LiBH₄-based Composites For Hydrogen Storage

In this thesis, the hydrogen storage properties of LiBH4-based composites were systematically investigated using XRD, SEM, DSC-TG-MS, FTIR and volumetric measurements. The effects of the additives including chlorides of Fe, Co, Ni, halides of rare earth metals Ce and La, as well as Cerium hydride on the hydrogen storage properties of LiBH4 or 2LiBH4/MgH2 composite were summarized as follows:(1) The dehydrogenation temperature of LiBH4 was considerably decreased to 230℃-300℃when it was manually mixed with FeCl2, CoCl2 and NiCl2 in a molar ratio of 2:1. Mixing LiBH4 with NiCl2 or FeCl2 led to complete hydrogen desorption from LiBH4, i.e.18.3wt% hydrogen was achieved with respect to the weight of LiBH4. However, the CoCl2 addition resulted in less hydrogen released due to the formation of diborane. The ball-milling treatment for the mixtures of LiBH4 and these chlorides further decreased hydrogen desorption temperatures. The results indicate that LiBH4 could be effectively destabilized by the chlorides of Fe, Co and Ni. Small doping of these chlorides into LiBH4 was effective in enhancing the dehydrogenation kinetics of the remaining LiBH4 due to the formation of metal borides.(2) The ball milling of LiBH4 with chloride of Ce or La in a molar ratio of 3:1 yielded Ce(BH4)3 and La(BH4)3, whereas fluoride of Ce or La did not react with LiBH4 during extended ball milling at room temperature. The dehydrogenation temperatures of the ball-milled mixtures were reduced to 220-320℃, which were much lower than that of pure LiBH4. The diborane emission during hydrogen release was observed at a low level. The dehydrogenation temperature is found to be affected by the composition of rare earth halides, but less influenced by ball milling time. The endothermic dehydrogenation reactions produced lithium halides, hydrides and borides of the corresponding rare earth element. Moreover, the LiBH4+1/3(Ce, La)(Cl, F)3 showed partial reversibility through the formation of an unknown borohydride, allowing for a potential hydrogen storage system.(3) The hydrogenation/dehydrogenation process of the 2LiBH4/MgH2 composite is reversible, but the dehydriding kinetics is impeded by the nucleation of MgB2.The results showed that only partial Mg reacted with LiBH4; after several hydrogenation/dehydrogenation cycles for the uncatalyzed 2LiBH4/MgH2 composite, which degraded the reversible capacity for hydrogen storage. The dehydrogenation kinetics and cyclic stability of the 2LiBH4/MgH2 composite were significantly enhanced by the addition of cerium compounds such as CeF3, CeCl3, and CeH3. During the dehydrogenation process, CeF3 reacted with LiBH4 to form CeB6. which functioned as the nuclei for MgB2 and thus enhanced the nucleation of MgB2. Therefore, the reaction kinetics and cycling stability of hydrogenation/dehydrogenation for the 2LiBH4/MgH2 composite were considerably improved.