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Posted: January 25, 2008
Towards bio-inspired hydrogen production without noble metals
(Nanowerk News) Using hydrogen as an energy vector and in fuel cells may provide solutions to the specific energy challenges of the 21st century. Hydrogen production is currently based on the catalytic properties of “noble” metals such as platinum. For the first time, researchers at the joint Laboratoire de chimie et biologie des métaux (metal chemistry and biology, CEA-CNRS-Université Joseph Fourier, CEA's Grenoble site) have succeeded in producing hydrogen with a molecular system that doesn't require a noble metal catalyst. This outcome has important implications for the financial future of hydrogen energy and was published on January 4, in the journal Angewandte Chemie International Edition.
Research to improve hydrogen production is based largely on chemical reactions observed during photosynthesis in plants. More specifically, certain micro-organisms produce hydrogen from water with the help of light. To reproduce and adapt these processes, researchers have developed molecular systems capable of both photosensitisation, which captures light energy, and catalysis, which uses the energy collected to liberate hydrogen from water.
To date, all the technological systems developed to produce or use hydrogen rely on noble metals such as platinum (historically, noble metals were the precious metals used to make jewellery– gold, silver, platinum. Chemists define them as metals which do not oxidise easily. Today this term is applied to metals present at low levels in the earth's crust, making them both rare and costly – palladium, rhodium, iridium, osmium and ruthenium). But platinum reserves are limited. The metal’s scarcity and cost are obstacles to the long-term financial prospects of hydrogen technologies, despite efforts to reduce the quantities used in electrolysers and fuel cells.
Current research focuses on alternatives to platinum, by developing catalysts based on metals which are naturally more abundant and less expensive, such as those used by natural organisms (iron, nickel, cobalt, manganese).
A new system has been developed using a cobalt-based catalyst. Supramolecular in nature, it plays the role of both the photosensitiser and the catalyst. With the help of light, the electrons from the organic molecule are used to liberate hydrogen from water. This is catalysed by cobalt with greater efficiency than comparable systems using noble metals (Pd, Rh and Pt). Ruthenium is still used as the photosensitiser (Ru, left side of the figure above); one of the next steps in this work will be finding an alternative.
While the ultimate goal is still to use water as a proton and electron source (to avoid adding an organic molecule), this outcome represents considerable progress towards the photoproduction of hydrogen.