PNNL

Abstract

Metal borohydrides are considered as promising materials for hydrogen storage applications due to their high volumetric and hydrogen densities. Recently, different solvents have been complexed with Mg(BH4)2 in efforts to improve its hydrogenation/dehydrogenation properties. Notably, Mg(BH4)2·xTHF adducts involving tetrahydrofuran (THF; C4OH8) have proven to be especially interesting. This work focuses on exploring structural, vibrational, and dynamical properties of the THF-richMg(BH4)2·3THF and THF-poor Mg(BH4)2·0.5THF adducts via various neutron-scattering methods and molecular DFT calculations. Structural analysis, based on neutron diffraction measurements, has confirmed lowering of the symmetry of Mg(BH4)2·3TDF (D - Deuterium) upon cooling, from monoclinic C2/c to P-1 via a triclinic distortion, around 200 K. Vibrational properties are strongly influenced by the THF environment; in particular, there is splitting in spectral features and a shift towards lower energies of the BH4- modes with increase in THF content as a result of changes in the bond lengths, force constants and lowering of the overall symmetry. Interestingly, the orientational mobilities of the BH4- anions obtained from quasielastic neutron scattering (QENS) are not particularly sensitive to THF present in either poor or rich THF adducts and compare well with the mobilities of BH4- anions in unsolvated Mg(BH4)2. Uniaxial 180°jump reorientations of the BH4- anions around a preferred C2 molecular symmetry axis is observed in the whole temperature range. The THF rings are also found to be orientationally mobile in the THF-rich samples, undergoing 180° reorientational jumps around their C2 molecular symmetry axis with jump frequencies about an order of magnitude lower than those for the BH4- anions. In contrast, no dynamical behavior of the THF rings is observed in the case of THF-poor samples. These results are further confirmed by the DFT calculations, showing a decrease in the binding energy with each additional THF ring in the adduct. Based on the combined experimental and computational results, we propose that using fractions of THF to Mg(BH4)2 is beneficial in (i) preventing weakly bound THF from coming free from the magnesium cation and reducing the concentration of an unwanted impurity in the hydrogen and (ii) disrupting the stability of crystalline phase leading to a lower melting point and enhanced kinetics for any potential hydrogen storage applications.