Investigation on Mg - Ni Alloy as Hydrogen Storage Material and the Effect of Alloy Element on its Properties Mg and Mg-based alloys have been considered as the negative materials in nickel/metal-hydride (Ni/MH) batteries because of high hydrogen storage capacity, light weight and low cost. However, their poor kinetic and low cyclic life makes a wide study necessary. Unlike the recent AB 5 -type and AB 2 -type hydrogen storage alloys such as LaNi 5 and ZrMn 2 , nanocrystalline Mg-based alloys provide the highest initial discharge capacity with no need to activation process. Initial discharge capacity and kinetic of charge and discharge can be improved by increasing the lattice defects, creating fresh surfaces and refining the grains for example through mechanical alloying (MA). In this paper, the effect of substitution of Ni in Mg 2 Ni alloy compound with Co was studied. The nominal Mg 2 Ni 1-x Co x (x=0, 0.1, 0.2, 0.3) alloys were synthesized by two method of MA. The speed of process and the ball-to-powder weight ratio were selected 500 rpm and 20:1, respectively. In method 1, adding Co to Ni and Mg powder at the first moments of MA process was led to the formation of some new phases (such as Mg 2 Co, MgCo 2 ) instead of a complete substitution. But, in method 2, powder of Co was added after formation of Mg 2 Ni phase and was milled again for 30 hours. In this way, the most of Co atoms dissolved in Mg 2 Ni lattice. Characterization of powder was carried out by SEM, TEM and X-ray diffractometer. Electrochemical measurements were used to evaluate the effects of Co substitution on storage capacity, cyclic life and kinetic of adsorbsion/desorption of Mg 2 Ni. The results showed that adding Co by method 1 causes to decrease hydrogen storage capacity, but improves cyclic life. Because, cobalt oxide serves as a protective film on the alloy surface, prevents the electrode from further corrosion in KOH solution and enhances the cycle performance. In contrast, adding Co by method 2 increase hydrogen storage capacity. Dissolved Co atoms in this method led to increase the lattice parameter. So, the diffusion rate of hydrogen in bulk of alloy improved and discharge capacity increased. The surface of the alloy particles after 8th charge/discharge cycle changes from smooth to rough. The kinetic of reactions was also characterized by electrochemical impedance spectroscopy
