Proton Exchange Membrane Fuel Cells (PEMFC)

Proton Exchange Membrane Fuel Cells
Proton Exchange Membrane Fuel Cells (PEMFC) First used in the 1960s for the NASA Gemini program, PEMFCs are currently being developed. PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. This membrane is small and light, and it works at low temperatures (about 80 degrees C, or about 175 degrees F). Other electrolytes require temperatures as high as 1,000 degrees C. Pems work well catalysts made of platinum and larger quantity of catalyst is required than other fuel cell.

Efficiency for a PEM cell reaches about 40 to 50 percent. An external reformer is required to convert fuels such as methanol or gasoline to hydrogen. Currently, demonstration units of 50 kilowatt (kw) capacity are operating and units producing up to 250 kw are under development. Peak power output is between 350-600 .

The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:

Anode Reactions:
2H₂ => 4H⁺ + 4e^-

Cathode Reactions:
O₂ + 4H⁺ + 4e^- => 2 H₂O

Overall Cell Reactions:
2H₂ + O₂ => 2 H₂O

Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell. This high-power density characteristic makes them compact and lightweight. In addition, the operating temperature is less than 100ºC, which allows rapid start-up. These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for vehicular power source to eventually replace the gasoline and diesel internal combustion engines.

Advantages