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New photocatalytic system gives hydrogen from water

An organic device that uses only sunlight to split water into hydrogen and oxygen has been developed by EU-funded researchers. The hydrogen produced, when used in fuel cells to provide electricity, will offer a promising alternative to fossil fuels.

Energy

In photoelectrochemical systems, hydrogen is produced by specialised semiconductors that use sunlight to dissociate water molecules into hydrogen and oxygen. The semiconductor materials currently used to convert solar energy to chemical energy in the form of hydrogen are similar to those used in photovoltaic solar power generation. There is, however, a significant difference. For photoelectrochemical water splitting, semiconductor electrodes are immersed in a water-based electrolyte. Although organic semiconductors offer great efficiency at a lower cost than inorganic materials they corrode when in contact with water. The EU-funded project PHOCS (Photogenerated hydrogen by organic catalytic systems) aimed to combine the light-absorption properties of organic with the charge-transport capabilities of inorganic semiconductors. Researchers therefore deposited a layer of nanometric titanium oxide over the photosensitive materials to act as a barrier between water and organic semiconductors. Moreover, the metallic nanomaterial electrically connected the electrodes to the platinum catalyst used. In this way, the photoelectrochemical light-harvesting system became more stable in the water/electrolyte environment. One of the main challenges facing the project was to demonstrate that organic materials can be used for photoelectrochemical hydrogen generation. With the new device, hydrogen production was sustained for three hours, thereby showing a stability that had not been reached before. Finally, a proof-of-concept device was built successfully fulfilling PHOCS targeted objectives in terms of photoconversion efficiency. The device clearly demonstrated how organic/inorganic hybrid interfaces, realised by low-cost and easily up-scalable techniques, can efficiently promote hydrogen production by using visible light, saline water and non-precious catalysts. The organic/inorganic systems developed can be also used in other photo-electrochemical applications, for instance for carbon dioxide reduction. Newly synthesized fullerenes have been proposed for promoting metal-free oxygen reduction reactions. PHOCS results can be applied to related fields such as photovoltaics, organic bioelectronics, photodetection in harsh environmental conditions and inorganic nanostructures processing techniques. The project will also contribute to the transition from the current energy model based on fossil fuels to a sustainable model that respects the environment.

Keywords

Hydrogen, fuel cell, semiconductor, photoelectrochemical, PHOCS, titanium oxide, fullerenes

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