Membrane Electrode Assembly

The electrodes (anode and cathode), catalyst, and polymer electrolyte membrane together form the membrane electrode assembly (MEA) of a PEM fuel cell.

*Anode. The anode, the negative side of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. Channels etched into the anode disperse the hydrogen gas equally over the surface of the catalyst.
*Cathode. The cathode, the positive side of the fuel cell, also contains channels that distribute the oxygen to the surface of the catalyst. It conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.
*Polymer electrolyte membrane. The polymer electrolyte membrane (PEM)—a specially treated material that looks something like ordinary kitchen plastic wrap—conducts only positively charged ions and blocks the electrons. The PEM is the key to the fuel cell technology; it must permit only the necessary ions to pass between the anode and cathode. Other substances passing through the electrolyte would disrupt the chemical reaction.

The thickness of the membrane in a membrane electrode assembly can vary with the type of membrane. The thickness of the catalyst layers depends upon how much platinum (Pt) is used in each electrode. For catalyst layers containing about 0.15 milligrams (mg) Pt/cm2, the thickness of the catalyst layer is close to 10 micrometers (μm)—less than half the thickness of a sheet of paper. This membrane/electrode assembly, with a total thickness of about 200 μm (or 0.2 mm), can generate more than half an ampere of current for every square centimeter of assembly area at a voltage of 0.7 volts, but only when encased in well-engineered components—backing layers, flow fields, and current collectors.