Dadi Þorsteinn Sveinbjörnsson

Ammonia dynamics in magnesium ammine from DFT and neutron scattering

Energy storage in the form of ammonia bound in metal salts, so-called metal ammines, combines high energy density with the possibility of fast and reversible NH3 ab- and desorption kinetics. The mechanisms and processes involved in the NH3 kinetics are investigated by density functional theory (DFT) and quasielastic neutron scattering (QENS). The crystal structures of Mg(NH3)nCl2 with n = 6, 2, 1, which contains up to 9.19 wt % hydrogen and 0.115 kg hydrogen L−1, are first analyzed using an algorithm based on simulated annealing (SA), finding all the experimentally known structures and predicting the C2/m structure for the uncharacterized low temperature phase of Mg(NH3)6Cl2. It is found from DFT that the rotation of ammonia in the hexammine complex (n = 6) requires an activation energy of 0.09 eV in the low temperature phase of Mg(NH3)6Cl2 and 0.002–0.12 eV in the high temperature phases; effectively having free rotors as observed experimentally. The findings are supported by the QENS data, which identify C3 rotations of NH3 in the low temperature phase with an activation energy of 0.09 eV. The calculated diffusion rates were found to be 106–107 Hz at the desorption temperatures for all n = 6, 2, 1 systems. DFT calculations involving bulk diffusion of NH3 correctly reproduces the trends observed in the experimental desorption enthalpies. In particular, for n = 6, 2, 1, there is a good agreement between activation barriers and experimental enthalpies. These results indicate that the desorption of NH3 is likely to be diffusion limited.

Ionic conductivity and the formation of cubic CaH{sub 2} in the LiBH{sub 4}–Ca(BH{sub 4}){sub 2} composite

Abstract LiBH 4 –Ca(BH 4 ) 2 composites were prepared by ball milling. Their crystal structures and phase composition were investigated using synchrotron X-ray diffraction and Rietveld refinement, and their ionic conductivity was measured using impedance spectroscopy. The materials were found to form a physical mixture. The composites were composed of α-Ca(BH 4 ) 2 , γ-Ca(BH 4 ) 2 and orthorhombic LiBH 4 , and the relative phase quantities of the Ca(BH 4 ) 2 polymorphs varied significantly with LiBH 4 content. The formation of small amounts of orthorhombic CaH 2 and cubic CaH 2 in a CaF 2 -like structure was observed upon heat treatment. Concurrent formation of elemental boron may also occur. The ionic conductivity of the composites was measured using impedance spectroscopy, and was found to be lower than that of ball milled LiBH 4 . Electronic band structure calculations indicate that cubic CaH 2 with hydrogen defects is electronically conducting. Its formation along with the possible precipitation of boron therefore has an effect on the measured conductivity of the LiBH 4 –Ca(BH 4 ) 2 composites and may increase the risk of an internal short-circuit in the cells.