11:13 06 February 2009 by Stephen Battersby
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Fuel cells have been hailed as saviours of the environment, because they can cleanly and efficiently turn hydrogen and other fuels into electricity. But so far this technology has been hobbled by the high cost of the platinum catalysts needed to make it work.
Now a new type of fuel cell based on carbon nanotubes promises to be much cheaper, as well as more compact and more efficient.
A team led by Liming Dai of the University of Dayton, Ohio, has discovered that a bundle of nanotubes doped with nitrogen can act as the catalyst, helping oxygen to react inside the fuel cell.
That is a vital stage of the fuel cell cycle. Rather than burning fuel to create heat to power a turbine, fuel cells turn chemical energy directly into a flow of electricity.
Hydrogen gas, for example, is pumped past one electrode (the anode), where it is split into its constituent electrons and protons. The electrons then flow out of the anode, providing electrical power, while the protons diffuse through the cell. Electrons and protons both end up at a second electrode (the cathode), where they combine with oxygen to form water.
Unaided, that reaction would happen only very slowly, so the cathode has to be formed of a chemical catalyst to speed up the reaction. Traditionally, the only substance that has worked well enough is platinum.
Carbon nanotubes had previously been shown to catalyse the fuel-cell reaction, but they were much less effective than platinum nanoparticles.
It had been thought that their slight catalytic properties were caused by traces of iron left over from the manufacturing process, but Dai’s group have discovered that the iron actually hinders catalysis.
They grew nanotubes doped with a trace of nitrogen using a process called chemical vapour deposition, in which nanotubes grow up from a base of iron nanoparticles. Then they removed the iron.
The original aim was to use these purified nanotubes in biosensors, but Dai also tried them out as catalysts – and found to his surprise that they worked very well.
“They are even better than platinum, long regarded as the best catalyst,” says Dai. The team’s device produces four times as much electric current as it would using platinum. And, while platinum nanoparticles can lose their effectiveness when they cluster together or become tainted by carbon monoxide, the nanotubes are immune to these degradations.
Dai thinks that it is presence of nitrogen in the nanotubes that makes them work so well. Calculations show that each nitrogen atom attracts electrons from neighbouring carbon atoms, which are then topped up by more electrons flowing from the anode. This means that when an oxygen molecule hits the cathode, there is a ready supply of electrons to react with.
While carbon nanotubes are an expensive material today, they are getting cheaper – and Dai says that the same effect could be produced with other forms of nitrogen-doped carbon. “Now we have discovered how this chemistry works, it may not be necessary to use nanotubes,” he told New Scientist.
“This is an interesting development. It would remove a major cost barrier for fuel cells,” says Di-Jia Liu of Argonne National Laboratory in Illinois, US, who found the earlier evidence for weakly catalytic nanotubes.
It’s too early to be sure that cars or phones of the future will be powered by nanotube, however. “The material has to be first tested in a real operating environment,” says Liu.
Journal reference: Science (DOI: 10.1126/science.1168049)