An Investigation On Hydrogen Storage Properties Of Mechanically Ball-milled Li-N-H System Hydrogen Storage Materials

This study focuses on the synthesis of LiNH₂ and the mixture of LiNH₂ with LiH based on the review of researches and development of the hydrogen storage materials. XRD, TG-DSC, TG-DSC-MS&SEM analysis, the P-C-T test method and desorption/absorption circulations under constant temperature are used to study the relationship among materials molar ratio, parameters of ball milling, temperature and desorption properties systematically.With XRD, Fullprof software and TG-DSC, the LiNH₂ in synthesized LiNH₂ was examined as 99.64wt%, which indicates that it is instability and decomposed to ammonia and Lithium amide while heating.The synthsiezed LiNH₂ was mixed with LiH in molar ratio 1:1 through ball milling. It is believed that the reaction between LiNH₂ and LiH in modified (LiNH₂ + LiH) and desorption begins at 230℃ and reaches the highest reaction rate at 230～340℃. The dehydriding temperature of cycled (LiNH₂+LiH) begins at 320℃ and reaches the highest reaction rate at 340～440℃. LiNH₂ decomposed and emitted out NH₃, and then NH₃ was captured by LiH to emit H₂ in the course of cycled (LiNH₂+LiH) dehydriding. The P-C-T curves show that both the hydrogen balance pressure and dehydriding content increase with the increase of temperature. The mixture has better cycle stability under lower temperature and worse cycle stability under higher temperature. Uneven diffusion of LiNH₂ with LiH, LiNH₂ decomposing ammonia and Li₂NH rehydride insufficiently during the dehydriding lead to the hydrogen content degradation. The XRD results of dehydrided products showed that the Li₂NH increased in the dehydrided remains when the temperature increased. That is to say that the modified (LiNH₂+LiH) dehydride keeps well in high temperature.The TG-DSC-MS results of LiNH₂ show that the LiNH₂ mainly decomposed to NH₃ in 80℃～450℃ during heating, and only minor H₂ and N₂ emmitted out simultaneously too.The Li₃N turned into LiNH₂+2LiH after it hydrided. The hydrided product dehydrided from 200℃, and reached the highest reaction rate near 275℃ and 360℃. Ranged from 200℃ to 320℃, the LiNH₂ reacted with LiH directly and emitted out H₂. However, in 320℃～450℃, LiNH₂ decomposed and emitted out NH₃, and NH₃ was then captured by LiH to emit H₂ in the course of dehydriding. The hydrided product of Li₃N emmited out H₂, minor NH₃ and N₂ simultaneously in the course of desorption. The TG and MS results suggested that Li-N-H emmited out ammonia during dehydriding is a universal problem.In the same molar ratio of LiNH₂ to LiH, the size of the sample decreased when the ball milling time increased. The temperature of the highest desorption rate was lower, the desorption content increased and the ammonia decreased as well. When the mixture of LiNH₂ and LiH was in different molar ratio ball milling in the same time, ammnia decreased with the increase of LiH percentage to LiNH₂. The result validated the second desorption mechanism, that is, LiNH₂ decomposed and emitted out NH₃, and then NH₃ was captured by LiH to emit H₂ in the course of desorption.