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A proton exchange membrane fuel cell (PEMFC) is a fuel cell that uses hydrogen and oxygen to liberate electrical energy. Its distinguishing feature is an electrolyte membrane called a membrane electrode assembly (MEA) that allows the passage of protons but not electrons. A proton exchange membrane fuel cell has potential applications as both a stationary and portable fuel cell.
An MEA has an anode side and a cathode side. An electrical current flows into the MEA's anode side and out of its cathode side. A proton exchange membrane fuel cell delivers hydrogen gas to the MEA's anode side, which splits the hydrogen atoms into electrons and protons, a process that can be shown by an equation: H2 -> 2e- + 2H+. The protons in this reaction travel through the MEA to its cathode side, and the electrons pass to the MEA's cathode side through an external circuit. The combination of these processes creates an electrical current.
A proton exchange membrane fuel cell also delivers oxygen gas to the MEA’s cathode side. If the molecules of diatomic oxygen (O2) can be split into oxygen atoms, the protons traveling through the MEA can react with these oxygen atoms to form molecules of water. This reaction also can be shown by an equation: O + 2H+ + 2e- -> H2O.
The MEA must meet several criteria in order to produce electricity. It can’t allow hydrogen or oxygen gas to travel through it. The MEA must also be able to resist the oxidative effect on the anode side and the reducing effect on the cathode side.
Platinum catalysts can be used to split hydrogen molecules with relative ease. Splitting oxygen molecules with platinum catalysts, however, produces significant electrical losses. An additional problem with platinum catalysts is that a very small amount of carbon dioxide will significantly degrade their performance. Scientists had not discovered a practical catalyst for splitting oxygen molecules as of 2010, but a catalyst composed of carbon, iron and nitrogen had shown the most promise. The primary difficulty with this catalyst is that its reaction rate drops quickly over a short period of time.
Water also causes electrical losses in a proton exchange membrane fuel cell. The fuel cell must prevent excess water from flooding the MEA but allow enough water to keep the MEA from drying out. Water management in a proton exchange membrane fuel cell is difficult, because the water is attracted to the cathode side of the MEA. Electro osmotic pumps are one possible solution to this problem.