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Billions of Fuel Tanks May be Better than One

Meeting the world’s future energy demands will be an enormous challenge on many fronts. While hydrogen gas has piqued the interest of researchers as a clean, renewable energy resource, a number of major obstacles remain before an integrated system of producing, storing, and using hydrogen can supplant fossil fuel usage in a meaningful way. Perhaps the biggest of these problems is how to store this plentiful gas. Viable techniques for producing and using hydrogen, while still years away, have been proven, at least in principle. But a workable option for containing—and therefore, distributing—hydrogen gas remains elusive.

"The problem now is that at the most basic level, the technology simply doesn't exist," said Anton Nikitin, a PhD student conducting research at the Stanford Synchrotron Radiation Laboratory (SSRL) under the guidance of Anders Nilsson. "And if you don't have all the pieces of the puzzle, you can't build the system."

Industrial storage usually involves high pressure tanks of liquid hydrogen. These tanks are extremely heavy and pressurized to 10,000 pounds per square inch—far too dangerous and unwieldy for use in passenger cars. If hydrogen were to leak from the tank, the danger of explosion would be very high, and a catastrophic rupture during an accident would be disastrous. Safely storing hydrogen for use in passenger vehicles will require a leak-proof, low pressure device that can pack enough of the gas to justify the weight of the storage medium.

Carbon nanotubes—microscopic cylinders of carbon molecules—have shown promise as a key component for safely and efficiently storing hydrogen. These hollow, minute strands of carbon are chemically grown on plates of silicon and can be produced in such a way as to have walls that are a single atom thick. Carbon's ability to form chemical bonds with hydrogen, coupled with the fact that every atom in a single-walled nanotube is on the surface, means that each atom of carbon theoretically has the capacity to bind with an atom of hydrogen.

Researchers call this state "100 percent absorption," but until now, that degree of absorption was only a theoretical possibility. No one knew if nanotubes could actually be made to absorb that concentration of hydrogen—that is, until Nikitin and colleagues made it happen.

"We started with just one fundamental question," said Nikitin. "Before this research, no one knew if 100 percent carbon–hydrogen complexes were possible."

Passenger vehicles account for as much as 95% of oil consumption in the United States. Cars that rely on high efficiency electric motors already roam the highways, but the electricity they consume comes largely from fossil fuels. A solution to automobile-induced greenhouse pollution, as well as minimizing the coming fossil fuel crisis, must address the need for passenger cars powered by carbon-free fuels such as hydrogen.

When nanotubes were first developed in the early 1990s, researchers immediately recognized their gas absorption potential, but using nanotubes to store hydrogen on board a passenger vehicle is still a long way off. However, by demonstrating 100% absorption is possible in practice, Nikitin and his team have exceeded the Department of Energy’s goal for this technology, potentially giving engineers a framework around which to design a workable and safe storage system. This breakthrough may open a new chapter of research that could finally solve the hydrogen storage problem, bringing us one step closer to a true hydrogen economy.

"Carbon is cheap and it's everywhere," Nikitin said. "But I have a feeling no one really understands how complicated nanotubes are as a system. It's a beautiful material."

—Brad Plummer
    SLAC Today, August 24, 2006

Above image: Anton Nikitin with a small sample of carbon nanotubes grown on a silicon plate. (Click on image for larger version.)