Hydrogen storage properties of magnesium based nanostructured composite materials

In this work, nanostructured composite materials Mg–Ni, Mg–Ni–La, Mg–Ni–Ce and Mg–LaNi5 have been synthesized using the mechanical alloying process. The new materials produced have been investigated by X-ray diffraction (XRD), transition electron microscope (TEM), scanning electron microscope (SEM) and electron energy dispersion spectrum (EDS) for their phase compositions, crystal structure, grain size, particle morphology and the distribution of catalyst element. Hydrogen storage capacities and the hydriding–dehydriding kinetics of the new materials have been measured at different temperatures using a Sieverts apparatus. The results show that amorphous/nanostructured composite material Mg 50 wt.%–Ni 50 wt.% absorbs 5.9 wt.% within 5 min and desorbs 5.1 wt.% hydrogen within 15 min at 250 °C, respectively. It is confirmed that mechanical alloying that reduces particle size and introduces mechanical stress accelerates the hydrogenation kinetics of the magnesium based materials even at low temperature. But, to release the absorbed hydrogen from the mechanical alloyed materials, a high temperature is still required. It is believed that the dehydriding temperature is largely controlled by the thermodynamic stability of magnesium hydride. It is found that doping Mg–Ni nano/amorphous composite materials with lanthanum, cerium and LaNi5 alloy reduces the hydriding and dehydriding temperatures. The composite material Mg 80 wt.%–LaNi5 20 wt.% absorbs 1.96% hydrogen and releases 1.75 wt.% hydrogen at 25 °C, which suggests the formation of unstable hydrides. Although the stability of MgH2 cannot be easily reduced by ball milling alone, the results suggest the thermodynamic behavior of Mg–Ni nano/amorphous composite materials can be alternated by additives such as La, LaNi5 or other effective elements.