Effect Of Alkali Metal Compounds On The Dynamical Properties Of Li-N-H System Hydrogen Storage Materials

As the increasing demand for energy, looking for the new-style, green, environmentally friendly, renewable energy to replace the disposable fossil fuel is imminent. As one of the ideal future energy carrier, hydrogen is a clean form of energy, the most abundant element in the universe and richest in energy per unit mass. In hydrogen development process, the most critical point is to deal with the problem of hydrogen storage. In various types of hydrogen storage materials, Metal-N-H hydrogen storage system have been attracted widespread attention since their large hydrogen storage capacity, low operating temperature, and excellent kinetic properties.Metal-N-H hydrogen storage systems have been investigated worldwide since it was firrst reported by Chen et al. in 2002, who indicates that Li3N reversibly stores over 10 wt% hydrogen in the two consecutive reactions (reaction 1).Later, the lithium amide (LiNH2)-lithium hydride (LiH) system was also proposed as a sound solid-state storage system, as it can more easily absorb/desorb 6.5 wt% of hydrogen (reaction 2).Recently, it was demonstrated that potassium compounds, including potassium hydride, potassium amide, potassium hydroxide and potassium halides, possess superior catalytic effects on the improvement of the hydrogenation/dehydrogenation kinetics of the Li-Mg-N-H system. In this study, we have investigated and discussed the hydrogen storage properties of alkali metal hydroxide, alkali metal hydride and alkali metal amide doped LiNH2-LiH system, and further discussed the reaction mechanism.1. Improved dehydrogenation properties of the LiNH2-LiH system by doping with alkali metal hydroxideThe hydrogen desorption properties of the LiNH2-LiH system were investigated and discussed by doping with alkali metal hydroxide (LiOH, NaOH, and KOH). It was determined that the three types of hydroxides are effective for enhancing the hydrogen desorption properties of the LiNH2-LiH system, among which, KOH shows the best effect. In comparison with the broad-shaped hydrogen desorption curve of the LiNH2-LiH system without an additive, the hydrogen desorption curve of the LiNH2-LiH-0.05 KOH composite becomes narrow. By doping with 5 mol% KOH, the dehydrogenation onset temperature of the LiNH2-LiH composite is decreased by about 36℃, and the dehydrogenation peak temperature is lowered by about 42℃. Detailed structural investigations reveal that during ball milling, the doped alkali metal hydroxide can react with LiH to convert to alkali metal hydride, which is responsible for the improvement in the hydrogen desorption properties of the LiNH2-LiH system.2. Improved dehydrogenation properties of the LiNH2-LiH system by doping with alkali metal hydrideThe hydrogen desorption properties of LiNH2-LiH system with alkali metal hydroxide (LiH, NaH, and KH) were investigated and discussed. It is found that the three kinds of hydrides are effective to enhance the hydrogen desorption property of LiNH2-LiH system, among which, KH shows the best effect. In comparison with the broad shaped hydrogen desorption curve of the LiNH2-LiH composite without additive, the hydrogen desorption curve of the LiNH2-LiH-0.05 KH composite becomes sharp. The onset temperature of the LiNH2-LiH-0.05 KH composite is decreased by about 20℃, and the peak temperature is lowered by about 30 ℃. The reason for the improvement is that KH with superior reactivity with NH3 plays the role of catalyst to immediately release hydrogen. In the following cycling properties test, compare with the no additive LiNH2-LiH sample, the LiNH2-LiH-0.05KH sample exhibited ideal cycling properties.3. Improved dehydrogenation properties of the LiNH2-LiH system by doping with alkali metal amideThis part we discussed the dehydrogenation properties of the LiNH2-LiH system added three kinds of alkali metal amides (LiNH2, NaNH2 and KNH2). The consequence shows that only KNH2 is effective to enhance the hydrogen desorption property of LiNH2-LiH system among the three kinds of alkali metal amides. In comparison with the broad shaped hydrogen desorption curve of the LiNH2-LiH composite without additive, the hydrogen desorption curve of the LiNH2-LiH-0.05 KNH2 composite becomes sharp. By doping with 5 mol% KNH2, the dehydrogenation onset temperature of the LiNH2-LiH composite is decreased by about 40℃, and the dehydrogenation peak temperature is lowered by about 34℃. Detailed structural investigations reveal that during ball milling, the doped alkali metal amide can react with LiH to convert to alkali metal hydride, which is responsible for the improvement in the hydrogen desorption properties of the LiNH2-LiH system. Finally, in the cycling properties test, compared with the sample without addition,5 mol% KNH2 added LiNH2-LiH sample exhibited ideal cycle performance.