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May 19, 2003
Volume 81, Number 20
CENEAR 81 20 p. 11
ISSN 0009-2347


MATERIALS SCIENCE

POROUS CRYSTALS SOAK UP HYDROGEN
Molecular design leads to materials with large H2 storage capacity

MITCH JACOBY

A key obstacle to using hydrogen as a fuel for transportation and other applications may soon be overcome, thanks to an advance in hydrogen-storage materials. Microporous materials designed by researchers at the University of Michigan, Ann Arbor, can be used to reversibly adsorb large quantities of hydrogen (a few percent by weight) at room temperature and low pressure. The family of metal-organic framework materials and the methods used to synthesize them may lead to even higher storage capacities, abating the hazards of storing hydrogen at high pressure.

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DESIGNERS Yaghi (left) and Rosi.
PHOTO BY ADAM MATZGER
.
Researchers in many countries have focused their efforts in recent years on methods for exploiting hydrogen's fuel benefits. Compared to hydrocarbons, its advantages include higher energy density and no pollution, because the sole by-product of combustion or electrochemical oxidation of hydrogen is water. In addition, the lightweight element is plentiful--although most is tied up as H2O.

Capitalizing on hydrogen's qualities--for example, in fuel cells for automotive applications--remains challenging, however. Among other reasons, there is no convenient way for the public to store and transport large quantities of hydrogen safely. Some advances in storage materials have been reported in connection with metal-hydride systems and carbon-based adsorbents, but both types of materials suffer from various shortcomings. And although safety certifications recently have been awarded in Europe for gas cylinders designed to hold 10,000 psi of hydrogen, safety concerns aren't expected to disappear anytime soon.

But now, Michigan graduate student Nathaniel L. Rosi and chemistry professor Omar M. Yaghi and their coworkers at Michigan, Los Alamos National Laboratory, and Arizona State University have designed and constructed promising storage compounds. Their crystalline materials, composed of metal-organic frameworks with a cubic three-dimensional extended porous structure, can adsorb up to 2% by weight of hydrogen at room temperature and about 10 atm of pressure [Science, 300, 1127 (2003)]. At lower temperatures, hydrogen uptake as high as 4.5% has been achieved, the group reports.

The materials, which remain stable even after solvent molecules are removed, are made up of cubes in which each corner is occupied by an OZn4(CO2)6 cluster that is bridged by six carboxylates of an organic linker. The group prepared crystals using various linkers including 1,4-benzenedicarboxylate, cyclobutylbenzene, and naphthalene.

The work is "an important advance toward safe hydrogen storage," notes Michael D. Ward, a crystal expert and professor of chemical engineering and materials science at the University of Minnesota, Twin Cities. The new materials fall short of the Department of Energy's target hydrogen-storage capacity of 6.5 wt %, Ward notes, but their architecture offers a rare opportunity to select solid-state properties through molecular design. The crystals can be manipulated through straightforward organic synthesis, Ward adds, "promising tailor-made hydrogen-storage materials."

LOTS O' SPACE Metal-organic framework crystals composed of OZn4(CO2)6 clusters (blue) and naphthalene linkers (gray) can adsorb several weight % of hydrogen (orange).
IMAGE BY NATHANIEL ROSI



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Copyright © 2003 American Chemical Society



 
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Related Person
Omar M. Yaghi

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