Electrolytes zinc/silver oxide cells

Unlike some other cathode materials, such as manganese dioxide, which are quite insoluble, silver oxide has a fair degree of solubility in alkaline electrolyte. If the soluble silver species were allowed to be transported to the zinc anode it would react directly with the zinc, and as a result the cell would self-discharge. In order to prevent this from happening, zinc—silver oxide cells use special separator materials such as cellophane [9005-81-6], that are designed to inhibit migration of soluble silver to the anode. 

Fig. 16. Comparison of battery efficiency for miniature zinc—silver oxide cells containing KOH or NaOH electrolyte (21).

The zinc/silver oxide cell consists of three active components a powdered zinc metal anode, a cathode of compressed silver oxide, and an aqueous electrolyte solution of potassium or sodium hydroxide with dissolved zincates. The active components are contained in an anode top, cathode can, separated hy a harrier and sealed with a gasket. 

The electrolytes used for zinc/silver oxide cells are based upon 20 to 45% aqueous solutions of potassium hydroxide (KOH) or sodium hydroxide (NaOH). Zinc oxide (ZnO) is dissolved in the electrolyte as the zincate to help control zinc gassing. The zinc oxide concentration varies from a few percent to a saturated solution. 

Because of the slight solubility of silver oxides in alkaline electrolyte, little work was done with zinc/silver oxide cells until 1941 when Andre suggested the use of a cellophane barrier. Cellophane prevents migrating silver ions from reaching the anode - " by reducing them to insoluble silver metal. The cellophane is oxidized and destroyed in the process, making it less effective for long-life cells. 

Electrolyte. The electrolyte used for reserve zinc/silver oxide cells is an aqueous solution of potassium hydroxide. High and medium discharge rate cells use a 31% by weight electrolyte solution because this composition has the lowest freezing point and is close to the minimum resistance which occurs at 28 wt. %. Low-rate cells may use a 40-45% solution since lower rates of hydrolysis of cellulosic separators occur with the higher KOH concentrations. 

The market for batteries is huge, with new types and applications being developed all the time. For example, a watch battery is a type of silver oxide cell silver in contact with silver oxide forms one half-cell while the other is zinc metal and dications. Conversely, a car battery is constructed with the two couples lead(IV) lead and lead(IV) lead(II). The electrolyte is sulphuric acid, hence this battery s popular name of lead-acid cell (see further discussion on p. 347). 

Silver oxide cells were developed in the 1960s. These cells use silver oxide mixed with carbon (to increase the electronic conductivity of the material) as cathode, amalgamated pellet zinc powder as anode, and a solution of potassium hydroxide or sodium hydroxide with dissolved zin-cates in water as electrolyte. Permion (a radiation graft of methacrylic acid onto a polyethylene membrane) is used as separator. The cell reactions are... 

Zinc/silver oxide button batteries are used in calculators and watches. Although the silver component makes them expensive, this is outweighed by their high performance. The battery uses KOH as the electrolyte, and the overall cell reaction is ... 

The impedance of a zinc/silver oxide battery is influenced primarily by the conductive diluents in the cathode, the barrier resistivity and the electrolyte type and concentration. These factors are balanced by battery manufacturers to obtain the desired values required to meet the applications. As the cell is discharged, the impedance will decline as the resistive silver oxide is reduced to conductive metallic silver (Fig. 12.13). 

The more familiar types of primary alkaline systems are the zinc/manganese dioxide, zinc/ mercuric oxide, and zinc/silver oxide batteries. These, typically, use potassium or sodium hydroxides, in concentrations from 25 to 40% hy weight, as the electrolyte, which functions primarily as an ionic conductor and is not consumed in the discharge process. In simple form, the overall discharge reaction for these metal oxide cells can be stated as... 

Both manually and automatically activated zinc/silver oxide batteries were developed to meet highly stringent requirements with regard to performance and reliability. The time and temperature of storage prior to use are of importance, and records should be maintained to ensure use within allowable limits. Special care must be exercised to ensure that the proper amount of the specified type of electrolyte is added to each cell of a manual-type battery and that, after activation, the unit is discharged within the shelf-life limitation at the proper temperature. Some battery containers have pressure-relief valves or heaters, or both, and these must be carefully maintained and monitored. 

Two common types of button batteries both use a zinc container, which acts as the anode, and an inert stainless steel cathode, as shown in Figure 11.11 on the next page. In the mercury button battery, the alkaline electrolyte paste contains mercury(II) oxide, HgO. In the silver button battery, the electrolyte paste contains silver oxide, Ag20. The batteries have similar voltages about 1.3 V for the mercury cell, and about 1.6 V for the silver cell. 

Various problems related to the construction and performances of these batteries, such as changes in materials of membranes and additives both to the electrode materials and to the electrolyte, were studied in recent years. Some instability of the silver electrode during such storage period and the ways of avoiding these difficulties were studied and discussed [347]. Reserve activated silver oxide-zinc cells were constructed [348] with synthetic Ag20 and Pb-treated zinc electrodes were produced by a nonelec-trolytic process. The cells were tested before and after thermally accelerated aging. 

---