14-09-2013, 04:20 PM
The Polymer Electrolyte Membrane Fuel Cell
As little as 10 years ago, vehicles powered by
fuel cells seemed more science fiction than fact.
Today, development of fuel cell technology for transpor-
tation is made possible due to the polymer electrolyte
membrane fuel cell. This type of fuel cell is also known
as the proton exchange membrane fuel cell, the solid
polymer electrolyte (SPETM) fuel cell and simply, poly-
mer electrolyte fuel cell. It is often referred to simply as
the “PEM” fuel cell. The center of the fuel cell is the
polymer electrolyte membrane. For all five families of
fuel cells, it is the electrolyte that defines the type of fuel
cell, so the discussion of the polymer electrolyte mem-
brane fuel cell should logically begin with its electrolyte,
the membrane.
The Polymer Electrolyte Membrane
An ordinary electrolyte is a substance that dissociates
into positively charged and negatively charged ions in
the presence of water, thereby making the water solu-
tion electrically conducting. The electrolyte in a
polymer electrolyte membrane fuel cell is a type of
plastic, a polymer, and is usually referred to as a mem-
brane. The appearance of the electrolyte varies
depending upon the manufacturer, but the most preva-
lent membrane, Nafion TM produced by DuPont,
resembles the plastic wrap used for sealing foods. Typi-
cally, the membrane material is more substantial than
common plastic wrap, varying in thickness from 50 to
175 microns. To put this in perspective, consider that a
piece of normal writing paper has a thickness of about 25
microns. Thus polymer electrolyte membranes have
thicknesses comparable to that of 2 to 7 pieces of paper.
In an operating fuel cell.
The Electrodes
All electrochemical reactions consist of two separate reactions: an oxidation half-reaction occurring at the anode and
a reduction half-reaction occurring at the cathode. The anode and the cathode are separated from each other by the
electrolyte, the membrane.
In the oxidation half-reaction, gaseous hydrogen produces hydrogen ions, which travel through the ionically con-
ducting membrane to the cathode, and electrons which travel through an external circuit to the cathode. In the
reduction half-reaction, oxygen, supplied from air flowing past the cathode, combines with these hydrogen ions and
electrons to form water and excess heat. These two half-reactions would normally occur very slowly at the low
operating temperature, typically 80 ̊C, of the polymer electrolyte membrane fuel cell. Thus, catalysts are used on
both the anode and cathode to increase the rates of each half-reaction. The catalyst that works the best on each
electrode is platinum, a very expensive material.
The final products of the overall cell reaction are electric power, water, and excess heat. Cooling is required, in fact,
to maintain the temperature of a fuel cell stack at about 80 ̊C. At this temperature, the product water produced at
the cathode is both liquid and vapor. This product water is carried out of the fuel cell by the air flow.