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Wednesday, April 2, 2008

Nano Tanks Could Store Hydrogen in Microscopic 'Soccer Balls'

While hydrogen gas and fuel cells remain far-off realities for solving the fuel crunch, new computer models of interlocked carbon chambers (above in gray) have proven to store hydrogen (white) at similar pressures to the cores of huge planets. (Photograph by Jennifer Bogo; illustrations courtesy of Nano Letters/ACS)

If engineering could keep up with the hype, we would all be driving hydrogen cars by now, whirring about in emissions-free wonderboxes like the Jetsons. But hydrogen, for all its potential, presents some serious technical challenges. As a gas, it requires gigantic, heavy tanks to store, and as a liquid, it must be kept impracticably cold—below 423 degrees Fahrenheit. Researchers at Rice University, however, recently tested a third option.

“What if, instead of one big tank, you have millions and billions of tiny little nanoscale containers? Would it be safer? How much pressure would they contain?” asks Dr. Boris Yakobson, lead author of a study that recently modeled hydrogen storage inside tiny hollow carbon structures called buckyballs. “We wanted to understand the limits—how many molecules of hydrogen, in principle, you can place inside the carbon cage before it just mechanically breaks.”

Picture a soccer ball a few hundred thousand times smaller than a grain of salt. This, essentially, is what buckyballs look like—strong, hollow cages of interlocked carbon. The idea that something could be held inside these cages isn’t new, but Yakobson and his colleagues are the first to accurately estimate how much pressure the walls could withstand, using computer modeling to push them to their breaking point.

The space inside a 60-atom buckyball would ordinarily be big enough for just a single atom of hydrogen. But as the researchers simulated adding more and more, the walls of the cage continued to hold, withstanding pressures close to those at the cores of Jupiter and Saturn. Ultimately, the team estimated its buckyball could hold 58 hydrogen atoms before bursting, squeezing the normally gaseous atoms into a near metallic state.

“This is not practical for storage, certainly, because it’s too small,” Yakobson says. “But then you say, okay, now what if I go to larger cages? The shell can still sustain the same tension.” Modeling a 60-atom buckyball enables researchers to scale up to cages made of thousands of carbon atoms and measuring several nanometers across. To the naked eye, a few billion of these would look like a kind of gray, metallic powder, similar to graphite dust.

Buckyballs do exist in nature—the soot from burning candles, for instance, contains buckyballs, split from vaporized wax by the heat of the flame—and scientists have been synthesizing them in labs since the mid 1980s. This study, though, is purely theoretical. No one yet knows how to put that much hydrogen inside a buckyball, or how to get it out afterwards.

Yakobson is currently working on other nanostructures he thinks might be better suited to hydrogen storage, including tiny cylindrical tubes that would compress hydrogen atoms inside with a sort of microscopic piston. In the future, such structures could also be used to contain radioactive isotopes for cancer treatment, or to subject tiny amounts of compounds to gigantic pressures in the lab for study.

“From a scientific point of view, hydrogen is a perfect material, but from an engineering point of view, you wind up having significant energy losses when you make it and store it and try to move it around,” says Dennis Witmer, a professor at the University of Alaska who regularly evaluates hydrogen fuel cells and other alternative energy sources.

“It looks to me like this is a really cool piece of science,” Witmer says, “but in terms of expecting to fuel a car with them anytime in my lifetime, I don’t expect it to happen.”

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