Issue Date: January 1, 2007
Hydrogen Storage Gets A Boost
Running cars on hydrogen-powered fuel cells might be commonplace in the future. But for now, it's still a technology on the drawing board, limited by the ability to store significant amounts of hydrogen in a safe and practical way.
Three papers published in the Journal of the American Chemical Society now report a significant milestone in hydrogen storage: the first definitive evidence for H2 binding to open metal coordination sites in nanoporous metal-organic frameworks (MOFs). The ability of H2 to bind to metal atoms allows the H2 molecules to pack more closely together and is expected to provide a major boost in storage capacity over simple H2 adsorption at nonmetal sites in previously prepared materials.
Jeffrey R. Long of the University of California, Berkeley, and his coworkers synthesized a manganese benzenetristetrazolate MOF with an observed H2 uptake of 6.9 weight %, or 60 g/L, at 77 K and 90 bar (J. Am. Chem. Soc. 2006, 128, 16876). This capacity is the highest yet reported for a MOF and is the first to exceed the Department of Energy's 2010 targets of 6.0 wt % and 45 g/L for H2 storage.
Neutron powder diffraction using deuterium (D2) in place of H2 further showed that the H2 enthalpy of adsorption of 10.1 kJ/mol-also a record for a MOF-is directly related to H2 binding to unsaturated Mn2+ centers within the framework. The data also showed H2 adsorption at several nonmetal pore sites.
In addition, Cameron J. Kepert of the University of Sydney, in Australia, and his coworkers have prepared a copper benzenetricarboxylate MOF and used neutron powder diffraction to study D2 adsorption (J. Am. Chem. Soc. 2006, 128, 15578). The data revealed six distinct D2 adsorption sites in the framework, with sites at Cu2+ atoms occupied first, followed by nonmetal sites in smaller pores and then in larger pores. The study provides "a very detailed structural understanding of the way in which D2 loading occurs," Kepert says.
A team co-led by Anthony K. Cheetham of UC Santa Barbara prepared a nickel sulfoisophthalate MOF and used neutron scattering spectroscopy to identify strong metal-H2 binding sites and weaker nonmetal adsorption sites in the material (J. Am. Chem. Soc. 2006, 128, 16846).
A significant hurdle for H2 storage using MOFs has been that the interaction between H2 and pore walls is by weak van der Waals forces of about 5 kJ/mol, Long explains. As a consequence, high loadings of H2 have been possible only under high pressure at liquid nitrogen temperature (77 K). Researchers in the field recognize that the goal of storing significant amounts of H2 at ambient temperature and closer to atmospheric pressure depends either on increasing the already high surface area of MOFs or, as the three papers show, on more of the tighter binding of H2.
"The observation of this effect in three materials that are structurally distinct and contain different transition metals indicates the very broad utility of this approach," Kepert says.
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