02.06.12

Frozen Cages and High-Energy Heroes

Lab research suggests hydrate 'cages' could be used to store hydrogen fuel.

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The methane trapped in frozen water burns easily, creating ice on fire. PNNL

The methane trapped in frozen water burns easily, creating ice on fire.

Heroes of the silver screen seem to have a habit of getting caught in impossible prisons and frozen cages. All hope seems lost until they somehow break free, saving the day (and presumably coming back for a sequel).

In a sense, researchers at the Office of Science's Pacific Northwest National Laboratory (PNNL) have recently uncovered how other energetic 'heroes' escape from their frozen cages, which may lead to lasting energy benefits.

PNNL chemist Sortiris Xantheas and his colleague Soohaeng Yoo Willow studied the icy substances known as gas hydrates. Found in great abundance deep in the ocean, hydrates consist of "cages" of ice surrounding natural gas (methane), which burns cleaner than gasoline and other oil-based fuels. Hydrates look like ice…but can also burn. The trick is breaking the fiery fuel free.

Using supercomputers at Lawrence Berkeley National Laboratory's National Energy Research Scientific Computing Center, the two made detailed simulations of the great molecular escape. This was the first time researchers had accurately described the interactions between the water and gas molecules, and it opened new insights and new ideas. While it had been thought that a great amount of energy would be needed to break the cages and free the gas, the simulation showed that the molecules can squeeze through another way, using far less energy.

The reason lies in the structure of the hydrate "cellblock." Individual hydrate cages are built from "bricks" of 20 or 24 water molecules, which stick together through a sort of mortar of positive and negative forces called hydrogen bonds. Rather than breaking multiple water molecules away – the rough equivalent of dynamiting a hole in the wall of the hydrate cage – PNNL researchers showed that breaking a single hydrogen bond opened the "cell door" just enough for the hydrogen and methane molecules to squeeze by and make their escape. Their simulation also showed that the cell door closed – and the broken hydrogen bond reestablished itself – after the gas molecule got through.

That's potentially important since it suggests that hydrogen hydrates – the gas hydrates that hold hydrogen – could be used as materials for hydrogen fuel storage. At the moment, hydrogen fuel is typically held in high pressure tanks, although experimental methods are also storing it in solid form. However, hydrogen hydrates might be an alternative, since PNNL's computer models showed a potential work-around to their problem of hydrogen fuel leaking out.

Specifically, the simulations suggested that adding a ‘cell mate' – say a molecule of methane –could block the hydrogen from leaking out. That could reduce the complications of storing hydrogen in hydrate cages, and increase their potential as a storage source. In addition, under ideal conditions, hydrates could potentially store hydrogen that is close to the Department of Energy’s (DOE) standards for such storage systems. Hydrates might even see another use, as a capture and storage system for carbon dioxide.

Someday, scientists might free methane from hydrates, and then lock down CO2 in those same frozen cages. It'll make for a brighter day, one made possible by the heroes of DOE's Office of Science.

For more information about PNNL, please go to: http://www.pnl.gov/External link. And for more about DOE's Office of Science, please go to: http://science.energy.gov/.

Charles Rousseaux is a Senior Writer in the Office of Science.

Last modified: 3/15/2013 5:23:07 PM