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FUEL CELLS, PEM & SOLID OXIDE
TWO DISTINCT types of fuel cells have been in the news lately. In a sense, both types are known for performing electrolysis in reverse: They take in hydrogen and oxygen and produce electricity. Their only “emissions” are trace amounts of water and heat. Beyond these basics, the two are somewhat different.
PEM cells (as in Proton Exchange Membrane or Polymer Electrolyte Membrane) are the ones of interest for automotive propulsion. Honda, Hyundai, Mercedes-Benz and Toyota have announced they plan to put fuel-cell cars into production, albeit in limited series, by 2015.
PEM cells operate at 50–100 degrees Celsius (122–212 degrees Fahrenheit). Their efficiency of operation is in the range of 40–60 percent, more than twice that of the most optimal internal combustion.
A PEM cell transforms hydrogen and oxygen into electricity, water and heat. It operates at around 170 degrees Fahrenheit, somewhat lower than a typical coolant temperature of an internal combustion engine.
The PEM membrane is a polymer electrolyte through which pass hydrogen ions, but not electrons (the latter, the electrical current). Before this magic can occur, a catalyst on the membrane is crucial in encouraging the breakup of hydrogen and oxygen molecules.
Thus far, just as it is in a car’s catalytic converter, platinum is the best catalyst. Alas, it’s also $1500/oz., so a lot of research has been devoted to minimizing the “catalyst loading” of a PEM cell.
The best deal is a platinum-free catalyst, an accomplishment that has recently been announced by Ilika Technologies. This materials specialist was a 2004 spinoff of the School of Chemistry at England’s University of Southampton. Since then, Ilika has partnered with the likes of Shell in improving hydrogen storage and Toyota with battery research.
Ilika has secured a U.S. patent on its metal-alloy catalyst, said to be 70-percent less expensive than the current PEM industry standard on a cost/performance basis. Ilika also has a partner in scaling up the material’s production.
On another fuel-cell front entirely, there’s growing application of solid oxide fuel cells (SOFCs) in large-scale stationary generation of electricity as well as auxiliary power units on vehicles. As its name hints, an SOFC differs from a PEM cell in the nature of its electrolyte, typically a ceramic solid.
One SOFC advantage is its relatively broad fuel diet. Any sulfur-free light hydrocarbon can provide the hydrogen—and without any platinum catalyst. Other benefits include long-term durability and high efficiency, with 60-percent levels having already been demonstrated.
However, an SOFC is characterized by hellaciously hot operating temperatures, 500–1000 degrees Celsius (932–1832 degrees Fahrenheit). To put these in perspective, aluminum melts at 1220 degrees Fahrenheit; copper, at 1983 degrees Fahrenheit.)
On the other hand, this heat can be tapped for enhanced efficiency. A “hybrid” SOFC facility produces electricity from two sources, its basic fuel cell as well as one or more microturbines run off the SOFC’s waste heat. Researchers talk about efficiencies of 75 percent and higher.
Auxiliary SOFC units are also being developed for supplying big-rig trucks with necessary electrical power, whether they’re running or not. These units use the same fuel as the truck’s, but their efficiency can return fuel savings of up to 85 percent. ds
© Dennis Simanaitis, SimanaitisSays.com, 2012
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