Thermodynamic and structural characterization of the Mg-Li-N-H hydrogen storage system

The Mg-Li-N-H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. Some of thermodynamic and structural properties for this system are characterized here. Pressure-composition isotherms are measured and presented in this paper for absorption-desorption at 220, 200 and 180 degrees C. Powder X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis were carried out for samples at various degrees of hydrogenation. These results provide information about the structural changes during absorption/desorption. The mixture of (2LiNH(2) + MgH2) partially converts to (Mg(NH2)(2) + 2LiH) when heated at 220 degrees C and 100 bar of hydrogen without undergoing desorption. Based on two distinct parts which appear in all of the pressure-composition isotherms (180-220 degrees C), two reactions taking place isothermally in hydrogen absorption/desorption are proposed for the material starting with (2LiNH(2) + MgH2) or (Mg(NH2)(2) + 2LiH). These reactions include a single solid-phase reaction, corresponding to the sloping region for hydrogen weight percent (Hwt%) smaller than 1.5%, and a multiple-phase reaction, corresponding to a plateau region for Hwt.% > 1.5 in the isotherms. During hydrogen absorption/desorption, the single-solid-phase reaction corresponds to the forming/consuming of -NH2 which is bonded to Li and the multiple-solid-phase reaction corresponds to forming/consurning Mg(NH2)(2) and LiH. A mechanism for the sorption reactions has been proposed. (c) 2005 Elsevier B.V. All rights reserved.