Structural Characterizations Of Complex Hydrides Upon Cycling

The Mg-based alloys possess good hydrogen storage properties and reversibility. In this work, Mg1.9NiTi0.1and Mg2Ni0.9Cr0.1alloys were synthesized by mechanical alloying and its cyclic hydrogen storage behaviors were investigated. It was found that Ti substituting in Mg2Ni alloy notably improves the absorption/desorption kinetics. After ball milling, the decrease of Mg-Ni atomic interaction lowers the stability of Ti-doped phases and has positive effect on the absorption/desorption behaviors. After20cycles, some amounts of Mg2Ni transform into MgNi2, which result in great decrease in effective hydrogen storage capacity. Mg2Ni0.9Cr0.1alloy shows stable absorption capacity, and its absorption/desorption rates further improve after cycling. The calculated activation energy for dehydrogenation was53kJ/mol after activation, and decreased to36kJ/mol after20cycles. In the ball-milled sample, the strong Cr-Ni bonds weaken the Cr-Mg bonds, thereby destabilizing Cr-doped phases. After20cycles, the stable Ni1-Mg1bonds may be dominant and control the structural stability of Mg2Ni phases.The microstructure and hydrogen storage properties of LiAIH4have attracted attention since it can liberate a pretty large theoretical quantity of7.9wt.%H2below250℃. In the present work, the dehydrogenation kinetics of LiAlH4-doped cerium oxide (CeO2), titanium oxide (TiO2), and zirconium chloride (ZrCl4) were studied. For stage I, from the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is51.5kJ/mol for Ce02-doped LiAIH4, compared to109.5kJ/mol for LiAlH4and57.2kJ/mol for Ti02-doped LiAlH4. For Ce02-doped LiAlH4, CeO2was present as a crystalline phase (CeO2-x) and remains without any reaction below130℃. After heating to180℃, however, a significant weight loss for CeO2were observed by PXD. Moreover, the EXAFS results showed that CeO2-x is formed in an amorphous state. Therefore, the CeO2may serve as a chemical catalyst to favor hydrogen release below130, but also a reactant to react with Li3AlH6or LiAlH4at high temperatures (180℃-220℃).Recently, lithium borohydride (LiBH4) have been proposed as a light weight hydrogen storage material with the high gravimetric (18wt%). The dehydrogenation kinetics of LiBH4incorporated within various carbon nanotubes has been studied. It is demonstrated that the desorption kinetics of LiBH4could be greatly promoted using a simple melt infiltration method and LiBH4confined in modified multi-walled carbon nanotubes (MWCNTs) shows the best desorption kinetics. High energy ball milling leads to a decrease in the average nanotube length and introduces a great amount of local disorder and structural defects in the CNTs, which may provide a considerable kinetic improvement.