Light metal amide/imide systems for on-board 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 similar to 6 mass% of hydrogen in temperature ranges from 150 to 250 degrees C under a He gas flow. However, the hydrogen desorption (H-desorption) temperature at P-H2 = 0.1 MPa was 250 degrees C which is too high for on-board applications. We investigated the H-desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H-2 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 similar to 7 mass% H-2 in range from 120 to 200 degrees C and the H-desorption pressure was higher than 5 MPa at 200 degrees 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.