Investigation On Hydrogen Storage Properties Of Li-Al-N-H System

Compared to traditional energy, hydrogen energy is featured by many compelling advantages, such as high energy density, renewability and environment-friendliness. Due to these advantages, hydrogen energy would be an ideal clear energy. Hydrogen storage technology is a crucial issue that should be well coped with before the wide application of hydrogen energy. Recently, due to their extremely high hydrogen storage capacity, metal complex hydrides (LiAlH₄) and metal-N-H compounds (LiNH₂) have been widely studied all round the world.Based on the review of the research and development on the metal complex hydrides/metal-N-H compounds for the hydrogen storage materials, the Li-Al-N-H system hydrogen storage materials, include LiAlH₄/xLiNH₂(x=1, 2) and Li₃AlH₆/xLiNH₂(x=1, 2, 3) system, were selected as the study object of this work. The hydrogen storage properties were studied by means of volumetric measurement and thermogravimetric measurement. The variation of the microstructures of the above mentioned Li-Al-N-H system hydrogen storage materials during hydrogen desorption and absorption processes, were characterized by means of Simultaneous thermogravimetric analysis (TG/DSC), Mass spectrum (MS), X-ray diffraction (XRD), Fourier transform infrared spectrophotometer (FTIR), Solid state nuclear magnetic resonance (NMR). The results showed that after addition of LiNH₂, the decomposition temperature of the system was decreased and thus as-prepared Li-Al-N-H system hydrogen storage materials showed up hydrogen desorption/absorption reversibility to some extent. Moreover, TiC, TiH₂, CeF₃, TiF₃ and Ti₂Ni were added into the composite hydrogen storage materials as catalysts respectively and their catalytic effects on the hydrogen storage properties of the Li-Al-N-H system were also investigated.For LiAlH₄/xLiNH₂(x=1, 2) system, the experimental results showed that the LiAlH₄/LiNH₂ system desorbed 5.97wt.% hydrogen after ball milling for 55h, and the final phase composition of this Li-Al-N-H system after hydrogen desorption were AlN and LiH. For the LiAlH₄/2LiNH₂ system, heating the post-milled sample from room temperature to 400℃at heating rate of 5℃/min induced the liberation of 5.62 wt.% hydrogen and the formation of Li₃AlN₂. The maximum hydrogen desorption capacity reached 7.25wt.%. After doped with TiC, TiH₂, and CeF₃ as the catalysts respectively, the hydrogen storage properties of LiAlH₄/2LiNH₂ system were improved, especially for CeF₃.For LiAlH₆/xLiNH₂(x=1, 2, 3) system, the experimental results indicated that as x increased, and the hydrogen desorption capacity increased. The total hydrogen desporption capacities of Li₃AlH₆/LiNH₂ and Li₃AlH₅/2LiNH₂ reached 5.27wt.% and 6.62wt.%, respectively, the corresponding hydrogen absorption capacities reached 0.86wt.% and 3.57wt.%, respectively. Li₃AlH₆/3LiNH₂ system showed the best hydrogen storage properties. The maximum hydrogen desorption capacity reached 8.15wt.% after heated from room temperature to 500℃at heating rate of 1℃/min and the end solid products were Li₃AlN₂, Li₂NH and LiH. During hydrogen absorption process, Li₃AlN₂ and Li₂NH could be reversibly hydrogenated at 400℃and 110bar hydrogen pressure, and the hydrogen absorption capacity reached 3.57wt.%. Unfortunately, Li₃AlN₂ could not return to Li₃AlH₆ and LiNH₂. The addition of CeF₃, TiF₃ and Ti₂Ni as catalyst could improve the kinetics and hydrogen storage capacity of Li₃AlH₆/xLiNH₂ system among which addition of 2mol% CeF₃ showed the best catalytic effect, and the hydrogen absorption capacity of Li₃AlH₆/3LiNH₂+2mol%CeF₃ system reached 4.75wt.%.