Wednesday, March 26, 2008

Polymer Electrolyte Membrane (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.

Diagram: How a PEM fuel cell works.  1. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channeled to the cathode on the other side of the cell.  2. At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons.  3. The Polymer Electrolyte Membrane (PEM) allows only the positively charged ions to pass through it to the cathode.  The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current.  4. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell.

Fuel Cell Stacks

Photo: GM fuel cell stack.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.

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