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

This paper investigates hydrogenation characterizations and effect of hydrogenation/dehydrogenation (H/D) cycling on the structural and morphological properties of MmNi4.22Co0.48Mn0.15Al0.15 (Mm=mischmetal) alloy. The alloy was prepared by arc melting method and it was re-melted several times under argon atmosphere. The results indicate that after several H/D cycles, the alloy is pulverized into fine particles, however, it keeps its hexagonal LaNi5-type structure. The pressure-composition isotherms of H/D reactions and absorption kinetics were measured in the temperature range 293-338K. The absorption plateau pressures were determined to be ~0.51, 1.22 and 2.49 bar at 293, 313 and 333K respectively, with a maximum hydrogen storage capacity of about 5.78 at 293K. The enthalpy (?H), entropy (?S) and the activation energy of reactions (Ea) were also calculated. The results show 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 results indicate that the MmNi4.22Co0.48Mn0.15Al0.15 alloy has potential for use in practical applications.