Dispersion and Integration of Dopants by Mechanical Alloying in Complex Hydrogen Storage Materials

Abstract

Prior work had found that MgH2, a thermodynamically hindered system, can be destabilized through the introduction of Si but no microstructural information has been presented. In this work, confirmation of the proposed forward dehydriding mechanism to Mg2Si was obtained in the MgH 2 + Si system using energy dispersive spectroscopy and electron diffraction. In addition, particle sizes were calculated for catalysts added to this system, and electron tomography was applied to determine the three-dimensional catalyst dispersion. In the kinetically hindered Ca(BH4)2 system, initial efforts to synthesize Ca(BH4)2 from CaB 6 in this work were shown to be unsuccessful both structurally and microchemically by using a combination of energy dispersive spectroscopy, electron energy loss spectroscopy, and X-ray diffraction, confirming results obtained via a volumetric Sievert's apparatus that showed no significant reversibility. However, reversibility is achieved if the reaction is started from Ca(BH 4)2. In this work, the degree of mixing of added catalysts to the reversible Ca(BH4)2 system was determined via energy dispersive spectroscopy. In addition, chemical and diffraction analysis of the amorphous intermediate phase indicated no significant segregation occurs on dehydriding. It is concluded from the work reported herein that, in this work that, although ball milling does provide a rapid method for introducing catalysts and for reducing grain size, it can hinder addressing the fundamental question, how the catalyst actually provides catalytic assistance to the material. This work has assisted the search for a viable hydrogen storage material for the automotive industry by addressing gaps in the current understanding regarding kinetic assistance of complex hydrogen storage materials with catalysts on a microstructural level.