Toyoto Sato

Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

A series of hydrogen rich Mg6–7TMH14–16 (TM = Ti, Zr, Hf, V, Nb and Ta) hydrides have been synthesized at 600 °C in a high pressure anvil cell above 4 GPa. All have structures based on a fluorite type metal atom subcell lattice with a ≈ 4.8 A. The TM atom arrangements are, however, more ordered and can best be described by a superstructure where the 4.8 A FCC unit cell axis is doubled. The full metal atom structure corresponds to the Ca7Ge type structure. This superstructure was also observed from electron diffraction patterns. The hydrogen atoms were found from powder X-ray diffraction using synchrotron radiation to be located in the two possible tetrahedral sites. One coordinates three Mg atoms and one TM atom and another coordinates four Mg atoms. These types of new hydrogen rich hydrides based on immiscible metals were initially considered as metastable but have been observed to be reversible if not fully dehydrogenated. In this work, DFT calculations suggest a mechanism whereby this can be explained: with H more strongly bonded to the TM, it is in principle possible to stepwise dehydrogenate the hydride. The remaining hydrogen in the tetrahedral site coordinating the TM would then act to prevent the metals from separating, thus making the system partially reversible.

Thermal decomposition of ammonia borane at high pressures

The effects of high pressure (up to 9 GPa) on the thermal decomposition of ammonia borane, BH3NH3, were studied in situ by Raman spectroscopy in a diamond anvil cell. In contrast with the three-step decomposition at ambient pressure, thermolysis under pressure releases almost the entire hydrogen content of the molecule in two distinct steps. The residual of the first decomposition is polymeric aminoborane, (BH2NH2)x, which is also observed at ambient pressure. The residual after the second decomposition is unique to high pressure. Presumably it corresponds to a precursor to hexagonal BN where macromolecular fragments of planar hexagon layers formed by B and N atoms are terminated by H atoms. Increasing pressure increases the temperature of both decomposition steps. Due to the increased first decomposition temperature it becomes possible to observe a new high pressure, high temperature phase of BH3NH3 which may represent melting.

Attempts to improve Mg2Ni hydrogen storage by aluminium addition

Abstract In an attempt to destabilize Mg 2 NiH 4 , the starting alloy, Mg 2 Ni, was doped with aluminium by ball milling before hydrogenation. The structure of Mg 2 NiH 4 is based on an electron-rich d 10 [NiH 4 ]-complex stabilized by a Mg ion framework and from theoretical calculations it had been suggested that a further destabilization of the hydride could be obtained by substituting some Mg for Al. After hydrogenation, the structure, color and phase transition temperature of Mg 2 NiH 4 was found to be affected by Al addition. The conventional monoclinic low-temperature (LT) structure of Mg 2 NiH 4 was not observed by X-ray diffraction for Mg 2− x Al x NiH 4 , which instead could be indexed with a cubic unit cell, a =6.519 A, that is very close to the dynamically disordered, cubic, high-temperature (HT) phase of Mg 2 NiH 4 ( a =6.490 A). In contrast, the Al-doped Mg 2 NiH 4 is stable at room temperature. Furthermore, the typical orange color of Mg 2 NiH 4 was not observed and differential scanning calorimetric measurements (DSC) of the Al-containing hydride showed a 10xa0°C decrease in phase transition temperature. This is interpreted as a transition from a statically disordered pseudo -HT structure to the dynamically disordered HT phase. However, the monoclinic structure of the LT phase of Mg 2 NiH 4 could again be observed at room temperature after 10 hydriding/dehydriding cycles, indicating that the Al-doping of Mg 2 NiH 4 is counteracted by cycling and only has a temporary influence on the properties.