How Stuff Works - PEM Fuel Cells
There are several kinds of fuel cells, but Polymer Electrolyte Membrane (PEM) fuel cells-also called Proton Exchange Membrane fuel cells-are the type typically used in automobiles. A PEM fuel cell uses hydrogen fuel and oxygen from the air to produce electricity.
The diagram below shows how these fuel cells work.
Most fuel cells designed for use in vehicles produce less than 1.16 volts of electricity-far from enough to power a vehicle. Therefore, multiple cells must be assembled into a fuel cell stack. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack and the surface area of the PEM.
Thanks EPA.GOV