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.
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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