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SCIENCE OBSERVER

Filling up with Hydrogen

David Schneider

In June, Honda leased its first hydrogen-powered, fuel-cell car to ordinary consumers, the Spallino family of Redondo Beach, California. That transaction marks an incremental step toward the hydrogen-fueled transportation system that President Bush championed in his 2003 State of the Union Address, when he announced a $1.2 billion "Freedom Fuel" initiative. That program, among other things, funds research on the longstanding problem of how to store hydrogen on a vehicle, one of the many possible showstoppers in the effort to nudge modern society toward a hydrogen-based economy. The trick is figuring out how to hold hydrogen safely and at sufficient density to allow a typical car to go 500 kilometers or so before having to tank up. While that requirement remains a significant hurdle, a new study indicates that the clever use of nanotechnology may give hydrogen storage a significant boost.

Honda's new hydrogen-powered, fuel-cell vehicle...Click to Enlarge Image

The surprising report, which appeared last June in the journal Angewandte Chemie, describes a way of storing hydrogen in the form of the compound ammonia borane, NH3BH3. Tom Autrey of the Pacific Northwest National Laboratory led the group of 12 authors who published the work. It builds on the decades-old idea of storing hydrogen in the form of ammonia, NH3. Unlike hydrogen gas, which requires cryogenic temperatures to liquefy, ammonia becomes a liquid at -34 degrees Celsius. It also does so at room temperature and 9 atmospheres pressure (it is similar to propane in this regard), making it much more convenient to use as a transportation fuel. Ammonia is comparatively inexpensive to produce, and the hydrogen can be separated out using catalysts without undue losses.

"It's a perfect fuel in many ways," says Ali T.-Raissi, head of the hydrogen research and development division at the Florida Solar Energy Center, part of the University of Central Florida. "The only problem it has is the fact that it's toxic." This consideration suggests that a better strategy might be to use the compound ammonia borane, which typically takes the form of a powdery solid. This chemical (and its cousin ammonium borohydride, NH4BH4) were first studied in the 1950s for their possible use in rocket fuel, a idea that was later abandoned. It largely fell off scientists' radar screens until the late 1990s, when Gert Wolf of the Technische Universität Freiberg realized that it might be a good medium for storing hydrogen in a vehicle. Indeed, ammonia borane contains almost 20 percent hydrogen by weight, giving the compound more hydrogen per unit mass or volume than even liquid hydrogen.

Getting the hydrogen out of ammonia borane isn't difficult and doesn't require additional energy: Once the compound is heated sufficiently, the decomposition reaction proceeds on its own. A third of the hydrogen is released at about 110 degrees, a second third at about 155 degrees (at which point ammonia borane is a liquid) and the final third at a higher temperature still, more than 500 degrees. Because the last increment requires awkwardly extreme temperatures, the new work of Autrey and his colleagues focused on the first two steps, whereby two-thirds of the hydrogen can be extracted.

Autrey's team infused ammonia borane into a nanometer-scale scaffolding of silica, a type of material often used as a substrate for catalysts because it provides an enormous surface area for reactions. Doing so allowed the hydrogen-release reaction to take place at lower temperatures and to give off less energy. In chemist Autrey's words: "It's just barely exothermic." That difference is important for two reasons. First, it allows engineers to consider using the waste heat from fuel cells to prompt the reaction (the most popular type of fuel cells heat up to about 85 degrees). More important, the change in thermodynamic properties means that driving the reaction in the opposite direction—regenerating the ammonia borane by somehow putting hydrogen back—becomes less difficult, at least in theory. As Autrey explains, "If you're going to regenerate the stuff, you don't have to go uphill so far."

But figuring out exactly how to regenerate ammonia borane from the residuum left after hydrogen has been extracted remains a stumbling block. And T.-Raissi stresses that being able to reconstitute the ammonia borane is necessary for this scheme to be economical for anything but niche applications. He notes also that it is going to be very challenging. "You've got to be smarter than Haber, smarter than Bosch," he says, referring to the German chemists Fritz Haber and Carl Bosch, who at the turn of the 20th century pioneered a system to synthesize ammonia from hydrogen and nitrogen by combining these gases at high temperature and pressure in the presence of osmium and uranium catalysts—the system used around the world today to manufacture synthetic fertilizer.

Autrey agrees that regeneration is critical and says that he and his colleagues are working on the problem in collaboration with others in the consortium of government, university and industrial labs that make up the Department of Energy's Center of Excellence for Chemical Hydrogen Storage. But he is otherwise tight-lipped about what avenues his group is investigating. Perhaps their best efforts will fail to resolve this critical issue. Or, just maybe, wielding computational, theoretical and experimental tools not available a century ago, Autrey's interdisciplinary team (or another one) will yet outwit Haber and Bosch. Doing so could help make hydrogen the fuel of choice in future vehicles.

Would such a change relieve the energy crunch and lower the amount of carbon dioxide released into the atmosphere? That all depends on how one gets the hydrogen, which, after all, is just serving as an energy carrier. Skeptics point out that it would probably come from natural gas, in which case the shift to a hydrogen-based transportation system would not fundamentally resolve current concerns. But at least the term "gas station" would finally make some sense.—David Schneider


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