To increase the number of these sites, the researchers used a commercially available form of carbon that already has a large number of similarly narrow pores. Filling these pores with a nitrogen-and-iron-containing material and then heating up the mixture resulted in the much improved reaction rates.
The catalyst is designed to work in proton exchange membrane (PEM) fuel cells, a type of fuel cell favored by automakers because it operates at relatively low temperatures and has high power density–that is, a relatively small fuel cell can produce enough electricity to propel a car. PEM fuel cells use catalysts at two electrodes. One catalyst splits hydrogen and the other promotes a reaction that combines protons and oxygen to produce water. The second reaction is more difficult to perform: in conventional fuel cells, platinum is used in both electrodes, but 10 times as much is needed on the water-producing side. The new catalyst replaces platinum on the water-producing side, eliminating almost all of the platinum in the fuel cell.
Recently, other nonprecious metal catalysts have been demonstrated in another type of fuel cell, called an alkaline cell, but these may not work in the acidic environment in PEM fuel cells. At the same time, many researchers are finding ways to reduce the amount of platinum needed, rather than replacing the material altogether. This could make fuel cells more affordable in the short term, although eventually, if fuel cells are to be used widely, a nonprecious metal catalyst will be needed, Adzic says.
Dodelet believes that while his group has “solved the problem” of increasing the activity of the catalyst, two more significant hurdles remain before it can be practical in fuel cells. First, the catalyst’s durability needs to be improved. After 100 hours of testing, the reaction rates decreased by half. Second, because the catalyst can only work as fast as the reactants are provided, the transport of oxygen and protons into the material needs to be improved, something Dodelet plans to leave to fuel-cell engineers. Adzic says that the first step toward addressing the materials’ durability will be closely studying the catalyst to better understand how it works.