Investigation On Hydrogen Storage Properties Of LiAlH₄/LiNH₂ Composite

Hydrogen energy would be an ideal form of clean energy because it islightweight, highly abundant and its oxidation product (water) is environmentallybenign. Hydrogen storage technology is the foundation for the widespread applicationof hydrogen energy. Recently, metal complex hydrides like NaAlH₄ and LiAlH₄, andmetal-N-H compounds like LiNH₂/LiH have been studied intensively as new types ofhydrogen storage materials because of their high hydrogen storage capacity. In orderto further improve their hydrogen storage properties, LiAlH₄/LiNH₂ composite wasprepared by mechanically ball-milling, and their hydrogen storage properties werecharacterized by using thermogravimetry (TG), differential scanningcalorimetry(DSC), and X-ray Diffraction(XRD). The results show that more than 2.6wt.% of hydrogen is released for the LiAlH₄/LiNH₂ composite during ball milling;and the total hydrogen desorption capacity reached 7 wt.% after heating to 400℃.Meanwhile, the ball-milling conditions, composite molar ratio, and the catalytic effectof metal Ti, Fe, Zn (or their mixed form) on the thermodynamics and kineticsproperties of the LiAlH₄/LiNH₂ composite are also investigated. In addition, thereaction mechanism of LiAlH₄/LiNH₂ composite was discussed. The following resultshave been obtained:(1) The hydrogen desorption capacity increases as the ball milling timeincreases. During the initial 3 h of the ball milling process the low temperaturechemical reaction was induced by mechanical energy, while during the 3 h-30 h ofthe ball milling process the particles distribution modification and particles sizereducement were mostly proceeded. The thermodynamics and kinetics properties ofthe LiAlH₄/LiNH₂ composite were remarkably affected by the ball milling time. Asthe ball milling time increases, the kinetics properties deteriorate but the peaktemperature values of the DSC curves decrease. In addition, the ball millingatmosphere and ball-stuff ratio also have great effect on the properties of theLiAlH₄/LiNH₂ composite. (2) The addition of transition metal Ti as catalyst could improve the kineticsproperties and increase hydrogen desorption capacity of the LiAlH₄/LiNH₂ composite.Meanwhile, it also lowers the decomposition temperature of LiAlH₄/LiNH₂ compositein low temperature area. The effective catalytic component is TiH_1.971 which is formedin the ball milling progress. Metallic Fe could also enhance the kinetics properties forthe initial reaction stage. Moreover, the catalytic effect of TiF₃ is better than metallicTi, Fe, Zn and the effect is eminent in the low temperature area for the decompositionof LiAlH₄ and in the high temperature area for the reaction of LiNH₂ and LiH.(3) The molar ratio of the LiAlH₄/LiNH₂ composite has great effect on itshydrogen desorption properties and reaction mechanism. The total hydrogendesorption capacity increases with increasing x value of LiAlH₄/LiNH₂ composite.