25 research outputs found
First-principles study on the intermediate compounds of LiBH
We report the results of the first-principles calculation on the intermediate
compounds of LiBH. The stability of LiBH and LiBH has been examined with the ultrasoft pseudopotential method based on
the density functional theory. Theoretical prediction has suggested that
monoclinic LiBH is the most stable among the candidate
materials. We propose the following hydriding/dehydriding process of LiBH
via this intermediate compound : LiBHLiBH LiH HLiH B H. The hydrogen content and enthalpy of the first
reaction are estimated to be 10 mass% and 56 kJ/mol H, respectively, and
those of the second reaction are 4 mass% and 125 kJ/mol H. They are in good
agreement with experimental results of the thermal desorption spectra of
LiBH. Our calculation has predicted that the bending modes for the
-phonon frequencies of monoclinic LiBH are lower than
that of LiBH, 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
Syntheses and Hydrogen Desorption Properties of Metal-Borohydrides M(BH 4 ) n
Metal-borohydrides M(BH 4 ) n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4) were synthesized by mechanical milling process according to the following reaction; MCl n þ nLiBH 4 /nNaBH 4 ! M(BH 4 ) n þ nLiCl/nNaCl. Then the thermal desorption properties of M(BH 4 ) n were investigated by gas-chromatography and mass-spectroscopy combined with thermogravimetry. The results indicate that the hydrogen desorption temperature T d of M(BH 4 ) n correlates with the Pauling electronegativity P of M; that is, T d decreases with increasing value of P . The components of desorbed gas for M ¼ Mg, Sc, Zr and Ti ( P 5 1:5) are hydrogen only, while that for M ¼ Zn ( P ¼ 1:6) contains borane besides hydrogen. The Pauling electronegativity P of M is an indicator to estimate T d of M(BH 4 ) n as candidates for advanced hydrogen storage materials with high gravimetric hydrogen densities and low desorption temperatures
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
Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: First-principles calculations and experiments
The thermodynamical stabilities for the series of metal borohydrides M(BH₄)n (M=Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; n=1–4) have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between Mⁿ⁺ cations and [BH₄]⁻ anions exists in M(BH₄)n, and the charge transfer from Mⁿ⁺ cations to [BH₄]⁻ anions is a key feature for the stability of M(BH₄)n. A good correlation between the heat of formation ΔHboro of M(BH₄)n and the Pauling electronegativity of the cation ϰP can be found, which is represented by the linear relation, ΔHboro=248.7ϰP–390.8 in the unit of kJ/mol BH₄. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for M(BH₄)n (M=Li, Na, K, Mg, Zn, Sc, Zr, and Hf), where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that LiBH₄, NaBH₄, and KBH₄ desorb hydrogen to hydride phases. Mg(BH₄)₂, Sc(BH₄)₃, and Zr(BH₄)₄ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn(BH₄)₂ desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature Td and the Pauling electronegativity ϰP is observed experimentally and so ϰP is an indicator to approximately estimate the stability of M(BH₄)n. The enthalpy change for the desorption reaction, ΔHdes, is estimated using the predicted ΔHboro and the reported data for decomposed product, ΔHhyd/boride. The estimated ΔHdes show a good correlation with the observed Td, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring M(BH₄)n with appropriate stability as hydrogen storage materials
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