A Monte Carlo study of hydrogen storage in charged metal-organic framework materials

Monte Carlo simulations were performed modeling hydrogen sorption in a recently synthesized metal-organic framework material (MOF) that exhibits large molecular hydrogen uptake capacity. Unlike most other MOF's that have been investigated for hydrogen storage, the MOF has a highly ionic framework and many relatively small channels. The simulations demonstrate that it is both of these physical characteristics that lead to relatively strong interactions between the MOF and diatomic hydrogen and ultimately large hydrogen uptake. Microscopically, hydrogen interacts with the MOF via three principle attractive potential energy contributions: Van der Waals, quadrupole-charge and induction. Previous simulations of hydrogen storage in MOF's and other materials have not focused on the role of polarization effects, but they are demonstrated here to be the dominant contribution to hydrogen physisorption. Indeed, polarization interactions in the MOF lead to two distinct populations of dipolar hydrogen that are identified from the simulations that should be experimentally discernible using, e.g. Raman spectroscopy. Because polarization interactions are significantly enhanced by the presence of a charged framework with narrow pores, this makes such MOF's excellent hydrogen storage candidates.