other, mostly a gas transmissive hydrophobic synthetic plastic foil, serves as the carrier for the catalyst layer and simultaneously separates the interior of the cell from the ambient atmosphere.

In a known air oxygen electrode of this type the carrier is a porous polytetrafluorethylene foil. The catalyst layer consists of a mixture of activated carbon and polytetrafluorethylene powder which is rolled onto a current take-off fabric of silvered nickel wire.

In accordance with prior conventional practice, the polytetrafluorethylene foil was pressed onto the catalyst layer by means of a quilting ram. For this purpose, the quilting ram was so shaped that regions of high compressive force, good adhesion and low permeability alternated with regions of low compressive force without adhesion and high permeability.

Thus, for firm joining of the two layers a nonhomogenous electrode material had to be taken into account. In addition this method was subject to the undesireable property of the rolled polytetrafluorethylene, which is that it exhibits in regions of high density a tendency to rupture in the direction of rolling even under very low deformation. This always led to lack of tightness of the cells.

In the regions of low adhesion or no adhesion at all, an electrolyte film can form between the polytetrafluorethylene foil and the catalyst layer which interrupts the air supply. Whenever the electrode then also drops below a predetermined operating potential, there can occur gas evolution and a lack of tight sealing due to build up of pressure.

Accordingly, it is an object of the present invention to join the rolled polytetrafluorethylene foil to the catalyst layer without destruction of its pore system in tight adhesion and to eliminate the above-mentioned threats to reliable electrode operation.

This and other objects that will appear are achieved in accordance with the invention by applying the polytetrafluorethylene foil to the catalyst layer between rollers in such manner that the pores of the polytetrafluorethylene foil are filled at least during the period of roller application with a wetting, easily volatilizable liquid.

In this roller process, the polytetrafluorethylene foil and the catalyst layer are removed in tape configurations continuously from respective storage reels and are united uniformly and tightly into an electrode tape.

Suprisingly it has been found that the pore system of the foil does not suffer any damage under the roller pressure when it is filled, before the foil enters the nip of the rollers, with a wetting, easily volatilizable liquid. For this purpose there are fundamentally suitable all liquids having low boiling point and low reactivity, preferably paraffin carbonates in the range C.sub.3 to C.sub.8. In particular, light benzene or petroleum ether are suitable, which volatilize without leaving a residue a few seconds after the finished electrode tape leaves the roller mechanism. As a minimum, there should remain within the electrodes no adsorbates which would cause marked reduction in catalytic activity of the active carbon in the catalyst mass.

To prevent partial pressing of the liquid out of the pores, during the roller treatment, it has proven advantageous to roughen the roller which faces the polytetrafluorethylene foil by sand blasting or the like. By these means the micropores in the foil surface are kept in existence to a great degree and it acquires a napped appearance.

By means of the roller pressure the permeability of the polytetrafluorethylene foil, which is preferably unsintered can be varied within wide limits. This is important in view of the exchange processes with the atmosphere and the type of load for which the cell is intended.

By means of the roller process embodying the invention, the embedding of the wire mesh serving as current take off in the catalyst layer is also appreciably improved, as compared with its condition in the tape-like starting material. This surprising effect is evidently also attributable to the pore filling liquid because it additionally serves as a lubricant.

For further details, reference is made to the accompanying drawing wherein the single FIGURE diagramatically illustrates a roller system embodying the invention.

In this FIGURE, the storage reel 1 has a polytetrafluorethylene foil 3 unwinding from it and the storage reel 2 has a catalyst tape 4 unwinding from it. These tapes are accurately positioned relative to each other by means of the closely adjacent guidance plates 5, 6 and are then supplied to rollers 7, 8. Guidance plates 5, 6 have upturned rims which impart to them a flat U-shaped profile. Roller 7 is slightly roughened. Both rollers are made of steel. Another suitable material for the rollers--preferably for the roller adjacent to the polytetrafluorethylene foil--is rubber or an elastic plastic. Before the polytetrafluorethylene foil 3 enters roller nip 9 there is lightly pressed onto it a sponge 11, soaked with a pore filling liquid 10. An adjustable supply 12 refills liquid 10 from a storage container 13. Catalyst tape 4, on the other hand, remains unaffected by this liquid treatment because it is covered by guidance plate 5 until just before entering between the rollers. The finished electrode tape 14 is taken up by storage reel 15.

In contrast with the method of ram pressing, the roller process embodying the invention provides several significant advantages. The overall thickness of the air oxygen electrodes is diminished by about 0.1 millimeters, the susceptibility of the polytetrafluorethylene foil to crack formation is substantially reduced, and the adhesion of the catalyst mass to the polytetrafluorethylene foil is significantly improved. The roller apparatus functions without much power and delivers the finished product via a single storage reel to the location at which electrode insertion takes place.