Silver-zinc cells

Dry cells (batteries) and fuel cells are the main chemical electricity sources. Diy cells consist of two electrodes, made of different metals, placed into a solid electrolyte. The latter facilitates an oxidation process and a flow of electrons between electrodes, directly converting chemical energy into electricity. Various metal combinations in electrodes determine different characteristics of the dry cells. For example, nickel-cadmium cells have low output but can work for several years. On the other hand, silver-zinc cells are more powerful but with a much shorter life span. Therefore, the use of a particular type of dry cell is determined by the spacecraft mission profile. Usually these are the short missions with low electricity consumption. Diy cells are simple and reliable, since they lack moving parts. Their major drawbacks are... 

The silver-zinc cell is a storage battery After discharge, it can be recharged by forcing through it an electric cnrrent in the reverse direction. In this process the two electrode reactions (19.3) and (19.4) as well as the overall reaction (19.2) go from right to left electrons flowing in the sense of arrow r in Fig. 19.1. 

The separator used in silver—zinc cells should be permeable to water and hydroxyl ions, stable in strong alkaline solutions, and not oxidized by the solid silver oxide or dissolved silver ions and should retard the migration of dissolved ions to the anode. 

Recently Serenyl used a flexible alkaline separator (FAS) in Silver—Zinc cells, which consists of a microporous polyolefin film, with inorganic filler. This can be folded around the silver and/or zinc electrodes to form conventional U wraps or heat sealed bags. They showed that the FAS was not attacked by the electrolyte and helps in inhibiting the shape change of zinc electrode. 

Lewis, H. L. Hammersley, V. L. Wharton, S. P. NSWC evaluation of cellophane separation in model rechargeable silver-zinc cell. Presented at the 38th Power Sources Conference, Cherry Hill, NJ, 1998. 

Lewis, H. Henderson, S. Danko, T. Separator composition evaluation in model rechargeable silver zinc cells. Proceedings of the 16th Annual Battery Conference on Applications Advances-, PEE-. New York, 2001. 

Silver peroxide (Ag O ) Used to manufacture silver-zinc cells (batteries). 

The study on the characterization of alkaline silver-zinc cells and composite electrodes for such cells was carried out [349, 350]. The improved silver-zinc battery with new developments in additives (Bi203) to the negative electrode and separator coatings for underwater... 

Cellophane or its derivatives have been used as the basic separator for the silver-zinc cell since the 1940s. The cellophane is the principal limitation to cell life. Oxidation of the cellophane in the cell environment degrades the separator and within a relatively short time short circuits may occur in the cell. 

Electrolyte. The electrolyte in silver-zinc cells is 30-45% KOH. The lower concentrations in tliis range have higher conductivities and are preferred for high rate cells. Higher concentrations have a less deleterious effect on cellulosic separators and are preferable for extended life characteristics. 

Charging. Charging of silver-zinc cells can be done by one of several methods. The constant-current method which is most common consists of a single rate of current, usually equivalent to a full input within the 12- 16-h period. 

Discharge. Silver—zinc cells have one of the flattest voltage curves of any practical battery system known, although there are two voltage steps caused by the two different valence states of silver oxide. 

Performance of silver-zinc, cells is normally considered to be adequate in the temperature range of 10—38°C. If a wider temperature range is desired silver-zinc cells and batteries may be used in the range 0-71 °C without any appreciable derating. 

Cell Life. Silver-zinc cells are usually manufactured as either low or high rate cells. Approximately 10-30 cycles can be expected for high rate cells, depending on the temperature of use, the rate of discharge, and methods of charging. Low rate cells have been satisfactorily used for 100-300 cycles under the proper conditions. In general, the overall life of the silver-zinc cells with the separator systems normally in use is approximately 1-2 yr. 

Cellophane or its derivatives have been used as the basic separator for the silver—zinc cell since the 1940s (65,66). Cellophane is hydrated by the caustic electrolyte and expands to approximately three times its dry thickness inside the cell exerting a small internal pressure in the cell. This pressure restrains the zinc anode active material within the plate itself and renders the zinc less available for dissolution during discharge. The cellophane, however, is also the principal limitation to cell life. Oxidation of the cellophane in the cell environment degrades the separator and within a relatively short time short circuits may occur in the cell. In addition, chemical combination of dissolved silver species in the electrolyte may form a conductive path through the cellophane. 

Charge acceptance of the silver—zinc system is normally on the order of 95—100% efficient based on coulombic (ampere-hour output over input) values. This is true of any of the charging methods when carried out in the proper manner. Thus overcharge is rarely necessary in charging silver—zinc cells and batteries. 

Fig. 12. Silver—zinc cell discharge curves at rates of A, 10 min B, 1 h and C, 10 h.

The packaging approach utilized for tliis battery is similar to that for nickel—hydrogen single cylindrical cells as shown in Figure 23. The silver electrode is typically the sintered type used in rechargeable silver—zinc cells. The hydrogen electrode is a Teflon-bonded platinum black gas diffusion electrode. 

In the simple case a battery (cell) consists of two electrodes made of different materials immersed in an electrolyte. The electrodes are conducting metal plates or grids covered by reactants active mass), the oxidizer is present on one electrode, the reducer on the other. In silver-zinc cells the electrodes are metal grids, one covered with silver oxide and the other with zinc. An aqueous solution of KOH serves as electrolyte. Schematically, this system can be written as... 

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