14-09-2013, 04:21 PM
The Polymer Electrolyte Membrane Fuel Cell Stack
Since fuel cells operate at less than 100% efficiency,
the voltage output of one cell is less than 1.16 volt.
As most applications require much higher voltages than
this, (for example, effective commercial electric motors
typically operate at 200 – 300 volts), the required voltage
is obtained by connecting individual fuel cells in series to
form a fuel cell “stack.” If fuel cells were simply lined-up
next to each other, the anode and cathode current
collectors would be side by side. To decrease the overall
volume and weight of the stack, instead of two current
collectors, only one plate is used with a flow field cut
into each side of the plate. This type of plate, called a
“bipolar plate,” separates one cell from the next, with
this single plate serving to carry hydrogen gas on one
side and air on the other. It is important that the bipolar
plate is made of gas-impermeable material. Otherwise
the two gases would intermix, leading to direct oxidation
of fuel. Without separation of the gases, electrons pass
directly from the hydrogen to the oxygen and these
electrons are essentially “wasted” as they cannot be
routed through an external circuit to do useful electrical
work. The bipolar plate must also be electronically
conductive because the electrons produced at the anode
on one side of the bipolar plate are conducted through
the plate where they enter the cathode on the other side
of the bipolar plate.