The neutron image of the hydrogen molecules adsorbed in MOF structure. The red-green circles are the H2 molecules. For clarity, the MOF-stucture is superimposed into the figure. Courtesy of Taner Yildirim
The future of sustainable transportation may be built on metallic-organic framework scaffolding.
Hydrogen, our lightest element, is a gas at atmospheric pressure and ambient temperatures. Because it yields only heat and water vapor when burned—rather than carbon dioxide—it is a promising alternative fuel.
However, hydrogen contains less energy per volume than liquid fuels, like gasoline or ethanol, so a great volume of it is required to generate enough energy to power a car or bus. It's as if you had to tote around the deluxe, 120-crayon box in order to access a single cornflower blue crayon.
Because of the small fraction of its useful energy, hydrogen presents problems for onboard fuel storage, which has constrained the creation of hydrogen-powered vehicles. For hydrogen to be an effective fuel, new materials that can store more hydrogen, per weight of the fuel storage system, are needed.
A joint venture between the Department of Energy and automakers, known as The FreedomCAR and Fuel Partnership, agreed that, in order to be economically viable, a vehicle's storage system must be at least 6% hydrogen by mass.
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Tamer Yildirim and Michael Hartman, of the National Institute for Standards and Technology's Center for Neutron Research, found that a man-made material known as MOF-5 may store up to 10% of its weight in hydrogen, as they report in the current issue of Physical Review Letters. Earlier studies reported that MOF-5 could hold only 2% of its weight in hydrogen, even at -200° C.
Metallic-organic framework compounds (MOFs) show great promise for hydrogen storage. MOFs are nano-porous materials in which metal-oxide clusters and organic molecules are arranged in a chicken-wire-like scaffolding. This framework leaves large spaces between its components, through which other molecules can enter. MOF-5's framework contains zinc-oxide clusters linked by central benzene rings.
Using neutron diffraction measurements, Yildirim and Hartman were able to image the locations where hydrogen joins the lattice of oxygen and zinc atoms within MOF-5. They observed that hydrogen molecules were packed similarly to the way apples or oranges fill a bowl. At high concentrations, the hydrogen molecules were closely arranged, with intermolecular distances as small as 3.0 angstroms (nearly 10 times less than a nanometer). They described the three-dimensional, densely-packed aggregation of hydrogen molecules as "nano-cages".
At present, 10% hydrogen storage by weight is only possible at very cold, cryogenic temperatures (-200° C).
"The fact that we can get this much storage at low temperatures indicates this material actually has enough room to hold up this much hydrogen," said Yildirim. "The next thing we have to do is close the channels so hydrogen can not easily leave the system at practical temperatures."
Yildirim and Hartman's work reveals that most of the adsorption of hydrogen molecules in MOF-5 is due to the metal-oxide cluster, rather than the benzene joints.
"The discovery/confirmation of primary hydrogen adsorption sites to be zinc oxide clusters, and secondary sites to be organic linker molecules in MOF-5 is very exciting," said Shuguang Deng, assistant professor of chemical engineering at New Mexico State University, via e-mail. "This makes it possible for scientists to optimize the MOF structures for hydrogen storage."
Scientists may try to develop even more efficient MOFs using different organic linkers, narrowing the channels through which hydrogen leaves the material. They may also create new MOFs, in which zinc is replaced with copper or another transition metal.
The findings have implications beyond hydrogen storage. MOFs could, possibly, be used as templates to link a series of hydrogen "nano-cages." A man-made material composed of tightly linked hydrogen molecules may exhibit interesting metallic properties, such as high melting and boiling points, and high electrical conductivity.