Hydrogen Storage Properties And Degradation Mechanisms Of Li-Mg-N-H Combined System

Li-Mg-N-H system is one of the most promising hydrogen storage materials because of its high reversible hydrogen capacity and nice reversibility. Nevertheless, the poor kinetics and the requirement of high operating temperature for dehydrogenation/hydrogenation makes it hard for practical applications. Recently, the effects of MgNH and KNH2on the hydrogen storage behaviors of Mg(NH2)2-2LiH, and the cycling stability of Mg(NH2)2-2LiH was systematically studied, as well as the roles played by the additives. The research achievements are of great significance for further improvement of hydrogen storage properties of the metal-N-H system.The hydrogen storage properties of the Li-Mg-N-H system modified by partial substitution of Mg(NH2)2by MgNH have been systematically studied. Results show that the thermal decomposition behavior of the sample with MgNH was obviously changed. Most of hydrogen can be desorbed at the temperature below180℃, but the overall hydrogen desorption amounts were slightly reduced from5.0to4.2wt%. The heat effects for hydrogen desorption of the0.8Mg(NH2)2-0.4MgNH-2LiH sample were calculated to be40.2and66.1kJ/mol for the first and second steps, respectively. The enthalpy change of the first step dehydrogenation is slightly lower than that of the pristine sample (42.4kJ/mol). Moreover, partial substitution of Mg(NH2)2by MgNH increased hydrogen absorption/desorption plateau pressures and improved the hysteresis. Further structural investigations reveal the different chemical compositions could result in different structures upon hydrogenation/dehydrogenation. For the pristine Mg(NH2)2-2LiH sample, the dehydrogenated sample mainly contains an orthorhombic imide structure, but the dehydrogenated sample of0.8Mg(NH2)2-0.4MgNH-2LiH possesses a cubic imide structure. After re-hydrogenation, the Mg(NH2)2-2LiH sample can go back to the starting chemicals, but for the sample with MgNH, the new ternary imide, Li2Mg2N3H3, can be obviously observed.The effects of KNH2on hydrogen storage property of the Mg(NH2)2-2LiH system were systematically studied. It showed that the presence of KNH2can significantly decreased the operating temperature of hydrogenation for the Mg(NH2)2-2LiH system, improve the kinetics of hydrogen performance, and inhibit the NH3byproducts from releasing in dehydrogenation process. The Mg(NH2)2-2LiH-0.07KNH2showed nice hydrogen storage properties, as its onset and terminal temperature for dehydrogenation decreased to75℃and220℃respectively, and its hydrogen desorption amounts was about5.09wt%. Further experiments on thermodynamics and kinetics of the sample showed that the overall heat effect for dehydrogenation was lowered by15.8%, in comparison with the pristine sample. And its apparent activation energy was69.9and95.0kJ/mol for the first and second-step dehydrogenation, both lower than pristine Mg(NH2)2-2LiH sample(118.3kJ/mol). Based on structural analyses, it was found that a metathesis reaction between KNH2and LiH had took placed to generate KH and LiNH2during ball milling. KH plays the role as catalyst in first-step of dehydrogenation, and it lowers the operating temperature. As the temperature rise, KH react with LiNH2into Li3K(NH2)4, which improves the dehydrogenation performances of the system, and lowers the operating temperature of dehydrogenation further.The effects of KNH2on the cycling stability of Mg(NH2)2-2LiH and the corresponding mechanisms were systematically studied. After30cycles, the hydrogen capacity of the pristine sample decreased from5.41wt%to4.18wt%with an average degradation rate of0.0205wt%per cycle. However, the degradation rate is only0.011wt%per cycle for the Mg(NH2)2-2LiH-0.07KNH2system as its hydrogen capacity decreased from5.08wt%to4.42wt%. The research indicates that the decline in NH3evolution in the heating process caused by the presence of KNH2is one of the important reasons for the improved cycling stability of the Mg(NH2)2-2LiH system.