A research team from the National Institute of Standards and Technology (NIST), the University of Maryland and the California Institute of Technology has demonstrated that the metal-organic framework material MOF-74 can absorb more hydrogen than any unpressurized framework structure studied to date, and packs the molecules in more densely than they would be if frozen in a block.
By achieving technologically relevant levels of gravimetric density for stored hydrogen without either the extremely high pressures for gaseous hydrogen or extremely low temperatures for liquid hydrogen, MOF-74 could enable practical mobile hydrogen storage. The researchers describe their work in a paper published online in the ACS journal Langmuir.
One of several classes of materials that can bind and release hydrogen under the right conditions, MOFs have some distinct advantages over competitors, such as not requiring the high temperatures (110° to 500° C) some other materials need to release hydrogen.
MOF-74 is a porous crystalline powder developed at the University of California at Los Angeles. MOF-74 resembles a series of tightly packed straws comprising mostly carbon atoms with columns of zinc ions running down the inside walls. A gram of the material has about the same surface area as two basketball courts.
The researchers used neutron scattering and gas adsorption techniques to determine that at 77 K (-196° C), MOF-74 can adsorb more hydrogen than any unpressurized framework structure studied to date. NIST Center for Neutron Research scientist Craig Brown says that, though his team doesn’t understand exactly what allows the hydrogen to bond in this fashion, they think the zinc center has some interesting properties.
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.—Craig Brown
Although the liquid-nitrogen temperature of MOF-74 is not exactly temperate, it’s easier to reach than the temperature of solid hydrogen (-269° C). One of the goals of this research is to achieve energy densities great enough to be as economical as gasoline at ambient, and thus less costly, temperatures.
MOF-74 is a step forward in terms of understanding energy density, but there are other factors left to be dealt with that, once addressed, could further increase the temperature at which the fuel can be stored. Fully understanding the physics of the interaction might allow scientists to develop means for removing refrigeration or insulation, both of which are costly in terms of fuel economy, fuel production, or both.
The work was funded in part through the Department of Energy’s Hydrogen Sorption Center of Excellence.
Y. Liu, H. Kabbour, C.M. Brown, D.A. Neumann and C.C. Ahn. Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks. Langmuir, ASAP Article 10.1021/la703864a. Published March 27, 2008.