Fuel cells have been one of the most promising, yet frustratingly elusive technologies of recent times.
A recent collaboration between researchers at the Universities of Oxford and Dundee promises to shed new light on the matter however.
The researchers, who worked in partnership with the Harwell Innovation Centre, discovered how bacteria rips hydrogen apart to produce energy. They believe that this new understanding will be a significant step towards a more efficient hydrogen economy.
The scientists used a number of techniques to try and uncover just what happens inside bacterial enzymes called nickel iron hydrogenase. These do two core things with hydrogen gas:
- turn it into protons and electrons
- recombine them to form hydrogen
This is usually the process used in fuel cells, albeit using platinum, but nature has evolved methods that can do the same thing with nickel and iron, which are considerably cheaper and more available than platinum.
With hydrogen fuel cells likely to play a big part in any low carbon future, developing a capability to produce them more cheaply and easily will be a big step forward. What’s more, this enzyme can do the reaction in reverse, producing hydrogen from electricity and water. As a result the study of these enzymes is a very active and important area of research.
The enzymes can typically replicate what fuel cells do at very high temperatures with expensive materials, at much lower temperatures with readily available materials.
Understanding nature
The researchers tested the natural process out by tinkering subtly to the amino acids in the part of the enzyme where the hydrogen reaction occurs. By doing this, they first produced a seemingly identical, although much less efficient, version of the original.
By using the Diamond Light Source, the UK’s synchrotron science facility, to work out and compare the structures of the original and changed enzyme using a technique called x-ray crystallography, the team hoped to understand the relative importance of form over function.
“We got very accurate structures of the mutant and basically nothing moved apart from the atoms of the amino acids. So the loss of activity is not down to a change in structure or the loss of the metals. Diamond was crucial in determining this,” they say.
This confirmed that reduction in activity had to be due to chemical, not physical, changes. The tiny change removed a nitrogen atom at its heart, one that was essential to make the hydrogen reaction work.
It emerged that the enzyme was using something known as the Frustrated Lewis Pair. A normal Lewis pair is made up of different chemicals that are keen to interact with each other and would so given the opportunity. In this case these are the atoms of nickel and iron together, and a particular nitrogen atom built into the enzyme. The frustration bit is that in the enzyme, these entities are held close enough to see each other, but not close enough to interact fully. This produces an area of tension between them, a bit like holding a dog back from its food bowl. And just as anything getting between a dog and its food is at risk of being mangled, so a hydrogen molecule fed into this area of tension gets split apart.
Interestingly however, the study suggests that the existing literature surrounding how these enzymes work is wrong and that the chemistry of things needs to be updated.
Now that this research has revealed just how the enzyme splits hydrogen the researchers are keen to go further and see if they can watch it in action.
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