Anode: Zinc powder
Cathode: Manganese dioxide (MnO2)
Electrolyte: Potassium hydroxide (KOH)
Applications: Radios, toys, photo-flash
applications, watches, high-drain applications
Alkaline cells use the same manganese dioxide and zinc
electrodes as the carbon-zinc, with a different electrolyte and construction.
This cell design gets its name from its use of alkaline aqueous solutions as
electrolytes. Alkaline battery chemistry was first introduced in the early
60s. The alkaline cell has grown in popularity, becoming the zinc-carbon
cell's greatest competitor. Alkaline cells have many acknowledged advantages
over zinc-carbon, including a higher energy density, longer shelf life,
superior leakage resistance, better performance in both continuous and
intermittent duty cycles, and lower internal resistance, which allows it to
operate at high discharge rates over a wider temperature range.
Zinc in a powdered form increases the surface area of the
anode, allowing more particle interaction. This lowers the internal resistance
and increases the power density. The cathode, MnO2, is synthetically
produced because of its superiority to naturally occurring MnO2.
This increases the energy density. Just as in the zinc carbon cell, graphite is
added to the cathode to increase conductivity. The electrolyte, KOH, allows
high ionic conductivity. Zinc oxide is often added to slow down corrosion of
the zinc anode. A cellulose derivative is thrown in as well as a gelling agent.
These materials make the alkaline cell more expensive than the zinc-carbon, but
its improved performance makes it more cost effective, especially in high drain
situations where the alkaline cell's energy density is much higher.
The half-reactions are:
Zn + 2 OH- > ZnO + H2O + 2
2 MnO2 + H2O + 2 e-
>Mn2O3 + 2 OH-
The overall reaction is:
Zn + 2MnO2 > ZnO +
Mn2O3 E=1.5 V
There are other cell designs that fit into the alkaline
cell category, including the mercury oxide, silver oxide, and zinc air cells.
Mercury and silver give even higher energy densities, but cost a lot more and
are being phased out through government regulations because of their high
toxicity as heavy metals. The mercury oxide, silver oxide, and zinc air (which
is being developed for electronic vehicles) are all discussed below.