Scientists in Greece say they have found a way to make so-called "carbon nanoscrolls" store more hydrogen than any other material.

By adding impurities to rolled sheets of carbon in detailed computer simulations, they found they could control how tightly the scrolls wind up and, hence, how much hydrogen they adsorb.

This result is very promising because it provides a potential solution to one of the major problems of hydrogen storage for mobile applications, says George Froudakis at the University of Crete, who led the work.

Hydrogen has been much touted as the clean fuel of the future for electric vehicles and portable devices. But, despite holding more energy than hydrocarbon fuels, its incredibly low density makes it difficult to store in sufficient quantity to make it worthwhile.
Under pressure

Liquefying hydrogen by placing it under great pressure is both expensive and potentially dangerous. Even then, with a density of just one tenth that of water, it would be necessary to store four times the volume of liquid to match the energy content of gasoline.

"Most of the scientists working on this field of research believe that the solution to this problem will arise from the synthesis of new materials," Froudakis says.

Indeed, in 2003 the US Department of Energy (DOE) set a target of developing novel materials capable of reversibly storing enough hydrogen to make up 6% of their total weight by 2010.

The idea is to find materials with high surface areas that soak up hydrogen at much higher densities than previously possible, and without the need for extreme cooling or pressurisation.
Adding impurities

To address this problem, Froudakis and colleagues carried out computer simulations to see how the hydrogen uptake of carbon nanoscrolls could be affected by adding quantities of different alkali metals. These impurities cause the atomic distance between the layers of a scroll to vary.

Their findings suggest that adding lithium ions should increase the uptake of hydrogen at atmospheric pressure and room temperature from 0.19% to 3.31%.

This is twice the amount that other materials have achieved. Furthermore, hydrogen uptake should increase as the temperature is reduced, the researchers say.

These are significant quantities of hydrogen, says Frantisek Svec, a researcher at Lawrence Berkeley National Laboratory, in California, US. but they still fall short of the DOE targets.

Also, as the study is only a simulation, the results will need to be confirmed experimentally. "Unfortunately, in practice, these carbon-based materials are most often much less encouraging," Svec says.

Journal reference: Nano Letters (doi:10.1021/nl070530u)