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The US Department of Energy has established guidelines (PDF) for various aspects of future hydrogen fuel cell vehicle systems. These standards cover everything from operational temperature to cost per gallon of gasoline equivalent to energy density. Meeting those standards is not easy; two of the crucial goals are volume density (how much hydrogen by weight can be put into a given space) and mass ratio (net useful energy to maximum system mass). These goals are intended to allow hydrogen fuel cell-based cars to function at least as well (in terms of range, weight, fueling convenience, storage, etc.) as current internal combustion engine cars.

Hydrogen storage is a particularly tough problem, for a variety of reasons, and work is progressing along a number of fronts to figure out how to put enough H2 into a gas tank sized space. Some people are working on using metals with a sponge-like nanostructure, but others have been working with single-wall carbon nanotubes. Early money was betting on the nanotubes, but results so far have proven disappointing.

Another form of carbon, graphite, was rejected as an option based on computer models of the interaction between carbon and hydrogen.

But researchers at the University of Saskatchewan in Saskatoon, the Steacie Institute for Molecular Sciences in Ottawa and the Technical University of Dresden in Germany have determined that those models were flawed, and that graphite may actually be the optimal material for hydrogen storage.

[The researchers] found the prior studies were incomplete when it came to exploring interactions between carbon and hydrogen on a quantum level.

They found graphite layers spaced slightly less than a nanometer apart can store hydrogen at room temperature and moderate pressures at close to a good weight. So the researchers contend graphite is a better option than carbon nanotubes, because it is far easier and less expensive to prepare.

Their report is a pre-print from the Proceedings of the National Academy of Sciences. They emphasize that work remains to be done -- researchers still need to figure out the optimal graphite structure for H2 storage, and (moreover) figure out how to actually get graphite into that structure.

But still, it's good news, and for a couple of reasons. Although one could consider batteries a better option for post-gasoline vehicles, hydrogen fuel cells have some advantages, too; even if we eventually decide to devote our attention to battery-electrics, progress on fuel cells means that we have options. Moreover, graphite is inexpensive, commonplace and environmentally benign -- and carbon nanotubes, at least, seem to be none of those three. This means that it will be that much easier for fuel cell automobile manufacturers to be able to hit target prices.

(Via Alternative Energy ~ Renewable Energy blog)