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The US Department of Energy has issued a directive to have systems able to store hydrogen in cars at a density of 5.5% in weight by 2017 and at 7.5% in 2020. As we know hydrogen is very light (55 times lighter than iron) and therefore creating a storage system that is safe and at the same time sufficiently light to reach those densities is not trivial.
To meet the challenge you need to use light atoms to trap hydrogen atoms and keep the ratio of trapping atoms to trapped hydrogen atoms to a minimum.
Researchers at the University of Maryland have managed to meet the challenge by creating tiny boxes made of graphene (a graphene layer is just one atom thick) to store hydrogen. These boxes can be closed or opened by applying a voltage, hence they can be used to store and release hydrogen on command.
They published the result of their research indicating that they have been able to store 1,468 atoms of hydrogen in a graphene box that can fold by itself like an origami (see drawing).
With this result they have already exceeded the demand of the US Department of Energy, reaching a density of 9.5% in weight.
Interesting to see how researchers are starting to manipulate graphene to custom build objects that can respond to external command, in this case through an electrical field.
