The effect of Sr(OH)2 on the hydrogen storage properties of the Mg(NH2)2–2LiH system

Abstract

The doping effect of Sr(OH)₂ on the Mg(NH₂)₂–2LiH system is investigated considering different amounts of added Sr(OH)₂ in the range of 0.05 to 0.2 mol. Experimental results show that both the thermodynamic and the kinetic properties of Mg(NH₂)₂–2LiH are influenced by the presence of Sr(OH)₂. The addition of 0.1 mol Sr(OH)₂ leads to a decrease in both the dehydrogenation onset and peak temperatures of ca. 70 and 13 °C, respectively, and an acceleration in the de/re-hydrogenation rates of one time at 150 °C compared to Mg(NH₂)₂–2LiH alone. Based on differential scanning calorimetry (DSC) analysis, the overall reaction enthalpy of the 0.1 Sr(OH)₂-doped sample is calculated to be 44 kJ per mol-H₂ and there are two absorption events occurring in the doped sample instead of one in the pristine sample. For the applied experimental conditions, according to the in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and Fourier Transform Infrared spectroscopy (FT-IR) analysis, the reaction mechanism has been finally defined: Sr(OH)₂, Mg(NH₂)₂ and LiH react with each other to form SrO, MgO and LiNH₂ during ball milling. After heating, SrO interacts with Mg(NH₂)₂ producing MgO and Sr(NH₂)₂. Then Mg(NH₂)₂, LiNH₂ and Sr(NH₂)₂ react with LiH to produce Li₂NH, SrNH, Li₂Mg(NH)₂ and Li₂Mg₂(NH)₃ in traces. After re-hydrogenation, LiSrH₃, LiH and LiNH₂ are formed along with amorphous Mg(NH₂)₂. The reasons for the improved kinetics are: (a) during dehydrogenation, the in situ formation of SrNH appears to increase the interfacial contacts between Mg(NH₂)₂ and LiH and also weakens the N–H bond of Mg(NH₂)₂; (b) during absorption, the formation of LiSrH₃ at around 150 °C could be the key factor for improving the hydrogenation properties.