Cadmium-mercuric oxide cells

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. 

The basic cell reaction for the cadmium/mercuric oxide cell is... 

Generally only potassium-based alkaline electrolytes are used in the cadmium/mercuric oxide cell. As cadmium is practically insoluble in il concentrations of aqueous potassium hydroxide solutions, the electrolyte can be optimized for low-temperature operation. 

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. 

Cadmium-Mercuric Oxide Primary Cells The scheme of the cell is ... 

An outstanding feature of the cadmium/mercuric oxide battery is its ability to operate over a wide temperature range. The usual operating range is from -55 to +80°C, but with the low gassing rate and thermal stability of the cell, operating temperatures to 180°C have been achieved with special designs. 

B. Berguss, Cadmium-Mercuric Oxide Alkaline Cell, Proc. Electrochem. Soc. Meeting, Chicago, Oct. 1965. 

Batteries. Many batteries intended for household use contain mercury or mercury compounds. In the form of red mercuric oxide [21908-53-2] mercury is the cathode material in the mercury—cadmium, mercury—indium—bismuth, and mercury—zinc batteries. In all other mercury batteries, the mercury is amalgamated with the zinc [7440-66-6] anode to deter corrosion and inhibit hydrogen build-up that can cause cell mpture and fire. Discarded batteries represent a primary source of mercury for release into the environment. This industry has been under intense pressure to reduce the amounts of mercury in batteries. Although battery sales have increased greatly, the battery industry has aimounced that reduction in mercury content of batteries has been made and further reductions are expected (3). In fact, by 1992, the battery industry had lowered the mercury content of batteries to 0.025 wt % (3). Use of mercury in film pack batteries for instant cameras was reportedly discontinued in 1988 (3). 

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 theoretical capacity of the cell is then given by the lowest total capacity of the two electrodes. The electrode with the lowest total capacity limits the capacity of the cell. For example, let us calculate the theoretical capacity of the Mercad cell (mercuric oxide/cadmium cell). The anodic reaction is... 

Accurate sorting relies on the identification of a number of different properties of a battery. These include the physical size and shape, the weight, the electromagnet properties and any surface identifiers such as colour or unique markings. These properties can be analysed in a number of different combinations in order to sort batteries into nickel cadmium, nickel metal hydride, lithium, lead acid, mercuric oxide, alkaline and zinc carbon batteries. Due to an voluntary marking initiative introduced by the european battery industry, it is now also possible to separate the alkaline and zinc carbon cells further into mercury free and mercury containing streams. 

The first devices to be implanted in humans used either rechargeable nickel-cadmium (NiCd) cells or alkaline zinc-mercuric oxide (Zn/HgO) cells [8]. 

This system utilizes two toxic materials, mercuric oxide and cadmium metal. Because of its low voltage, however, it is thermodynamically stable, and if it is well sealed, it has a very long lifetime. It also has a very stable voltage and can be used as a voltage reference in electronic circuits. It is seeing less and less usage because of disposal issues and the availability of stable lithium cells with equivalent lifetime and voltage stability. 

Other cells, based on zinc anodes or on mercuric oxide cathodes are known. Among them are the silver-zinc battery, zinc-copper oxide battery, mercury-cadmium battery etc. 

The classical zinc-corrosion inhibitor has been mercuric or mercurous chloride, which forms an amalgam with the zinc. Cadmium and lead, which reside in the zinc alloy, also provide zinc anode corrosion protection. Other materials like potassium chromate or dichromate, used successfully in the past, form oxide films on the zinc and protect via passivation. Surface-active organic compounds, which coat the zinc, usually from solution, improve the wetting characteristic of the surface unifying the potential. Inhibitors are usually introduced into the cell via the electrolyte or as part of the coating on the paper separator. Zinc cans could be pretreated however, this is ordinarily not practical. 

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