Mercuric oxide-zinc cell

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

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. 

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... 

Lithium-iodine (Li/12) was proposed as a power source for implantable cardiac pacemakers in 1971 [12]. The first pacemaker run by a cell was implanted in 1972 [6]. These cells were originally developed as more reliable and longer lived alternative to the zinc-mercuric oxide cells (see below) used in implantable cardiac pacemakers since they were introduced in 1960 and on into the mid-1970s. However, Li/l2 cells have been the dominant power source for implantable cardiac pacemakers for more than 30 years. 

It is generally accepted that the basic cell reaction for the zinc/mercuric oxide cell is... 

Some types of zinc/mercuric oxide cells exhibit open-circuit voltages between 1.40 and 1.55 V. These cells contain a small percentage of manganese dioxide in the cathode and are used where voltage stability is not of major importance for the application. 

Two types of alkaline electrolyte are used in the zinc/mercuric oxide cell, one based on potassium hydroxide and one on sodium hydroxide. Both of these bases are very soluble in water and highly concentrated solutions are used zinc oxide is also dissolved in varying amounts in the solution to suppress hydrogen generation. 

Materials of construction for the zinc/mercuric oxide cells are limited not only by their ability to survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility with the electrode materials. As far as the external contacts are concerned, these are decided by corrosion resistance, compatibility with the equipment interface with respect to galvanic corrosion, and, to some degree, cosmetic appearance. Metal parts may be homogeneous, plated metal, or clad metal. Insulating parts may be injection-, compression-, or transfer-molded polymers or rubbers. 

With the exception of the anode contact (where slight modification of the top/anode interface is necessary), materials for the cadmium/mercuric oxide cell are generally the same as for the zinc/mercuric oxide cell. However, because of the wide range of storage and operating conditions of most applications, cellulose and its derivatives are not used, and low-melting-point polymers are also avoided. Nickel is usually used on the anode side of the cell and also, conveniently, at the cathode. 

The performance of the zinc/mercuric oxide cell at various temperatures and loads is summarized in Figs. 11.12 and 11.13 on a weight and volume basis. These data, based on the performance of a 800 mAh battery with a dispersed anode, can be used to approximate the performance of a zinc/mercuric oxide battery. 

S. A. G. Karunathilaka, N. A. Hampson, T. P. Haas, R. Leek, and T. J. Sinclair. The Impedance of the Alkaline Zinc-Mercuric Oxide Cell. 1. Behaviour and Interpretation of Impedance Spectra. J. Appl. Electrochem. 11 (1981). 

Table 55.2 Union Carbide Eveready zinc-mercuric oxide cells, 1.35 and 1,40 V ... 

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