Nanoscale Mg-based materials for hydrogen storage

Hydrogen storage materials research has entered a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, we study experimentally the structure and electrochemical properties of nanocrystalline Mg2CuMg2Cu, (Mg1-xMx)2Ni(Mg1-xMx)2Ni alloys, as well as Mg2Cu/M′Mg2Cu/M′ and (Mg1-xMx)2Ni/M′(Mg1-xMx)2Ni/M′ (x=0x=0, 0.5; M=AlM=Al, Mn; M′=CM′=C, Pd) nanocomposites. These materials were prepared by mechanical alloying (MA). In the nanocrystalline Mg2CuMg2Cu powder, discharge capacity up to 26mAhg-1 was measured. It was found that nickel substituting copper in Mg2Cu1-xNixMg2Cu1-xNix alloy greatly improved the discharge capacity of studied material. In nanocrystalline Mg2NiMg2Ni powder, discharge capacities up to 100mAhg-1 were measured. Additionally, it was found that Al or Mn substituting magnesium in Mg2-xMxNiMg2-xMxNi alloy greatly improved the discharge capacity of studied material. For example, in nanocrystalline Mg1.5Mn0.5NiMg1.5Mn0.5Ni powder, discharge capacities up to 241mAhg-1 were measured. On the other hand, mechanically coated Mg-based alloys with graphite or palladium have effectively reduced the degradation rate of the studied electrode materials. Finally, the properties of nanocrystalline alloys and their nanocomposites are compared to that of microcrystalline samples. The substitution of Mg by transition metals leads to significant modifications of the shape and width of the XPS valence band of the nanocrystalline as well as microcrystalline samples. Especially, the valence bands of the MA nanocrystalline alloys are considerably broader compared to those measured for the microcrystalline samples. Results also showed that the strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence their hydrogenation properties.