Studies On The Hydrogen Storage Performance And Reaction Mechanism Of Li-Na Mixed Alkali Alanate Composite System

Environmental pollution and energy crisis highlight the importance of such clean and renewable secondary energies as hydrogen.The bottleneck of hydrogen utilization is hydrogen storage.The solid hydrogen storage materials are very promising because of its high energy density,simple operation and high safety.Alkali alanates possess high hydrogen storage capacity.Among them,lithium sodium mixed alanates exhibit excellent hydrogen storage properties.The onset dehydrogenation temperature of LiNa₂AlH₆ is relatively lower?190^oC²10^oC?than many other complex hydrides,and the dehydrogenation capacity is relatively high?7.6wt.%?.Lithium sodium mixed alanate owns many advantages over other alanates,such as perfect reversibility,excellent kinetics,and dehydrogenation without fusion.For the bimetallic aluminum hydride,only a few Li-Na-Al-H compounds such as LiNa₂AlH₆have been obtained before,but the formation range and structure and performance of Li-Na-Al-H system are still unknown.However,there are still some problems in the system.Compared with the targets of practical applications,the system still has a large gap.The reason is that the thermal stability of hydride dehydrogenation system is still high.The process is multi-step dehydrogenation and complex steps lead to some defects in reaction kinetics.In view of the above problems and on the basis of the previous work,this thesis is designed to study lithium sodium mixed alanates from the following aspects:?1?A single phase Li-Na-Al-H compound was prepared by a convenient and simple mechanical alloying method.By adjusting the mole ratio of Li₃AlH₆ over Na₃AlH₆,the formation range of single phase Li_xNa_3-xAlH₆ can be determined as Li x=0.9¹.3..When the lithium amount is x=1.3,the initial dehydrogenation temperature is the lowest?423K?,the hydrogen storage capacity is the highest?3.45wt.%?,and the dehydrogenation reaction enthalpy change is the lowest(49.7 KJ mol H₂^-1).?2?Introducing Ti₃C₂ into LiNa₂AlH₆ would result in the initial dehydrogenation temperature decreasing with the increase of doping amount.When the dopnant amount attains5wt.%,the onset dehydrogenation temperature would be reduced to the lowest 385K,which is68K lower than the onset temperature of the pristine LiNa₂AlH₆.The reaction enthalpy of dehydrogenation of LiNa₂AlH₆+5wt.%Ti₃C₂ is obviously reduced to 60.96 KJ mol H₂^-1,which is 7.16 KJ mol H₂^-11 lower than that of pure LiNa₂AlH₆.XPS measurments inform that the Ti elements changed from Ti-C and Ti²⁺to Ti⁰ and Ti³⁺,went through Ti⁰ to Ti³⁺,and finally reached Ti³⁺to Ti²⁺through the whole process of ball milling,dehydrogenation and hydrogen absorption,respectively.Because of the reaction between two phases,the thermodynamic performance of the sample was effectively improved.?3?On the basis of the optimized Li_1.3Na_1.7AlH₆,the titanium dopant would be introduced as well.First,the Ti₃C₂ and Li_1.3Na_1.7AlH₆ are combined with different mass ratios.When the amount of doping is 5wt.%,the onset dehydrogenation temperature of the sample is 388K,which is 35K lower than the onset dehydrogenation temperature of pristine Li_1.3Na_1.7AlH₆.At the same time,Ti₃C₂ can effectively reduce dehydrogenation enthalpy to 46.94 KJ mol H₂^-1.According to the XPS results,it can be found that Ti₃C₂ and Li_1.3Na_1.7AlH₆ have been reacted during the ball milling process,converting Ti-C and Ti²⁺into Ti³⁺and Ti⁰,respectively.After dehydrogenation,partial Ti⁰ changes to Ti³⁺,this destabilizes the thermodynamics of the sample.