Influence of aluminium content on the dynamic characterstics of mischmetal based hydrogen storage alloys

Mischmetal (Mm) nickel (Ni) alloys with different aluminium (Al) contents are tested for hydrogen storage characteristics under dynamic operating conditions. The compositions considered are MmNi5, MmNi4.7Al0.3, MmNi4.5Al0.5, MmNi4.2Al0.8 and MmNi4Al. Pressure concentration isotherms (PCI), plateau slopes and absorption rates are measured. Activation energies are calculated by fitting Arrhenius equation to the absorption data. As expected, the plateau pressure of MmNi5 decreased with increase in Al content. Apparent reaction enthalpy values (as distinct from enthalpy of formation calculated from static measurements), plateau slope, and activation energies increased, and absorption rate decreased with increase in Al content. The study confirms MmNi4.5Al0.5 as a preferred candidate for reversible hydrogen storage at room temperature with nominal supply pressure and fast kinetics.