| Complex metal hydrides represent a potential storage material for the hydrogen economy and specifically for automotive on-board regenerative storage. What has emerged from the studies performed to date is that catalysts must often be added in order to aid hydrogen uptake or release. However, although effective catalysts have been identified, the mechanisms by which these additions affect the properties are not well understood. In this work, four individual complex hydride systems, NaAlH4, AlH3, MgH2, and borohydride-based systems have been analyzed on a microscopic level using electron microscopy to determine the spatial distribution and the local chemical environment of the added catalysts.;To date, NaAlH4 has been a prototype material for the development of other complex hydrides but the precise catalytic role of Ti has remained elusive. In this work, electron energy loss spectroscopy has revealed the presence of inactive Al3Ti at the nanoscale, reinforcing the view that catalytically active Ti in this system is a minority species and much of the customarily added ∼ 2 mol % Ti is locked up as inert Al3Ti. In a related system, AlH3, which plays a role in NaAlH4 decomposition as well as being a potential storage material in its own right, recent work had found that reversibility is possible with complexing by a Lewis base, potentially stabilizing the electron-poor AlH3, but definite knowledge of the mechanism of the Ti catalyst in this system is also unclear. In this work, energy dispersive spectroscopy has revealed that no Ti is present in the complexed AlH3-triethyl diamine, indicating that the action of the Ti catalyst in this system is enhanced by scavenging free AlH3.;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. |
