Hydrogen is an ideal clean carrier for storage and conversion of energy which holds great prospects in order to meet zero-emission mobile application and eliminate dependence on fossil. Magnesium hydride (MgH2) has attracted huge interest for hydrogen storage in the last several decades due to its high hydrogen capacity (7.6 wt%), excellent reversibility, abundance, low cost and non-toxicity. However, its use as a commercial hydrogen storage material is impeded by high dehydrogenation thermal stability (ΔH=75 kJ/mol H2) and sluggish sorption kinetics. This thesis provides the strategy to store hydrogen in a metal hydride form, particularly magnesium hydride, to achieve an efficient and reasonable storage performance in acceptable operating conditions for mobile application. Transition metals of titanium, vanadium, nickel and cobalt were employed in various approaches and the possibility of coupling these catalysts with carbon nanotubes was studied.
Mg (MgH2)-based composites, using carbon nanotubes (CNTs) and pre-synthesized titanium based complex (TCat) as the catalysts, were prepared by high energy ball milling technique. The use of both catalysts demonstrated markedly improved the hydrogen storage performance, e.g. a significant increase of hydrogen release rate and decrease of desorption temperature. The synthesized composites can absorb almost 6 wt% of hydrogen within 3 minutes at 200°C and desorb 6 wt% hydrogen in 10 minutes at 310°C. The influence of CNTs and TCat on desorption temperature was also investigated by using temperature programmed desorption (TPD). The TPD results reveal that the peak desorption temperature and the onset temperature can be lowered by 109°C and 155°C, respectively, compared to the non-catalyzed MgH2.
The synergistic effect of coupling CNTs and vanadium-based complex catalyst in MgH2 was observed when an ultra-fast absorption rate of 6.00 wt% of hydrogen is achieved in 1 minute and 6.00 wt% of hydrogen released in 10 minutes at 200 and 300 °C respectively. In addition, the presence of both reduces the enthalpy and entropy of desorption of about 7 kJ/mol H2 and 11 J/mol H2•K respectively as compared to the commercial MgH2. From the study of the effect of CNTs milling time, it is shown that partially destroyed CNTs (shorter milling time) further enhance the hydrogen sorption performance of the composites.
The potential of MOF and CNTs as catalyst in MgH2 storage system was investigated by the combination of both MIL-47(V) with CNTs exhibited a significantly low temperature reduction of both hydrogen absorption and desorption. The composite exhibits the best hydrogen absorption rate of 4.3 wt% and released 4.0 wt% of hydrogen within 10 minutes at 100 °C and 280 °C respectively. These results confirm that MOFs can be an excellent candidate as a hydrogen storage catalyst in magnesium based nanocomposites.
An investigation on coupling Ni-MOF-74 and CNTs in hydrogen storage of MgH2, reveals a significant temperature reduction towards both hydrogen absorption and desorption that suits for practical application. Particularly, the MgH2+5 wt% (Ni-MOF-74)+5 wt% CNTs composite exhibits the best hydrogen absorption rate of 6.5 wt% within only 30 s and almost reaching 7 wt% after 2 min. Meanwhile, the same composites can reversibly desorbed 6.5 wt% hydrogen within 8 min at 300 °C. Furthermore, the microstructure characterizations of Ni-MOF-74 displays a surface area of 614 m2 g-1 and a uniform microporous size of 0.93 nm increases the Mg(MgH2)/Ni-MOF-74 catalytic interfaces thus enhances hydrogen diffusion. The presence of nickel multi-valence of Ni (2+) from Ni-MOF-74 and Ni (0) from in-situ reduction during ball milling enhanced both absorption and desorption process in which this unique in-situ Ni reduction may promote a smaller size of Ni nanoparticle as well as distribute it fairly well in the Mg(MgH2)/Ni-MOF-74 composite matrix, respectively.
Bi-metal MOF of Ni-Co-MOF-74 was introduced in to the MgH2 system, revealing that pre-reduced Ni-Co-MOF-74 provides better catalytic effect for both hydrogen absorption and desorption. A higher ratio of reduced metal species in M5h-NiCof provided more active sites thus better sorption properties and altered the thermodynamic stability with ΔH and ΔS of 69.68 ± 1.16 kJ/mol H2 and 123.39 ± 2.24 J/mol H2•K respectively. The unique metal coordination in the pre-reduced MOF enhanced the dispersion of the catalyst in the composite thus contributes to the improved sorption properties; 4.7 wt% of hydrogen was released in 30 minutes at 280 °C, while at 200 °C the composite displayed an outstanding hydrogen absorption rate of 6.3 wt% in 2 minutes.