Formation and Equilibrium of Ammonia in the Mg(NH2)2−2LiH Hydrogen Storage System

Abstract

The equilibrium concentration of NH3 in the volatile products of the reaction between Mg(NH2)2 and 2LiH and the decomposition of Mg(NH2)2 and LiNH2, respectively, depends strongly on the reaction temperatures. The enthalpy changes of NH3 generation in the decomposition of LiNH2 and Mg(NH2)2 are deduced from the slopes of van't Hoff plots to be ca. 43.4 and 40.8 kJ/mol, respectively. When 600 psi hydrogen pressure was applied, the corresponding enthalpy changes increased to ca. 54.2 and 50.5 kJ/mol, respectively. For the Mg(NH2)2−2LiH system, the enthalpy change of NH3 generation in hydrogen desorption was calculated to be ca. 80.3 kJ/mol, which is substantially higher than those of the decomposition of Mg(NH2)2 and LiNH2. When high H2 pressure (higher than equilibrium pressure) was applied, the corresponding enthalpy change decreased to ca. 51.3 kJ/mol, which is close to that of the decomposition of Mg(NH2)2 under high H2 pressure.