Shin-ichi Towata

Experimental studies on intermediate compound of LiBH4

The formation condition of an intermediate compound of LiBH4 during the partial dehydriding reaction and its local atomistic structure have been experimentally investigated. LiBH4 changes into an intermediate compound accompanying the release of approximately 11mass% of hydrogen at 700–730K. The Raman spectra indicate that the B–H bending and stretching modes of the compound appear at lower and higher frequencies, respectively, as compared to those of LiBH4. These features are consistent with the theoretical calculation on the monoclinic Li2B12H12, consisting of Li+ and [B12H12]2− ions, as a possible intermediate compound of LiBH4.

First-principles study on the stability of intermediate compounds of LiBH(4)

Note: Times Cited: 110 Reference EPFL-ARTICLE-206017doi:10.1103/PhysRevB.74.075110View record in Web of Science URL: ://WOS:000240238800042 Record created on 2015-03-03, modified on 2017-05-12

Formation of an intermediate compound with a B12H12 cluster: experimental and theoretical studies on magnesium borohydride Mg(BH4)2.

Experimental and theoretical studies on Mg(BH4)2 were carried out from the viewpoint of the formation of the intermediate compound MgB12H12 with B12H12 cluster. The full dehydriding and partial rehydriding reactions of Mg(BH4)2 occurred according to the following multistep reaction: Mg(BH4)2 -->1/6MgB12H12 + 5/6MgH2 + 13/6H2 <--> MgH2 + 2B + 3H2 <--> Mg + 2B + 4H2. The dehydriding reaction of Mg(BH4)2 starts at approximately 520 K, and 14.4 mass% of hydrogen is released upon heating to 800 K. Furthermore, 6.1 mass% of hydrogen can be rehydrided through the formation of MgB12H12. The mechanism for the formation of MgB12H12 under the present rehydriding condition is also discussed.

Chemical bonding of hydrogen in MgH2

MgH2 is one of the promising base materials for hydrogen storage, which is a key technology of clean energy source. In this study, the bonding nature of hydrogen in MgH2 was fully uncovered by examining the charge density distribution of this substance obtained by the maximum entropy method from the synchrotron radiation powder data. MgH2 can be expressed as Mg1.91+ H0.26−, which is much weaker ionicity than the theoretical expectations. It also shows weak covalence between Mg and H. Though the bonding nature of hydrogen in MgH2 is rather complex, i.e., the mixture of ionic and covalent bonding, it is certain that hydrogen is weakly bonded to Mg, which must be a big advantage of hydrogenation–dehydrogenation of this substance.

Charge density measurement in MgH2 by synchrotron X-ray diffraction

Abstract Magnesium and its alloys are considered one of the most promising materials for reversible hydrogen storage. However, their high thermodynamic stability is unfavorable for dehydrogenation processes. In order to improve their properties, understanding of the bonding nature of Mg and H is essential. In this study, we reveal the precise experimental charge density distribution in MgH 2 . X-ray powder diffraction data were obtained using the synchrotron radiation. The charge density distribution was calculated by the MEM (maximum entropy method)/Rietveld analysis. At room temperature, the charge density distribution around Mg is spherical, whereas the lower charge density distribution around H is non-spherical and slightly spread in the direction of the nearest neighbor Mg and H atoms. The number of electrons within the sphere around the Mg and the H atoms were estimated from the obtained distribution. As the result, Mg is almost fully ionized as Mg 2+ , whereas hydrogen is very weakly ionized. The ionic charge of hydrogen is lower than the theoretical value.

Reversible hydriding and dehydriding properties of CaSi: Potential of metal silicides for hydrogen storage

We found that CaSi reversibly absorbs and desorbs hydrogen. First-principles calculations theoretically indicated that CaSi hydride is thermodynamically stable. The hydriding and dehydriding properties of CaSi were experimentally determined using pressure-composition (p‐c) isotherms and x-ray diffraction analysis. The p‐c isotherms clearly demonstrated plateau pressures in a temperature range of 473–573K. The maximum hydrogen content was 1.9wt% under a hydrogen pressure of 9MPa at 473K. The reversible hydriding and dehydriding properties of CaSi suggest the potential of metal silicides for hydrogen storage.

Structure determination of structurally complex Ag36Li64 gamma-brass

The crystal structure of the Ag(36)Li(64) gamma-brass was determined by analyzing the powder diffraction pattern taken using a synchrotron radiation beam with wavelength 0.50226 A. It turned out that the compound contained 52 atoms in its unit cell with the space group I43m and that the Li atom enters exclusively into inner tetrahedral (IT) and cubo-octahedral (CO) sites, whereas the Ag atom enters into those on outer tetrahedral (OT) and octahedral (OH) sites in the 26-atom cluster. Small amounts of Li also exist in OT and OH sites, resulting in chemical disorder. We discovered that the volumes of the IT and CO polyhedra shrink, while those of the OT and OH polyhedra expand relative to those of the corresponding polyhedra in the original b.c.c. (body-centered cubic) structure. This feature is universal and is found in other gamma-brasses such as Cu(5)Zn(8) and Al(8)V(5), for which the structure data are available. Among these gamma-brasses, we revealed the unique bond-length distribution for pairs connecting the atom on OH sites and that on CO sites, depending on the degree of d-p orbital hybridization between the transition metal elements such as Ag, Cu and V on OH sites, and the non-transition metal elements such as Li, Zn and Al on CO sites. It is suggested that this may hold a clue to resolving why some gamma-brasses such as the present Ag-Li and Cu-Zn possess a finite solid solution, but others such as Al(8)V(5) and Mn(3)In exist as line compounds.

Microscopic indicator for thermodynamic stability of hydrogen storage materials provided by muon-spin spectroscopy

In search of a high-capacity hydrogen storage system, we have investigated the thermodynamic properties of borohydrides [M(BH4)n]. Using positive muon-spin rotation and relaxation (μ+SR), we have acquired experimental data for a wide range of different powder samples below ambient temperature. Zero-field μ+SR measurements indicate the formation of the H-μ+-H system in LiBH4, NaBH4, KBH4 and Ca(BH4)2, but not in Mg(BH4)2. It is also found that the amplitude of the HμH signal (AHμH) varies with the electronegativity (χP) of Mn+. This is because, when χP of Mn+ is small, [BH4]− should be more negative, resulting in an increase in the electron-density of H− ions. Therefore, AHμH increases with decreasing χP. Since the thermodynamic stability of M(BH4)n also depends on χP, AHμH is thought to be a microscopic indicator for the stability of borohydrides.

Structural investigation of metal borohydrides by X-ray/neutron diffraction and computational study

Ammonia borane contains more than 19wt% hydrogen and has received significant attention as a promising hydrogen storage material. Although the decomposition temperature of NH3BH3 is relatively low, its propensity to release deliterious decomposition products such as borazine has mitigated against its development as a hydrogen storage material. Here we present results that show that improved thermal desorption parameters may be obtained when one of the protic hydrogens on the nitrogen is replaced by either a lithium or a sodium cation. Lithium and sodium amidoboranes have recently been shown to release two molar equivalents of H2 at around 92°C. Structural analysis has shown that both LiNH2BH3 and NaNH2BH3 are isostructural and that the dihydrogen bond that is evident in ammonia borane is not present in these amidoboranes. We present neutron and X-ray powder diffraction measurements and provide a detailed comparison of ammonia borane with the alkali metal amidoboranes and other amidoboranes that show promising decomposition behaviour close to room temperature.