A membrane electrode assembly (MEA) is an assembled stack of proton exchange membranes (PEMs), catalyst and electrode used in a fuel cell. The PEM is sandwiched between two electrodes which have the catalyst imbedded in them. The electrodes are electrically insulated from each other by the PEM. These two electrodes make up the anode and cathode respectively.

The PEM is a proton permeable but electrical insulator barrier. This barrier allows the transport of the protons from the anode to the cathode through the membrane but forces the electrons to travel around a conductive path to the cathode. Companies such as DuPont, Dow and E-TEK produce PEM. DuPont PEM are offered under the trade name Nafion. The commonly used Nafion PEM are Nafion 112, 115, 117, 105.