DE2853740A1 - Primary zinc air cell - with oxygen reduction activating layer and hydrophobic layer forming the positive air electrode - Google Patents

Abstract

A primary galvanic cell with an alkaline electrolyte includes a negative electrode (zinc) and a positive air electrode. The latter consists of the electrolyte side of a layer which activates the oxygen reduction and, firmly connected it a hydrophobic layer on the gas side. The hydrophobic layer is pref. a thermoplastic material such as PTFE. Its thickness can vary between 25 and 100 mu and it has a permeability factor for air from 10-8 and 10-5cm3/cm.s.bar. The electrolyte pref. is caustic soda soln. A metallic bleeder grid is embedded in the layer for the activation of the oxygen reduction. Such a cell can supply small currents of up to a few 100mA over long working and periods so that it ideal for button cells required in wristwatches or pocket computers.

Description

Galvanic primary element

The invention relates to a galvanic primary element with alkaline Electrolytes, a negative electrode and a positive air electrode, the electrolyte side from a layer that activates the oxygen reduction and a firmly connected, the gas side facing hydrophobic layer consists.

Alkaline air / zinc systems have been known for a long time.

According to their current level of development, they can have energy densities of own over 700mWh / cm3. They belong to the high-energy systems, e.g. the lithium cells, with which usually only 500 mWh / cm³ to 700 mWh / cm³ is achieved will. Their discharge voltage is between 1.4 and 1.2 volts. It therefore corresponds that of the HgO / Zn system. In contrast to this, however, is the air / zinc system significantly more environmentally friendly as it contains practically no mercury.

Known button cells of the air / zinc system have constructions of Air electrode, which in principle always consists of two layers, namely one electrolyte-accessible - the actually active - layer and a hydrophobic layer facing the outside air.

DE-OS 2 454 890 describes such a layer arrangement in which of the hydrophobic gas-permeable layer also has an absorbent layer Paper is assigned to act as an air distribution layer and at the same time as an absorber to serve for electrolytes that may leak through. The hydrophobic layer is a microporous, unsintered polytetrafluoroethylene film (PTFE) o Die from the DE-OS 2 743 078 known air electrode has a hydrophobic PTFE membrane which has a density of 0.5 to 1.5 g / cm3 and a thickness between 75 and 300ru.

From DE-OS 2 535 269, finally, an air electrode with a Also unsintered PTFE film of 200> i thickness and 40, ~ porosity known.

The positive electrodes of these known air / zinc systems are for high currents or current densities (over 10 mA / cm2). A high current density requires a correspondingly high air supply or high air exchange. A high one However, air exchange with the surrounding atmosphere carries the risk of dehydration or the inadmissible water absorption of the air / zinc system in itself, above In addition, a high exchange of air means an increased carbonation of the alkaline Electrolytes due to the C02 content of the air.

The invention is based on the object of a galvanic primary element indicate with a positive air electrode which one for such a low air exchange is designed that from the cell equipped with this electrode only load currents, for example up to a few 100 ~ u A, but this can be taken over long periods of operation can.

The ones for small consumers such as electronic pocket calculators or wristwatches already very largely miniaturized air / zinc elements in the form of a button cell should also meet the actual needs of these small devices with regard to power output be adjusted.

The object is achieved according to the invention in that the hydrophobic Layer a thickness between 25 and 100 to and a permeability coefficient K between 10 8 and 10 ## 5 cm3 / cm x sec. x bar (for air).

Layers of the permeability and thickness according to the invention can be Manufacture from unsintered plastic films by rolling, pressing or sintering. In addition to the preferably used PTFE, thermoplastics such as polyethylene, Polypropylene or polystyrene or mixtures of PTFE with a thermoplastic are used will. By rolling or pressing the compacted foils onto the catalytic active layer one obtains the two-layer air electrode.

Criteria for the suitability of a film as a hydrophobic film according to the invention Electrode layers, especially in clock and computer cells, result from the considerations below. The starting point is always a 700 r thick PTFE film. In the commercially available, unsintered state, this has a density of 1.62 g / cm² a pore volume of 26%. The limit current iG is 300 mA / cm2.

Unsintered foils that allow such a high limit current will be Used in the air electrodes for heavy duty button cells. Close to this Normal air / zinc hearing aid cells work in the power range.

The limit current iG is proportional to a certain volume of air, which must be brought to the electrode in a unit of time to generate it, whereby the following applies to the air throughput V: As can be seen from the relationship, different flow values can be achieved by varying the film cross-section Q, the film thickness d and the pressure difference tp. However, if one assumes the external atmospheric pressure against the air element and thus Ap as constant and continues to agree constant values for Q (= 1 cm2) and d (= 100> 1), the permeability coefficient K (cm3 / cm x sec x bar) a direct measure of air flow.

Under the above conditions, even the permeability coefficient becomes K can be calculated from the limit current iG, because iG is proportional to the air flow rate V, with the proviso that 1 cm3 of air must be supplied to the electrode to generate a current of 1 mA.

This fact can be determined experimentally by measurements on PTFE foils confirm the step-by-step - of the density each = 1.62 with the pore volume 26 96 starting out - rolled down or sintered to ever higher densities (up to Jo = 2.23) and which, in addition to the density value, by corresponding values for the limit current iG and the permeability coefficient K for air could become.

The unsintered starting film has a coefficient of permeability K = 4.4 x 10 3 cm3 / cm x sec x bar. At the density # = 1.83 the pore volume is still 15% and the limit current iG 165 mA / cm2 (compared to 300 mA / cm2 with 26% porosity). With a density of p = 2.06, a pore volume of 2.8% corresponds to a limit current iG = 30 mA / cm² and a permeability coefficient K = 9.75 x 10 6 cm3 / cm x sec x bar.

It has now been found that in the largest density range from # = 2.15 to 2.23 the sheet material depending on its treatment by continued rolling or has two different states through sintering and quenching, namely a transparent, highly crystalline one in the first and an amorphous one in the other Cases. In both forms the measurable porosity is practically 0 5 '.

According to their porosity, they are no longer distinguishable, but they can due to their permeability coefficient, which in the crystalline form is 8 x 10 -9 and in the amorphous form 8 x 10 8 cm3 / cm x sec bar.

Corresponding to these values are also - although the porosity is practically 0% - limit currents of 10 x 10 # 3mA / cm2 (crystalline) or 36 x 10 cm2 (amorphous) measurable.

By carefully selecting the film material for the air electrode it is possible to meet the minimal energy requirements of electronic wristwatches and calculators, whose power consumption is in the microampere range, appropriately adapted.

However, it must be taken into account that the limit current is a current at which the load voltage practically has the value zero. Should the current therefore be at a useful voltage flow, it may only flow a fraction of the limit current, be for example 1/10 of it.

For foils of the density range p = 2.06 to 2.23 and with corresponding Permeability coefficients of approx. 10 5 to 10 8 cm3 / cm x sec x bar this means that real utility currents between 3 mA and 1 »can be assigned to them.

These numerical values always relate to a film with the thickness d = 100 to and the cross-section Q = 1 cm2 and apply to a pressure difference that corresponds to atmospheric pressure.

The upper limit of the use currents when using the invention Foils, however, is reduced to about 700 # iA when one takes into account that Q at the built-in foil because of the cell diameter, which is only a few mm 1/4 to 1/3 cm2 reached. On the other hand, the lower limit of the usage currents from approx.

if necessary move to higher currents by the film thickness reduced from 100 u down to 25 P.

For the design of the air electrode with regard to the necessary air exchange there is thus a sufficiently large leeway, assuming something like this is allowed that the base current for a quartz watch hardly exceeds 9 #iA and the pocket calculator no more current than 200 is required.

The subject matter of the invention is set out below with reference to FIGS to 8 explained in more detail.

Figure 1 shows a partial view of an air / zinc cell with typical Dimensions of 11.6 mm ~ and 5.4 mm in height and Figure 2 in a partial view Enlargement of the section of the figure delimited with a dotted line 1. The air electrode is made up of the catalytically active layer 1 and the hydrophobic layer 2 3 composed. The layer 2 is here one by sintering or rolling in its original thickness reduced, i.e. compressed PTFE film. In the active layer 1, the electrical arrester 4 is embedded in the form of a metal mesh, e.g. made of nickel. Furthermore, the cell consists of the zinc 5 with the electrolyte and the cover 6, where appropriate in the lid space or in another known manner a sufficient Expansion space is available to accommodate the increase in volume of the zinc that occurs during discharge. This is followed by the electrolyte source sheet 7, the separator 8 and the contact disk 9, with the help of which between the cup 10 and the air electrode 4 a safe electrical contact is established.

The seal 11 connects the lid 6 and cup 10 to the air inlet hole 12. With 13 is the sealing zone between the air electrode 4 and the cup 10 designated. In addition, the cell according to FIG. 2 contains a highly porous layer 14 for air distribution. It also serves to improve the electrolyte tightness the cell.

In principle, however, this is always due to the highly compressed layer 2 guaranteed.

With this cell construction, the air inlet and the air distribution achieved in that the cup is slightly curved bottom. This creates between the cup base and the air electrode 3 an air gap.

This construction makes sense for currents up to approx. 500fluA / cm2. The air gap itself is in the range of about 1/10 mm.

FIG. 3 shows, also in a partial view, a particularly flat one Variant of the cell described for FIG. 1, suitable for cell heights of less than 2 mm.

The aforementioned air gap is missing here, but the air distribution is used via a micro-roughness of the cup base or the PTFE layer on the cup base is present, guaranteed. The contact ring 9 is also missing; the contact is made here at 9 'over the electronically conductive, catalytically active layer 1 with the Cup rim 10.

FIG. 4 again corresponds to an enlargement of the dotted section of Figure 3, but deviating from this shows one with contact ring 9 and arrester 4 equipped cell. In particular, FIG. 4 shows the borderline case for an extreme Compaction of the hydrophobic layer 2 shown, which is so strong that the film no longer contains any pores, i.e. this is where the lowest gas permeability coefficients arise. Layer 2 is then a film that is fully sintered through to transparency. Man also recognizes that in the sealing area 13 between the highly compressed layer 2 and the micro-roughness of the cup base due to the sealing forces 15 disappears. FIG. 5 shows a particularly advantageous form of air supply in the bottom of the cup 10 again. The sealing zone 13 is located between the outer diameter of this cell and the inner dashed circle 10. Drawn in became 3 areas, which are separated from each other by dashed lines. The air holes 12 are in the center of gravity of the individual areas 16. This is given a particularly even air distribution.

Forms between the electrode layers 1 and 2 during rolling a solid bond. It has been found to be advantageous and important that the metallic arrester 4 is not in the area of the boundary zone which the transition from the active to the hydrophobic layer. Apparently there will be mutual liability particularly favored by this arrangement. In addition, the hydrophilic leads on the electrolyte side oriented electrical arrester 4 after contacting the electrolyte to form a Particularly rapid wetting of the active layer as a result of an existing one Binder for the catalyst is originally also hydrophobized.

It is also possible based on sufficient electron conduction of the catalyst material used in each case to completely dispense with a discharge network 4. This allows an air electrode to be built up with a total thickness of about 0.1 to 0.15 mm. This construction is only for To use very low currents, it does not have the pulse load capacity of the air electrodes according to Figures 2 and 4, in which the layer 1 is made somewhat stronger.

Air electrodes according to FIGS. 1 and 2 have catalytically active layers for example 0.3 mm thick. Capacity of the volume of air stored in it a buffer capacity that you can use for a short time to supply power to e.g. a LED display enables without the voltage dropping significantly. the The pulse load required for this is 10 mA for 2 seconds, corresponding to an amount of current from 5.55 # Ä # iAh. If one takes into account the relationship between Current and amount of air to be exchanged, after which the air volume 10 jil is supplied for 1 # Ah must be, and if the catalytically active layer has a porosity of 50 5 ', the circumference and thickness of this layer can be used to refer to the Close buffer capacity. By dimensioning the layer thickness can also in turn affect the pulse load capacity of the cell, i.e. the thicker the layer 1, the better the pulse load capacity.

The electrolyte used is always caustic soda. Figures 6, 7 8 and 8 show voltage-current diagrams showing the load behavior of the various Identify cell designs. The diagrams of Figures 6 and 7 were created by discharge obtained from button cells (zinc / air) measuring 11.6 mm ~ x 5.4 mm. Demonstrated figure 6 first of all the - not according to the invention - case of the use of a 200 P strong, unsintered and only on the strength 150> u r compressed PTFE film, their Porosity (approx. 15 5 ') is such that only above 25 mA does the voltage suddenly begins to fall off. A current of e.g. 27 mA is no longer possible.

The voltage drop is due to the flow resistance, the through the PTFE film 2, the air inlet opening 12 and through the between the cup base and film layer 2 lying air distribution space is formed. With unhindered If air advances to the PTFE film, the limit current increases from 25 mA to around 50 mA.

Button cells according to the invention, on the other hand, have the load diagrams of FIGS. 7 and 8. The PTFE film used according to FIG. 7 has a density of 2.13 g / cm3, and the air electrode only allows a limit current of about 25O # iA. Here the current is controlled exclusively by the PTEE> foil.

Figure 8 refers to the flat cell 11.6 mm # x 1.7 mm after Figure 3, in this case with a 50> 1 thick, fully sintered PTFE film is equipped. This only allows currents of approx. 9 # 2A in the long term.

If the current is increased to 12 µA, the voltage of the cell collapses. The breakdown of the voltage takes place here again due to the gas passage resistance the slide.

All of the above results were obtained at about 250C. as Electrolyte of the cells served in the case of FIG. 6 8n KOH and in the case of the figures 7 and 8 8n NaOH, which in cells of the type according to the invention because of their lower Creep is beneficial.

Claims (8)

Claims 1 Galvanic primary element with alkaline electrolyte, a negative electrode and a positive air electrode, the electrolyte side from a layer that activates the oxygen reduction and a firmly connected, the gas side facing hydrophobic layer, characterized in that the hydrophobic layer has a thickness between 25 and 100> 1 and a permeability coefficient K between 10 8 and 10 5 cm3 / cm x sec. X bar (for air). 2. Galvanic primary element according to claim 1, characterized in that that in the layer which activates the oxygen reduction there is a metallic discharge network is stored. 3. Galvanic primary element according to claims 1 and 2, characterized characterized in that the metallic arrester network-outside the boundary zone between the oxygen reduction activating layer and the hydrophobic layer is. 4. Galvanic primary element according to claims 1 to 3, characterized characterized in that the hydrophobic layer consists of polytetrafluoroethylene. 5. Galvanic primary element according to claims 1 to 3, characterized characterized in that the hydrophobic layer consists of a thermoplastic. 6. Galvanic primary element according to claims 1 to 5, characterized characterized in that the alkaline electrolyte is sodium hydroxide solution. 7. Galvanic primary element according to claims 1 to 6, characterized characterized in that the cup base provided with at least one air inlet opening or the hydrophobic layer adjacent to it along the mutual contact surface is roughened. 8. Galvanic primary element according to claims 1 to 6, characterized characterized in that the bottom of the cup is slightly curved.