Modification And Dehydrogenation Performance Study On LiBH₄and Its Ammoniate For Hydrogen Storage

Complex hydrides as hydrogen storage systems have the advantage of higher hydrogen capacity over the traditional hydrogen storage alloys. Amongst which LiBH4is currently under consideration for solid-state on-board hydrogen storage thanks to its comparatively high hydrogen content, which reaches up to18.5wt.%. However, the pure LiBH4suffers from a lot of shortages, since they feature inferior kinetics, thermodynamics, and are irreversible under moderate conditions. Fortunately, modifying by anion substitution, reactive hydride composites(RHC) and catalyst addition et al., have been demonstrated to be effective approaches to improve the hydrogen storage propeorteis of metal borohydrodes. Moreover, nanosizing and nanocomfinement techniques can effectively control the particle size distribution of active phase within a desired narrow range, expedite the decomposition kinetics and potentially for some systems, change the thermodynamics and make them "rehydridable".One of my research projects during the graduate study is the preparation and characterization of the nanocomposites of lithium borohydride (LiBH4) or ammine lithium borodydride (LiBH4·NH3) loaded on different nanoscaffolds with different methods (e.g., wet infiltration, melting infiltration, or energetic ball milling).On the basis of the previous research results concerned with modifications of hydrogen storage properties of LiBH4through catalyst doping and self-micronizing, in the present study, the method of producing nano-sized LiBH4combined with nano SiO2templates in the solvent at comparatively modest temperature has been proposed. In probing into the preparation conditions and constraining the particle growth in order to fulfill its micronization, the micronized LiBH4catalyst composite is prepared; and of concern to such a study is giving full play to both the characteristics of nanosized support and the micronization of LiBH4in improving the dehydrogenation/hydrogenation properties of LiBH4. The multicomponent LiBH4/SiO2material synthesized by the wet method has been found to dehydrogenate at much lower temperatures than the pure LiBH4, as well as LiBH4/SiO2mixtures prepared by ball milling. For example, the onset of dehydrogenation is decreased to about200℃for a wet-treated LiBH4/SiO2mixture with a mass ratio of1:1, and the majority of hydrogen can be released below350℃.Moreover, confining LiBH4·NH3into another oxide nanoframeworks Al2O3/CNTS via melting infiltration is investigated. As a consequence, it was found that the dehydrogenation of the loaded LiBH4·NH3is remarkably enhanced, showing an onset dehydrogenation at temperatures below100℃, in particular a dominant dehydrogenation at115℃is achieved for the LiBH4·NH3/Al2O3composite with a mass ratio of1:4, superior to LiBH4-NH3catalyzed by hydrides. These results may provide some insights into the nanosize effect of the oxide nanoframeworks supported hydrogen storage materials.Another research focus of this study is deciphering the decomposition mechanisms for the ammine metal borohydrides (AMBs) and the related RHC systems. LiBH4has been proved to have the potential to work in coordination with other hydrogen storage candidates with positively charged H, notably Mg(NH2)2-Mixtures of Mg(NH2)2and LiBH4are catalyzed with various chlorides of different fractions, where LiBH4-Mg(NH2)2+20wt.%NiCl2is found to be the optimal composition. Characterizations of that complex reveal a probable in-situ stoichiometric formation of Ni(BH4)2occurring during ball milling that effectively stabilizes the [NH2]-as its ancillary ligands by forming a postulated N-Ni-B-H complex upon milling, which is responsible for the facilitated B-N polymer formation upon heating, and enables this modified system to release7.3wt.%hydrogen in the temperature range of60-400℃.