Complex metal hydride holds potential for hydrogen storage

Vitalij Pecharsky, scientist at the U.S. Department of Energy’s Ames Laboratory, is working to develop a mass-marketable hydrogen-powered vehicle. He has also won several awards in his field.

Cristóbal Matibag

The dream of developing a mass-marketable hydrogen-powered vehicle has eluded automotive designers for decades. But the work of Vitalij Pecharsky, senior scientist at the U.S. Department of Energy’s Ames Laboratory, may someday make it a reality.

Since 2005, Pecharsky, also a distinguished professor of materials science and engineering at Iowa State, has been researching hydrogen-rich solid metal compounds called complex hydrides.

He believes that these materials could provide a means of hydrogen storage that is more efficient than gaseous or liquid storage — and someday allow the world to end its dependence on fossil fuels. The official name of his project supported by the U.S. Department of Energy Materials Science and Engineering Division of the Office of Science is “Complex Hydrides — A New Frontier for Future Energy Applications.”

Thus far, hydrogen has attracted considerable attention from researchers and the public because the only byproduct from its combustion as a vehicle fuel is water.

This makes it a very attractive alternative to conventional fossil fuels like gas and oil. Vehicles powered by fossil fuels are widely believed to contribute to global warming, due to the high levels of carbon dioxide they emit.

A 2004 study by the Environmental Defense Fund showed that cars in the United States alone produced half of the carbon dioxide emitted by automobiles worldwide. But cost-effective alternatives to fossil fuels have been slow to develop.

Today, the main obstacle to hydrogen’s use as a vehicle fuel is the high cost of storing it. Pecharsky said the two most frequently used methods for storing the element — as a pressurized gas or as a liquid — are highly inefficient. Because hydrogen gas has such a low energy density by volume, a car fueled by it would need either an impractically large tank or an energy-gobbling off-board mechanism to compress it.

Liquid hydrogen requires a very energy-intensive cooling process. It’s also subject to what Pecharsky calls “boil off,” which occurs when heat leaks into the storage mechanism, causing some of the liquid to evaporate.

There are also safety considerations associated with using hydrogen gas as a fuel source.

Complex hydrides sidestep these problems because as solids they have higher energy density by volume than gas or liquid hydrogen. Thus, they wouldn’t need to be stored in giant or pressurized tanks. Nor would they need to be cooled to extremely low temperatures.

Boil off wouldn’t be an issue with the solid. And finally, solid hydrogen fuel wouldn’t pose the same safety risks as hydrogen gas, which can burst into flames when mixed with oxygen.

Until now, scientists have had difficulty extracting hydrogen from a hydride without heating it or using a chemical reaction with a solvent. Pecharsky said these processes either require high temperature and, therefore, too much energy, or change the chemical properties of the hydride so fundamentally that it is difficult to “get hydrogen back into the system” after it’s been extracted.

In search of a suitable process for hydrogen extraction, Pecharsky and his group have been experimenting instead with what he calls “mechanochemistry.” This process releases hydrogen from the material by agitating it in a vial with a quantity of metal balls. After the agitation and hydrogen release, the material can easily be re-infused with hydrogen.

Pecharsky thinks an adapted form of this technology could someday be used to develop a rechargeable storage medium for hydrogen fuel — which would make the idea of a mass-production hydrogen-powered car far more plausible. But he said he doesn’t want to be too bold in his predictions.

“We’re funded through this fiscal year, at least. But who knows the future?” Pecharsky said.