| Abstract |
Several experimental studies have found improved thermodynamic, kinetic, and reversibility properties when LiBH4 is confined in nano-porous materials. In this study, we used Monte Carlo method and genetic algorithm to search the conformations of reactant (LiBH4)n and one of products (LiB)n. Other products, Lin, Bn, and Li2BnHn, were obtained from literatures. We used the density functional theory to calculate the total energies of reactants and all possible products, and found that the stability of LiBH4 nanoclusters have only little difference compared with that of bulk-LiBH4. However, for the products, the stabilities decrease with reducing sizes. This phenomenon indicates nano-LiBH4 would have higher reaction temperatures. We also investigated the reaction paths of nano-LiBH4, and the results show that the reaction path change from bulk-phase reaction (LiBH4 → LiH + B + 3/2H2) to ((LiBH4)n → (LiB)n + 2nH2) after reducing size. We also found intermediate compound Li2BnHn might appear dehydrogenation. The possible intermediate compounds of (LiBH4)m(m=2,3,4,6) were further included in the calculations to understand the hydrogenation releasing processes. Our results show that different (LiBH4)m have different hydrogen release properties. When m=3 and 6, under 1 bar, both of them release hydrogen rapidly in small temperature interval near their starting points of hydrogen release. When m=2 and 4, we found the amount of releasing hydrogen has a linear relation with temperature near their starting points. However, our results do not replicate experimental results due to the exclusion of the substrate effects i.e. to the nano-porous materials used in experiments which improve the thermodynamic properties of nano-LiBH4. |
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