Shigehito Isobe

Rechargeable hydrogen storage in nanostructured mixtures of hydrogenated carbon and lithium hydride

A hydrogen storage ability caused by the interaction between nanostructured carbon (CnanoHx) and lithium hydride (LiH) is demonstrated, which should be recognized as Li-C-H system in the H-storage materials. Especially, the 2:1 mixture of CnanoHx and LiH exhibited promising hydrogen storage properties with a rechargeable hydrogen capacity of more than 4 mass% below 350 °C, preserving the nanostructural feature in the mixture even after hydrogen release. On the other hand, the 1:2 and 1:1 mixtures exhibited the crystal growth of Li2C2 after hydrogen desorption, leading to poorer hydrogen rechargeability.

Study on reaction mechanism of dehydrogenation of magnesium hydride by in situ transmission electron microscopy

In situ observation on dehydrogenation of MgH2 was performed by using transmission electron microscope (TEM). The dehydrogenation of MgH2 with 1 mol % Nb2O5 and formation of nanosized Mg particles were observed at 150 °C. Nb2O5 was not confirmed in diffraction patterns and TEM images probably due to wide dispersion. On MgH2 with 10 mol % Nb2O5, the high resolution TEM could recognize the dehydrogenation at the interface between MgH2 and Nb2O5, proceeding with increasing temperature. This suggests that hydrogen atoms could diffuse from MgH2 phase to the interface between Mg and Nb2O5, resulting in formation of hydrogen molecules at the interface.

Low temperature hydrogenation of iron nanoparticles on graphene.

Hydrogenation of iron nanoparticles was performed both computationally and experimentally where previously chemically-bonded iron hydride is considered to be unachievable under ordinary conditions. Density functional theory (DFT) calculations predict that hydrogenated iron nanoparticles are stabilized on a single-layer graphene/Cu substrate. Experimentally, iron nanoparticles were deposited onto a graphene/Cu substrate by vacuum deposition. Hydrogenation was done at 1atm of hydrogen gas and under liquid nitrogen. Mass spectrometry peak confirmed the hydrogen release from hydrogenated iron nanoparticles while a scanning transmission electron microscopy is used in order to link a geometrical shape of iron hydride nanoparticles between experimental and theoretical treatments. The hydrogenated iron nanoparticles were successfully synthesized where hydrogenated iron nanoparticles are stable under ordinary conditions.

Chemisorption of hydrogen on Fe clusters through hybrid bonding mechanisms

The interaction of H and Fe clusters of up to nine atoms were investigated within a density functional theory. Calculations indicate that one gas-phase Fe atom can absorb ten H atoms, an amount 2.5 times more than methane (CH4). The magnetic state of Fe atoms non-uniformly decrease by increasing the number of H. The bonding of Fe-H in FeH clusters consists of charge transfer and electron pairing. Thus, two types of bondings are involved. The bond mechanism is general in nature within transition metal clusters, bringing insight for the development of heterogeneous catalyst and hydrogen storage materials.

Plastic bag method for active sample loading into transmission electron microscope

A plastic bag method was developed to observe air-sensitive samples on microstructure and phase distribution without exposure to air during the holder transfer process into the transmission electron microscope (TEM). As an example, a type of lithium aluminum hydride (Li(3)AlH(6)) was observed in the TEM to demonstrate the effectiveness of this method. Results show that the plastic bag method is a simple and practical TEM transfer method utilized to reduce air contact for a series of air-sensitive materials.

A Homogeneous Metal Oxide Catalyst Enhanced Solid–Solid Reaction in the Hydrogen Desorption of a Lithium–Hydrogen–Nitrogen System

In this study, LiTi2O4 was synthesized as a possible catalyst in the Li‐N‐H system. The properties of hydrogen desorption in the Li‐N‐H system with a homogeneous catalyst have been investigated. The X‐ray diffraction and X‐ray photoelectron spectroscopy results indicated that the single phase of LiTi2O4 was successfully synthesized and it was stable in the sample after high energy ball‐milling and heat treatment. LiTi2O4 exhibited a catalytic effect in the Li‐N‐H system according to the thermogravimetry differential thermal analysis results. During dehydrogenation, a storage capacity of 5.7 wt % was obtained under moderate temperature. A sharp peak of thermal gas desorption mass spectrometry curve occurred at 227 °C. Furthermore, the catalytic mechanism of LiTi2O4 in the Li‐N‐H system was discussed in accordance with the experimental results.

Identifying catalyst in Li-N-H system by x-ray absorption spectroscopy

Chemical bonding states of titanium compounds in LiH and LiNH2 mixture, which have been a candidate for a hydrogen storage material, have been examined by x-ray absorption spectroscopy measurement as the characterization of the catalysts. The results of x-ray absorption near-edge structure indicated that the Ti atoms in the Ti compounds, which had the catalytic effect on the kinetics of the hydrogen desorption properties, had a common electronic (chemical bonding) state. Additionally, this common electronic state of the Ti catalysts agrees with that of TiCl3·5NH3. These results indicated that TiCl3·5NH3 could act as the catalyst.

The structural properties of amides and imides as hydrogen storage materials

Abstract We discuss crystal structure of amides and imides, which are focusing on promising hydrogen storage materials. They are ionic crystal with anion of (NH2)– or (NH)2– and cation of Li+, Na+, Mg2+, Ca2+, and so forth. We also discuss reaction mechanisms on hydrogen ab/desorption of amide/imide hydrogen storage materials.

H2 dissociation over NbO: the first step toward hydrogenation of Mg.

Niobium-based oxide nanoparticles have proven to be catalytically effective toward hydrogenation of Mg where H2 dissociation over the niobium-oxides is considered to be a crucial reaction step. However, the role of niobium oxides toward H2 dissociation still remains unclear as to what atomic configurations are responsible for the catalytic activity. H2 dissociation over different surface planes of Nb, NbO, and Nb2O5 as well as small NbO clusters is performed by using a density functional theory. The calculations reveal that H2 dissociation, adsorption energy, and the bond type between H and surfaces (clusters) depend on the atomic configurations of Nb and O. In particular, H2 adsorption on NbO(111) is enhanced by O atoms without forming O-H bond where the bond type of H and surface is found to be an electron pairing. Thus, NbO(111) could not only be a effective catalyst but also potentially prevent the formation of MgO during the hydrogenation of Mg. The results should be helpful in developing and tailoring the efficient catalyst toward H2 dissociation and hydrogenation of Mg.

Raman Scattering Study of Hydrogen Storage Material LiNH2

Raman scattering spectra of LiNH 2 have been measured from 3.4 to 673 K. Precise polarization dependence of the single crystalline LiNH 2 and first principles calculation have successfully assigned all observed peaks. Li vibration with the lowest energy at 133 cm -1 shows the anomaly, where its energy decreases with decreasing temperature. This anomaly shows that the Li vibration is highly anharmonic with large amplitude. In addition, the energy of 133 cm -1 gives the very small force constant of 0.05 mdyn/A between Li and NH 2 in the diatomic model. This weak interaction suggests that LiNH 2 easily decomposes to Li and NH 2 . Below 100 K, we have found new peaks in the energy range from 100 to 700 cm -1 . No additional degrees of freedom for vibrations conclude that rotational motion of NH 2 molecule freezes below 100 K. With increasing temperature, the bond angle of H–N–H and bond length N–H in NH 2 become narrow and long, respectively. At the reaction temperature region, the correlation between the int...