Complex hydride systems must fulfill a wide range of requirements in order to function as hydrogen storage materials. Metal borohydrides are considered as promising materials for hydrogen storage applications due to their high volumetric and hydrogen densities. Among the alkali- and alkaline-earth-based borohydrides, magnesium borohydride (Mg(BH4)2) displays very interesting properties both from fundamental and applicative points of view. Recently, different solvent adducts have been added to Mg(BH4)2 in efforts to improve its hydrogenation/dehydrogenation properties. Among these, tetrahydrofuran (THF) adducts of Mg(BH4)2 have proven to be especially interesting, as it was found that THF binds to the Mg2+ and appears to increase both selectivity and kinetics for H2 release (and reversible uptake). However, the total effect of THF on the hydrogenation and dehydrogenation processes is still unclear, and as such, in-depth fundamental studies of the THF intermolecular interactions with the Mg2+ and BH4- ions are desirable. This work focuses on exploring structural, vibrational, and dynamical properties of these compounds via various neutron-scattering methods in order to begin to understand why this adduct enhances the hydrogen-cycling properties of these materials. A series of Mg(BH4)2 samples with different concentrations of THF and TDF (deuterated THF) solvents were prepared and characterized. Neutron powder diffraction results have shown significant changes in the structures at room temperature and 4 K, which were confirmed by neutron vibrational spectroscopy and DFT calculations. Vibrational properties are strongly influenced by the THF environment; in particular, there is a red shift in the band energies of the BH4- librational and bending modes with increasing THF concentration as a result of changes in the bond lengths and force constants. Splitting in spectral features is also observed and correlated with the influence of the THF adducts. Interestingly, at a given temperature, the orientational mobilities of the BH4- anions obtained from quasielastic neutron scattering measurements are not particularly sensitive to the amount of THF or TDF present in the material and compare well with the mobilities of BH4- anions in unsolvated Mg(BH4)2. The correlation among the vibrational and the dynamical properties with the hydrogen storage capabilities of these materials will be discussed.

Role of solvent adducts in hydrogen dynamics of metal borohydrides: Neutron-scattering characterization

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.