Composite materials based on light elements for hydrogen storage

In this paper, we review our recent experimental results on hydrogen storage properties of light elements Li, C and Mg based nano-composite materials. The results are summarized as follows: In the Li-N-H system, such as the ball milled 1:1 mixture of Li amide and Li hydride containing a small amount of TiCl₃ (1 mol%), a large amount of hydrogen (∼6 mass%) is absorbed and desorbed in the temperature range from 150 to 250°C with good reversibility and high reaction rate. Furthermore, in the ball milled mixture of 3Mg(NH₂)₂ and 8LiH, ∼7 mass% of hydrogen is reversibly stored in the temperature from 140 to 220°C, indicating one of the suitable hydrogen storage materials. In graphite containing a small amount of nanometer sized Fe (∼2 at.%), a large amount of hydrogen (∼7 mass%) is chemisorbed by ball milling for 80 h under less than 1 M Pa of H ₂-gas pressure. However, the chemisorbed hydrogen capacity decreases with increase in the milling pressure for the 80 h ball milled graphite (down to ∼4.1 mass% at 6 MPa), while the physisorbed hydrogen capacity in graphite increases with increase in the milling pressure, reaching up to 0.5-1.0mass% at 6 MPa. Unfortunately, the desorption temperature of chemisorbed hydrogen is higher than 300°C. Therefore, some break-through is necessary for the development of carbon-based materials as one of the hydrogen storage systems. On the other hand, some nano-composite Mg catalyzed by Ni nano-particle or Nb oxide reveals superior reversible hydrogen storage properties: ∼6.5mass% of hydrogen is reversibly stored in the temperature range from 150 to 250°C. Especially, the Nb metals uniformly dispersed in nanometer scale on the surface of MgH₂, which was produced by reduction of Nb₂O ₅, is the best catalyst we have studied so far. Thus, it seems that some Mg nano-composites catalyzed by nano-particles of d-electron transition metals is acceptable for practical applications.