Structural Changes Observed during the Reversible Hydrogenation of Mg(BH4)2 with Ni-Based Additives

The decomposition and rehydrogenation of gamma-Mg(BH4)(2) ball milled together with 2 mol% of Ni-based additives, Ni-nano, NiCl2, NiF2 and Ni3B, has been investigated during one hydrogen desorption-absorption cycle. Under the applied ball milling conditions, no mechanochemical reactions between gamma-Mg(BH4)(2) and Ni-add were observed. Hydrogen desorption carried out at temperatures of 220-264 degrees C resulted for all samples in partial decomposition of Mg(BH4)(2) and formation of amorphous phases, as seen by powder X-ray diffraction (PXD). PXD analysis after rehydrogenation at temperatures of 210-262 degrees C and at pressures between 100 and 155 bar revealed increased fractions of crystalline beta-Mg(BH4)(2) indicating a partial reversibility of the composite powders. The highest amound of [BH4](-) is formed in the composite containing NI3B. Analysis by X-ray absorption spectroscopy performed after ball milling, after desorption, and after absorption shows that the additive remains unaffected, whereas NiCl2 and NiF2 additives react with Mg(BH4)(2) during the hydrogen desorption-absorption and form compounds with a local structure very similar to that of amorphous Ni3B Multinuclear NMR-spectroscopy confirms-the partial-reversibility of the system as well as the formation of during hydrogen absorption. The presence of (n = 10, 12) was also detected by infrared (IR) spectroscopy of the dehydrogenated and rehydrogenated samples. The IR measurements give no clear indication that ions containing B-H-B bridged hydrogen groups were formed during the H-sorption cycle.