Preparation And Hydrogen Storage Properties Of An Li-N-H System

Metal-N-H-based system has been considered the most promising hydrogen storage materials, due to its high storage capacity, good reversibility and mild conditions. In which the Li-N-H system is the research hot. But the hydrogen absorption temperature and kinetic performance of the system is still to be further improved, especially the dehydrogenation kinetic performance of the system is still rarely reported. Therefore, an LiNH2-Li3N system has been synthesized by a ball-milling process in present work, and its dehydrogenation properties, the cause of the deteriorated dehydrogenation performances and reaction mechanism were investigated by means of XRD, SEM, TPD, BET, FTIR, hydrogen storage performance test and in situ gas chromatograph. In addition, the system was modification by adding triphenyl phosphite.The Li3N-LiNH2 sample was prepared by high energy ball milling and the dehydrogenation performance of it was investigated. TPD results show that the sample start to release hydrogen at 175℃, the desorption rate reach the maximum value at 280℃and end at 400℃. The sample desorbes 5.6 wt.%hydrogen when temperature programmed to 280℃, the amount of hydrogen is decreased sharply to 4.0wt.%was after two dehydrogenation cycles. In situ gas chromatograph (GC) results showed that no NH3 could be observed during the dehydrogenation process. PCT kinetic tests show that:the sample reaches highest dehydriding rate of 0.096 wt.%/min after 50 min, the amount of dehydrogenation is reached 4.8 wt.%within 100min in the first dehydrogenation cycles, but that is only 1.9 wt.%within 100min and the highest dehydriding rate is less than 0.03 wt.%/min after two dehydrogenation cycles at 280℃.Combining XRD, FTIR, in situ GC, SEM and BET analyses, we consider that the grain growth and sintering badly should be responsible for the deteriorated dehydrogenation performances of the Li3N-LiNH2 system. To improve the dehydrogenation cycling performance, the Li3N-LiNH2 system was modification by the addition of triphenyl phosphite. XRD and SEM results showed that:the triphenyl phosphite play the role of lubricant and dispersing agent in the milling process, which improve the effect of ball milling, effectively prevent the sample sintering and grain growth, In addition, the triphenyl phosphite also play stabilizer and protective agent role in the process of hydrogen absorption and desorption cycles, which reduce the desorption temperature, significantly improve the desorption kinetics performance and cyclic stability. The Li3N-LiNH2+20 wt.%TPP sample start to release hydrogen at 130℃, the desorption rate reache the maximum value at 230℃and end at 300℃. PCT kinetic tests show that:the amount of dehydrogenation is reached 4.6 wt.% within 100min in the first dehydrogenation cycle, that is still 4.0 wt.%and kinetic performances almost no decline after twenty dehydrogenation cycles at 230℃.