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Biologically inspired catalyst could lead to cheaper fuel cells

18 February 2013

Researchers have reported a fast and efficient iron-based catalyst that converts hydrogen directly to electricity. The result brings promise of more affordable fuel cells.

A new iron-based catalyst might help make those fuel cells less expensive (image courtesy of PNNL)

"A drawback with today's fuel cells is that the platinum they use is more than a thousand times more expensive than iron," says chemist R. Morris Bullock, who is leading the research at the US Department of Energy's Pacific Northwest National Laboratory.

His team at the Centre for Molecular Electrocatalysis has been developing catalysts that use cheaper metals such as nickel and iron. The one they report here can split hydrogen as fast as two molecules per second with an efficiency approaching those of commercial catalysts. A molecule, which exists in nature, called a hydrogenase, uses iron to split hydrogen.

Bullock and his PNNL colleagues, took inspiration for their iron catalyst from a hydrogenase, creating several potential molecules for the team to test. To do this, they need to split hydrogen molecules unevenly in an early step of the process. One hydrogen molecule is made up of two protons and two electrons, but the team needed the catalyst to remove one proton first and send it to a proton acceptor, such as oxygen. Once the first proton is gone, the electrode is able to attract the first electron. Then another proton and electron are similarly removed, with both of the electrons being transferred to the electrode.

The team determined the shape and size of the catalyst and also tested different proton acceptors. With the iron in the middle, in a configuration that is able to draw out the protons. The best acceptors sequestered these drawn-off protons quickly. The catalyst was able to split about two hydrogen molecules per second, thousands of times faster than the closest, iron-based competitor. In addition, they determined its overpotential, which is a measure of how efficient the catalyst is. Coming in at 160 to 220 millivolts, the catalyst revealed itself to be similar in efficiency to most commercially available catalysts.

The team is now working to make the catalyst work faster, as well as determining the best conditions under which it performs.

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