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Abstract
Nanostructuring metal hydride has been identified as a potential approach to overcome kinetics and thermodynamic limitations due to the large surface area and high surface energy of nanomaterials. However, in practice the synthesis of such nanosized materials with controlled properties is a real challenge. In particular, the high reactivity of magnesium – a promising material for hydrogen storage - challenges its synthesis at the nanoscale. Hence, this thesis aims to explore different strategies based on wet synthesis methods to synthesize and stabilize magnesium hydride (MgH2) nanoparticles.
Thermal decomposition of organomagnesium is a promising method to obtain magnesium nanoparticles in simple steps and with high yield. Yet, the resulting decomposition products would depend on the precursors, conditions, and medium during decomposition. Di-n-butylmagnesium is the best precursor investigated in the study. The mediums also determined the physical properties of MgH2 from di-n-butylmagnesium; hydrogenolysis in dry solid conditions which led to materials capable of storing 7.1 wt% hydrogen capacity with fast desorption kinetics at 300 °C. Similar kinetics were also observed in the material obtained from hydrogenolysis of di-n-butylmagnesium in cyclohexane but with only 5.5 wt% capacity due to more hydrocarbon residue from solvent. Another promising method is through catalytical hydrogenation of MgAnthracene.3THF complex which could produce MgH2 nanoparticles in high yield and with good economical value.
Despite having kinetic improvements, the thermodynamic limitations still cause a high temperature requirement for hydrogen desorption from these materials. Indeed some theoretical studies showed that significant destabilization can only occur when the nanoparticles sizes are less than 5 nm. The methods were further extended by introducing other compounds such as surfactants and polymers to obtain much smaller sized nanoparticles. Herein, the hybrid magnesium polystyrene nanocomposite was successfully synthesised and proven to give protection against oxidation. However, the size did not become smaller with polystyrene but we found the thermodynamic could be altered by functional groups on the polystyrene. To achieve smaller particle size, polystyrene with different nanostructures such as star, dendrimers, and hyperbranched were synthesised and used as templates for limiting the particle growth.