The Power and Transportation Future - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 261

of hydrogen and oxygen.
An electric current is produced as electrons are stripped from hydro-
gen atoms at catalysis sites on the membrane surface. The charge carriers
are hydrogen ions or protons and they move through the membrane to
combine with oxygen and an electron to form water which is the main by-
product.
Trace amounts of other elements may be found in this water, de-
pending on the cell construction. In most cells the water is very pure and
fit for human consumption. Individual cells are assembled into modules
that are called stacks.
PEM fuel cells can convert about 55% of the fuel energy fed into
them into actual work. The comparable efficiency for IC engines is in the
range of 30%. PEM cells also offer relatively low temperature operation at
80°C. The materials are used to make them reasonably safe with low main-
tenance requirements.
The emergence of commercial fuel cell cars will depend on develop-
ments in membrane technology, which are about one third of the fuel cell
cost. Improvements are desired in fuel crossover from one side of a mem-
brane to the other, the chemical and mechanical stability of the membrane,
undesirable side reactions, contamination from fuel impurities and over-
all costs.
One breakthrough occurred in membrane technology when PolyFuel,
in Mountain View, CA, produced a hydrocarbon polymer membrane with
improved performance and lower costs than the current perfluorinated
membranes. This cellophane like film has performed better than more
common perfluorinated membranes, such as DuPoint's Nafion material.
The hydrocarbon membrane can also operate at higher tempera-
tures, of up to 95°C, which allows the use of smaller radiators to dissipate
heat. It also lasts 50% longer, while generating up to 15% more power and
operating at lower humidity levels. Fluorocarbon membranes can cost
about $300 per square meter, the PolyFuel materials cost about half of this.
While hydrocarbon membranes may have to prove themselves to many,
Honda's FCX fuel cell cars use them.
CATALYSTS
Another key part of a PEM membrane is the thin layer of platinum-
based catalyst coating that is used. It makes up about 40% of the fuel cell
cost. The catalyst prepares hydrogen from the fuel and oxygen from the