Hydrogen cycling by enzymes: electrocatalysis and implications for future energy technology

Hydrogenases provide an inspiration for future energy technologies. The active sites of these microbial enzymes contain Fe or Ni and Fe coordinated by CO and CN ligands: yet they have activities for hydrogen cycling that compare with Pt catalysts. Is there a future for enzymes in technological H-2 cycling? There are obviously going to be disadvantages, perhaps overwhelming, as enzymes are notoriously fragile; yet what are the positive aspects and can we learn any chemistry that might be applied to produce the electrolytic and fuel cell catalysts of the future? We have developed a suite of novel electrochemical experiments to probe the chemistry of hydrogenases. The reactions are controlled and monitored at the surface of a small electrode, and characteristic catalytic properties are discernible from tiny amounts of sample material, so this approach can be used to search the microbial world for the best catalysts. Although electrochemistry does not provide structural information directly, it does give a "roadmap" by which to navigate the pathways and conditions that lead to particular states of the enzymes. This has prompted many interdisciplinary collaborations with other scientists who have provided microbiological, spectroscopic and structural contexts for this work. This article describes how these electrochemical experiments are set up, the data are analysed, and the results interpreted. We have determined mechanisms of catalysis, electron transfer, activation and inactivation, and defined important properties such as O-2 tolerance and CO resistance in physical terms. Using an O-2-tolerant hydrogenase, we have demonstrated a "proof of concept" miniature fuel cell that will run on a mixed H-2/O-2 feed in aqueous solution.