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August 5, 2005

Paving the Way for the Hydrogen Future
Berkeley Lab Group Receives $4.5 Million to Develop Hydrogen Storage Materials

Someday, hydrogen-fueled cars could zip along America's highways. Hydrogen fueling stations could be as ubiquitous as today's gas stations. And petroleum-sputtering cars could be as quaint as the horse and buggy.

Scientists must overcome a few hurdles before hydrogen powered cars are widely used. One of the most urgent is to find a convenient way of storing it.

Sounds ideal, but a future with zero-emission vehicles powered by a renewable source of energy won't happen unless scientists overcome several daunting technological hurdles. Can large amounts of hydrogen be produced without also creating carbon dioxide, a greenhouse gas? And can enough hydrogen be stored aboard a car to power fuel cells for hundreds of miles without refueling?

For now, the answer is no on both counts. But a team of Berkeley Lab scientists is working to turn the corner on the latter problem.

"Pressurized hydrogen cylinders take up a lot of volume in a car. They don't leave much room for luggage if you want to drive 300 miles without refueling," says Jeff Long of Berkeley Lab's Materials Sciences Division. "Hopefully that will change, but for now it is difficult to store a lot of hydrogen in a small volume without cooling it or placing it under very high pressure."

Long heads a group of nine Berkeley Lab scientists who are investigating new classes of materials that can efficiently store hydrogen — a very light and volatile gas — aboard cars under less extreme temperatures and pressures. The team is among the recipients of $64 million in DOE funding aimed at making hydrogen fuel cell vehicles and refueling stations available, practical, and affordable for U.S. consumers by 2020. The funding was announced in May 2005, by Secretary of Energy Samuel Bodman and is divided among 70 R&D projects at more than 50 institutions.

Their project will receive $4.5 million in DOE funding over the next four years. Another Berkeley Lab project, headed by Lutgard DeJonghe of the Materials Sciences Division, also received DOE funding to explore the development of nanocomposite proton conductors, which are used in hydrogen fuel cells.

For his project, Long assembled a materials sciences dream team of leading experimentalists and theoreticians. Like Long, most of them hold joint appointments at Berkeley Lab and UC Berkeley's departments of chemistry or physics. Collectively, the group has a formidable range of experience when it comes to materials discovery.

Paul Alivisatos will examine how hydrogen storage properties change with a material's size, from the nanoscale to the bulk scale. Martin Head-Gordon will help predict what kinds of metals and ligands bind to hydrogen. Jean Fréchet, a polymer scientist, will try to create nanoporous polymers that absorb hydrogen onto tiny cavities. Alex Zettl, a pioneer in developing nanotubes, will explore how boron nitride nanotubes can be used to store hydrogen.

Theoreticians Marvin Cohen and Steven Louie will predict new boron nitride nanostructures that can more efficiently capture hydrogen. UC Berkeley's Tom Richardson will synthesize and characterize metal hydrides. And to test these new materials, UC Berkeley's Samuel Mao will set up instrumentation that measures hydrogen uptake under a range of temperatures and pressures.

The future of hydrogen storage may look like this: materials scientist Jeff Long is working to develop nanoporous solids in which exposed metal sites capture hydrogen molecules under less extreme temperatures and pressures.

"The idea is to get people who are highly skilled in materials development to start thinking about the hydrogen storage problem," says Long.

For his part, Long is working on a storage material that has an incredibly large surface area enfolded into a small amount of space. Called a nanoporous coordination solid, some materials of this type boast 4,500 square meters of surface area per gram, meaning it has the potential to store a lot of hydrogen.

The material is composed of hollow cubes that are woven into an expansive three-dimensional framework, with each cube a potential home for several hydrogen molecules. It is created during a simple solution reaction in which the hollow cubes are filled with a solvent. After it's completed, the material is heated and the space-filling solvent escapes. What's left is a honeycomb of empty cubes, a ready-made storage material for hydrogen molecules under certain conditions. The trick is in getting hydrogen molecules to bond to the framework without having to cool it to low temperatures or expose it to high pressures.

"But so far the affinity of the hydrogen is too weak, so our goal is to increase the binding energy by creating the framework from new materials," says Long.

In this vein, Long and colleagues are working to expose metal sites in the interior of each cube, because they believe hydrogen will bond more strongly to the metal. They've investigated coordination sites composed of metal cyanide, magnesium, and carbonyl compounds, and in doing so have increased the hydrogen binding energy substantially, but not enough to work in a storage material destined for use in a car.

"We're making progress, but we're still at the very early materials-discovery stage," says Long. "The idea would be to load a fuel tank with a storage material that can absorb enough hydrogen to power the car for miles. The fueling process would need to be fast and fully reversible."

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