Abstract
Hydrogen stored in metal hydrides improves significantly its volumetric density (from ~ 0.08 kg H2/ m3 at gaseous state up to ~ 150 kg H2/ m3 chemically bonded in metal host). Metal hydrides also surpass by far volumetric densities of hydrogen stored in compressed form (40 kg H2/ m3) or in liquid state (~ 70 kg H2/ m3), as well as their storage safety aspects. Therefore, metal hydrides are potential candidates for emission-free energy storage systems in future applications. Boron-based light-weight metal complex hydrides are of particular interest for mobile applications, owing to their high gravimetric hydrogen storage capacities at high volumetric densities. However, their reaction enthalpies are rather high to be applied in mobile storage systems. In order to reduce their reaction enthalpies by maintaining their high gravimetric hydrogen capacities, an additive metal hydride can be mixed with the respective complex hydride. During the dehydrogenation process of the composite system the additive metal hydride can destabilize the complex hydride by an exothermic reaction, leading to an overall reduction of the reaction enthalpy. This approach is termed as “Reactive Hydride Composites” (RHC). Indeed, the reaction enthalpies of numerous hydride composite systems could be reduced by using this destabilization approach. However, the dehydrogenation/rehydrogenation reaction kinetics of RHCs suffers along their reaction paths by various activation barriers.
In the present work, LiBH4-MgH2 (Li-RHC) and Ca(BH4)2-MgH2 (Ca-RHC) were investigated, in detail, which are the two most promising members of boron-based RHC family with their respective gravimetric capacities of ~ 11 wt. % H2 and ~ 8.4 wt. % H2. By addition of a small amount of NbF5, the dehydrogenation/hydrogenation reaction kinetics of both systems was considerably enhanced.
Using several experimental methods at large scale research facilities (synchrotron radiation facilities and neutron research reactor facilities (in situ SR-PXD, EXAFS, SAXS/ASAXS and SANS/USANS)) and lab scale apparatus (Sievert type apparatus, DSC/DTA, SEM/TEM, MS, and NMR) enabled characterization of crystalline, amorphous and/or nanoscopic structures in both composite systems from Ångstrom region to nanoscopic range and up to macroscopic sizes. The combination, inter-pretation and condensation of the results obtained by such reach methods allowed gaining unique insights in to complex interactions between the additives, their chemical states, size distribution and the hydride matrix and their correlations, in turn, with dehydrogenation/hydrogenation kinetics of the systems.
Additionally, for the first time, the decomposition products of Ca(BH4)2 could be stabilized by destabilizing MgH2, hence using Mg2NiH4 as a destabilization agent for Ca(BH4)2. Furthermore, two new synthesis methods were proposed to produce transition metal boride nanoparticles and complex borohydrides, respectively. Lastly, the RHCs were organized and divided, according to their specific thermodynamical behaviours, in the following three distinct subclasses: “mutually destabilized-RHC” (m-RHC), “single-RHC” (s-RHC), and “additive-RHC (a-RHC).
Prior to this research less was known about the reasons hidden behind the positive effects of transition metal halides on hydrogen release and uptake kinetics of RHCs. The insights gained by this work contributes to a much deeper understanding of structural effects of transition metal halides with respect to kinetic behaviour of composite systems and, thus, it provides a guidance for upcoming investigations in this field or of hydride composite systems, in general. This work also offers new destabilization route to further optimize the thermodynamic properties of RHC systems. In addition, this thesis provides a new pathway to synthesize transition metal boride nanoparticles, which opens possibilities to study their effects on dehydrogenation/hydrogenation kinetics in RHC model systems and their own properties with regard to other application fields such as super conductivity etc. at the molecular level.
