The effects of ball milling and molar ratio of LiH on the hydrogen storage properties of nanocrystalline lithium amide and lithium hydride (LiNH_2 + LiH) system

摘要

High-energy ball milling was applied to the mixtures of LiNH_2 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 LiNH_2 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 100h due to the excessive agglomeration of powder particles into lumps. A single-phase LiNH_2 decomposes through melting and the release of ammonia (NH_3). A just mixed LiNH_2 + LiH mixture still mostly decomposes through the melting of LiNH_2 and release of NH_3. For the hydrogen to be effectively released from the mixture of (LiNH_2 + 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 (LiNH_2+LiH) decreases with increasing SSA of powders up to ~26 m~2/g and then levels off with further increase of SSA. For the ball milled mixture of LiNH_2 :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 NH_3 + LiH -> LiNH_2+H_2 and creates the major obstacle to the hydrogen desorption from the ball milled mixture of LiNH_2 + LiH. At the molar ratio 1:1.2 of LiNH_2 :LiH the mass of the active LiH is the largest one which leads to the largest quantity of desorbed hydrogen (~5 wt.%). The amount of hydrogen desorbed from LiNH_2+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.