Synthesis, characterization and hydrogen storage properties of Mm(Ni,Co,Mn,Al)5 alloy

The hydrogenation characterizations of the hydrogen storage alloy MmNi4.22Co0.48Mn0.15Al0.15 (Mm= mischmetal), and the effect of hydrogenation/dehydrogenation (H/D) cycling on its structural and morphological properties are investigated. The results indicate that after several H/D cycles the alloy was pulverized into fine particles, but it kept its hexagonal CaCu5-type structure. The pressure-composition (PC) isotherms for hydrogen absorption/desorption and absorption kinetic were measured in temperature range of 293-338 K. The absorption plateau pressures were determined to be ~ 0.51, 1.22 and 2.49 bar at 293, 313 and 33 K respectively, with a maximum hydrogen storage capacity of about 5.78 at 293 K. The enthalpy (H), entropy (S) and the activation energy of reactions (Ea) were also calculated. The results show that the hydrogenation reaction rate increases with an increase in the operating temperature or pressure. The Jander diffusion (JDM) and Johnson-Mehl-Avrami (JMA) models were employed and the kinetic of hydrogenation was analyzed in detail for hydriding reaction (rate controlling steps) mechanism. The obtained results indicate that the MmNi4.22Co0.48Mn0.15Al0.15 alloy has potential to be suitable for use in practical applications.