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There are a number of different types of fuel cells, and each offers its own operating characteristics and application opportunities. Fuel cells are divided into five main groups: PEM, or proton exchange membrane fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and alkaline fuel cells. All work on the same basic principle: electrons are stripped from hydrogen atoms and sent off as current, or electricity. The remaining ions – hydrogen protons – pass through an electrolyte to a region where it can combine again with the electron and with oxygen to produce water. Fuel cells are generally classified according to the type of electrolyte used.
PEM Fuel Cells
IdaTech's systems use PEM (proton exchange membrane) fuel cells. These operate at relatively low temperatures, about 80 degrees C, and generally come in sizes ranging from 1 kW up to 250 kW. PEM fuel cells can readily adjust their electric output to meet shifting power demands, and offer a high energy density. PEM fuel cells are fast starting and can begin delivering electricity within milliseconds of activation. They are light and potentially inexpensive and durable. The Department of Energy identifies PEM fuel cells as the preferable fuel cell for automotive, portable, residential and small commercial applications.
The membrane, or electrolyte, is a thin plastic sheet coated on both sides with a metal-based catalyst, primarily platinum. Hydrogen reacts with the catalysts to strip off its electrons, which are then sent off in the form of electricity to do some useful work. The hydrogen protons (hydrogen atoms minus their electrons) cross the membrane and then, again in the presence of the platinum catalyst, combine with oxygen and the returning electrons to form water and heat.
Phosphoric Acid Fuel Cells (PAFC)
PAFCs were the first fully commercial fuel cells, and hundreds of 200 kW PACF systems are in use today, powering motels, office complexes, military facilities, a U.S. Post Office and other large buildings and facilities. Phosphoric acid is the electrolyte, a medium that requires high temperature to efficiently transport the hydrogen ion. They are large, heavy and very expensive systems.
Molten Carbonate Fuel Cells (MCFC)
The electrolyte for these large, high-temperature fuel cells use a liquid solution of lithium, sodium and/or potassium carbonates, soaked in a matrix material. They operate at 650 degrees C. They are generally large systems with power ranges that extend to 2 mW. Their large size and mass limits the technology to large stationary applications.
Solid Oxide Fuel Cells (SOFC)
The electrolyte is generally a hard ceramic material made up mainly of zirconium oxide with small amounts of ytrria. The systems operate at about 1000 degrees C. The high temperature means these are slow starting systems and most are designed for larger, stationary applications in the 25 kW to 100 kW range.
Alkaline Fuel Cells (AFC)
NASA uses alkaline fuel cells on its space missions, though there are few commercial targets. They operate a 300 to 400 degrees C, and use a liquid alkaline solution of potassium hydroxide for an electrolyte. Alkaline fuel cells tend to be very costly, which currently limits their commercial appeal.
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