Zinc-mercuric oxide

Miniature zinc—mercuric oxide batteries have a zinc anode and a cathode containing mercuric oxide... 

Eig. 11. Typical discharge curve comparison for zinc—mercuric oxide batteries (-) model 325, HgO, and (-... 

Miniature zinc—mercuric oxide batteries may be made with either KOH or NaOH as the electrolyte. Cells having KOH operate more efficiently than those having NaOH at high current drains (Eig. 12) because of the higher conductivity of KOH. On the other hand, batteries with KOH are more difficult to seal, cells with NaOH are more resistant to leakage. 

Eig. 12. Comparison of battery efficiency for miniature zinc—mercuric oxide cells containing KOH or NaOH electrolyte (21). 

There are two major types of household batteries (a) Primary batteries are those that cannot be reused. They include alkaline/manganese, carbon-zinc, mercuric oxide, zinc-air, silver oxide, and other types of button batteries, (b) Secondary batteries are those that can be reused secondary batteries (rechargeable) include lead-acid, nickel-cadmium, and potentially nickel-hydrogen. 

Zinc-mercuric oxide, cadmium-mercuric oxide, zinc-silver oxide and related systems... 

Fig. 3.24 Cross-section of a typical zinc-mercuric oxide button cell...

Power sources based on the zinc-mercuric oxide system are particularly suited to a wide range of applications, mainly concerned with miniature portable electronic equipment, where a relatively constant voltage is required throughout long discharge periods. In addition, such cells are used as voltage reference standards in regulated power supplies, potentiometers, chart... 

Fig. 3.25 Discharge characteristics of 1 Ah zinc-mercuric oxide button cell under continuous load at room temperature...

Replacing zinc with cadmium produces a cell with an OCV of 0.90 V, with characteristics very similar to those of the zinc-mercuric oxide system described above, but which is able to be stored and operated at extreme temperatures (—55 to 80°C) due to the low solubility of cadmium oxide even in concentrated KOH. Cells have been successfully operated at 180°C. Note that hydrogen generation does not occur at a cadmium anode. Because of cost and disposal problems, such cells are used only for applications where their special properties can be exploited, e.g. telemetry from internal combustion, jet or rocket engines. 

The main features of zinc-silver oxide cells are similar to those of the zinc-mercuric oxide system, except for a higher OCV and significantly increased cost. The overall cell reaction is... 

The primary objective of miniature battery design is to maximize the energy density in a small container. A compromise must be reached, however, since volumetric energy density decreases as cell volume decreases and the dead volume due to containers, seals, etc. becomes increasingly significant. A plot of energy density as a function of total volume is given in Fig. 3.28 for the zinc-mercuric oxide and zinc-silver oxide systems. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Miniature zinc-mercuric oxide batteries function efficiently over a wide range of temperatures and have good storage life. 

Although die zinc-mercuric oxide battery has many excellent qualities, increasing environmental concerns have led to a deemphasis in the use of this system. The main environmental difficulty is in the disposal of the cell. Both the mercuric oxide in the fresh cell and the mercury rcducrion product in the used cell have long-term toxic effects. 

Ruben cell — This is a zinc-mercuric oxide alkaline cell, more commonly called a mercury -> battery, a type of primary (nonrechargeable) cell, developed by Samuel Ruben during World War II in response to a requirement for batteries with a high capacity-to-volume ratio which would withstand storage under tropical conditions. It was licensed to the RR. Mallory Co., which... 

Zinc-mercuric oxide alkaline cell -> Ruben cell... 

FIGURE 17.7 A zinc-mercuric oxide dry cell, used in electric watches and cameras. 

A third primary dry cell is the zinc-mercuric oxide cell depicted in Figure 17.7. It is commonly given the shape of a small button and is used in automatic cameras, hearing aids, digital calculators, and quartz-electric watches. This battery has an anode that is a mixture of mercury and zinc and a steel cathode in contact with solid mercury(II) oxide (HgO). The electrolyte is a 45% KOH solution that saturates an absorbent material. The anode half-reaction is the same as that in an alkaline dry cell,... 

Calculate the standard voltage of the zinc-mercuric oxide cell shown in Figure 17.7. (Hint The easiest way to proceed is to calculate AG° for the corresponding overall reaction, and then find A%° from it.) Take A° (Zn(OH)2(s)) = -553.5 kjmorh... 

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