Hydrogen has long been viewed by technologists and environmentalists alike as a clean, abundant fuel for the future. We've reported before on Celsias about new technologies to store solar energy chemically as hydrogen and new ways to produce hydrogen from bacteria. It seems that government too has been keeping its eye on this futuristic fuel. As new ways become available to utilise hydrogen to power everything from cars to mobile phones, new regulations are also emerging. The U.S. Department of Energy (DOE) has established requirements for reversible hydrogen storage cells that must be met by 2010. These requirements dictate that volumetric capacity of such cells should be at least 42g of hydrogen per litre.

Now, new research from the University of Crete is coming close to designing a material to meet these goals. Until recently, it had been thought that the only way to meet the DOE targets was to use certain metal alloys capable of bonding with hydrogen to form hydrides. Hydrogen could then be released from the hydrides for use. However, bonding hydrogen to metallic compounds and releasing it for use is a relatively slow process and unsuitable for some commercial applications. Metal alloys are also heavy and expensive, making them a less than desirable option.

Using computer simulations, the University of Crete researchers have designed a material comprised of parallel graphene layers, just one atom thick, supported by carbon nanotubes (CNTs) positioned at right angles to the graphene planes. The nanotubes are roughly 50,000 times smaller than the width of a human hair. Nanoporous materials can store hydrogen by physisorption, where the hydrogen adheres to the surface through relatively weak molecular interactions, allowing for fast storage and retrieval of the fuel. Until this recent breakthrough, such materials were only capable of storing small quantities of hydrogen due to lack of appropriate pore size and low surface area.

Initially the researchers believed that CNTs would be easily applied to hydrogen storage. However, upon investigation it was discovered that unadulterated CNTs were not that promising. Further analysis revealed that doping the CNTs with lithium atoms increased their hydrogen storage capacity considerably.

Computer simulations on such materials showed that hydrogen storage was substantially increased, but still fell far short of DOE requirements. They needed to find a way to increase the amount of absorbed molecules. Hydrogen absorption in a material depends largely on the porosity of that material. In order to improve hydrogen absorption the researchers adjusted their design to allow porosity to be tailored in the new material, literally allowing the size of the pores in the material to be tunable.

The research by Georgios K. Dimitrakakis, Emmanuel Tylianakis, and George E. Froudakis, was published in the October 8 issue of ACS' Nano Letters, a monthly journal. The researchers have tentatively named the new material "pillared graphene". Their calculations show that the new material is capable of storing up to 41 grams of H2 per litre, just falling short of the DOE target of 45 grams of hydrogen per litre. Should the researchers be able to improve their design to meet DOE requirements, it will then be up to technologists to develop new manufacturing techniques to produce this material.