| Hydrogen energy has the advantages of high safety and efficiency as well as low environment impact,and the storage technology of hydrogen is one of the key problems for its practical application.Based on the introduction of solid-state hydrogen storage technology,this dissertation focuses on the research status of coordination hydrides,especially the development and existent problems of LiBH4 and LiAlH4.Then the6LiBH4/YbH2.5,x LiAlH4/yLiBH4/MgCl2 and 3LiAlH4/g-C3N4 composite systems were constructed and prepared.The hydrogen storage performance and mechanism of these systems were studied by means of pressure-temperature-composition?p-c-T?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,differential scanning calorimetry?DSC?,scanning electron microscopy?SEM?and energy dispersive spectrometer?EDS?.The YbH2.5.5 hydride was prepared by ball milling and high temperature hydrogenation,and the 6LiBH4/YbH2.5.5 composite system was obtained by mechanical milling.It was found that the system starts to release as much as 6.6 wt.%of hydrogen at about 300oC with the ending dehydrogenation temperature of about 500oC.The addition of YbH2.5.5 can promote significantly the dehydrogenation process of Li BH4,and the dehydrogenation activation energy of the 6LiBH4/YbH2.5.5 system was determined to be 68 kJ/mol.Through p-c-T test,it was found that the 6LiBH4/YbH2.5.5 system has two dehydrogenation platforms,impling that the reaction can be divided into two steps.The higher platform reaction is:6LiBH4+YbH2.5?1/2LiYb?BH4?4+1/4YbB4+1/4YbB12+11/2LiH+13/2H2 with the reaction enthalpy and entropy are 74 kJ/mol H2 and 126 J/?mol·K?H2,respectively.The lower platform reaction is:1/4YbB4+1/2LiYb?BH4?4+1/4YbB12?YbB6+1/2LiH+15/4H2.Its reaction enthalpy and entropy are 53 kJ/mol H2 and 84 J/?mol·K?H2,respectively.The x LiAlH4/yLiBH4/MgCl2 composite systems were constructed.It was found that the ball-milling time has a significant effect on the phase component of products.When the ball-milling time reaches up to 30 h,the LiMg?AlH4?2?BH4?and LiCl would form completely for the 2LiAlH4/LiBH4/MgCl2 system.By changing the raw material ratio,it was found that when x changes from 2 to 1?y=3-x?,the amount of LiMg?AlH4?2?BH4?in the product decreases gradually,while the amount of LiMg?AlH4??BH4?2 increases gradually.Compared with Mg?AlH4?2?at LiCl?and Mg?BH4?2?at LiCl?,the dehydrogenation process of LiMg?AlH4?2?BH4?and LiMg?Al H4??BH4?2 can be divided into three steps.However,their dehydrogenation property was improved with an initial dehydriding temperature of about 100oC.The g-C3N4 with high purity was prepared by calcining dicyandiamide at high temperature,and then the 3LiAlH4/g-C3N4 composite system was obtained by ball milling g-C3N4 and LiAlH4.It was found that dehydrogenation of the 3LiAlH4/g-C3N4 system starts at about 120oC and ends at about 200oC,with about 5.8 wt.%of hydrogen desorbed.Compared with pure LiAlH4,the onset and ending desorption temperatures are both obviously decreased.The kinetic performance test indicates that dehydrogenation of the system is exothermic,and the activation energy of hydrogen desorption reaction is 121kJ/mol.The large amount of heat released during the dehydrogenation process can further promote the dehydrogenation reaction and improve the kinetic performance of the3LiAlH4/g-C3N4 system.The XRD and FTIR analyses of the dehydrogenation products imply that the addition of g-C3N4 completely changed the dehydrogenation reaction path of LiAlH4.By means of SEM and EDS analyses,it was further confirmed that ball milling can make the composition of 3LiAlH4/g-C3N4 system homogeneous,thus improving the dehydrogenation performance. |
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