Direct synthesis and hydrogen storage behaviors of nanocrystalline Na₂LiAlH₆

Nanocrystalline Na₂LiAlH₆ was directly synthesized by mechanical milling 2NaH/LiH/Al mixture with TiF₃ catalyst under hydrogen pressure of 3.0 MPa. The synthesized Na₂LiAlH₆ exhibits a dehydriding capacity of 3.09 wt% in the first cycle, which is higher than that of Na₃AlH₆. Because of the complexity of mass transfer, the rehydrogenation process of the dehydrided Na₂LiAlH₆ is more intricate than that of the dehydrided sodium alanate, causing the formation of Na₃AlH₆ and the reduction of rehydriding capacity in the following cycles. As temperature increases from 70 to 120 °C, hydrogen absorption kinetics is extremely enhanced. The dehydrided material can reabsorb 80% of the reversible hydrogen capacity within 5 min when the temperature is above 100 °C with an initial hydrogen pressure of 4 MPa. The scanning electron microscopy and energy dispersive X-ray spectroscopy show that the as-synthesized Na₂LiAlH₆ along with the catalyst form a much homogeneous composite with a spherical particle size of 200 nm–2 μm.