Synthesis of Mg (NH2)2 and Hydrogen Storage Properties of Mg (NH2)2–LiH System

Abstract:

Mg(NH2)2 was synthesized first by high energy milling MgH2 powder in a 99.995% NH3 atmosphere and then via crystallization heat treating at 300 ℃, and the hydrogen storage properties of Mg(NH2)2+2.2LiH (molar ratio) were investigated in the temperature range of 150~240 ℃. It is found that mechanical milling of Mg(NH2)2 and LiH with molar ratio 1:2.2 followed by heat treatment under static hydrogen pressure and dehydrogenating at 208.5 ℃ yields the desired reversible hydrogen storage phase:Li2Mg(NH)2. Desorption kinetics reveal that the reversible hydrogen capacity for the system can reach 4.5wt% in the temperature range of 200~240 ℃.The system starts to dehydrogenate at 150 ℃ and the Arrhenius activation energy Ea of desorption reaction can be determined to be 51.7 kJ/mol H2 based on the data of kinetics. Additionally, the dehydrogenating reaction enthalpy (△H) and entropy (△S) are calculated to be 42.8 kJ/mol H2 and 149.2 J·K-1/mol H2, respectively, by P-C-T measurements.