Powering up fuel cells

A hydrogen fuel cell that uses carbon nanotubes to increase the amount of electrocatalyst attached to electrodes has been designed by UK scientists. This arrangement offers an order of magnitude improvement in power density over existing designs, they say.

Membrane-less hydrogen fuel cells based on enzyme electrocatalysts offer a clean and sustainable source of energy. In contrast to conventional hydrogen fuel cells, the high specificity of enzyme active sites means they can work on a mixed feed of hydrogen and oxidant so they don't require protons transported across a membrane; however, fuel cells depend on the performance of their electrocatalysts and enzymes are bulky, which limits the power output.  

In an effort to improve the power density, Sadagopan Krishnan and Fraser Armstrong from the University of Oxford attached carbon nanotubes to graphite electrodes. They then modified the carbon nanotubes with two different enzymes (hydrogenase-1 at the anode and bilirubin oxidase at the cathode) using 1-pyrenebutyric acid. This 3D arrangement greatly increases the amount of enzyme attached to the electrode. 'Increasing the amount of enzyme leads to greater power,' explains Armstrong.

There are still problems to overcome before this type of fuel cell finds a practical use, such as improving the long term stability and further increasing the power from a given size of fuel, but Armstrong hopes that they'll eventually find a use in devices that don't require a high power output. Yi-Heng Percival Zhang, an expert in bioengineering at Virginia Tech, US, agrees. 'This is a beautiful example of an enzymatic fuel cell,' he comments, adding that 'this micro-power source with reasonably high power outputs could be used for powering niche electronic devices'.  

Russell Johnson