Future energy technologies that use hydrogen as an energy carrier offer the tantalizing prospect of operating essentially free of pollutant and greenhouse gas emissions while utilizing hydrogen produced from a diverse range of renewable sources. To realize these technologies, such as hydrogen proton exchange membrane (PEM) fuel cells, improved approaches are needed for high-capacity storage of hydrogen at temperatures ranging from near ambient to about 100 °C and at pressures below about 100 bar. These conditions favour storage based on the interaction of hydrogen with solid materials, rather than storage based on compressed or liquid hydrogen, which require high pressures (700 bar) or low temperatures (20 K), respectively. Significant advances have been made recently both in materials that store hydrogen as H2 molecules adsorbed on suitable supports, and in materials that bind hydrogen chemically in the form of atoms, protons (cations), or hydride anions.

Advances in molecular storage have come largely from a detailed understanding of the structures and bonding processes in traditional adsorbents and the development of new high-surface-area adsorbent materials with structures tailored on the molecular scale. Much of the emphasis has been on further increasing the number of adsorption sites to improve storage capacity. The low adsorption energies of current materials present another challenge because the weakly bound H2 can achieve technologically significant capacities only at cryogenic temperatures (50-80 K). Ongoing efforts to improve the thermodynamics of adsorption are primarily focused on composition and structure modifications. For chemically bound hydrogen, advances have come from investigation into light-element binary and complex hydrides, which inherently have high hydrogen capacities. The polar covalent bonding that characterizes these hydrides leads to very slow kinetics for hydrogen exchange, so here the emphasis is on improving kinetics through structures and catalyst systems that enhance diffusion. These and other issues concerning both molecularly and chemically bound hydrogen storage materials have begun to be addressed through an understanding of their behaviour and their manipulation on the nanoscale.

Special issue
A current survey of this endeavour can be found in Nanotechnology . The themes covered in this special issue include the thermodynamics and kinetics of hydrogen storage materials at the nanoscale; the structure of nanoporous adsorbents; the structure of hydrogen adsorbed in nanosized pores; and the behavior of nanoparticulate, nanocrystalline, and nanoconfined metal and complex hydrides, including the form and effects of catalysts. These themes are addressed through theoretical, computational, and experimental approaches.

Although an ideal hydrogen storage material has not yet been identified, the papers in this issue indicate that an ideal material will likely be highly structured on the nanometer scale. To optimize the capacity and interaction energy of adsorbents, the pore size, shape, and volume will need to be carefully controlled. Similarly, the diffusion lengths in hydride materials will need to be matched to crystallite and particle sizes. Furthermore, the diffusion lengths themselves will need to be tailored through the use of dopants, the placement of catalysts, and control of interface energies.

•  The Nanotechnology special issue on nanoscale phenomena in hydrogen storage is free to read until the end of 2009.