Page 24 - Fuel Cell Teacher Edition

The PEM fuel cell consists of two sheet electrodes, by NASA for use on the Apollo spacecraft. [4,6]
an electrolyte (the proton exchange membrane),
a sealed gas input on each side, and a provision The alkaline fuel cell functions differently than the
for draining water that accumulates in the cell. PEM fuel cell. Rather than having hydrogen ions
When hydrogen flows across the anode side of migrate through a membrane, hydroxyl groups
the cell the bonded diatomic hydrogen molecules migrate from the cathode to the anode through the
lose an electron and separate to become hydrogen liquid electrolyte (potassium hydroxide). When the
ions. These ions flow through the proton exchange hydroxyl ions contact hydrogen at the anode, they
membrane and the electrons flow through the combine to produce water and release electrons. They
wire connecting the anode and cathode plates. At return to the cathode through an electrical circuit
the cathode, the electrons and two hydrogen ions outside the cell. Electrons react with oxygen and water
combine with one oxygen to create heat and water. at the cathode to produce more hydroxyl ions.

In 1959, Francis Thomas Bacon developed a 5kW Building an efficient fuel cell that is safe,
fuel cell at 60% efficiency. [3] The Bacon cell, now inexpensive, damage resistant, and reliable is
called an alkaline fuel cell, was used most famously quite complicated. There are over 22 types of
on the Apollo and space shuttle missions. His cell fuel cells in existence, but only the zinc-air cell
also operated on compressed hydrogen and oxygen (which is mechanically rechargeable) is produced
but used a solution of potassium hydroxide in water on a wide scale. [5] The primary application for
as its electrolyte. The temperatures of these cells zincair alkaline cells is military communications
were much greater, operating at 150 to 200˚C. The and unmanned aerial vehicles as they are small,
alkaline fuel cell offered longer life than the PEM lightweight, and safer in combat situations than
fuel cell that was used on Gemini and was selected lithium batteries.

8 TOTAL REDOX™ – FUEL CELLS TOTAL REDOX™ – FUEL CELLS 9