MOF-74 is a unique carbon metal compound, which can store hydrogen unpressurized at densities higher than solid hydrogen. The green atoms are zinc ions, while the white/gray atoms are carbon.  (Source: NIST)
Carbon based material offers hydrogen storage densities better than solid hydrogen without pressure

With the rise in interest of a so-called "hydrogen-economy" and switching to vehicles burning hydrogen or using it in fuel cells, a critical challenge has become how to store hydrogen.  Many experts think that in order for hydrogen-powered vehicles to be competitive in terms of range with modern gas vehicles, hydrogen will need to be stored at densities greater than that of liquid hydrogen.  Such super compression brings up complex problems of safety and costs.

A recent DailyTech article describes how scientists are developing plans to store hydrogen in carbon fullerene structures, such as "buckyballs".  Now, the National Institute of Standards and Technology’s Center for Neutron Research (NCNR) demonstrated a promising new storage technique using another exotic class of carbon materials.

NIST, the University of Maryland and the California Institute of Technology all collaborated in the study, which examined metal-organic frameworks (MOFs).  This class of compounds is considered highly promising as it can store and release hydrogen easily with simple changes in conditions.  This might allow for a fuel pump-style approach one day.  MOFs also are advantageous in that they don't require as high temperatures to release hydrogen.  Other materials frequently require between temperatures ranging from 110 to 500 C before they release the hydrogen.

The team focused most of their research on MOF-74, a porous crystalline powder developed at the University of California at Los Angeles.  The compound is curiously shaped with carbon arranged in straw like molecules with columns of zinc ions running along the insides of the tubes.  The straws are densely packed and form a powdery material.  Just one gram of the substance has as much surface area as a tennis court, due to its unique design.

The structure packs hydrogen in amazingly well.  Researchers discovered that at 77 K (-196 C), MOF-74 absorbs more hydrogen than any non-pressurized material to date.  The material packs hydrogen more densely than even a block of solid hydrogen.  The researchers used a mixture of neutron scattering and gas adsorption techniques to come to these findings.

NCNR scientist Craig Brown states that the team remains unsure exactly what makes MOF-74 so great at grabbing hydrogen.  He says the team is speculating it may be due to some interactions with the zinc ions.  Brown enthuses about MOF-74's promise stating, "When we started doing experiments, we realized the metal interaction doesn’t just increase the temperature at which hydrogen can be stored, but it also increases the density above that in solid hydrogen.  This is absolutely the first time this has been encountered without having to use pressure."

The temperature needed for storage in MOF-74 is easily obtainable using cheaper liquid nitrogen.  This is favorable in comparison with solid hydrogen, which requires lower temperatures of -269 C.  The researchers hope further studies will allow them to modify the material or find similar materials that will allow storage at even higher temperatures.  This will allow for the removal of refrigeration and/or insulation systems, lowering the production costs and improving fuel economy.

The new research was funded by the Department of Energy's Hydrogen Sorption Center of Excellence.

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