Primary Zn-air batteries, in their button form factor, are especially designed to provide power to hearing aids. In most hearing aid applications, these batteries can be directly substituted for Zn/Ag2O and give the longest service of any common battery system. Some of their characteristics, also useful for other electronic devices, are:
Zn-air batteries use O2 from the air as the active cathode material. O2 diffuses through the cathode and, after reaching the cathode interface with the alkaline electrolyte, is catalytically reduced. because one active electrode material (O2) is outside the cell, the other (Zn) occupy most of the cell volume. The capacity of the cell only depends on the anode, and, since the amount of Zn that can be stored in a given volume is about twice that of Zn/MnO2 or Zn/Ag2O cells, the energy of the Zn-air cell is very high both on a gravimetric and a volumetric basis.
The anode is formed by high surface area Zn powder mixed with the electrolyte and, in some cases, a gelling agent. The cathode region is quite complex: the holes allow air access; the air diffuser layer distributes O2 uniformly over the cathode; the hydrophobic Teflon layer is O2-permeable but limit water vapour access; and the air cathode is formed by a metallic mesh supporting the catalyst layer (carbon blended with Mn oxides and Teflon powder). The maximum current the cell can support depends on the O2 availability. If air access were not regulated, excess O2 could reach the cathode and high discharge rates would be possible. In fact, strict regulation is necessary to limit the inlet of H2O vapour and other gases that would degrade the cell. The electrolyte is typically a 30% KOH solution in water. In dry days, it tends to lose water, while the reverse occurs in wet days. In both cases, the service life of the cell is affected; this explains why holes and diffusion membrane are necessary.
Zn-air button cells have capacities in the range 40-600 mAh, which are delivered at low rates (0.4-2 mA). At these rates, flat discharge curves are obtained, with excellent capacity retention even at 0C. Operating lives of 1-3 months are typical: this is not surprising if one considers that the cells are in contact with the atmosphere, this favouring direct Zn oxidation, carbonation of the electrolyte and gas transfer. So, they are better used in continuous-drain applications.
Apart from hearing aids, primary Zn-air cell can be used, for instance, in:
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1. Alkaline Batteries
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