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In order to clarify the reaction mechanism of the hydrogenation process in the Li2NH system, we have observed the partially hydrogenated samples by transmission electron microscopy (TEM). From the TEM images of partially hydrogenated samples, it was shown that the LiNH2 phase was located between Li2NH phase, which was almost at the center of the particle, and LiH particles with the size of 100 nm. This result indicated that LiH was generated and grew up at the outside of the complex particle of Li2NH and LiNH2 with Li moving from the center to the surface of the particle.

Two kinds of nano-structural graphite were prepared from graphite powder by ball-milling under hydrogen or argon atmosphere, where each product is denoted as HG or AG, respectively. And then, non-treated graphite powder (G), HG or AG was milled with Li to synthesize lithium-graphite intercalation compounds, which are denoted as Li-G, Li-HG and Li-AG, respectively. In XRD profiles of Li-G and Li-HG, it was revealed that the peaks correspond to LiC6 and LiCl2, while no peaks were observed in the case of Li-AG. However, similar thermal decomposition reactions were observed for all compounds at about 450°C under an inert gas, indicating that Li-AG includes the lithium-carbon (Li-C) compound. After heat treatment under hydrogen pressure, the formation of LiH was confirmed by XRD measurements for all compounds, indicating that the intercalated lithium reacted with hydrogen. Furthermore, all hydrogenated compounds desorbed hydrogen with heating up to 500°C. In addition, the hydrogenated Li-HG and Li-AG formed lithium carbide (LiC) after hydrogen desorption. On the other hand, in the case of the hydrogenated Li-G, it is confirmed that not only LiC but also LiC12 were formed after dehydrogenation.

Hydrogen storage properties of the ball-milled mixtures composed of hydrogenated nano-structural graphite (CnanoHx) and lithium hydride (LiH) were examined from thermodynamic and structural points of view, where the CnanoHx was synthesized by ball-milling graphite powder under hydrogen atmosphere. Since hydrogen atoms in the CnanoHx and LiH are strongly bonded with each host atom, heating up to >600°C is necessary to release hydrogen from each solid product. The CnanoHx and LiH composites desorbed hydrogen and hydrocarbon gases below 400°C with about 9.4 mass% weight loss. The 2: 1 composite of the CnanoHx and LiH reversibly stored hydrogen with an effective capacity of about 4.5 mass % at 350°C. No evident peaks were observed in XRD profiles after the dehydrogenation, indicating that the nano-structural feature remained after dehydrogenation due to formation of (CLi) nano clusters. However, only the LiH phase was crystallized after rehydrogenation at 350°C. From these results, it is concluded that hydrogen in the composites is destabilized by a novel interaction between the CnanoHx and LiH in a nanometer scale, and is desorbed at lower temperatures than each of the components. Therefore, this Li-C-H system can be recognized to be a new family of hydrogen storage materials.

We have investigated hydrogen storage materials composed of light elements such as MgH2, Metal-N-H, Metal-C-H and Metal-B-H (Metal=Li, Na, Mg, Ca) for automobile applications. Particularly, our recent research results on H-storage properties of the metal-N-H system are reviewed in this paper. The mixture of LiH and LiNH2 catalyzed with titanium compound desorbed ~6 mass% of hydrogen in temperature ranges from 150 to 250°C under a He gas flow. However, the hydrogen desorption (H-desorption) temperature at PH2=0.1 MPa was 250°C which is too high for on-board applications. We investigated the H-desorption mechanism in the reaction from LiH+LiNH2 to Li2NH+H2 by Thermal Desorption Mass Spectroscopy (TDMS) and Fourier Transform Infrared (FT-IR) analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results indicated that the H-desorption reaction progresses through two-step elementary reactions mediated by ammonia. On the basis of the ammonia mediated model, we successfully designed a new Li-Mg-N-H system composed of 8LiH and 3Mg(NH2)2. The mechanically milled composite desorbed ~7 mass%H2 in range from 120 to 200°C and the H-desorption pressure was higher than 5 MPa at 200°C, being suitable for on-board applications. Moreover, for understanding the role of titanium compounds as catalysts, the chemical state of the titanium compounds doped in the mixture was examined by X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements.