HYDROGEN STORAGE PROPERTIES OF MAGNESIUM BASED NANOSTRUCTURED/AMORPHOUS COMPOSITE MATERIALS

摘要

In this work, nanostructured composite materials Mg-Ni, Mg-Ni-La, Mg-Ni-Ce and Mg-LaNi_5 have been synthesized using the mechanical alloying process. The new materials produced have been investigated by X-ray diffraction (XRD), TEM, SEM and EDS for their phase compositions, crystal structure, grain size, particle morphology and the distribution of the catalyst elements. 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 amorphousanostructured composite material Mg50%-Ni50% absorbs 5.89wt% within five minutes and desorbs 4.46% hydrogen within 50 minutes at 250℃ respectively. Adding 5% La into Mg-Ni composite materials reduces the starting temperature of hydrogen absorption and desorption from 200℃ to 25℃ which suggests the formation of unstable hydrides. The composite material Mg80%-LaNi_5 20% absorbs 1.96% hydrogen and releases 1.75 wt% hydrogen at 25℃. It is observed that mechanical alloying accelerates the hydrogenation kinetics of the magnesium based materials at low temperature, but a high temperature must be provided to release the absorbed hydrogen from the hydrided magnesium based materials. It is believed the dehydriding temperature is largely controlled by the thermodynamic configuration of magnesium hydride. Doping Mg-Ni nano/amorphous composite materials with lanthanum reduces the hydriding and dehydriding temperature. Although the stability of MgH_2 can not be easily reduced by ball milling alone, the results suggest the thermodynamic properties of Mg-Ni nano/amorphous composite materials can be alternated by additives such as La or other effective elements. Further investigation toward understanding the mechanism of additives will be rewarded.