Mercuric Oxide Battery | Chemistry Construction Advantages Uses

During World war II it became essential to have constant voltage, high capacity, long life battery system for using under extreme tropical condition. The technology of zinc mercuric oxide battery had been known for more than 100 years, but it was first practically used by Samuel Ruben during World war II. Because of its constant and stable voltage characteristics, it is particularly advantageous to use in watches, cameras, and other small electronic devices. It was also used in some early model of pacemakers.
Because of its extremely stable output voltage characteristic, the mercuric oxide battery was widely used as a voltage reference source in electrical measuring instrument. In addition to these, the battery was also used in small scatterable satellite mines, radio sets, and early satellites.

Now a days these battery are becoming obsolete because of its environmental problem in association with mercury. There are mainly two types of mercuric oxide battery – one zinc mercuric oxide battery and two cadmium mercuric oxide battery. Environmental problems are also associated with cadmium. The market of this battery has been occupied by alkaline manganese dioxide, zinc-air, silver oxide and lithium battery.

Advantages of Zinc Mercuric Oxide Battery

1. It has very high energy density. It is about 450 Wh/L
2. It has a very long storage life.
3. Remains stable under wide range of current density.
4. It is highly electrochemically efficient.
5. It is very robust and generally non sensitive to mechanical impact and vibration.
6. It gives stable 1.35 V of open-circuit voltage which is an important advantage of zinc mercuric battery.
7. It gives stable voltage throughout long range of current drain operating period.

Disadvantages of Zinc Mercuric Oxide Battery

1. These batteries are highly expensive. That is why they have limited usage.
2. Although the energy to volume ratio of the battery is high but energy to weight ratio is moderate.
3. Performance of this battery is not very good at low temperature.
4. Due to presence of mercury, disposal of used zinc mercuric oxide battery creates a problem.

Advantages of Cadmium Mercuric Oxide Battery

1. It has longer storage life.
2. It has more flat discharge curve over a long range of current.
3. Unlike zinc mercuric oxide battery, it operates efficiently at low temperature.
4. Gas evolution label of cadmium mercuric oxide battery is low.

Disadvantages of Cadmium Mercuric Oxide Battery

1. It is costlier than zinc mercuric oxide battery because of cadmium.
2. Standard open circuit voltage of this battery is 0.9 V which is much lower than that of zinc mercuric oxide battery.
3. Its energy to volume ratio is moderate, and energy to weight ratio is low.
4. Disposal of cadmium mercuric oxide battery also creates environmental problem due to presence of both cadmium and mercury.

Construction of Mercuric Oxide Battery

The construction of mercuric oxide batteries includes various shapes like bottom, flat, and cylindrical. In a typical bottom configuration, the top cover, made of a copper alloy inside and nickel or stainless steel outside, is insulated from the bottom container by a nylon grommet. Inside the top cover, zinc powder amalgamated with graphite—enhancing conductivity—is used as the anode. The bottom contains mercuric oxide mixed with graphite as the cathode. A potassium hydroxide or sodium hydroxide soaked porous barrier separates these components. The assembly is sealed by crimping the container’s edges, ensuring a tight fit. Similarly, in the flat configuration, the structure involves double-plated top covers and an adapter tube that absorbs electrolytes and allows gas escape during discharge, ensuring operational stability and safety.

Chemistry of Mercuric Oxide Battery

Two types of alkaline electrolyte are used in the zinc / mercuric cell, one based potassium hydroxide and other is sodium hydroxide. The sodium hydroxide electrolyte is generally used where low temperature operation and high current drain are not essential. This electrolyte is generally used in zinc mercuric oxide cell, whereas potassium hydroxide based electrolyte is only used in cadmium mercuric oxide cell. Cadmium is insoluble in potassium hydroxide solution and that is why the cadmium mercuric oxide cell is quite suitable for low temperature operation.

Anode Reaction in Zinc Mercuric Oxide Battery

The anode reaction can be written as,

This reaction can be simplified as,

Anode Reaction in Cadmium Mercuric Oxide Battery

The anode reaction can be written as,

This reaction does not produce any water hence; the electrolyte used in this cell should have desired high percentage of water.

Cathode Reaction in Mercuric Oxide Battery

The cathode reaction of the battery can be written as,

Rated Voltage of Mercuric Oxide Battery

The open-circuit voltage of zinc mercuric oxide batteries is a stable 1.35 V across various temperatures, maintaining this consistency within 1% for several years. It fluctuates minimally, only by about 2.5 mV, across a temperature range of -20°C to +50°C. These batteries exhibit a flat discharge curve, meaning their voltage remains nearly constant throughout extended use, regardless of varying current levels. Similarly, the cadmium mercuric oxide battery’s voltage stands at 0.9 V, stable across a wide temperature range from -55°C to +80°C, with specialized designs supporting up to +180°C.

Storage of Mercuric Oxide Battery

The storage characteristic of this Battery is quite good, and store life is given in below table,

	Period of Storing	Temperature of Storing	Capacity loss
Zinc Mercuric Oxide Battery	2 years	20oC	10 to 20%
	1 years	45oC	20%
Cadmium Mercuric Oxide Battery	10 years	20oC	20%
	1 years	80oC	15%