New insights into the mechanism of activation and hydrogen absorption of (2LiNH2–MgH2)

2LiNH2–MgH2 is considered an attractive material for reversible hydrogen storage. In an attempt to improve the hydrogen storage characteristics of the 2LiNH2–MgH2 system, the activation mechanism of the material, as well as the improvement in hydrogen absorption rates through doping were explored. Differential Scanning Calorimetry (DSC) investigations reveal that the initial and irreversible conversion process 2LiNH2+MgH2⇒2LiH+Mg(NH2)2 is exothermic, indicating it is energetically favorable for this initial conversion of the starting material. The exothermicity of this first step explains why the original starting material (2LiNH2 + MgH2) is never regenerated during re-hydrogenation of the desorbed product. Adding catalytic amounts (<4 mol %) of potassium hydride (KH) significantly increases the hydrogen absorption rate of the desorbed material, and has a less dramatic effect on the kinetics of hydrogen desorption. Pressure-Composition-Temperature (PCT) studies for the KH-catalyzed material indicate a substantial hydrogen equilibrium pressure of 20 atm at 180 °C. The fast absorption rate obtained via using KH catalysis allows a more accurate equilibrium measurement. The changes of enthalpy and entropy for the conversion of catalyzed (2LiH + Mg(NH2)2) to Li2Mg(NH)2 with hydrogen release were determined from the van’t Hoff plot. These values for the enthalpy and entropy of hydrogen desorption are ∆H = 40 kJ/mol H2, and ∆S = 99 J/K-mole H2, respectively. This ∆H value is similar to that reported previously by Wang et al. for the K-catalyzed material and previous measurements for the un-catalyzed material. The similarity of the ∆H values for both K-doped and un-doped material confirms KH is acting catalytically and not thermodynamically.

► The conversion of 2LiNH2+MgH2⇒2LiH+Mg(NH2)2is exothermic, confirmed by DSC. ► H2 absorption rate by Li2Mg(NH)2 increases significantly by 4 mol% KH-addition. ► Pressure-Composition isotherms for Li–Mg–N–H at 220 and 180  °C are reported. ► 2H2 + Li2Mg(NH)2 sorption enthalpy of 42 kJ/mol and entropy of 99 J/K-mol are reported.