MgH_2 was mechanically milled with nano-Ni (nano-Ni/Al_2O_3/C) and Ni catalysts (Ni), yielding Mg-based nano-composite materials. X-ray and TEM measurements revealed that nano-Ni particles, which are 6-20 nm size, were dispersed in the MgH_2 matrix. The nano-composite material with nano-Ni/Al_2O_3/C showed excellent properties as compared to that with Ni, a ball-milled MgH_2 and the mixture of MgH_2 and nano-Ni/Al_2O_3/C in terms of the H_2 desorption and absorption. The nano-composite material with nano-Ni/Al_2O_3/C desorbed H_2 of 4.9-5.8 wt% at 423-473 K, while the mixture could not desorb H_2 at the temperature. The H_2 absorption capacity at 9 MPa and room temperature in 6 hr. increased from below 0.1 wt% for the mixture to 5.0 wt % for the nano-composite material, approaching a maximum of 6.5 wt% in 70 hr. The catalyst activity was improved with decreasing Ni size. The improved kinetics is indicated by the small activation barrier, the short diffusion path length and the high driving force.
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