Effects of Al-based additives on the hydrogen storage performance of the Mg(NH2)2–2LiH system

Abstract

The Mg(NH₂)₂–2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li₃AlH₆ was introduced to the Mg(NH₂)₂–2LiH system. Experimental results show that a 3.2% mol Li₃AlH₆-modified Mg(NH₂)₂–2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li₃AlH₆-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (E_a) of ca. 96.3 ± 9 kJ mol⁻¹ which was significantly decreased by 31 kJ mol⁻¹ from that of the Li₃AlH₆-free sample. The interaction of Li₃AlH₆ and Mg(NH₂)₂ during ball milling results in the formation of LiAl(NH)₂, LiNH₂ and Mg₃N₂. LiAl(NH)₂ was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.