| Storage of hydrogen in a safe, efficient and compact manner is the key issue of utilizinghydrogen as an alternative energy carrier. Light-wight metal complex hydrides have attactedrather more attention owning to their intrinsic high hydrogen storage capacity. However, theirpractical applications are limited by the high thermodynamic stability, sluggish kinetics andpoor reversibility. Based on the review of the research progress and existing problems of LiBH4complex hydride, some novel LiBH4hydrogen storage systems, i.e.6LiBH4/SrH2, xLiBH4/YF3(x=3,4) and yLiBH4/CeF3(y=3,6), were constructed in this thesis, and their de-/hydrogenationproperties were characterized by the Sievert method. Then, the de-/hydrogenation reactionmechanisms as well as the kinetic and thermodynamic characteristics of the composite systemswere analyzed by means of X–Ray diffraction (XRD), pressure–composition–temperature (p–c–T) measurement, fourier transform infrared spectroscopy (FTIR) and diffferential scanningcalorimetry (DSC). Moreover, the mechanism involved in the effect of SrH2, YF3and CeF3onthe hydrogen storage properties of LiBH4was discussed.The hydrogen storage properties of6LiBH4/SrH2system were studied firstly. It was foundthat hydrogen desorption of the6LiBH4/SrH2system starts arount220°C and processes almostcompletely at about450°C with a dehydrogenation amount of8.7wt.%was totally achieved.The dehydrogenation reaction can be expressed as6LiBH4+SrH2â†'SrB6+6LiH+10H2. Thedehydrogenation experiment demonstrates that the dehydrogenated product can reversibly store6.0wt.%of hydrogen with the formation of LiBH4and LiSrH3at450°C under an initialhydrogen pressure8.0MPa. The studies on the kinetic characteristics indicate that thedehydrogenation reaction of the6LiBH4/SrH2system is mainly controlled by the three-dimensional phase boundary migration, with an activation energy of64kJ/mol. Furthermore,the enthalpy and entropy changes for the dehydrogenation of the6LiBH4/SrH2system weredetermined to be48kJ/mol H2and82J/(mol·K) H2, respectively. In comparison with thepristine LiBH4, the decrease in both dehydrogenation activation energy and enthalpy change canbe regarded as the main reasons for the improved properties of the6LiBH4/SrH2system.The researches on the hydrogen storage properties of xLiBH4/YF3(x=3,4) compositesystems show that these two systems start to desorb hydrogen at about200°C, and the mostamount of hydrogen is released at about320°C. The amounts of hydrogen totally desorbedfor the3LiBH4/YF3and4LiBH4/YF3systems are5.0and6.1wt.%, respectively. Afterdehydrogenation, the phases of YB4, YH2and LiH(F) were formed, and the ball milling timehas almost no positive effect on the dehydrogenation behavior of the xLiBH4/YF3(x=3,4) systems. The4LiBH4/YF3system dehydrogenated at500°C can reabsorb2.4wt.%of hydrogenwith10h at350°C under an initial hydrogen pressure of8MPa, and the phases of LiBH4andYH3were formed after rehydrogenation for the3LiBH4/YF3system. The p–c–T analysisindicates that two plateaus are present in the dehydrogenation p–c–T curves for the xLiBH4/YF3(x=3,4) systems. The dehydrogention enthalpy changes for the higher and lower plateaus of the3LiBH4/YF3system are-88and-86kJ/mol H2, respectively, and those are-89and-69kJ/molH2for the4LiBH4/YF3system. The dehydrogenation activation energies for the3LiBH4/YF3and4LiBH4/YF3system are reduced to88and101kJ/mol, respectively.The hydrogen storage properties of yLiBH4/CeF3(y=3,6) systems were investigated finally.The results show that the addition of CeF3can also decrease the dehydrogenation stability ofLiBH4. For the3LiBH4/CeF3system, most amount of hydrogen can be desorbed around300°C,with4.1wt.%of hydrogen released totally. The6LiBH4/CeF3system can desorb6.1wt.%ofhydrogen, and the dehydrogenation process can be divided in two stages around300and400°C,respectively. The3LiBH4/CeF3and6LiBH4/CeF3systems can reabsorb1.5and4.0wt.%ofhydrogen within20h, respectively, at320°C and8MPa hydrogen pressure, with the formationof LiBH4, CeF3and CeF2. The activation energies for the dehydrogenation of3LiBH4/CeF3system and the first-step dehydrogenation of6LiBH4/CeF3system are90and98kJ/mol,respectively. Those are lower than that of the pristine LiBH4. The p–c–T analysis indicates thatone plateau is present in the dehydrogenation p–c–T curves for the yLiBH4/CeF3(y=3,6)systems in the pressure range of0~6MPa. The dehydrogenation enthalpy changes for the3LiBH4/CeF3and6LiBH4/CeF3systems were determined to be-88and-89kJ/mol H2,respectively. In comparison with the pristine LiBH4, the decrease in dehydrogenation activationenergy turns into another reason for the improved properties of the3LiBH4/CeF3and6LiBH4/CeF3systems. |
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