First-principles study on the intermediate compounds of LiBH4_4

We report the results of the first-principles calculation on the intermediate compounds of LiBH$_4$. The stability of LiB$_3$H$_8$ and Li$_2$B$_n$H$_n (n=5-12)$ has been examined with the ultrasoft pseudopotential method based on the density functional theory. Theoretical prediction has suggested that monoclinic Li$_2$B$_{12}$H$_{12}$ is the most stable among the candidate materials. We propose the following hydriding/dehydriding process of LiBH$_4$ via this intermediate compound : LiBH$_4 \leftrightarrow {1/12}$Li$_{2}$B$_{12}$H$_{12} + {5/6}$ LiH $+ {13/12}$H$_2 \leftrightarrow$LiH $+$ B $+ {3/2}$H$_2$. The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass% and 56 kJ/mol H$_2$, respectively, and those of the second reaction are 4 mass% and 125 kJ/mol H$_2$. They are in good agreement with experimental results of the thermal desorption spectra of LiBH$_4$. Our calculation has predicted that the bending modes for the $\Gamma$-phonon frequencies of monoclinic Li$_2$B$_{12}$H$_{12}$ are lower than that of LiBH$_4$, while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds.Comment: 7 pages, 5 figures, submitted to PR

Hysteresis phenomena on the crystal lattice of Ti0.8Zr0.2Mn1.5 in the hydrogenation and dehydrogenation process

The structural changes in the hydrogenation and dehydrogenation reaction process on Ti0.8Zr0.2Mn1.5 alloy have been investigated by the in-situ synchrotron radiation X-ray diffraction experiments to elucidate the mechanism of hysteresis phenomena in equilibrium hydrogen pressure.The lattice constant and unitcell volume of both hydrogen solid solution phase and hydride phase in the dehydrogenation pricess are smaller than those in hydrogenation process despite of lower equilibrium pressure. This result is opposite to the general relation between the lattice volume and pressure. The hysteresis behavior also exhibits in the diffraction peak width. The hysteresis behavior may be related to lattice difects and hydrogen concentration

First-principles study on the stability of intermediate compounds of LiBH₄

We report the results of the first-principles calculation on the intermediate compounds of LiBH₄. The stability of LiB₃H₈ and Li₂BnHn (n=5–12) has been examined with the ultrasoft pseudopotential method based on the density-functional theory. Theoretical prediction has suggested that monoclinic Li₂B₁₂H₁₂ is the most stable among the candidate materials. We propose the following hydriding (dehydriding) process of LiBH₄ via this intermediate compound: LiBH₄ 1/2Li₂B₁₂H₁₂+5/6LiH+13/12H₂ LiH+B+3/2H₂. The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass % and 56 kJ/mol H₂, respectively, and those of the second reaction are 4 mass % and 125 kJ/mol H₂. They are in good agreement with experimental results of the thermal desorption spectra of LiBH₄. Our calculation has predicted that the bending modes for the Γ-phonon frequencies of monoclinic Li₂B₁₂H₁₂ are lower than that of LiBH₄, while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds

Guidelines for Developing Amide-Based Hydrogen Storage Materials

An effective method for developing amide-based high-performance hydrogen storage materials is to prepare appropriate combinations of amides and hydrides. We have proposed that a mixture of an amide with a low decomposition temperature and a hydride showing rapid reaction to ammonia would be an appropriate combination. According to this proposal, the mixture of Mg(NH 2 ) 2 (Mg amide) and LiH (Li hydride) was investigated. The dehydriding temperature of the mixture of Mg(NH 2 ) 2 and 4ÁLiH is lower than that of the mixture of LiNH 2 (Li amide) and 2ÁLiH. A method for preventing ammonia release is increasing the LiH ratio in the mixtures, which results in a reduction in the amount of desorbed hydrogen. The homogeneous dispersion between Mg(NH 2 ) 2 and LiH might be also an important factor for preventing ammonia release

Role of Nb in Improving the Cyclic Stability of V-Ti-Cr Alloys

V-Ti-Cr合金の水素の繰返し吸蔵放出能の劣化は僅かなNbの添加で劇的に改善するが、Nbの繰り返し耐久性への寄与は明らかではない。本研究ではNbの添加効果を明らかにするために、 V0.25Ti0.25Cr0.5H2と V0.25Ti0.25Cr0.45Nb0.05H2の水素吸蔵放出サイクルによる局所構造の変化をX線原子二体分布関数とEXAFSによって調べた。本研究によって水素吸蔵放出の繰返し過程で、転位によるひずみを低減させるために大きなTi原子が転位中心へ引きずられることが示唆された。これは、水素吸蔵に高い水素ガス圧が必要になるため転位近傍のTiが少ない領域が残ることになるが、NbはTiの偏析を抑制している可能性が高い