A new technique for storing hydrogen has been proposed by scientists in Canada and Germany. The method, which involves storing the gas between layers of graphite just nanometres deep, could help in the quest for practical hydrogen-storage devices for fuel cells. Graphite can store hydrogen better than other materials, such as carbon nanotubes, because it is cheap, non-toxic and easy to prepare (Proc. Natl. Acad. Sci. to be published).
Hydrogen fuel cells could be an environmentally friendly alternative to conventional fossil fuels. By oxidizing molecular hydrogen, they produce only energy and water. Fuel cells could therefore cut pollution and slash emissions of man-made greenhouse gases. What is holding them back is a way of efficiently storing the hydrogen they need.
Previous work on hydrogen-storage compounds has focused mainly on carbon nanotubes and hydrogen clathrate hydrate compounds. However, these materials only work in fuel cells at low temperatures or high pressures. Although physicists have investigated graphite in the past, theoretical models suggested that the material was not very good at storing hydrogen.
Now, however, John Tse (now at the University of Saskatchewan) and colleagues at the Steacie Institute for Molecular Sciences in Canada and the Technical University of Dresden in Germany have reanalyzed graphite using mathematical models. They found that previous studies did not take into account the interactions between carbon and hydrogen on the quantum scale, which led to misleading conclusions for the absorption capacity of this material. Including such interactions involves solving the Schrödinger equation for the motion of the hydrogen atoms on the complicated potential energy surface of graphite.
According to their calculations, thin layers of graphite or graphene — two-dimensional sheets of carbon atoms — spaced between 6 and 7 Angstroms apart can store hydrogen at room temperature and moderate pressures of just 10 MPa. Moreover, the amount of hydrogen stored comes close to a practical goal of 62 kilograms per cubic metre set by the US Department of Energy. Another advantge of the graphite is that the hydrogen gas can be released by moderate warming.
The Canada-Germany team says it could create “tuneable” graphite nanostructures with different hydrogen storage properties by interposing “spacer” molecules between the graphite layers. These spacers would have the added advantage of keeping out contaminants, such as nitrogen and carbon monoxide, which can reduce hydrogen storage capacity.
“The technological challenge is to now synthesise graphenes with the appropriate interplanar spacing for maximum hydrogen adsorption,” says Tse. “Once this is achieved and our theoretical prediction confirmed, I foresee that graphene would be a strong contender for practical hydrogen storage.”
