Hiroki Miyaoka

Solid state NMR study on the thermal decomposition pathway of sodium amidoborane NaNH2BH3

The thermal decomposition pathway of sodium amidoborane (NaAB; NaNH2BH3) has been investigated in detail by using solid state NMR spectroscopy. 23Na MAS/3QMAS NMR spectra suggested that NaH and an amorphous Na–N–B–H phase started to be formed as decomposition products even at 79 °C, although NaAB was prepared from NaH and NH3BH3 by ball milling at room temperature. Based on the quantitative analyses of the 23Na MAS spectra, we proposed a decomposition reaction to 200 °C to be NaNH2BH3 → Na0.5NBH0.5 + 0.5NaH + 2.0H2. The hypothetical phase Na0.5NBH0.5 is amorphous, where the basic molecular unit of the original NaAB is polymerized into a [–BN–]n network structure. It was also found that the diammoniate of diborane (DADB) and polyaminoborane (PAB) were not formed during the decomposition of NaAB, which are both key compounds on the pyrolysis of ammonia borane (AB).

The reaction process of hydrogen absorption and desorption on the nanocomposite of hydrogenated graphite and lithium hydride

The lithium-carbon-hydrogen (Li-C-H) system is composed of hydrogenated nanostructural graphite (C(nano)Hx) and lithium hydride (LiH). C(nano)Hx is synthesized by ball-milling of graphite under a hydrogen atmosphere. In this work, the reaction process of hydrogen absorption and desorption on the Li-C-H system is investigated. The C(nano)Hx-LiH composite can desorb about 5.0 mass% of hydrogen at 350 degrees C with the formation of Li2C2 until the second cycle. However, the hydrogen desorption amount significantly decreases from the third cycle. Furthermore, it is shown by using gas chromatography that a considerable amount of hydrocarbons is desorbed during the rehydrogenation process. These results indicate that the amount of reaction between the polarized C-H groups in C(nano)Hx and LiH is reduced due to a decrease in the C-H groups by losing carbon atoms under the hydrogen absorption and desorption cycles.

Characterization of hydrogen absorption/desorption states on lithium-carbon-hydrogen system by neutron diffraction

The nanostructural hydrogenated graphite (CnanoHx) was synthesized from graphite by ball milling under hydrogen (H2) atmosphere. In this product, characteristic hydrogenated states in the form of polarized hydrocarbon groups (CH, CH2, and CH3) are realized in the nanoscale. By synthesizing the composite of CnanoHx and lithium hydride (LiH), known as the LiCH system, hydrogen was desorbed at 350°C, which is a lower temperature compared to the decomposition temperature of each component. It is considered that this hydrogen desorption would be induced by destabilization of each hydrogen absorbed state due to an interaction between the polarized CH groups in CnanoHx and LiH. Therefore, in order to understand the hydrogen absorption/desorption mechanism of the LiCH system, it is an important issue to investigate the change in the CH groups during hydrogen absorption/desorption reactions in the composite. The correlations among atoms contained in this composite are examined by neutron diffraction measu...

How does TiF4 affect the decomposition of MgH2 and its complex variants? – An XPS investigation

Magnesium hydride and its complex variants, i.e. Mg(AlH4)2 and Mg(BH4)2 are known for their high hydrogen capacity. The theoretical capacities of these three are 7.6 wt%, 9.3 wt% and 14.4 wt%, respectively. In spite of having very attractive H2 capacities, their high operating temperature, i.e. more than 300 °C and sluggish kinetics are big issues to be solved before these can be realized for a practical hydrogen storage system. There are several efforts devoted to reduce the working temperature and enhance the sorption kinetics using various additives. Ti-based additives have always been interesting contenders in enhancing the kinetics as well as altering thermodynamics thus reducing the working temperature for various hydrides. Recently, TiF4 has shown superior catalytic activity on the decomposition of Mg(AlH4)2. This motivates us to study its effect on the decomposition of all the above-mentioned Mg-based hydrides. It is observed that the addition of 10 wt% TiF4 to the above materials greatly influences the decomposition temperature. The decomposition temperature for all three reactions is shifted to the lower temperature side. The oxidation state of the catalyst surface has been investigated using the XPS technique and then the detailed mechanism associated with this improvement is proposed herein.

Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH2 Composite Electrode

Magnesium hydride, MgH2, a recently developed compound for lithium-ion batteries, is considered to be a promising conversion-type negative electrode material due to its high theoretical lithium storage capacity of over 2000 mA h g-1, suitable working potential, and relatively small volume expansion. Nevertheless, it suffers from unsatisfactory cyclability, poor reversibility, and slow kinetics in conventional nonaqueous electrolyte systems, which greatly limit the practical application of MgH2. In this work, a vapor-grown carbon nanofiber was used to enhance the electrical conductivity of MgH2 using LiBH4 as the solid-state electrolyte. It shows that a reversible capacity of over 1200 mA h g-1 with an average voltage of 0.5 V (vs Li/Li+) can be obtained after 50 cycles at a current density of 1000 mA g-1. In addition, the capacity of MgH2 retains over 1100 mA h g-1 at a high current density of 8000 mA g-1, which indicates the possibility of using MgH2 as a negative electrode material for high power and high capacity lithium-ion batteries in future practical applications. Moreover, the widely studied sulfide-based solid electrolyte was also used to assemble battery cells with MgH2 electrode in the same system, and the electrochemical performance was as good as that using LiBH4 electrolyte.

Structural and Electronic Interplay in the Gap Formation in CeRhAs1-xSbx (0 ≤x ≤1)

Both CeRhAs and CeRhSb are known to open a narrow pseudogap at the Fermi level. For the alloys CeRhAs 1- x Sb x (0 ≤ x ≤1), we report the magnetic, transport and x-ray diffraction measurements. For...

Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System

Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH2)2) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH and Mg(NH2)2 lead to the fast hydrogen desorption. To preserve the potential hydrogen desorption properties, thermochemical and mechanochemical rehydrogenation processes are investigated. Although the only thermochemical process results in slowing the reaction rate due to the crystallization, the ball-milling can recover the original hydrogen desorption properties. Furthermore, the mechanochemical process at 150 °C is useful as the rehydrogenation technique to preserve the suitable crystalline size and mixing state of the reactants. As a result, it is demonstrated that the 8LiH and 3Mg(NH2)2 system is recognized as the potential hydrogen storage material to desorb more than 5.5 mass% of H2 at 150 °C.

Thermodynamic Characterization on Hydrogen Absorption and Desorption Reactions of Lithium – Silicon Alloy

Lithium hydride LiH is one of the attractive hydrogen storage materials, because it stores 12.7 mass% of H2. However, H2 desorption reaction occurs over 600 °C due to the large enthalpy change of H2 desorption Ho = 181 kJ/mol H2. The purpose of this work is to control the enthalpy change of LiH to much lower value by a mechanical alloying with Si, where the Li-Si alloy is thermodynamically more stable than Li. The alloy was synthesized from Li granule and Si powder by a mechanical alloying method. The H2 absorption and desorption properties were characterized by High-Pressure Differential Scanning Calorimetry and Thermogravimetry - Differential Thermal Analysis - Mass Spectroscopy, and X-ray diffraction measurement. Pressure - Composition - Isotherm measurements were performed at 400, 450, and 500 °C to estimate the enthalpy change. From the results, it was confirmed that reversible H2 ab/desorption reactions of the Li-Si alloy were expressed as 7LiH + 3Si ↔ (3/7)Li12Si7 + (13/7)LiH + (18/7)H2 ↔ Li7Si3 + (7/2)H2 (theoretically 5.0 mass% H2) at 400 °C. From van’t Hoff plot obtained by the results of PCI measurements, the enthalpy change of the former reaction was estimated to be Ho = 103 kJ/mol H2, which is lower than that of LiH.

Microscopic characterization of metal-carbon-hydrogen composites (metal = Li, Mg)

Li-C-H system, which can store about 5.0 mass% of rechargeable H2, has been reported as a promising hydrogen storage system by Ichikawa et al. [Appl. Phys. Lett. 86, 241914 (2005); Mater. Trans. 46, 1757 (2005)]. This system was investigated from the thermodynamic and structural viewpoints. However, hydrogen absorption/desorption mechanism and the state of hydrogen atoms absorbed in the composite have not been clarified yet. In order to find new or better hydrogen storage system, graphite powder and nano-structural graphite ball-milled under H2 and Ar atmosphere were prepared and milled with Li and Mg under Ar atmosphere in this study. Microstructural analysis for those samples by transmission electron microscope revealed that LiC6 and/or LiC12 were formed in Li-C-H system. On the other hand, MgC2 was found in Mg-C-H system ball-milled under H2 atmosphere, but not in the system ball-milled under Ar atmosphere. These results indicated that nano-structure in composites of nano-structural graphite is differe...

Hydrogen Storage Properties of Hydrogenated Graphite and Lithium Hydride Nanocomposite

Recently, hydrogen storage and transportation are being studied all over the world as the key technology to establish clean and renewable energy systems for a sustainable society. In the case of an on−board application for a vehicle, hydrogen should be stored in a compact, light, safe, and reasonable vessel. Hydrogen storage materials can safely store higher density of hydrogen compared to the gaseous and liquid hydrogen storage systems. Therefore, the systems using hydrogen storage materials are considered as the most suitable technique (Akiba, 1999, Grochala & Edwards, 2004, Sandrock, 1999, Schlapbach & Zuttel, 2001, Zuttel, 2007). Particularly, much attention has been paid to materials based on the light elements because these materials are expected to realize high gravimetric and volumetric densities of hydrogen (Orimo & Nakamori et al., 2007, Schuth & Bogdanovic et al., 2004, Zuttel, 2004). Carbon is one of the attractive light elements because of abundant resource and low cost. Therefore, a lot of carbon based materials have been investigated as a hydrogen storage material since Dillon et al. reported on single−walled carbon nano−tubes in 1997 (Dillon & Jones et al., 1997). Among the carbon based materials, the hydrogenated nano−structural graphite (CnanoHx) can stably store large amount of hydrogen. The hydrogen ab/desorption properties of CnanoHx have been investigated so far (Chen et al., 2003, Ichikawa et al., 2004, Majer et al., 2003, Miyaoka et al., 2010, Orimo et al., 1999, Orimo et al., 2001, Stanik et al., 2005). CnanoHx is synthesized from graphite by ball−milling method under hydrogen atmosphere. With respect to the hydrogen absorption site in this product, the hydrogen atoms are chemisorbed as the stable C−H bonds at the graphene edges and defects induced by ball−milling (Fukunaga & Itoh et al., 2004, Itoh & Miyahara et al., 2003, Ogita & Yamamoto et al., 2004, Smith & Miyaoka et al., 2009). On the other hand, this product needs a high temperature of more than 700 °C to release the hydrogen, and desorbs a considerable amount of hydrocarbons, such as methane (CH4) and ethane (C2H6), together with hydrogen. Furthermore, it is quite difficult to recharge this product with hydrogen under moderate conditions of pressure and temperature for the on−board application. In order to improve the hydrogen absorption and desorption properties of CnanoHx, Ichikawa et al. have paid attention to the chemical reaction between NH3 and LiH, which is one of the elementary reactions in the Li−N−H system (Ichikawa & Hanada et al., 2004). This reaction proceeds even at room temperature and the hydrogen is released, indicating that the stable ionic crystal of LiH is