12 July 2005: Lo-tech beats hi-tech in hydrogen storage

Canadian and German researchers have discovered that a relatively lo-tech material - graphite - might help solve the hi-tech problem of hydrogen storage. Hi-tech materials, such as carbon nanotubes and clathrate hydrates, have generated far more attention in the past. 

Scientists who studied the hydrogen storage capabilities of graphite had generally concluded it would be unable to store sufficient quantities of molecular hydrogen. But they had not fully considered the quantum behaviour of hydrogen in graphite, say researchers at the Steacie Institute for Molecular Sciences (SIMS), Ottawa, and Technische Universit?t Dresden.

The quantum behaviour of hydrogen influences its adsorption onto graphite through mechanisms such as quantum sieving, where molecules with high zero-point energies are unable to adsorb into narrow pores. Team leader John Tse at SIMS and colleagues developed a model for hydrogen storage that took these quantum effects into account. This involved solving the Schr?dinger equation, which calculates a molecule’s wave-function, for molecular hydrogen adsorbed onto graphite.

The team’s model suggests that a single layer of graphite could store additional hydrogen, but still not enough to be useful on a practical basis. However, the model also showed that much more hydrogen could be stored by trapping it between two layers of graphite. 

The model showed that, for graphite layers separated by 6-7?, a hydrogen storage density of 62 kg/m³ could be achieved at room temperature and moderate pressures. This is a greater storage density than has so far been achieved with either carbon nanotubes or clathrate hydrates.

The challenge now is to develop a material containing such finely separated graphite layers, although this will need to be conducted by other research teams. ’At this moment, we are not engaged in a practical realisation of this theoretical prediction,’ Tse told Chemistry World.

Nevertheless, Tse and his team have suggested two possible graphite-based materials: graphite intercalation compounds, where guest molecules such as alkali metal atoms are inserted between layers of graphite; and carbon foam, which is made up of tiny carbon clusters connected in a web-like foam. Jon Evans