FUEL cells could become smaller, more efficient and cheaper, if carbon nanotubes replaced the expensive platinum catalysts the cells currently rely on.
Fuel cells have been hailed as a saviour of the environment, because they can turn hydrogen and other fuels into electricity cleanly and efficiently. But the technology has been hindered by the high cost of the platinum catalysts they require.
Hydrogen fuels cells, for example, work by pumping hydrogen gas past one electrode (the anode), where it is split into its constituent electrons and protons. The electrons then flow through 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 recombine with oxygen from the atmosphere to form water.
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Unaided, that oxidising reaction happens only very slowly, so to speed it up the cathode has to be formed of a chemical catalyst. The only material that has worked well enough so far is based on platinum nanoparticles.
Now a team led by of the University of Dayton, Ohio, has discovered that bundles of nanotubes doped with nitrogen work even better.
Carbon nanotubes were already known to catalyse this reaction, but only mildly. Researchers had thought that the catalytic properties were the result of traces of iron left over from the nanotube-manufacturing process, but Dai鈥檚 group has discovered that the iron actually hinders catalysis. The team 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 (Science ).
Their iron-free nanotubes were 鈥渆ven better than platinum鈥�, says Dai. The team鈥檚 device produces four times as much electric current as an equivalent 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 sorts of degradations.
Dai reckons the nitrogen is the key. Calculations show that each nitrogen atom attracts a cluster of 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 are ready pools of electrons to react with.
Carbon nanotubes are an expensive material for now, but Dai says that the same effect could be produced with other forms of nitrogen-doped carbon. 鈥淣ow we have discovered how this chemistry works, it may not be necessary to use nanotubes,鈥� he told New Scientist.
