Zinc-mercury oxide cells

Primary alkaline cells use sodium hydroxide or potassium hydroxide as tlie electrolyte. They can be made using a variety of chemistries and physical constructions. The alkaline cells of the 1990s are mostly of the limited electrolyte, dry cell type. Most primary alkaline cells are made sing zinc as the anode material a variety of cathode materials can be used. Primary alkaline cells are commonly divided into tW o classes, based on type of construction the larger, cylindrically shaped batteries, and the miniature, button-type cells. Cylindrical alkaline batteries are mainly produced using zinc-manganese dioxide chemistry, although some cylindrical zinc-mercury oxide cells are made. 

The first portable radios operated by batteries were developed in the 1930s. The invention of the first portable hearing aids followed soon after in 1937 [47]. It could be carried by the hearing aid user, but since it used vacuum tubes it was the size of a lunch box. Wearable hearing aids became possible with the development of smaller electronic components that replaced vacuum tubes and the commercialization of small zinc-mercury oxide cells after World War II. 

The main features of the zinc silver oxide cell are similar to those of the zinc mercury(II) oxide system. The principal difference apart from cost is the higher open circnit voltage the emf calculated from the standard free energies of formation of ZnO and Ag20 is 1.593 V, in close agreement with the open circuit voltage of commercial cells of 1.60 V. 

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,... 

The zinc-mercury oxide button cell (Fig. 23.8) uses a pellet of mercury oxide with a little graphite added to it for better conductivity as cathode. The anode of this battery is zinc powder (pressed or amalgamated). The electrolyte, a concentrated ZnO saturated potassium hydroxide solution, is on a cellulose felt. The following shows the simplified processes at the electrodes ... 

Fig. 23.8 Cross section of a zinc-mercury oxide button cell.

Zinc/Mercury oxide 1.35 400 520 Primary battery in button cell design... 

Other alkaline primary cells couple zinc with oxides of mercury or silver and some even use atmospheric oxygen (zinc—air cell). Frequendy, zinc powder is used in the fabrication of batteries because of its high surface area. Secondary (rechargeable) cells with zinc anodes under development are the alkaline zinc—nickel oxide and zinc—chlorine (see Batteries). 

Another important primary battery is the mercury cell. It usually comes in very small sizes and is used in hearing aids, watches, cameras, and some calculators. The anode of this cell is a zinc-mercury amalgam the reacting species is zinc. The cathode is a plate made up of mercury(II) oxide, HgO. The electrolyte is a paste containing HgO and sodium or potassium hydroxide. The electrode reactions are... 

Mercury batteries (cells) consist of a zinc anode and a mercuric oxide cathode. These cells produce a steady 1.3 volts throughout the cells hfetime. 

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. 

Closely related to the alkaline dry cell is the mercury battery, often used in watches, heart pacemakers, and other devices where a battery of small size is required (Figure 18.10). The anode of the mercury battery is zinc, as in the alkaline dry cell, but the cathode is steel in contact with mercury(II) oxide (HgO) in an alkaline medium of KOH and Zn(OH)2. Zinc is oxidized at the anode, and HgO is reduced at the cathode ... 

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... 

The Ruben-MaUory zinc mercury(11) oxide battery, which revolutionized the battery industry, uses the cell... 

Alkaline mercury-zinc batteries were manufactured as sealed cells of low capacity (0.05-15 Ah). They contain mercury oxide HgO and a limited amount of electrolyte (about 1 ml/Ah) absorbed in a porous matrix, so they operate only according to the secondary process of the zinc electrode. Modern mercury-zinc batteries were developed by S. Ruben in the beginning of the 1940s. His button construction was so effective that large-scale production started in the United States as early as World War II and after the war in other countries. A schematics of the button construction is shown in Figure 4.1... 

As another major contribution to the reduction of hazardous substances, the European battery manufacturers had already decided in mid-1999 to cease sales of mercury oxide round cell batteries, which are mainly used in hearing aids. As an alternative, zinc/air batteries with a low mercury content (far less than 1 percent of weight in Hg) are used. Advances made in hearing aids and battery technology now make it possible to use these batteries even in hearing aids for the extremely hard of hearing. Zinc/air batteries have been on offer for ordinary hearing aids for more than a decade now. 

Cadmium/Mercuric Oxide Battery. The substitution of cadmium for the zinc anode (the cadmium/mercuric oxide cell) results in a lower-voltage but very stable system, with a shelf life of up to 10 years as well as performance at high and low temperatures. Because of the lower voltage, the watthour capacity of this battery is about 60% of the zinc/mercuric oxide battery capacity. Again, because of the hazardous characteristics of mercury and cadmium, the use of this battery is limited. 

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