Only a single metal centre is needed to catalyse the reduction of oxygen to produce water, opening the door to more efficient fuel cells in the future, say researchers in the US.

Converting solar energy to chemical energy using solar fuel cells and releasing stored energy from hydrogen fuel cells involves two key multielectron redox reactions - oxidising water to evolve oxygen and the reverse, reducing oxygen to water. It is the second reaction that limits the application of hydrogen fuel cells at the moment, as it generally requires expensive metal catalysts, such as platinum.

Nature achieves similar results in many different catalytic systems using metalloenzymes that contain bi- or multimetallic reaction sites, which has provided inspiration for development of bimetallic porphyrin catalysts. Now Daniel Nocera and colleagues at Massachusetts Institute of Technology have shown for the first time that single centre cobalt porphyrins anchored on carbon nanotubes can efficiently catalyse the reduction of oxygen, as long as they also contain a proton transfer group.

The positioning of the proton transfer group - in this case a carboxylic acid - the correct distance away from the cobalt is essential to stops the catalyst from partially reducing the oxygen, which is often a key problem in maintaining the efficiency of these reactions, explains Nocera.

Nocera's porphyrins are much more efficient than existing cobalt catalysts and are made easily in two steps, so could invigorate the design of future fuel cells using cobalt over its more costly metal cousins.

Minhua Shao, an expert in fuel cell technologies at UTC Power in the US, believes that the results are 'important to guide the design and development of non-precious metal electrocatalysts for oxygen reduction reaction in fuel cells'.

This is something Nocera is keen to develop, saying that he is now 'focusing on improving the catalysts by lowering the amount of energy needed for the reaction'.

Jon Watson

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