The effects of ball milling and molar ratio of LiH on the hydrogen storage properties of nanocrystalline lithium amide and lithium hydride (LiNH2 + LiH) system

High-energy ball milling was applied to the mixtures of LiNH2 and LiH having the molar ratio 1:1, 1:1.2 and 1:1.4LiH. During a high-energy ball milling of the 1:1 molar ratio mixture the grain (crystallite) size of LiNH2 and LiH constituent decreases monotonically with increasing milling time while the specific surface area (SSA) of powder increases up to 25 h of milling duration and then decreases after milling for 100 h due to the excessive agglomeration of powder particles into lumps. A single-phase LiNH2 decomposes through melting and the release of ammonia (NH3). A just mixed LiNH2 + LiH mixture still mostly decomposes through the melting of LiNH2 and release of NH3. For the hydrogen to be effectively released from the mixture of (LiNH2 + LiH) a high-energy ball milling is necessary which makes an intimate contact between both constituents. The activation energy for hydrogen desorption from the ball milled mixture of (LiNH2 + LiH) decreases with increasing SSA of powders up to similar to 26m(2)/g and then levels off with further increase of SSA. For the ball milled mixture of LiNH2:LiH the lowest activation energy is observed for the molar ratio of 1:1.2LiH. The hydrolysis/oxidation of the fraction of LiH into LiOH in the mixture makes a fraction of LiH inactive in the intermediate reaction NH3 + LiH -> LiNH2 + H-2 and creates the major obstacle to the hydrogen desorption from the ball milled mixture of LiNH2 + LiH. At the molar ratio 1:1.2 of LiNH2:LiH the mass of the active LiH is the largest one which leads to the largest quantity of desorbed hydrogen (similar to 5 wt.%). The amount of hydrogen desorbed from LiNH2 + LiH slightly decreases with increasing milling time from 5 to 100 h due to the reduction in grain (crystallite) size of LiH which renders it more sensitive to hydrolysis and the formation of LiOH. (C) 2009 Elsevier B.V. All rights reserved.