Investigation on hydrogen storage properties of Mg-based nanostructured composites

Hydrogen storage is one of the most important aspects when hydrogen is considered as a future secondary energy carrier, especially as fuel for vehicles and portable devices. Three concepts for hydrogen storage have been intensively studied, viz. pressurised gaseous hydrogen, liquefied hydrogen or metal hydrides as solid storage media. Metal hydrides provide more safety and a higher volumetric storage density. However, the gravimetric densities of conventionally employed materials (e.g. TiFe, LaNi5) are comparably quite low (1-2 wt.%). MgH2 is a very promising hydride for hydrogen storage due to its high reversible storage capacity of 7.6 wt.%. However, pure Mg reacts with hydrogen only at a temperature higher than 300 °C.

In the frame of this dissertation, possibilities were investigated to make the attractive high storage capacity of Mg useful for practical applications. The work emphasises the decrease of the reaction temperature and the increase of the reaction kinetics. Nano-structured Mg-based materials have been synthesised by ball milling. The influence of two intermetallic compounds, TiFeMn and LaNi5, has been studied by varying their composition. Moreover, additional influence of different catalysts on the MgH2-TiFeMn and MgH2-LaNi5 systems has been investigated. The applied catalysts were CrCl3, Cr2O3, Fe3O4, Ti and V. The focus of investigations was on thermodynamic and kinetics properties via measuring pressureconcentration isotherms (PCIs) and reaction kinetics. To understand the hydrogenation behaviour of the investigated materials, microsturcture and morphology have been examined by physical analysis methods, e.g. scanning electron microscopy (SEM), X-ray diffraction (XRD), laser granulometry (LG), etc. Among the investigated materials, the ball milled MgH2-5 wt.% TiFeMn-5 wt.% V and MgH2-5 wt.% LaNi5-5 wt.% Ti have been found to be the best materials. Long-term cycle stability tests have also been carried out for these two materials (500 cycles).