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CARBON-ZINC & ALKALINE CELLS

The carbon-zinc cell was invented in the 1860s by a French chemist named Georges Leclanche, and is sometimes called the "Leclanche cell". It is also sometimes called the "dry cell", but this term is somewhat misleading as it can also be legitimately applied to similar storage cell technologies.

The carbon-zinc cell consists of zinc cylindrical "cup" that makes up the anode -- which is separated from the external casing of the battery by an insulating spacer -- and a central carbon rod. The electrolyte is powdered ammonium chloride (NH4Cl), an acid, in water, mixed with along with powdered manganese dioxide (MnO2) and graphite to form a paste.

Zinc chloride (ZnCl2) can also be used in place of ammonium chloride, providing longer service life at higher cost, and in fact carbon-zinc cells often use a small amount of zinc chloride along with the ammonium chloride. However, for simplicity this discussion assumes that only ammonium chloride is used.

The anode reaction of the carbon-zinc cell involves the double oxidation of a zinc atom, releasing two electrons into the external circuit:
Zn --> Zn++ and 2 e- The cathode reaction is supported by the manganese dioxide. The graphite powder is mixed to the manganese dioxide powder as manganese dioxide is not very conductive. While some sources call the central carbon rod the cathode, it is more properly the "cathode collector", as it is inert and simply provides a conductive path to the positive contact. In fact, this type of cell should more properly be known as a "manganese-zinc acidic" cell, but that's not the useage that was adopted.
The cathode reaction involves reduction of MnO2 to Mn2O3 on the inert carbon cathode:

2 NH4+ and 2 MnO2 and 2 e- --> Mn2O3 and 2 OH-

Carbon-zinc batteries will go dead prematurely if discharged too quickly, due to the buildup of reaction products around the carbon cathode, but they will "rejuvenate" if allowed to rest for a while, allowing the reaction products to disperse.
* The alkaline cell operates on similar principles, with a zinc anode and manganese dioxide mixed with graphite for the cathode. However, the electrolyte is potassium hydroxide (KOH), which is alkaline rather than acidic, again mixed with manganese dioxide and graphite. It should be properly known as the "manganese zinc alkaline" cell, but once more that's not the usage that was adopted.

The anode reaction is:

Zn and 2 OH- --> ZnO and H2O and 2 e-

The cathode reaction is;
2 MnO2 and H2O and 2 e- --> Mn2O3 and 2 OH-

Sodium hydroxide (NaOH) can also be used as the electrolyte. This is the case for almost all cells that use potassium hydroxide as an electrolyte.
Despite the similarity in operation, the alkaline cell's structure is very different from that of the carbon-zinc cell. The alkaline cell is enclosed in a nickel-plated steel can that forms the positive cathode contact, which is separated from the bottom cap, which is the negative anode contact, by a cardboard spacer. The can contains the potassium hydroxide / manganese dioxide / graphite paste for the cathode reaction, separated from a core of powdered zinc by a fabric separator.

A tin-plated brass "nail" connected to the bottom cap is inserted up into the powdered zinc to conduct current to the cap. A plastic plug seals the bottom of the can and supports the fabric separator.

Of course, all cells are covered with a plastic sheath to provide protection, insulation, and labeling. Alkaline cells have about twice the power density of carbon-zinc cells, but several times the cost. The carbon-zinc cell's virtue, probably its only virtue, is that it is dirt cheap.
Both carbon-zinc and alkaline cells have a cell voltage of about 1.5 volts, and are both regarded as environmentally benign, at least by the standards of storage cells. Carbon-zinc and alkaline storage cells are not in general rechargeable, but Raytheon did introduce a rechargeable alkaline battery series under the name Renewal in the late 1990s, though this product line was not commercially successful.

* An interesting variation on this technology has been promoted by a company in Israel named "Power Paper", after their product, which is literally printed onto cardboard boxes or similar substrates using silkscreen technology. The Power Paper cell is not actually made of paper, of course. In essence, it consists of a five-layer structure of silkscreened materials:

A conductor layer.
A zinc anode layer.
An electrolyte / separator layer.
A manganese dioxide cathode layer.
A conductor layer.

The whole assembly is sealed under a protective layer of plastic. The cell provides 1.5 volts, but multiple layers could be used to construct a battery with higher voltage if necessary. Storage capacity is 2.5 milliampere-hours per square centimeter. It has a shelf life of two years.

The Power Paper cell could be used for point-of-sale gimmicks to sell CDs or movies; for singing greeting cards; or for toys and novelties. The company has recently been promoting "smart notebooks", with a calculator, electronic memo taker, or even a simple computer game embedded on the cover. Another possible application is for a temperature-logging system to be built into packages for the transport of fresh fish, fowl, or produce. The system would include a microcircuit and a cheap temperature sensor, driven by a Power Paper cell, that would log temperatures while the package is in transport, allowing the buyer to determine who is responsible for a spoiled delivery.

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