Lithium-Amide Systems for Hydrogen Storage: cation/anion substitution

L. FERNANDEZ ALBANESI; G. AMICA; N. S. GAMBA; P. ARNEORDO LAROCHETTE; F. C. GENNARI

Praga

Congreso; Hydrogen Days 2018; 2018

Light metal amides and hydrides are promising materials for solid hydrogen storage due to their light weight, favourable thermodynamic stability and hydrogen storage reversibility. It has been demonstrated that 6.5 wt% of hydrogen can be reversibly stored in the Li-N-H system according to this equation: Li2NH + H2 ↔ LiNH2 + LiH BH=-44.5 kJ/molH2However, the temperature required for rehydrogenation is still high for the application of the Li-N-H system as storage material.New ternary metal-N-H systems such as Li-Al-N-H and Li-Mg-N-H were developed tomodify the thermodynamic properties with positive effect on the kinetics. These were obtained adding hydrides and chlorides of Al and Mg by ball milling. The hydrogen storage properties were investigated and compared with the Li-N-H system. The new systems enhance the hydrogenation / re-hydrogenation kinetics, reduce the operation temperature and improve the stability under H2 cycling.In order to understand the role of additives on the Li-N-H system, the samples during milling, after heating under hydrogen pressure, after dehydrogenation, or rehydrogenation, have been systematically investigated by differential scanning calorimetry, hydrogen volumetric measurements, X-ray powder diffraction and FTIR spectroscopy. Synthesis of Mg(NH2)2 was promoted by reactive milling of MgCl2 and LiNH2. After LiH addition, formation of structural phases type- Li4(NH2)3Cl were detected under hydrogen cycling. The formation of structural phases type-Li4(NH2)3Cl, product of the interaction of MgCl2 and LiNH2-LiH, demonstrated the Lithium cation substitution by magnesium ion, and/or NH2 - anion substitution by chlorine. On the other hand, the reaction between AlCl3 and LiNH2-LiH occurs during milling and later dehydrogenation and rehydrogenation. For low quantities of AlCl3 the Al3+ was incorporated into the LiNH2 lattice, whereas for high quantities a new amide-chloride, Li3Alx(NH2)3Cl3x, phase was produced. In these systems, the effect of anion/cation substitution promotes N-H bond destabilization and induces structural defects into LiNH2 lattice improving the lithium cation mobility.