Improvement Of Hydrogen Storage Properties And Catalytic Mechanisms In The Li-Mg-N-H System

In Li-Mg-N-H system,2LiNH₂-MgH₂ and LiMgN hydrogen storage materials possess the theoretical hydrogen storage capacities of 5.5 wt%and 8.2 wt%,respectively,which are considered as the most promising hydrogen storage materials.However,their poor thermodynamics,kinetics and cycle stability limit practical application in the field of onboard hydrogen storage.In the 2LiNH₂-MgH₂ hydrogen storage material,the carbon nanotubes,boron hydride and boron hydride/carbon nanotubes composite are used as additives in order to improve the hydrogen storage performance,while their catalytic mechanisms of the material are studied.In the LiMgN hydrogen storage material,the synthesis methods,formation mechanism and its absorption hydrogen reaction mechanism are reavealed.In the present study,it provides a theoretical and experimental guidance for the development of Li-Mg-N-H system hydrogen storage materials with a high capacity,good cycle stability,suitable thermodynamics and kinetics.The 2LiNH₂-MgH₂ and LiMgN hydrogen storage materials were prepared by ball milling.The composition and phase structure of the ball-milled and dehydrogenated samples were analyzed by X-ray diffraction?XRD?and Fourier infrared spectrum?FTIR?.The microstructure of the samples were observed using scanning electron microscopy?SEM?.The hydrogen storage properties of the samples were tested by the pressure-component-temperature?PCT?and thermal analyzer?STA?.The catalytic effects of the carbon materials?carbon nanotubes,carbon black and graphite?,boron hydride?magnesium borohydride,lithium borohydride,sodium borohydride?on the hydrogen storage performance for the 2LiNH₂-MgH₂ are firstly studied.The hydrogen absorption-desorption cycling stablity is significantly improved by the addition of carbon nanotubes,and the capacity retention is increased from the original R5 = 6.3%to R5 = 71.7%.The onset desorption temperature is significantly lowered from 150 ? to?130 ? by boron hydride addition.A considerable improvement of hydrogen storage capacity and kinetics of hydrogen is achiveved.The phases transformation during the hydrogen desorption is also studied.The dehydrogenation reaction process and the catalytic mechanism of the Mg?BH₄?2-added samples are mainly revealed.The effects of boron hydride?Mg?BH₄?2,LiBH₄?/carbon nanotubes on the dynamics,thermodynamics and the cycling stability of 2LiNH₂-MgH₂ hydrogen storage materials are particularly studied.The amount of small particles increases,which is consisted of the nanoscale porous particle,and the degree of agglomeration of particles reduces.The thermodynamics,dynamics,hydrogen storage capacity and cycling stability of the Mg?BH₄?2/CNT-added sample are significantly improved.The Mg?BH₄?2 is converted into in-situ Li4?BH₄??NH₂?3 during ball milling process,which is the intermediate product in the process of hydrogen production,and accelerates the reaction rate and increases the hydrogen desorption capacity.CNTs prevent the sample from agglomeration and provide a channel for the rapid diffusion of hydrogen.Therefore,under the synergistic effect of Mg?BH₄?2 and CNTs,the Mg?BH₄?2/CNT-added sample has the best hydrogen storage performance:the hydrogen storage capacity is up to 4.8 wt%,the capacity retention rate is R5=100%,and the initial hydrogen desorption temperature is kept at a temperature of 147 ?.The prepare methods of LiMgN in different reaction systems are explored,including Li3N/Mg3N2 sintering synthesis,LiNH₂/MgH₂?1:1?and Li3N/MgH₂ ball milling.The synthesis of LiMgN prepared by ball milling of Li3N/MgH₂ is simple,the time is short and the repeatability is good.The content of the main phase of LiMgN is up to 70 wt%.On this basis,the formation mechanism of LiMgN and its reaction mechanism during hydrogen absorption-desorption of Li3N/MgH₂ with the different molar ratios are studied.