"It would be very realistic to say a solution for the solar generation of hydrogen could be forthcoming in three to four years," said Craig Grimes, a materials scientist at Penn State University in University Park, Pa.

The chemical reaction that would power hydrogen vehicles is simple: Combine hydrogen with oxygen in a fuel cell and they produce energy and water, with none of the dirty mix of toxins and global-warming gases that gasoline combustion spews.

In addition, hydrogen is the most common element, making up roughly three-quarters of the known universe and thereby constituting a virtually unlimited fuel source.

Convinced hydrogen is the key fuel of the future, political leaders are supporting hydrogen development. In 2003, the Bush administration adopted a five-year, $1.2 billion initiative to develop fuel cells.

The United States also has entered the Cooperation in the Area of Fuel Cells agreement with the European Union. Industry giants are focused on hydrogen as well, with Shell Oil committing $1 billion over five years, according to plans announced in 2001.

Still, the challenges remain daunting. Grimes said for hydrogen to replace oil completely, the United States alone would need to generate more than 250,000 tons of the gas daily, enough to lift 13,000 Hindenburg airships.

Current methods for producing hydrogen on a large scale require natural gas - methane - the processing of which yields substantial volumes of carbon dioxide, the primary suspect greenhouse gas.

One alternative is to use sunlight to split water molecules placed on semiconductors into their constituent hydrogen and oxygen, a process called photolysis.

Research has shown that billionth-of-a-meter-scale nanotubes can channel the hydrogen separated from water molecules and keep the atoms apart before they can recombine with oxygen, explained John Shelnutt, a chemist at Sandia National Laboratories in Albuquerque.

Grimes and colleagues have created titanium-dioxide cylinders that are 224 nanometers long with 34-nanometer-thick walls.

The nanotubes are 85 percent efficient at harvesting the ultraviolet portion of sunlight and 12.8 percent efficient at extracting hydrogen from water.

They also are easy to make, inexpensive and stable after repeated use, Grimes said. "The nanotube architecture is perfect."

The problem is only 5 percent of the sun's energy is ultraviolet light, so Grimes and colleagues are working to make the nanotubes operate in the visible spectrum.

"If we can take the efficiency we saw with ultraviolet light and move it into the visible, it would be a really good thing for society," Grimes said. "A 10 percent conversion efficiency is basically the threshold where hydrogen becomes a very tenable idea with respect to cost."

Research by chemist Michael Graetzel and colleagues at the Swiss Federal Institute of Technology in Lausanne suggests nanocolumns of iron oxide, or rust, could act in the visible light range in photolysis.

"It doesn't get any cheaper than iron oxide," Grimes said.

Shelnutt and colleagues also are working on nanotubes made with organic molecules known as porphyrins.

"They're used in biology for absorbing light to convert to chemical energy, so they're a natural choice for us to work with," he said.

The team employs porphyrins containing gold on the inside and platinum on the outside that naturally self-assemble into nanotubes, and they can work both in the visible and ultraviolet ranges, although their efficiencies remain uncertain.

"Ultimately, we want to move away from gold and platinum to less-expensive metals," Shelnutt said.

He said he and other researchers are developing fuel-cell catalysts with nano-scale features that increase their surface area, enhancing their ability to extract electricity from the hydrogen-oxygen reaction.

Storage remains the potential Achilles heel of a hydrogen economy, noted Sandia researcher Jay Keller in Livermore, Calif.

The challenge is to create a device that is no heavier and takes up no more space than a traditional automotive fuel tank, but provides enough hydrogen to power a vehicle for roughly 300 miles before refueling.

So far, most prototype hydrogen vehicles employ liquid hydrogen, which is expensive to store because it requires temperatures of roughly minus 423 degrees Fahrenheit (minus 253 degrees Celsius), while 1-liter bottles of compressed hydrogen gas typically only hold 15 grams of the element at a pressure of roughly 5,000 pounds per square inch.

The Department of Energy's Office of Energy Efficiency and Renewable Energy current operates three centers that work on nanotech solutions for hydrogen storage:

- Pacific Northwest National Laboratory in Richland, Wash., focuses on chemical methods to store hydrogen;

- Sandia National Laboratory leads the effort in metal hydrides, which sponge up hydrogen, and

- The National Renewable Energy Laboratory in Golden, Colo., examines carbon-based methods, such as carbon nanotubes.

Karl Johnson, a chemical engineer at the University of Pittsburgh, and colleagues are working on metal hydrides with the hope nanotech can help metal hydrides release hydrogen at lower temperatures than currently needed.

"Typically a metal hydride doesn't want to give up hydrogen without a lot of heat," Johnson explained.

The question of whether carbon nanotubes can be a viable hydrogen storage material remains open.

Michael Heben, a materials scientists at the NREL, and colleagues continue work in the field, although Johnson said the theoretical simulations and experimental work to date suggest carbon nanotubes will not play a large role in hydrogen storage.

Tom Autrey, a chemist at PNNL, and colleagues are working on ammonia borane, which scientists examined in the 1950s for use in rocket propellants because it can hold nearly 20 percent of its weight in hydrogen.

Ammonia borane normally releases hydrogen very slowly at temperatures below about 170 degrees F (77 degrees C), but researchers found the hydrogen separated roughly 100 times faster when the ammonia borane was built on a nanoporous silica scaffold.

They want the hydrogen cell to work at 170 degrees F or below - roughly the level of waste heat generated by a fuel cell.

"You can take the waste heat from the fuel cell and hopefully use that to remove the hydrogen from the storage tank - talk about the perfect system," Autrey said.

He said his team will confer with chemical engineers on hydrogen fuel cell requirements for vehicles, and hope to have non-commercial applications ready in three to five years, potentially as portable power for military hardware.

Nano World is a weekly series examining the exploding field of nanotechnology, by Charles Choi, who covers research and technology for UPI Science News. E-mail: [email protected]

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