With mounting concern over the environmental impact of oil as a fuel, hydrogen increasingly looks like a useful alternative. In principle, hydrogen can be generated in a clean way from water by using sunlight in combination with solar cells. Moreover, it is non-polluting, and forms an environmentally benign by-product — water — on combustion. Hydrogen is thought to be an ideal fuel for vehicles, but its widespread use is limited by the lack of a safe, efficient system for on-board storage. The density and the condensation temperature of hydrogen are very low (-252 °C at 1 atmosphere), which makes it difficult to use conventional storage systems such as high-pressure gas containers or cryogenic liquid-gas containers. Therefore, the development of safe and convenient methods for hydrogen storage is an active research area1. In Angewandte Chemie, Weller and co-workers (Brayshaw et al.2) report that hydrogen may be stored and released using molecular clusters, in a process that is easily controlled by a simple chemical reaction or by an electric current.

One of the most successful and extensively studied methods for hydrogen storage is to keep the gas in a 'hydride' form3,4. In this approach, an alloy absorbs and holds a large amount of hydrogen by chemically bonding with the gas to form metal hydride compounds4. But although a hydrogen-storage alloy can absorb and release hydrogen without compromising its own structure, heating is required to promote the release, as this process takes up energy. The alloys studied so far usually require a temperature of about 300 °C to provide hydrogen at 1 atmosphere pressure.

In contrast, the method now reported by Weller and colleagues2 enables hydrogen storage and controlled release without a large input of energy. Their system is based on an organo-metallic compound that contains a core of six rhodium atoms5, as part of a complex that also includes 12 hydrogen atoms (Fig. 1). This cluster absorbs two molecules of hydrogen (H2) to produce a compound holding 16 hydrogen atoms. The absorption process takes 10 minutes at room temperature under 1 atmosphere pressure of hydrogen, and is almost instanta-neous under 4 atmospheres of hydrogen6,7. The absorbed hydrogen molecules are retained at room temperature for weeks without any external hydrogen pressure (under an inert atmosphere of argon), but can be removed under vacuum to quantitatively regenerate the 12-hydrogen cluster, although this takes a long time (several days) compared with the uptake process.

Remarkably, Weller and colleagues2 have found that hydrogen release from the 16-hydrogen cluster can be dramatically accelerated simply by changing the cluster's oxidation state. Adding a reducing agent to a solution of the 16-hydrogen compound releases one molecule of hydrogen from each cluster, so yielding a product containing 14 hydrogen atoms (Fig. 1). The original 12-hydrogen cluster is then easily regenerated by treating the 14-hydrogen cluster with an oxidizing agent, liberating another hydrogen molecule and completing the hydrogen uptake–release cycle.

Another crucial finding by Weller and colleagues2 is that a rapid hydrogen uptake–release cycle can also be accomplished electrochemically — that is, the reduction and oxidation steps are achieved directly by electron transfers at electrodes. Adding one electron to the 16-hydrogen cluster liberates a hydrogen molecule, giving the 14-hydrogen cluster described above. This rapidly loses another hydrogen molecule to yield a 12-hydrogen cluster, which differs from the original starting material by having only one positive charge — the original 12-hydrogen cluster has two positive charges. The electrochemical hydrogen-release process occurs in a matter of milliseconds on a glassy carbon electrode at ambient temperature and pressure. The product of this process is easily converted back to the original 12-hydrogen cluster by electrochemical oxidation (electron removal at an electrode; Fig. 1). The overall electrochemical process of hydrogen uptake and release can be repeated at will.

With the aid of theoretical calculations, the authors2 inspected the electronic structures of the starting 12-hydrogen cluster and of the 16-hydrogen cluster in this hydrogen-storage system. They found that the energy level of the lowest-energy molecular orbital that lacks electrons (known as the lowest unoccupied molecular orbital, or LUMO) in the starting material is only slightly higher in energy than that of the highest electron-filled orbital (the highest occupied molecular orbital, or HOMO). This is why the LUMO readily accepts electrons donated from hydrogen molecules. In contrast, the 16-hydrogen cluster has a large HOMO–LUMO gap; the addition of an electron into the LUMO destabilizes the molecule, and induces the release of a hydrogen molecule.

The hydrogen-storage capacity of this rhodium system, expressed as the ratio of the mass of releasable hydrogen to that of the storage system, is only 0.1%. This is clearly not sufficient for practical applications — the US Department of Energy wants hydrogen-storage systems to have a capacity of 6% weight-for-weight by 2010. Improved materials must be developed with a greater number of usable hydrogen molecules, bound to clusters of metals with an overall lower molecular mass. Never-theless, this work2 provides a well-defined mol-ecular model and a worthwhile strategy for the development of hydrogen-storage materials with high efficiency and convenience.

References
1.  Schlapbach, L. & Züttel, A. Nature 414, 353–358 (2001). 
2.  Brayshaw, S. K. et al. Angew. Chem. Int. Edn 45, 6005–6008 (2006). 
3.  Schüth, F., Bogdanovi, B. & Felderhoff, M. Chem. Commun. 2249–2258 (2004). 
4.  Sandrock, G. J. Alloys Compounds 293–295, 877–888 (1999).
5.  Ingleson, M. J. et al. J. Am. Chem. Soc. 126, 4784–4785 (2004). 
6.  Brayshaw, S. K. et al. Angew. Chem. Int. Edn 44, 6875–6878 (2005). 
7.  Brayshaw, S. K. et al. J. Am. Chem. Soc. 128, 6247–6263 (2006). 

Masanori Takimoto and Zhaomin Hou

Masanori Takimoto and Zhaomin Hou are at RIKEN, the Institute of Physical and Chemical Research, Hirosawa 2-1, Wako, Saitama 351-0198, Japan.