What form fits?
Finding the right storage material may be the toughest challenge.
The light and fluffy carbon material in Peter Eklund's lab has nearly 20,000 square feet of surface area crammed into each half-teaspoon worth.
"It has lots of internal spaces. When you think in terms of football fields, you realize how much space we're talking about." A football field, he notes, is about 32,000 square feet.
Arc apparatus
Within the nooks and crannies covering that carbon surface, Eklund, professor of physics at Penn State, is trying to "stick" as many hydrogen molecules as possible. His goal is to create a hydrogen-rich solid that could serve as a fuel tank for hydrogen-powered cars.
What might this fuel tank of the future look like? "Sort of like your water purifier cartridge but bigger — a big can filled with fluffy stuff," he says.
The push for hydrogen
In 2003, shortly after President Bush announced his hydrogen energy initiative, the Department of Energy challenged researchers across the country to develop innovative ways to store hydrogen aboard cars. At first glance, the DOE goal might not seem too hard to achieve: the storage system has to pack enough fuel to give the car a 300-mile driving range without infringing on passenger or cargo space. These are the basic standards we expect from our gasoline-powered cars.
The problem, explains Eklund, is that "when it comes to energy density, gasoline blows hydrogen away." While hydrogen packs more energy per pound than gasoline — roughly three times more — it fills four times the space. To visualize: A standard 15-gallon fuel tank holds about 90 pounds of gasoline. To get the same amount of energy from hydrogen, you'd only need about 34 pounds of fuel, but holding it would take a 60-gallon tank.
Most prototype hydrogen-powered vehicles solve the problem by using high-pressure tanks. The Toyota SUV that appeared on Penn State's campus during Hydrogen Day last November carried two such tanks in its trunk, each filled at 5,000 pounds per square inch. But safety and space remain significant concerns, says Eklund. Even tanks of compressed hydrogen are big and bulky. And, "if you puncture one of those compressed tanks, you release a lot of gas in a hurry."
Other hydrogen-powered cars, like the newest BMW model, store hydrogen as a liquid in super-cooled tanks nestled near the driver's seat. Cooling the hydrogen increases its density, but a tremendous amount of energy is required both to keep the tanks cold and, when needed, to turn the liquid back into a gas that can be delivered to an engine or fuel cell.
While some researchers are working on tanks that will safely hold both gas and liquid hydrogen at pressures up to 10,000 pounds per square inch, "we're reaching the technological limits of gas and liquid storage" of hydrogen, says Angela Lueking, assistant professor of energy and geo-environmental engineering at Penn State. "But we're just scraping the surface of what we can do with solid state."
Football fields and cages
Eklund's "high surface area carbon material," as he calls it, is just one type of solid that could potentially store significant amounts of hydrogen. Other candidates include carbon nanotubes, lightweight metals, and silicon structures. The key will be finding the one that is cheap, lightweight, and capable of storing and releasing hydrogen at convenient temperatures and pressures.
Eklund's research group spent years trying to store hydrogen inside carbon nanotubes — porous rolls of carbon, each a fraction of the diameter of a strand of hair. "But we found that carbon nanotubes could only absorb hydrogen at very cold temperatures," he reports. "We couldn't make it work at higher temperatures."
The fluffy, high surface area carbons that Eklund and Penn State collaborators Hank Foley, Mike Chung and Vin Crespi are working with now might solve the problem. That's partly because of the unique surface structure. It looks disorganized and random, "but it has a local order — five or six hexagons of carbon lined up next to each other, and then a break or a seam, and then another group of hexagons." A weak physical attraction, called a van der Waals force, holds the hydrogen on the surface of the carbon. Adding a small amount of a lightweight metal like boron to the surface might increase the attraction. "We're trying to come up with ways to increase what's called the binding energy of the material. If we can do that, more hydrogen will stick to the surface at higher temperatures," says Eklund. "That's our goal, but we don't know if we can reach it."
Eklund is also looking at structures he calls "hydrogen cage" materials. "You can create a cage with a metal oxide or silicon," he explains. "At room temperature, hydrogen molecules can't fit into the cage. But if you heat the system, the openings in the material expand, and you can force the hydrogen molecules inside." Cooling the system causes the openings to shrink, trapping the hydrogen until it is needed.
Sponges
If you steep certain metals in a flow of hydrogen gas, the metal will gradually suck the hydrogen into its lattice structure, much like a sponge absorbs water. "It's primarily a chemical reaction," explains Digby Macdonald, distinguished professor of materials science and engineering. The hydrogen molecules break up and dissolve into the metal and may then react with the metal to form tight ionic bonds. The resulting solid is called a metal hydride.
Metal hydrides could potentially hold more hydrogen than any other solid storage system, up to 12 percent by weight. But metal hydrides tend to be heavy, and it often takes a lot of energy and very high temperatures to break those ionic bonds to release the hydrogen. "Once you get hydrogen into a solid, you have to be able to get it out," Macdonald explains. The system must be reversible, or it's not practical."
Macdonald is part of a research team that is focusing on boron, a silvery solid at room temperature that has properties of metals and non-metals alike. Boron is lighter than carbon, and "with the possible exception of carbon," it combines with hydrogen in more ways than any other element on the periodic table, forming chains, sheets, and cages -- structures that are known collectively as boranes. Boranes, Macdonald says, could hold more hydrogen than any other kinds of hydride, and they could release the hydrogen at close to room temperature.
Doorways
Cartoon shows carbon nanotubes "doped" with clusters of metal. Hydrogen gas is catalyzed by the metal to stick in the nanotubes' honeycomb structures.Graphic courtesy of J.K. Johnson, University of Pittsburgh
Lueking is studying special kinds of carbon nanotubes that are covered with clusters of metal. "The metals act like doorways, letting the hydrogen in," she explains. Some of the hydrogen is absorbed by the metals to form metal hydrides, but most of it leaks onto the surface of the nanotubes and sticks in the nanotubes' honeycomb structures.
By themselves, carbon nanotubes absorb hydrogen at extremely low temperatures, too low to be practical, says Lueking. But adding metals to the system significantly increases the temperature at which the hydrogen is absorbed. "The temperature depends on the type of metal. Right now, we're using platinum as a model because it's well characterized -- we know a lot about its properties. But platinum is heavy, so we hope to apply what we learn to lighter metals like nickel and magnesium." Ultimately, the goal is to use as little metal as possible and channel most of the hydrogen onto the surface of the carbon nanotubes.
One of the problems with nanotubes, however, is cost, says Lueking. A little over two pounds of the material sells for about $50,000.
Lueking and her graduate students are also examining alternative types of carbon — graphite and coal, for example. Graphite has already been shown to absorb hydrogen when combined with lightweight metals, she notes. The structure of anthracite coal is very similar to graphite and could yield the same results.
"This is a first step in exploring cheaper, natural carbon materials."
Driving ahead
So far, however, the only storage systems that have met the DOE targets are compressed tanks of gas and liquid. A 2004 report by the American Physical Society concluded that even the most promising solid-storage technologies are still several breakthroughs away from practical use.
"It's a matter of finding the right material with the right physical properties and figuring out how to make lots of it," says Eklund. "There will definitely be a market for that material."
Peter Eklund, Ph.D., is professor of physics and materials science in the Eberly College of Science; pce3@psu.edu. Digby MacDonald, Ph.D., is distinguished professor of materials science and engineering in the College of Earth and Mineral Sciences; ddm2@psu.edu. Angela Lueking, Ph.D., is assistant professor of energy and geo-environmental engineering in the College of Earth and Mineral Sciences; adl11@psu.edu. Eklund and MacDonald have funding from the Department of Energy's Hydrogen Center of Excellence Program for their research. Lueking's research is funded by Penn State's Institutes for the Environment and the Department of Energy's Consortium of Premium Carbon Products from Coal.
