| The Mg?NH2?2-2LiH system was regarded as one of the promising hydrogen storage materials due to its high reversible hydrogen capacity and favorable thermodynamics.However,a large kinetic barrier?Ea:?102-120 kJ mol-1?results in high operating temperatures for hydrogen storage in this reaction.Considerable efforts have been devoted to lowering the operating temperature and enhancing the dehydrogenation kinetics of Mg?NH2?2-2LiH system.In this work,we systematically investigate the effects of the addition of alkli metal hydrides on hydrogen storage properties of Mg?NH2?2-2LiH complex system.Moreover,the modification mechanisms of KH,RbH and CsH were also studied and discussed.CsH was successfully synthesized by ball milling Cs under a hydrogen atmosphere of 50 bar at 50 ? for 12 h in our own laboratory,and its effects on Mg?NH2?2-2LiH system were investigated and discussed.The results show that adding small amount of CsH reduced significantly the operating temperature of the Mg?NH2?2-2LiH system and enhanced the hydrogen desorption and absorption kinetics.The Mg?NH2?2-2LiH-0.08CsH composite exhibited optimal hydrogen storage properties because it reversibly stores approximately 4.62 wt%hydrogen with a dehydrogenation onset temperature of 70 ?.The fully dehydrogenated CsH-containing sample began to absorb hydrogen at 55 ? and took up approximately 4.58 wt%hydrogen at 130 ?.Moreover,the fully dehydrogenated CsH-containing sample took up 4 wt%hydrogen at 120 ? and 100 bar of H2 within 20 min.Detailed mechanistic studies revealed that during the initial ball milling process,CsH gradually reacted with Mg?NH2?2 to afford CsNH2,and CsNH2 acted as a catalyst to reduce the activation energy barrier of the first dehydrogenation step.As the operating temperature increased,CsNH2 as a reactant participated in the second dehydrogenation step to decrease the desorption enthalpy change.This reasonably explains the significantly improved hydrogen storage properties of the CsH-containing Mg?NH2?2-2LiH system.Based on the above study,CsH was partially replaced with KH to further improve the overall hydrogen storage properties of the Mg?NH2?2-2LiH-0.08CsH system.It was found that the available hydrogen storage capacity and the dehydrogenation kinetics were effectively improved with the presence of KH,whereas the hydrogenation kinetics was slightly decreased.In particular,the Mg?NH2?2-2LiH-0.04CsH-0.04KH sample exhibited the optimal hydrogen storage performance;it could reversibly store 4.9 wt%H2 when dehydrogenation operates at 130 ? and hydrogenation at 120 ?.Further thermodynamic and kinetic analyses revealed that the co-addition of CsH and KH lowered the reaction enthalpy changes and the values of Ea together,which is responsible for the improved overall hydrogens storage properties.More importantly,the Mg?NH2?2-2LiH-0.04CsH-0.04KH sample also has a good cycling stability,as more than 4.2 wt%of hydrogen was remained after 30 cycles.Moreover,the roles played by KH,RbH and CsH on imporved hydrogen storage properties of the Mg?NH2?2-2LiH system were further analyzed and disucced.After adding 0.08 mol either KH,RbH or CsH,the onset temperature for hydrogen desorption was lowered to 75 ?,and the dehydrogenation terminated at 215 ?.The dehydrogenation capacities of the 0.08 mol CsH-,RbH-and KH-added samples were determined to be 4.6,4.8 and 5.15 wt%,respectively,because of the different molecular weight of Cs,Rb and K.Interestingly,the RbH-added sample exhibited the superior hydrogenation kinetics and the cycling stability to the KH-or CsH-added samples,as the dehydrogenated Mg?NH2?2-2LiH-0.08RbH sample absorbed approximately 90%of hydrogen within 10 min at 120 ? with 97%of capacity retention after 10 cycles.Furtehr Kissinger and DSC analyses indicated that the addition of KH reduced the activation energy barrier of the first dehydrogenation step,and the addition of RbH and CsH lowered the desorption enthalpy change of the second dehydrogenation step.As a result,it is believed that CsH-,RbH-and KH play slightly different roles in imporving hydrogens storage behivours of the Li-Mg-N-H system. |
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