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Neutron-scattering image reveals where hydrogen
molecules (red-green circles) connect to a metal organic framework
(MOF), a type of custom-made compound eyed for hydrogen storage
applications. The ball-and-stick model of the MOF is superimposed
on the neutron image.
Image credit: T. Yildirim/NIST
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Using beams of neutrons as probes, NIST
scientists determined where hydrogen latches onto the lattice-like
arrangement of zinc and oxygen clusters in a custom-made material
known as a metal-organic framework, or MOF. Called MOF5, the
particular nanoscale material studied by Taner Yildirim and Michael
Hartman has four types of docking sites, including a "surprising"
three-dimensional network of "nano-cages" that appears to form after
other sites load up with hydrogen.
This finding, reported in Physical Review Letters,*
suggests that MOF materials might be engineered to optimize both the
storage of hydrogen and its release under normal vehicle operating
conditions. It also suggests that MOFs might be used as templates for
interlinking hydrogen nano-cages, creating materials with unusual
properties due to a phenomenon known as quantum confinement. In a
sense, this discovery is a bonus.
Yildirim and Hartman found that the two most stable
sites in the scaffolding already offer considerable room for storing
hydrogen, accounting for the interest MOFs already have attracted.
Earlier studies reported that, at about –200 degrees Celsius, MOF5
could hold less than 2 percent of its weight in hydrogen.
The NIST research indicates ample room for
improvement. At very low temperatures, hydrogen uptake approached 10
percent of the material's weight. (The FreedomCar and Fuel Partnership
involving the Department of Energy and the nation's "Big 3" automakers
has set a level of about 6 percent as a minimum capacity for
economically viable hydrogen storage.) The bulk of the hydrogen was
held in nanometer-scale cavities inside the box-like arrangements of
zinc and oxygen clusters.
"Neutron diffraction measurements clearly show that
the molecules are packed in a fashion similar to the way apples or
oranges fill a bowl," Yildirim explains. The unexpected nano-cages
introduce the potential for spillover capacity, so to speak.
Hydrogen storage levels of 10 percent are
encouraging, but these results were achieved at impractically low
temperatures. Yildirim and Hartman say they hope better understanding
of how hydrogen molecules tether to MOFs will ultimately lead to
improved materials suitable for practical applications. |
