| Hydrogen energy is an important secondary energy,while the hydrogen storage is crucial to the utilization of hydrogen energy.Solid-state hydrogen storage materials have been intensively studied during last decades because of their advantages of safety,high hydrogen storage density and easy operations,etc.Among them,the Mg-based hydrogen storage materials have the advantages of low cost,abundant resources,high capacity of hydrogen storage(MgH2 owns a mass hydrogen storage density up to 7.6wt.%).Meanwhile,some light metal complex hydrides also possess high gravimetric and volumetric density(LiAlH4 occupies a mass hydrogen storage density up to 10.5wt.%),and becomes potentially applicable hydrogen storage material.However,MgH2 and LiAIH4 still meet poor kinetics of dehydrogenation or rehydrogenation,poor reversibility and some other issues,which greatly hampers its application.In recent years,researchers all ove the world have devoted many works on the kinetics and thermodynamics modifications of MgH,and LiAIH4.Although significant progress has been made,still many shortcomings are unsolved,for example:initial dehydrogenation temperature,but the working temperature of on-board hydrogen storage needs 80?.It's difficult to meet the actual needs for the hydrogen release rate due to the poor dehydrogenation kinetics.The overhigh stability of thermodynamically.Poor reversibility means harsh conditions are required when hydrogen is being absorbed,and the reversible hydrogen absorption capacity is only a few.Targeting to these problems,we implemented optimization works and mechanism studies on the MgH,-LiAlH4 in following aspects:(1)MgH2 and LiAIH4 were combined in a certain molar ratio by ball milling,and the catalyst TiH2 was added in the composite system.TiH2 was not involved in the reactions whether in de/re-sorption,but served as catalyst to improve the kinetics of 4MgH2-LiAlH4,the apparent activation energy of the decomposition of LiAIH4 was reduced from 111.4kJ/mol to 88.1kJ/mol.Similarly,the activation energy for rchydrogenation was also reduced by 49.9kJ/mol due to the addition of TiH2.(2)Based on the analyses on 4MgH2-LiAlH4-TiH2 composite system,we found that,in addition that the grain refinement and doping catalytic could modificate the hydrogen storage properties,the in-situ produced A1 and LiH can react with MgH2,destabilize the system,and improve the hydrogen storag properties.Therefore,the auther intended to study the effects separately caused by Al and LiH on the dehydrogenation of MgH2.(3)Based on the 12Al-17MgH2 system,we doped the catalyst TiH2 and achieved 12Al-17MgH2-TiH2-The research found that TIH2 catalyzes the reaction between Mgl-2 with Al of the reactive system during the dehydrogenation below 280?,which is the major reason for the enhancment of dehydrogenation kinetics.When the temperature is higher than 280?,TiH2 can directly participate in the dehydrogenation process,which is the crucial reason for the destabilization of the system.The kinetics of dehydrogenation of 12Al-17MgH2 was improved due to the additive TiH2,the apparent activation energy was decreased from 168.11kJ mol-1(12Al-17MgH2)to 107.42kJ mol-1(12Al-17MgH2-TiH2).Similarly,the additive TiH2 reduced the dehydrogenation enthalpy from 81.9kJ mol-1 H2(12Al-17MgH2)to 71.7kJ mol-1 H2(12Al-17MgH2-TiH2).(4)Based on the 3LiH-7MgH2 system,we doped the catalyst TiH2 and achieved 3LiH-7MgH2-TiH2.It was found that TiH2 was not involved in de/re-sorption reactions but served as catalyst to improve the dehydrogenation kinetics.The apparent activation energy was decreased from 195.88kJ mol-1(3LiH-7MgH2)to 164.04kJ mol-1(3LiH-7MgH2-TiH2).Similarly,the performance of hydrogen-absorption was improved due to the additive TiH2. |
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