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MERCURY, ZINC-AIR, & SILVER OXIDE BUTTON CELLS
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Calculators, hearing aids, and other small electronics devices use tiny nonrechargeable "button" cells. The original technology for button cells was the mercury cell, which had a mercuric oxide (HgO) cathode, an anode made of an amalgam of mercury and zinc, and an electrolyte consisting of potassium hydroxide mixed with zinc hydroxide (or Zn(OH)2). The anode reaction is:
Zn and 2 OH- --> ZnO and H2O and 2 e-
The cathode reaction is:
HgO and H2O and 2 e- --> Hg and 2 OH-
Mercury cells had a highly constant cell voltage of 1.35 volts. A similar cell could be made with cadmium instead of zinc, providing a cell voltage of 0.91 volts. As mercury is toxic, mercury cells are now banned in the US and some other countries and they are now only a curiosity.
* Modern zinc-air button cells are similar to alkaline cells. The anode is powdered zinc mixed in a gel, the electrolyte is a layer of potassium hydroxide, and the cathode is a carbon disk, designed to support cathode reactions through the oxygen in the air. A porous Teflon membrane allows air into the cell while preventing electrolyte from leaking out.
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The anode reaction is:
Zn and 2 OH- --> Zn(OH)2 and 2 e-
The cathode reaction is:
O2 and 2 H2O and 4 e- --> 4 OH-
Zinc-air batteries have a cell voltage of about 1.65 volts. They have a very high energy density, but also have a high internal resistance and are not well suited to high-current applications. They have to be sealed in storage to keep the air out, but as long as they are kept sealed they have a long shelf life.
Large zinc-air cells have been used in consumer equipment, at least on a limited basis, and very large zinc-air batteries have experimentally used in vehicular applications.
* The silver oxide cell is similar in construction to the zinc-air type, with an anode of powdered zinc in gel with a potassium hydroxide electrolyte, except that instead of having a cathode made of carbon and exposed to the air, it is a silver screen pasted with silver oxide (Ag2O).
They have a cell voltage of 1.55 volts, a flat discharge curve, and long shelf life. They can be recharged a limited number of times, but they are not generally recharged in practice.
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